Hi Dave,

OK, thanks for walking me through this, one step at a time; appreciate
your help. If I've got this right, with such a large number of ovens
in use, we can reasonably assume no more than one-third of these
elements will be energized at any one time, regardless of the length
of each "on" cycle, be it 1/120 of a second or one to two minutes. I
had envisioned this load would be more variable at these longer cycles
and that we could, in effect, "chop peak" and "fill valley" by slicing
it into increasingly finer increments. Perhaps with just 100 ovens
that might be possible, but with 100,000 it would make no discernable
difference. That seems to makes sense.

In terms of ensuring high power quality (and reduced appliance cost),
one alternative might be dual-wattage elements. One high power
element for quick start-up and a second, low-density companion that
would maintain the oven at its set temperature (similar to how some
hot water tanks operate). If, for whatever reason, this secondary
element couldn't keep up (e.g., repeated door openings), it would
temporarily throw things back to the primary element, then once again
resume command; it would still cycle on and off as required, but it
would be sized to more or less to run continuously and minimize any
further need for its bigger brother.

While such an arrangement might not reduce peak demand, it could still
offer some benefits in terms of reducing the strain on the local
distribution system. Would that sound reasonable?

Cheers,
Paul

On Tue, 13 Feb 2007 01:53:38 +0000 (UTC), (Dave
Martindale) wrote:

No, the load on the utility (averaged over 100000 houses) is still 100
MW, no matter whether the ovens elements are cycling on and off every
1/120 sec or every 2 minutes. Because of the randomness of the
mechanical thermostat open/close, you'll never get more than about 1/3
of those ovens on at any one instant.

In fact, I will bet that the utility would be mightily *unhappy* to
have 100 MW of load all switch on for the last 1/3 of every half-cycle
of the line. That will distort the waveform on the grid.

Thus, if we can effectively reduce peak demand from by just ***ONE***
MW, the capital savings to the utility is a minimum of $367,000.00 US
($436,730.00 CDN); at 67 MW, the savings amount to $CDN 29.3 million.

But you haven't reduced peak demand at all. In fact, you've increased
it slightly due to losses in the triacs of the electronic control, and
distorted the current waveform.

Dave

"Sam E" wrote in message
...
On Mon, 12 Feb 2007 22:13:12 GMT, "JoeSpareBedroom"
wrote:

"Dave Martindale" wrote in message
...
"Bill" writes:

Well I installed a woodstove and tried cooking on it. I cooked eggs and
noticed they came out perfect!

When I cook eggs on my electric range, they will tend to stick to the
bottom
of the pan or overheat / underheat.

Anyway the difference between cooking on the woodstove and on my
electric
range is amazing! The difference of course is the "steady heat" of the
wood
stove as opposed to the "on/off" heat of the electric range.

There are many possible explanations for this. Perhaps you just pay
more attention when cooking on the woodstove. Perhaps the large flat
iron cooking surface of the wood stove heats your pan more evenly than a
coil element on the electric stove. Or maybe the cyclic temperature
variations do matter. You haven't provided any evidence for the latter
explanation.

It would be interesting to measure the amount of temperature swing at
the surface of your electric element as the element cycles on and off.
Then measure it on the inside surface of the pan. I'll bet the
temperature range is not very large.

Dave


Coming to conclusions while missing 90% of the pertinent information is a
great American pastime, apparently.


That's normal. It's a lot easier to ignore 90% or more of what you
heard, and make up stuff to fill the gap.


"Contrary to recorded weather data from 7 independent scientifical sources,
and reports from over 4,300 farmers, we has conclusiatious evidences that Mr
Al Qaeda is responsiblatious for the shortage of them brocollis from
California during the last past two or couples of weeks". - George W. Bush

In article ,
says...
[...]

In terms of ensuring high power quality (and reduced appliance cost),
one alternative might be dual-wattage elements. One high power
element for quick start-up and a second, low-density companion that
would maintain the oven at its set temperature (similar to how some
hot water tanks operate). If, for whatever reason, this secondary
element couldn't keep up (e.g., repeated door openings), it would
temporarily throw things back to the primary element, then once again
resume command; it would still cycle on and off as required, but it
would be sized to more or less to run continuously and minimize any
further need for its bigger brother.

Many (most?) electric ovens already do this. The pre-heat stage
energizes both the bottom and broiler elements. The idea, though, is
to pre-heat the oven faster, not to help the power company balance
its load.

On Feb 12, 10:38 pm, Paul M. Eldridge
wrote:
Hi Dave,

OK, thanks for walking me through this, one step at a time; appreciate
your help. If I've got this right, with such a large number of ovens
in use, we can reasonably assume no more than one-third of these
elements will be energized at any one time, regardless of the length
of each "on" cycle, be it 1/120 of a second or one to two minutes. I
had envisioned this load would be more variable at these longer cycles
and that we could, in effect, "chop peak" and "fill valley" by slicing
it into increasingly finer increments. Perhaps with just 100 ovens
that might be possible, but with 100,000 it would make no discernable
difference. That seems to makes sense.

In terms of ensuring high power quality (and reduced appliance cost),
one alternative might be dual-wattage elements. One high power
element for quick start-up and a second, low-density companion that
would maintain the oven at its set temperature (similar to how some
hot water tanks operate). If, for whatever reason, this secondary
element couldn't keep up (e.g., repeated door openings), it would
temporarily throw things back to the primary element, then once again
resume command; it would still cycle on and off as required, but it
would be sized to more or less to run continuously and minimize any
further need for its bigger brother.

While such an arrangement might not reduce peak demand, it could still
offer some benefits in terms of reducing the strain on the local
distribution system. Would that sound reasonable?

Cheers,
Paul


No, it doesn't sound reasonable, because, as Dave and I have
repeatedly tried to explain to you:

1 - In your example, with 100,000 stoves cycling on and off randomly,
the load is already randomly distributed, at least after the initial
heat up period of 15 mins or so. You can use one element, two
elements, or 300 elements and it doesn't do anything to affect the
peak load or power distribution as long as the heating elements are
the same size and the duty cycle is the same.

2 - Assuming a lot of ranges/ovens come on around 5-7pm, if you wanted
to reduced the load at this time, you can do it by either:
a - Using smaller heating elements
b - Keeping the duty cycle from being 100% during the heat up period.

Either of those will reduce the heating capacity of the stove.
Option a permanently and option b during startup, meaning the oven
will take longer to get hot.

And I think the problem you're trying to solve here doesn't exist to
begin with. There are generally two problems that utilities are
concerned with regarding peak demand. One is they need a generator
and system big enough to handle the peak, requiring more capital
investment. And/or they need to buy power from somewhere else during
peak time and that power may cost more. AFAIK, none of these issues
typically occurs at 5-7PM due to home ranges. Around here, they
typically occur during very hot summer afternoon periods, when
commercial use is high and everyone has their home AC units running,
etc. Most people don't have their ovens going then, because it's hot
and they aren't planning on making a roast turkey to make the house
even hotter.






On Tue, 13 Feb 2007 01:53:38 +0000 (UTC), (Dave



Martindale) wrote:
No, the load on the utility (averaged over 100000 houses) is still 100
MW, no matter whether the ovens elements are cycling on and off every
1/120 sec or every 2 minutes. Because of the randomness of the
mechanical thermostat open/close, you'll never get more than about 1/3
of those ovens on at any one instant.

In fact, I will bet that the utility would be mightily *unhappy* to
have 100 MW of load all switch on for the last 1/3 of every half-cycle
of the line. That will distort the waveform on the grid.

Thus, if we can effectively reduce peak demand from by just ***ONE***
MW, the capital savings to the utility is a minimum of $367,000.00 US
($436,730.00 CDN); at 67 MW, the savings amount to $CDN 29.3 million.

But you haven't reduced peak demand at all. In fact, you've increased
it slightly due to losses in the triacs of the electronic control, and
distorted the current waveform.

Dave- Hide quoted text -

- Show quoted text -

Hi Mike,

My apologies for asking this, but is this something relatively new or
has it always been this way? I don't have a lot of experience with
electric ovens because I've always used gas, but this time around I
had to settle for a combination unit because an all gas version wasn't
available.

I did experiment with mine just now and here's what I found. There
are basically three heating options I can choose: "bake", "broil" and
"convection" and as far as I can tell all three work independently of
each other. When I turn on "broil" I can safely leave my hand on the
lower bake element and it remains cool to the touch; likewise, the
broiler never comes on when I select "bake" and neither of these two
elements are used when I pick "convection" (that appears to be a third
element hidden somewhere inside the oven's back wall).

In any event, this second element arrangement I had imagined would be
used in bake (or convection) mode and would be very low wattage --
just the minimum required to maintain a steady operating temperature
and effectively "lock out" the primary element that is used during the
initial warm-up. It may be that we only need 500 or 1,000-watts
during this extended cooking phase to keep things moving along.

There doesn't appear to be any real benefit in terms of utility-wide
peak shaving (sorry to say it took a couple blows to the head to drive
that point home), but there may be some benefit in terms of reducing
the strain on the local distribution system. If, for example, my
neighbour and I share the same pole transformer and our ovens both
cycle on at roughly the same time (and we can safely assume there will
be at least some overlap during their operation), the combined load of
these two appliances might be 6 or 8 kW. However, if we don't turn
our ovens on within ten minutes of each other (i.e., during that
initial warm-up phase), with this dual wattage arrangement, our
combined load may never exceed 4 or 5 kW and our steady-state
operation may drop to just 1 or 2 kW.

It would seem that as we move closer to the point of use (i.e., from
sub-station/feeder to local line, to individual pole transformer) the
potential benefits to the utility become increasingly more attractive.
And in the case of a large condo or apartment complex, I imagine the
potential load reduction could result in some capital savings (i.e.,
smaller service requirements) and perhaps reduced monthly demand
charges. Ideally, if dual wattage elements added an extra $50.00 to
the cost of each appliance, this extra cost would be fully offset by
these other capital savings and any additional savings in terms of
reduced monthly demand charges would simply add extra gravy in the
pot.

Cheers,
Paul

On Tue, 13 Feb 2007 06:43:18 -0600, Mike Hartigan
wrote:

Many (most?) electric ovens already do this. The pre-heat stage
energizes both the bottom and broiler elements. The idea, though, is
to pre-heat the oven faster, not to help the power company balance
its load.

On Feb 13, 10:14 am, Paul M. Eldridge
wrote:
Hi Mike,

My apologies for asking this, but is this something relatively new or
has it always been this way? I don't have a lot of experience with
electric ovens because I've always used gas, but this time around I
had to settle for a combination unit because an all gas version wasn't
available.

I did experiment with mine just now and here's what I found. There
are basically three heating options I can choose: "bake", "broil" and
"convection" and as far as I can tell all three work independently of
each other. When I turn on "broil" I can safely leave my hand on the
lower bake element and it remains cool to the touch; likewise, the
broiler never comes on when I select "bake" and neither of these two
elements are used when I pick "convection" (that appears to be a third
element hidden somewhere inside the oven's back wall).

In any event, this second element arrangement I had imagined would be
used in bake (or convection) mode and would be very low wattage --
just the minimum required to maintain a steady operating temperature
and effectively "lock out" the primary element that is used during the
initial warm-up. It may be that we only need 500 or 1,000-watts
during this extended cooking phase to keep things moving along.

There doesn't appear to be any real benefit in terms of utility-wide
peak shaving (sorry to say it took a couple blows to the head to drive
that point home), but there may be some benefit in terms of reducing
the strain on the local distribution system. If, for example, my
neighbour and I share the same pole transformer and our ovens both
cycle on at roughly the same time (and we can safely assume there will
be at least some overlap during their operation), the combined load of
these two appliances might be 6 or 8 kW. However, if we don't turn
our ovens on within ten minutes of each other (i.e., during that
initial warm-up phase),

And how are you going to coordinate not turning on ovens with your
neighbor at the same time? Use a flag hanging out the window?
This is simple physics. If you expect to limit the power, then the
oven isn't going to heat up as fast. And that could be solved just as
simply by just putting in an oven element that was say 25% smaller to
begin with. You really only need the max when you're trying to take
the oven from cold to operating temp. After, that, it cycles anyway
and could easily get by with probably 1/2 the existing element
capacity. Of course the downside is that instead of waiting 15 mins
for the oven to heat up, now you're gonna wait a half hour and few
people will put up with that to solve a problem that doesn't exist to
begin with.



with this dual wattage arrangement, our
combined load may never exceed 4 or 5 kW and our steady-state
operation may drop to just 1 or 2 kW.

You still don't get it. The amount of energy that it takes to
operate an oven is independent of whether you have 1 element or 40.
You could have a 4000 watt element on for 15 mins or a 2000 watt
element for 30 and it uses exaclty the same amount of energy. You
can't just decrease the steady state amount of power and have the oven
be just as hot.





It would seem that as we move closer to the point of use (i.e., from
sub-station/feeder to local line, to individual pole transformer) the
potential benefits to the utility become increasingly more attractive.
And in the case of a large condo or apartment complex, I imagine the
potential load reduction could result in some capital savings (i.e.,
smaller service requirements) and perhaps reduced monthly demand
charges.

You're focusing on one small nit here and ignoring everything else.
Condos typically have all kinds of loads, AC, heat pumps, furnace
blowers, electric water heaters, etc. Reducing some oven power
isn;t going to be a big factor that now means smaller gauge wire or a
smaller transformers can be used. And to reduce the oven loads,
what you fail to realize is that you are either asking people to:

a - Wait longer for their ovens to warm up, because the power into
them is being reduced to limit peak during start up.

b - Hang flags out the window so unit A can't start cooking dinnner at
the same time as unit B

Who is going to put up with that?






Ideally, if dual wattage elements added an extra $50.00 to
the cost of each appliance, this extra cost would be fully offset by
these other capital savings and any additional savings in terms of
reduced monthly demand charges would simply add extra gravy in the
pot.

Cheers,
Paul



What reduced monthly charges? You still need the same amount of
energy going into the oven to make it hot. In fact, your idea could
take MORE energy. By reducing the max power, its' going to take
longer for the oven to get to 400 deg. While it's talking the extra
10 or 15 mins, heat is being lost out of the oven throught the walls,
or even worse, people opening the door to see what's going on.







On Tue, 13 Feb 2007 06:43:18 -0600, Mike Hartigan



wrote:
Many (most?) electric ovens already do this. The pre-heat stage
energizes both the bottom and broiler elements. The idea, though, is
to pre-heat the oven faster, not to help the power company balance
its load.- Hide quoted text -

- Show quoted text -

On Tue, 13 Feb 2007 01:53:38 +0000 (UTC), (Dave
Martindale) wrote:

Paul M. Eldridge writes:

Nowhere did I say these ovens would operate at reduced power upon
start-up. Each could continue to operate at full power for as long it
takes to come up to temperature, then drop to the lowest wattage
required to maintain a constant set temperaturer; if the oven element
is rated at 3,000 watts and it normally cycles on one-third of the
time, then it's fair to say a constant 1,000 watts is all that's
needed to maintain a steady temperature from this point forward.
There would be absolutely no inconvenience to the consumer whatsoever
and the utility would still benefit from reduced aggregate load.

No, the load on the utility (averaged over 100000 houses) is still 100
MW, no matter whether the ovens elements are cycling on and off every
1/120 sec or every 2 minutes. Because of the randomness of the
mechanical thermostat open/close, you'll never get more than about 1/3
of those ovens on at any one instant.


There's nothing to prevent all of then from switching on at the same
moment. The chance of this happening at times will be nearly 100%.

Peak demand is still 300MW, but most peaks can be expected to be very
short. Well within the utility's capability.

In fact, I will bet that the utility would be mightily *unhappy* to
have 100 MW of load all switch on for the last 1/3 of every half-cycle
of the line. That will distort the waveform on the grid.

Thus, if we can effectively reduce peak demand from by just ***ONE***
MW, the capital savings to the utility is a minimum of $367,000.00 US
($436,730.00 CDN); at 67 MW, the savings amount to $CDN 29.3 million.

But you haven't reduced peak demand at all. In fact, you've increased
it slightly due to losses in the triacs of the electronic control, and
distorted the current waveform.

Dave
--
Mark Lloyd
http://notstupid.laughingsquid.com

"Unlike biological evolution. 'intelligent design' is
not a genuine scientific theory and, therefore, has
no place in the curriculum of our nation's public
school classes." -- Ted Kennedy

On Tue, 13 Feb 2007 02:04:41 GMT, "wff_ng_7"
wrote:

"ms_peacock" wrote:
I've had numerous electric stoves over the years and the elements don't go
on and off on any of them. They already use a "dimmer switch." The heat
is constant at whatever setting you put the dial.

I had one stove that had an element that was thermostatically controlled
and it did vary the heat. But it didn't just go off and on, as the temp
of the food came up the element would lower the heat output to maintain
the temp. I still miss that stove, it also had an oven and a half.

In reality, those electric stoves were going on and off the whole time, and
you never noticed! If you have a very quiet kitchen and you listen very
carefully, you can hear the switch turn the burner off and on. The "dimmer
switch" is adjusting how long the "on" time is versus the "off" time. The
owner's manual on my 1982 GE electric range even mentioned the noise the
switch made in the troubleshooting section, to put to rest the minds of
people who noticed the sound.

Even dimmer switches for lights are in a way turning the light on and off to
adjust the light intensity. The dimmer switch is varying the amount of time
the light bulb filament is turned on versus turned off. Only it is happening
60 times a second versus every several seconds as on an electric stove
burner. The principle is basically the same, but on dimmers the controls are
solid state electronics, while on a stove burner the controls are
mechanical. It would be costly to make a solid state electronic control to
handle the power required for a surface burner. Most light dimmers are 300
watts capacity. A surface burner is about 2,500 watts.

This cycling of the burner is different than thermostatic control. What it
is doing is keeping the burner on for a percentage of the total time, giving
a proportional heat output, regardless of how hot the pan ends up getting.
There are thermostatically controlled surface burners out there, but they
are not that common.

The old electric stove my grandmother had had one burner that was
thermostatically controlled. The other burners didn't have knobs, but
rows of buttons (labeled something like "high", 'med-high", "medium",
"med-low", "low", "simmer", "warm", "off"). BTW, it also had a 120V
outlet on it. I guess people usually didn't have enough countertop
outlets then.
--
Mark Lloyd
http://notstupid.laughingsquid.com

"Unlike biological evolution. 'intelligent design' is
not a genuine scientific theory and, therefore, has
no place in the curriculum of our nation's public
school classes." -- Ted Kennedy

Ï "Bill" Ýãñáøå óôï ìÞíõìá
...
Does anyone manufacture a "variable heat" electric range, where when you
select the heat setting, it would have a constant heat at a certain
temperature? (Like you can do with a gas range...)

This would be sort of like a dimmer switch for a light where you can
adjust
how much light is output from the bulb.

The way electric ranges work now is they go on and off, on and off.

Less heat means the "burner" goes on for a little while, then off for
quite
awhile. Then with more heat, the "burner" is on for a long time, then off
for a little amount of time.

Well, traditionally stoves (or ranges) here in EU (certainly in Greece) have
3 elements for each hob, and a dial for each hob, that is numbered from 0 to
3 with 1/2 subdivisions(thus 0-1/2-1-11/2...)and the three elements are
turned on and off, respectively.So, for full heat, all 3.For 1/2 set.the
smallest one etc.
With a gas range, you can adjust the heat so it is constant - no off and
on.
Seems they could do this with an electric range as well....



--
Tzortzakakis Dimitrios
major in electrical engineering
mechanized infantry reservist
dimtzort AT otenet DOT gr

Ï "Sharon" Ýãñáøå óôï ìÞíõìá
...
In article , Peter A
writes:
In article ,
says...
Does anyone manufacture a "variable heat" electric range, where when
you
select the heat setting, it would have a constant heat at a certain
temperature? (Like you can do with a gas range...)

Why do you want this? The on/off technique works just fine in my
experience. The thermal mass of the burner and the pan even things out.
For example, when I am simmering a soup on low, the soup simmers at an
even, constant rate even though the element is on for 2 seconds then off
for 10 (more or less).

I'm with the OP. I was just commenting that this kind of thing would
be nice to my husband yesterday as I was making our week's dinners. We
have a
****-poor glass-topped electric stove. We think it's crappy because it
might
be low-watt, but don't know for sure. It can't boil a gallon of water
unless
it's tightly lidded, and even then it takes over a half an hour.
Last weekend, I was making a roux, and I really noticed how poor it is
there too. I had trouble getting the correct temp to cook the roux - it
cooked
fine while the burner was on, but all cooking stopped when the burner
cycled
off. We HATE the thing.
Well, excuse me, but in EU (at least in Greece)glass-topped electric stoves
are state-of-the-art, and very expensive, and very robust, and efficient,
too.The traditional stove has four hobs with the elements inside a ring of
iron (not steel),ours is glass-topped (or better, glass-ceramic, and cost
like, 700 euros, while a cheap iron one, like 400-500 euros.

Mark Lloyd writes:

Because of the randomness of the
mechanical thermostat open/close, you'll never get more than about 1/3
of those ovens on at any one instant.

There's nothing to prevent all of then from switching on at the same
moment. The chance of this happening at times will be nearly 100%.

There is no *technical device* to prevent them all from coming on at
once. But the simple statistics of the situation mean that almost all
the time, the number of ovens drawing power is 1/3 of the total plus or
minus a few percent. The odds of having all of them on at once is
incredibly tiny, so tiny that nobody would estimate or plan a peak load
based on that event.

Just look at the probability distribution of a 100,000 random events
each with a probability of being on of 1/3. The odds of having exactly
33,333 of them on is quite small, but the odds of somewhere between 30
and 35000 being on is nearly 1.

Dave

Mark Lloyd writes:

The old electric stove my grandmother had had one burner that was
thermostatically controlled. The other burners didn't have knobs, but
rows of buttons (labeled something like "high", 'med-high", "medium",
"med-low", "low", "simmer", "warm", "off").

My mother used to have a stove like that, but with knobs instead of
buttons. The elements in that stove were solid metal discs, not coils,
and they had multiple resistances built into them. The multi-position
switch achieved its different heat outputs by connecting various
combinations of terminals on the element to the 240 V line (it might
have used 120 V in some of the lower positions too; I no longer
remember).

So this stove did have several different continuous heat outputs,
without switching the element on and off. But modern stoves with
"infinite heat" controls are better. The coil element has low mass and
heats up (or cools down) faster, and you can have almost infinite
control over the amount of heat via the modulating control.

Most other writers in this thread are talking about infinite-heat
controls on a modern burner (or about the oven, which is
thermostatically controlled).

Dave

Hi Mark,

Thanks for volunteering to be my "human shield" on this one; maybe I
can use this time to lick a few of these self-inflicted wounds. ;-)

I suspect Dave's right. As you move into these very large numbers,
these loads, on a system-wide basis, tend to "self-balance" (no doubt
someone could quickly develop a computer model to confirm this). I'm
now thinking their greatest impact may be in terms of the local
distribution system, especially in predominately residential
neighbourhoods, as their relative size and random behaviour would hold
proportionately greater weight.

BTW, I picked 100,000 as our working number because NSP serves about
420,000 residential customers in this province and when you add in the
contributions of the smaller municipal utilities, that final tally
might reach upwards of 450,000 households; thus, we're expecting one
out of every four and a half households to be operating their ovens
during the suppertime peak and that estimate is likely to be a bit on
the high side, even though we Nova Scotians are your stereotypical
"supper-waiting-on-the-table-when-we-get-home-from-work" type.

And, hey, don't forget. I owe you one!

Cheers,
Paul

On Tue, 13 Feb 2007 12:55:47 -0600, Mark Lloyd
wrote:

There's nothing to prevent all of then from switching on at the same
moment. The chance of this happening at times will be nearly 100%.

Peak demand is still 300MW, but most peaks can be expected to be very
short. Well within the utility's capability.

On Mon, 12 Feb 2007 13:58:55 -0600, Chris Friesen
wrote:

Lou Decruss wrote:
On Mon, 12 Feb 2007 10:22:38 -0600, "Steve Barker"
wrote:

I think the main question here is why would anyone who does any serious
amount of cooking want an electric range to begin with? No real cooking can
be done on them.

Nonsense. Maybe 50 years ago, but today electric smoothtops have
just as much heat and control as gas.

There's an element of truth to it.

Nice pun.

Unless you use an induction element,
you cannot turn an electric element *down* quickly...it takes some time
for the heat in the element to dissipate.

I did mention smoothtop. The disk on a good quality pan will hold
heat longer than the glass. The element is on or off.

Also, commercial-grade gas ranges have heat outputs that far exceed
electric ranges (and indeed most residential gas ranges). This can be
useful for some types of cooking.

This is very true. But I don't think the OP was referring to a
commercial setting.

Lou

On Feb 12, 1:13 pm, Paul M. Eldridge
wrote:

On 12 Feb 2007 08:18:13 -0800, "dpb" wrote:

....
they're switching thermostats, too, ...
....

Hmm, good point. Because it uses a triac, I had assumed (incorrectly)
that it works pretty much like a standard household dimmer.
....

The triac is essentially just a bi-directional gated switching circuit
able to be controlled for either voltage polarity, so unless there is
more internally than that, it is essentially just an enhanced switch.
Less expensive dimmers are essentially the same, more expensive may
include other circuitry to modify the waveform and phase to provide
nearer a sinusoidal voltage, but I'd guess these thermostats don't
have that sophistication.

"Mark Lloyd" wrote:
The old electric stove my grandmother had had one burner that was
thermostatically controlled. The other burners didn't have knobs, but
rows of buttons (labeled something like "high", 'med-high", "medium",
"med-low", "low", "simmer", "warm", "off"). BTW, it also had a 120V
outlet on it. I guess people usually didn't have enough countertop
outlets then.

I know I've lived on one or more houses as a kid that had the push button
controls for the surface elements. The last one I remember my parents
replaced in 1965, so the stove must have been from the 1950s or even late
1940s. I think push button controls were gone by the mid 1960s.

I do have a 120V outlet on my gas stove, circa 1973. It comes in handy since
the nearest outlet on that side of the kitchen is six feet away. The house
was built in 1963.

"Tzortzakakis Dimitrios" wrote:
Well, traditionally stoves (or ranges) here in EU (certainly in Greece)
have
3 elements for each hob, and a dial for each hob, that is numbered from 0
to
3 with 1/2 subdivisions(thus 0-1/2-1-11/2...)and the three elements are
turned on and off, respectively.So, for full heat, all 3.For 1/2 set.the
smallest one etc.

They used to have burners with multiple elements here in the USA, but I
think they disappeared by the mid 1970s. I have a catalog of home and
apartment repair parts that lists a few replacement burners that have two
elements in the burner. The listings for these say for GE through 1975.

On Tue, 13 Feb 2007 20:06:58 +0000 (UTC), (Dave
Martindale) wrote:

Mark Lloyd writes:

Because of the randomness of the
mechanical thermostat open/close, you'll never get more than about 1/3
of those ovens on at any one instant.

There's nothing to prevent all of then from switching on at the same
moment. The chance of this happening at times will be nearly 100%.

There is no *technical device* to prevent them all from coming on at
once. But the simple statistics of the situation mean that almost all
the time, the number of ovens drawing power is 1/3 of the total plus or
minus a few percent. The odds of having all of them on at once is
incredibly tiny, so tiny that nobody would estimate or plan a peak load
based on that event.

Just look at the probability distribution of a 100,000 random events
each with a probability of being on of 1/3. The odds of having exactly
33,333 of them on is quite small, but the odds of somewhere between 30
and 35000 being on is nearly 1.

Dave

That's a very common statistics mistake. That gives you the
probability that all stoves are on at a particular time. What matters
is if all stoves are on at ANY time. There's a really big difference
there (as big as the difference between a millimeter and the width of
the galaxy).
--
Mark Lloyd
http://notstupid.laughingsquid.com

"Unlike biological evolution. 'intelligent design' is
not a genuine scientific theory and, therefore, has
no place in the curriculum of our nation's public
school classes." -- Ted Kennedy

On Feb 12, 5:52 pm, Paul M. Eldridge
wrote:
Hi Dave,

You're right. Clearly I was a couple neurons short in my thinking.
Let me see if I can move closer to the mark this time or, failing
that, embarrass myself further trying, as the case may be.

Our basic assumption is that these elements will operate 33 per cent
of the time, once the oven reaches its set temperature. But this
cycling will be random in nature, so our 100,000 ovens won't be
cycling "perfectly" in the sense that only one-third will be energized
at any one time. As the total number of ovens increase, I take it
we'll move ever closer to this ideal scenario, but it's probably fair
to say their combined load will fluctuate due to the unevenness in
this cycling. If we were to take a series of snap shots, we might
find that perhaps 50 per cent of these elements are energized, in
which case our load at that particular moment in time is closer to 150
MW and not the 100 MW I had stated.

The point of this exercise was to determine if it might be possible to
"smooth out" or flatten this load, so its net contribution to peak can
be lowered. If we have 100,000 ovens running at a constant 1 KW each
once they reach their set temperature, their combined load should
remain fairly close to 100 MW (slightly more to account for the higher
demand during start-up). Again, my thinking is that energy
consumption should remain constant (or perhaps slightly more due to
control related losses, as you suggest), but peak demand should be
reduced.

Your concerns related to power quality are well taken. There may be
ways to address that but I'm afraid I'm not very knowledgeable in this
area.

Please let me know if I'm a little more successful this time out, or
if I should be hiding my face. :-0

Cheers,
Paul


How exactly is peak demand reduced? Are you using flags to signal
the neighbor not to start dinner, while you're starting yours? Or
are we doing it by ripping out the X Kwatt element and putting in one
that is 30% smaller, so we can wait longer for the oven to heat up?

Do they teach any basic science or probability where you live? Or
are you just stupid?

Thanks. I had only a sketchy understanding of how this technology
works so this helps me considerably. And given these thermostats are
connected to resistance loads that should happily work with pretty
much anything you throw at them, they most likely lack any
sophisticated circuitry, as you suggest.

Which brings me to this question: these thermostats are becomming
increasingly popular and they do work extremely well from a consumer's
point of view, but I wonder what impact they may have on power
quality. I understand triacs can generate some nasty THD numbers; one
thing to dim a 60-watt incandescent bulb but 6,000 watts of electric
heat has to kick things up a notch or two. Any thoughts?

Cheers,
Paul

On 13 Feb 2007 15:00:43 -0800, "dpb" wrote:

The triac is essentially just a bi-directional gated switching circuit
able to be controlled for either voltage polarity, so unless there is
more internally than that, it is essentially just an enhanced switch.
Less expensive dimmers are essentially the same, more expensive may
include other circuitry to modify the waveform and phase to provide
nearer a sinusoidal voltage, but I'd guess these thermostats don't
have that sophistication.

On Wed, 14 Feb 2007 00:22:40 GMT, "wff_ng_7"
wrote:

"Mark Lloyd" wrote:
The old electric stove my grandmother had had one burner that was
thermostatically controlled. The other burners didn't have knobs, but
rows of buttons (labeled something like "high", 'med-high", "medium",
"med-low", "low", "simmer", "warm", "off"). BTW, it also had a 120V
outlet on it. I guess people usually didn't have enough countertop
outlets then.

I know I've lived on one or more houses as a kid that had the push button
controls for the surface elements. The last one I remember my parents
replaced in 1965, so the stove must have been from the 1950s or even late
1940s. I think push button controls were gone by the mid 1960s.

I do have a 120V outlet on my gas stove, circa 1973. It comes in handy since
the nearest outlet on that side of the kitchen is six feet away. The house
was built in 1963.

My grandmother got this stove in 1967, but it was used at the time.
I'm not sure exactly when it was made, but I'd guess around 1960.
--
Mark Lloyd
http://notstupid.laughingsquid.com

"Unlike biological evolution. 'intelligent design' is
not a genuine scientific theory and, therefore, has
no place in the curriculum of our nation's public
school classes." -- Ted Kennedy

On Tue, 13 Feb 2007 20:14:02 GMT, Paul M. Eldridge
wrote:

Hi Mark,

Thanks for volunteering to be my "human shield" on this one; maybe I
can use this time to lick a few of these self-inflicted wounds. ;-)

I suspect Dave's right. As you move into these very large numbers,
these loads, on a system-wide basis, tend to "self-balance" (no doubt
someone could quickly develop a computer model to confirm this).

Just how are these stoves interacting with each other? Something can't
happen unless there is actually some way for it to happen.

I'm
now thinking their greatest impact may be in terms of the local
distribution system, especially in predominately residential
neighbourhoods, as their relative size and random behaviour would hold
proportionately greater weight.

BTW, I picked 100,000 as our working number because NSP serves about
420,000 residential customers in this province and when you add in the
contributions of the smaller municipal utilities, that final tally
might reach upwards of 450,000 households; thus, we're expecting one
out of every four and a half households to be operating their ovens
during the suppertime peak and that estimate is likely to be a bit on
the high side, even though we Nova Scotians are your stereotypical
"supper-waiting-on-the-table-when-we-get-home-from-work" type.

And, hey, don't forget. I owe you one!

Cheers,
Paul

On Tue, 13 Feb 2007 12:55:47 -0600, Mark Lloyd
wrote:

There's nothing to prevent all of then from switching on at the same
moment. The chance of this happening at times will be nearly 100%.

Peak demand is still 300MW, but most peaks can be expected to be very
short. Well within the utility's capability.

When a stove is turned on, in generates a force which is distributed
on the power line. This force is known as AWASAF (Area-Wide Anti-Stove
Activation Force). The force generated by one stove is so small that
it can be detected only with sophisticated instruments, but it is
cumulative. So much so that if 35,000 stoves are on, there is so much
AWASAF present that there is only a 1% probability that anyone can
turn on another stove. This means that the chance of 100,000 stoves
being on at once is infinitesimal.

Recognize nonsense?

In article ,
"Matthew L. Martin" wrote:

wrote:


Exactly right. I think what the OP might want to look at is
an inductive cooktop. Expensive, and I'm not sure how they
achieve their variable heat settings, it may just be a duty cycle
switching type of control also, but they are supposed to have
very steady heat control.

The very few experiences I have had with induction cook tops showed me
that they have an on/off duty cycle that controls the heat production.

Matthew



The Luxine units claim to cycle at variable power so even though they
pulse off and on have greater range of control.

This is illustrated by them in their "chocolate test" where they melt a
bar of chocolate on the lowest setting of their burner vs that of a
competitor. The competitors seized the chocolate because it was pulsing
at 3500 Watts while the Luxine did not because it was pulsing at 700 W.
I think Viking markets Luxine induction units so that might be a place
to research this.

The above is found buried in the text of this article:

http://www.appliancedesign.com/CDA/A...0VgnVCM100000f
932a8c0____

I do not know it this applies to all units or the specific model in the
article. The OP might want to call Viking.

Roland

Mark Lloyd wrote:
The old electric stove my grandmother had had one burner that was
thermostatically controlled. The other burners didn't have knobs, but
rows of buttons (labeled something like "high", 'med-high", "medium",
"med-low", "low", "simmer", "warm", "off").

GE and Hotpoint ranges of the 50s and 60s typically had dual coil
surface units with five switched heat levels:

High -- 240V across both coils in parallel
Second -- 240V across one coil
Third -- 240V across both coils in series
Low -- 120V across one coil
Warm -- 120V across both coils in series

The "infinite level" time regulated controls in modern ranges require
much less wiring than the old style (less cost) and provide more user
control.

In article ,
says...
Hi Mike,

My apologies for asking this, but is this something relatively new or
has it always been this way? I don't have a lot of experience with
electric ovens because I've always used gas, but this time around I
had to settle for a combination unit because an all gas version wasn't
available.

When I was a kid (1960's), our old Frigidaire did this. Today (still
a kid only bigger), our new Thermador does this. (ours is dual fuel,
not because all gas wasn't available, but because we wanted the
advantages of dual fuel.)

I did experiment with mine just now and here's what I found. There
are basically three heating options I can choose: "bake", "broil" and
"convection" and as far as I can tell all three work independently of
each other. When I turn on "broil" I can safely leave my hand on the
lower bake element and it remains cool to the touch; likewise, the
broiler never comes on when I select "bake" and neither of these two
elements are used when I pick "convection" (that appears to be a third
element hidden somewhere inside the oven's back wall).

Our Thermador uses the upper for broiling (duh!), both the upper and
lower to preheat, switching to lower only to maintain temperature.
The convection setting has its own element.

[...]

On Mon, 12 Feb 2007 19:23:02 +0000 (UTC), (Dave
Martindale) wrote:


In fact, the electronic control wastes a bit of power in the switching
element, and consumes slightly *more* power than the non-electronic
oven. The triac control also distorts the utility waveform into
something that is less of a sine wave, which the utility also will not
like (the power factor gets worse, so they need higher current capacity
for the same billable watts).

How can that be? Any distortion by the triac appears beyond the
triac.

The electricity is billed by what goes through the meter, where it is
undistorted.

Mark Lloyd writes:

That's a very common statistics mistake. That gives you the
probability that all stoves are on at a particular time. What matters
is if all stoves are on at ANY time. There's a really big difference
there (as big as the difference between a millimeter and the width of
the galaxy).

It was a thought experiment. It's clearly *possible* for many more than
the average number of stoves to be on, but as this number increases the
probability of it happening gets vanishingly small - too small to worry
about.

If you want to be more precise, what really matters is the amount of
momentary extra load the system can tolerate (which in turn is a
function of the duration of the overload) and how often the randomness
of the load will cause it to exceed that overload threshold.

For example, if it turns out that increasing the load due to ovens from
100 MW to 150 MW for (say) 10 seconds is enough to take down part of
the distribution system by blowing a fuse or tripping a breaker, and
that event is likely to happen once a year on average, that's a
problem. If this event is likely once every thousand years, you can
ignore it.

Repeat this calculation for different load levels and durations. If
all possible random variation in oven load have virtually no effect on
grid reliability, then it can be ignored.

Dave

Hi Mike,

That's interesting. I wonder if your Thermador being dual fuel works
a little differently from your standard, run of the mill electric
range. We have a dual fuel Heartland Legacy, but the original range
(another Frigidaire) was all electric and the circuit supplying it is
equipped with a 40-amp breaker which, for the moment, I'll assume is
the norm. If we abide by the 80 per cent rule and assuming a full 240
volts, that gives us roughly 7,700 watts to work with.

A standard 30-inch range would have four cook-top burners and two oven
elements (bake and broil). I guess the question we must ask ourselves
is do we have enough capacity at 40-amps to supply all or a reasonable
combination of these elements without tripping the breaker? I realize
it's not likely we'd have all four burners turned on high and the oven
pre-heating, but it would be interesting to see just how far we might
push our luck.

Quoting from the Whirlpool's website, "Electric coil ranges usually
have two high-output elements (8-inch coils rated 2,600 Watts) and two
low-output elements (6-inch coils rated 1,500 Watts)." Using these
numbers, if all four burners were turned on high, our combined load
would be 8,200-watts (34 amps) or just slightly over 85 per cent of
our circuit's capacity.

Now I'm guessing a standard bake element is 3,000-watts and a broil
element is about the same or perhaps a little higher. If we assume
the two elements total 6,000-watts, we stand at 25 amps or just a
little over 60 per cent of total capacity.

If we have our two large burners turned on and both oven elements
operating, demand exceeds 11,000-watts (47 amps) and our breaker
trips. However, these same two burners and just the bake element
drops us back down to 8,200 watts/34 amps which should keep the power
flowing.

So, realistically speaking, operating both oven elements on a 40-amp
circuit doesn't seem feasible. With dual fuel, it won't be a problem
but with an all electric range, you would have to bump things up to 50
or even 60 amps. Any idea what size breaker is normally used for a
standard 30-inch range? I'm guessing 40 only by what I see in my own
panel, but perhaps 50 or 60 amps is more common.

Cheers,
Paul

On Tue, 13 Feb 2007 22:00:36 -0600, Mike Hartigan
wrote:

When I was a kid (1960's), our old Frigidaire did this. Today (still
a kid only bigger), our new Thermador does this. (ours is dual fuel,
not because all gas wasn't available, but because we wanted the
advantages of dual fuel.) ....

Our Thermador uses the upper for broiling (duh!), both the upper and
lower to preheat, switching to lower only to maintain temperature.
The convection setting has its own element.

[...]

(Dave Martindale) writes:

Just look at the probability distribution of a 100,000 random events
each with a probability of being on of 1/3. The odds of having exactly
33,333 of them on is quite small, but the odds of somewhere between 30
and 35000 being on is nearly 1.

It's been a very long time since I did any statistics with real numbers,
but here's how I think you might work this out. Any given oven has a
probability p=1/3 of being on at any given time. Checking many ovens at
the same time gives a result with a binomial distribution. With a large
number of ovens, the binomial distribution approaches a normal
distribution with a mean of n*p = 33,333 and a variance of n*p*(1-p)
= 22,222 and standard deviation of 149.

This distribution has a very steep narrow peak around the mean of
33,333. 68% of the time, the actual load will be within one standard
deviation of the mean, i.e. between 33,184 and 33,482 - a change in load
of less than half a percent. 95% of the time, the load will be within
two standard deviations of mean, less than a 1% change. And it will be
within three standard deviations, still only +- 1.3 percent load change,
99.7 percent of the time.

If we only care about unusually high load, not unusually low load, we
look at the one-sided cumulative distribution. The load will exceed
33,333 ovens 50% of the time, as you'd expect. The load will go above
101% of mean (i.e. a 1% increase, to 33,667 ovens) only 1.3 percent of
the time. The load will be above 102% of mean (= 34000 ovens on) only
4 parts per million of operating time, or about 2 minutes per year if
the ovens were left turned on 24 hours/day.

An increase of 3% above mean can be expected only 1 part per 100 billion
of time - essentially it will never happen. At 4% increase above mean
load, Excel becomes unable to calculate the probability at all.

In other words, with 100,000 of anything participating, variations of
more than a couple of percent from mean are extremely unlikely.

Dave

Paul M. Eldridge writes:

Our basic assumption is that these elements will operate 33 per cent
of the time, once the oven reaches its set temperature. But this
cycling will be random in nature, so our 100,000 ovens won't be
cycling "perfectly" in the sense that only one-third will be energized
at any one time. As the total number of ovens increase, I take it
we'll move ever closer to this ideal scenario, but it's probably fair
to say their combined load will fluctuate due to the unevenness in
this cycling. If we were to take a series of snap shots, we might
find that perhaps 50 per cent of these elements are energized, in
which case our load at that particular moment in time is closer to 150
MW and not the 100 MW I had stated.

See my other recent post in this thread. You underestimate how strongly
large numbers of things tend to produce results that cluster around the
mean. According to my calculations, with 100,000 ovens, the likelihood
of even a 2% increase in instantaneous load due to random fluctuation is
a few parts per million. A 50% change in load is unimaginably
unlikely.

Dave

Paul M. Eldridge writes:
Thanks. I had only a sketchy understanding of how this technology
works so this helps me considerably. And given these thermostats are
connected to resistance loads that should happily work with pretty
much anything you throw at them, they most likely lack any
sophisticated circuitry, as you suggest.

Yeah. There is just a small heater in them that is energized whenever
the stove element is energized, a bimetallic strip thermostat, and a
setting knob. Essentially, you are controlling the temperature inside
the housing of the stove control by setting the knob position. The
heater inside the control operates from zero to 100 percent of the time,
whatever is required to maintain the set temperature. The stove elment
operates off a different contact of the same switch, and this allows you
to continuously vary the average power to the element.

Which brings me to this question: these thermostats are becomming
increasingly popular and they do work extremely well from a consumer's
point of view, but I wonder what impact they may have on power
quality. I understand triacs can generate some nasty THD numbers; one
thing to dim a 60-watt incandescent bulb but 6,000 watts of electric
heat has to kick things up a notch or two. Any thoughts?

The "infinite heat" stove controls have simple mechanical switches that
are either on or off. They have no effect on power waveform (unlike
triac dimmers).

Dave

On Feb 14, 1:07 am, (Dave Martindale) wrote:

....

The "infinite heat" stove controls have simple mechanical switches that
are either on or off. They have no effect on power waveform (unlike
triac dimmers).

That depends on what you mean by "no effect" ---

They chop the AC sinusoidal waveform to turn power off and on.
Whether they do it randomly in the cycle or as w/ diac/triac switches
at or very near the crossing voltage makes some difference in what the
resulting waveform is, but in either case the output isn't continuous
and is a chopped sine. The "more expensive" triac dimmers mentioned
earlier have some additional components (usually an RC to introduce a
time delay tied into another diode to bleed the cap while the main
triac isn't conducting to contribute a portion during the "off"
cycle. For incandescent lights, it reduces flicker and "singing"
caused by the harmonics generated in the simple "bang-bang" chopped
control case.

For the heater, (and the cooktop range element) the resulting
difference in input waveform would be pretty much immaterial owing to
the higher thermal mass as compared to a bulb filament and the
likelihood of objectionable generated mechanical vibration is much
less again owing to the size/mass.

If, otoh, by no effect you meant "the power waveform is just a
sinusoid with some variable fraction missing" referring to there being
no attempt to compensate, then I agree. Wasn't sure which
interpretation you were intending...

Hopefully, that will help Paul more than confuse further.

On Feb 13, 6:51 pm, Paul M. Eldridge
wrote:
Thanks. I had only a sketchy understanding of how this technology
works so this helps me considerably. And given these thermostats are
connected to resistance loads that should happily work with pretty
much anything you throw at them, they most likely lack any
sophisticated circuitry, as you suggest.

Which brings me to this question: these thermostats are becomming
increasingly popular and they do work extremely well from a consumer's
point of view, but I wonder what impact they may have on power
quality. I understand triacs can generate some nasty THD numbers; one
thing to dim a 60-watt incandescent bulb but 6,000 watts of electric
heat has to kick things up a notch or two. Any thoughts?
....

That's at least theoretically true -- to what extent it is a real
problem I don't know -- it's not quite as bad as a chopped DC in terms
of the generated harmonics and not as much of a problem from high
frequency as a switching power supply owing to the base 60 Hz
frequency, but I don't have any real information at hand on what sort
of problems one might cause in the practical sense.

As I noted in another response, the noticeable effect w/ dimmers is
owing to the small inertia of the filament so that flicker can be
visible and "singing" may sometimes be heard. That's not nearly as
likely w/ the heaters so unless there's something nearby that is
susceptible to the radiated harmonics (AM radio is one likely
candidate, perhaps), it shouldn't cause too much problem. Large
heaters like you're talking about tend to be on dedicated circuits so
there isn't as much likelihood of direct contamination of some
sensitive input supply.

Thanks for describing this in greater detail. I'm seeing more of
these new electronic thermostats used in electrically heated homes and
so I was curious what impact, if any, they might have on power quality
(those nasty third harmonics et al.). I have three in my own home
controlling my in-floor radiant heat and have been quite pleased with
their performance.

Cheers,
Paul

On 14 Feb 2007 06:38:12 -0800, "dpb" wrote:

That's at least theoretically true -- to what extent it is a real
problem I don't know -- it's not quite as bad as a chopped DC in terms
of the generated harmonics and not as much of a problem from high
frequency as a switching power supply owing to the base 60 Hz
frequency, but I don't have any real information at hand on what sort
of problems one might cause in the practical sense.

As I noted in another response, the noticeable effect w/ dimmers is
owing to the small inertia of the filament so that flicker can be
visible and "singing" may sometimes be heard. That's not nearly as
likely w/ the heaters so unless there's something nearby that is
susceptible to the radiated harmonics (AM radio is one likely
candidate, perhaps), it shouldn't cause too much problem. Large
heaters like you're talking about tend to be on dedicated circuits so
there isn't as much likelihood of direct contamination of some
sensitive input supply.

Actually, there were electric cooktops that did this--sorta- the old
GE's with the 7 button pushbutton switches. They used coils that were
actually 2 separate coils of different wattages in one., and also had a
neutral to the switch. Highest setting was 240 to both segments of the
coil, then 240 to one and 115 the other, then 240 to one only ( and am
not sure about the exact sequence) it would put the 2 circuits in series
with 240 volts, etc, and finally on the lowest would put 115 to both in
series. In the early 70's we worked on a few ranges, and I remember the
service manager explaining this setup. I do remember going out on one
where the lady said that on certain settings none of the burners would
work right. She also said "the same time the trouble started, I found
this in the drawer underneath the cooktop", and handed me a wirenut.
The incoming power was just spliced right there wide open and something
in the drawer snagged the neutral and pulled it loose. I lived in an
apartment about that same time that had that type of range. It seemed to
work OK, though I really couldn't say it was any better than a regular
type cooktop. Granted my experience with electric was limited (as it
still is) so it wasn't much of a comparison. Larry

On Feb 14, 9:00 am, Paul M. Eldridge
wrote:
Thanks for describing this in greater detail. I'm seeing more of
these new electronic thermostats used in electrically heated homes and
so I was curious what impact, if any, they might have on power quality
(those nasty third harmonics et al.). ...

The "third harmonic" thing is the result of chopping a DC supply, and
not the same as a chopped AC supply. I'm sure there is some 3rd-
harmonic content, but with a chopped sine the theoretical waveform
won't be the "all odd harmonics in 1/N magnitude" of the chopped DC.
OTTOMH I don't recall the characteristics of the transform for the
chopped sinusoidal case and was/am too lazy to get up and look for it
(and definitely too lazy to work it out ), but it's different--just
how different was what I was hemming and hawing about. It is, of
course, dependent on the phase angle as well as the discontinuity
changes characeristics as the chopping point moves through the cycle.
Actually, as I think about it, while the zero-switching is
advantageous from the standpoint of switching small currents, it is
the steepest gradient of voltage change w/ time, so in fact, the worst
from the standpoint of generating harmonics. But, the fact that it
isn't a square wave means it isn't the odd-harmonics only case.

mm writes:
On Mon, 12 Feb 2007 19:23:02 +0000 (UTC), (Dave
Martindale) wrote:

In fact, the electronic control wastes a bit of power in the switching
element, and consumes slightly *more* power than the non-electronic
oven. The triac control also distorts the utility waveform into
something that is less of a sine wave, which the utility also will not
like (the power factor gets worse, so they need higher current capacity
for the same billable watts).

How can that be? Any distortion by the triac appears beyond the
triac.

The electricity is billed by what goes through the meter, where it is
undistorted.

The triac, by switching on part way through each half-cycle of line
voltage, severely distorts the voltage and current waveform to the load
in the process of doing its job. In addition, you now have a load that
is drawing current only for the later portion of each half-cycle, and
that distorts the waveform of the *current* drawn by your house, at the
connection from the pole.

If you're dimming one 100 W bulb, this doesn't matter much, but a triac
dimmer feeding a 3 kW range element is more significant. If you had any
substantial fraction of 100,000 ovens using triac power control, the
current waveform distortion might be visible all the way back at the
generator. With *all* ovens operating this way, you're talking about
100 MW of load (on each of 3 phases) turning on part way through each
half-cycle of AC.

Also, when the current waveform departs from a sine wave and becomes
more pulse-like, resistive losses increase for the same average current.
A thought experiment to show this: suppose you draw 1 W from a DC
source by drawing 1 A at 1 V continuously. Now change to drawing 2 A
50% of the time and nothing the rest of the time. The average current
is still 1 A, and the power is still 1 W. But the resistive losses in
the wiring are proportional to current *squared*. When the switch is
turned on and you're drawing 2 A, the losses are 4 times as large as
when you were drawing 1 A. You're only drawing current half the time,
so the losses the other half are now zero, so the average loss is twice
what it was before. And that means you need twice as large a wire for
the *same* voltage drop at the same power and the same average current.

Now, you don't care about this effect. Your wiring is sized to carry
the current when the load is fully on. When you turn down the dimmer,
the total power drops and the total losses are reduced. And your meter
only bills you for the actual watts used - even though the current
waveform is distorted.

But the utility cares. It sizes its generators and lines and
transformers for the *average* load plus a safety factor, not the peak
possible load. It depends on 2/3 of the ovens being off at any given
time due to thermostat cycling. As long as any given oven or range
element is either on or off, the voltage and current waveforms at the
generator remain nice sine waves. But if all those ovens switched to
using triac controls, the current to the ovens would be zero for the
first half+ of the half-cycle, and *three times higher than average*
for the last half- of each half-cycle of the AC waveform. That requires
heavier conductors and larger transformers to deliver the same average
power to the load with the same transmission losses. It costs the
utility more to deliver the same amount of billable power, so they're
not going to be happy.

This is similar to the effect of power factor in motors. Most motors
draw current that is somewhat out of phase with the voltage. Because of
this, the power consumed by the motor is somewhat less than the volts
applied times the amps consumed. Said another way, the motor current is
*higher* than what you'd expect from the motor power and efficiency.
But the size of transformers and lines feeding a factory depends on the
amps and volts needed, not the watts. So utilities bill large factories
by the volts times amps they use, *not* watts. And factories try to
keep their power factor as close to 1 as possible.

Dave

"dpb" writes:
On Feb 14, 1:07 am, (Dave Martindale) wrote:

The "infinite heat" stove controls have simple mechanical switches that
are either on or off. They have no effect on power waveform (unlike
triac dimmers).

That depends on what you mean by "no effect" ---

First, I was talking about their effect on the current waveform at the
input to the house, or at the output of the utility generator, not the
output to the element.

What I meant was that the waveform may be disturbed for one half-cycle
as the mechanical switch opens or closes at some random time, but then
the switch remains open or closed for many hundreds of cycles before
changing state again. So a fraction of one percent of the waveform
half-cycles are distorted, but the remainder are unmodified sine waves.
To a utility, that's an undistorted waveform.

But a triac dimmer adjusts power by turning on part way through *every*
half cycle, so *every* cycle is distorted. That's what I was comparing
to, and I think what the original poster was referring to.

There's yet another type of modulating control that uses a triac switch
turned on at zero-crossing of the waveform. It can be cycled on or off
quite rapidly to control power - it can let through a few cycles of AC,
then turn off for a few more. So its cycling rate is somewhere between
that of a conventional triac dimmer and a conventional mechanical
"infinite heat" control. I don't know if these are used in any stoves,
but they are used in industrial furnaces. These don't distort the AC
waveform at all.

Dave

On Tue, 13 Feb 2007 22:00:36 -0600, Mike Hartigan
wrote:

In article ,
says...
Hi Mike,

My apologies for asking this, but is this something relatively new or
has it always been this way? I don't have a lot of experience with
electric ovens because I've always used gas, but this time around I
had to settle for a combination unit because an all gas version wasn't
available.

When I was a kid (1960's), our old Frigidaire did this. Today (still
a kid only bigger), our new Thermador does this. (ours is dual fuel,
not because all gas wasn't available, but because we wanted the
advantages of dual fuel.)

I did experiment with mine just now and here's what I found. There
are basically three heating options I can choose: "bake", "broil" and
"convection" and as far as I can tell all three work independently of
each other. When I turn on "broil" I can safely leave my hand on the
lower bake element and it remains cool to the touch; likewise, the
broiler never comes on when I select "bake" and neither of these two
elements are used when I pick "convection" (that appears to be a third
element hidden somewhere inside the oven's back wall).

Our Thermador uses the upper for broiling (duh!), both the upper and
lower to preheat, switching to lower only to maintain temperature.
The convection setting has its own element.

[...]

My built-in oven (old Frigidaire electric) uses both elements when
baking. I found out then the upper element quit working (not a bad
element, but the connector). Food would burn on the bottom and still
be raw on top.
--
Mark Lloyd
http://notstupid.laughingsquid.com

"Unlike biological evolution. 'intelligent design' is
not a genuine scientific theory and, therefore, has
no place in the curriculum of our nation's public
school classes." -- Ted Kennedy

Paul M. Eldridge writes:
Thanks for describing this in greater detail. I'm seeing more of
these new electronic thermostats used in electrically heated homes and
so I was curious what impact, if any, they might have on power quality
(those nasty third harmonics et al.). I have three in my own home
controlling my in-floor radiant heat and have been quite pleased with
their performance.

I'll bet that if you look at the output waveform on an oscilloscope,
you'll find that the thermostat is either on or off at any given point
in time, and that it cycles between on and off every few seconds in
order to modulate the heat. The switching could happen at random times
during the AC cycle if a mechanical relay is used, or it might be at
zero-crossings if an electronic relay is used.

But a heater has enough thermal inertia that there's no point in
switching the current 120 times per second, like a lamp dimmer does, and
switching only every few seconds reduces any electrical interference and
avoids creating non-sinusoidal current waveforms.

Dave