On Wed, 14 Feb 2007 05:51:08 GMT, Paul M. Eldridge
wrote:

[snip]


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.


What I have here is built-in, where the oven and cooktop are separate
units. Each unit is on a separate 30A breaker.

The coils on the cooktop are the same size you mentioned (2 6 inch and
2 8 inch). That would seem to mean that would all require 35A. I have
had all 4 on high and it hasn't tripped the breaker.

That breaker (Square D) has a red trip indicator that is visible no
matter what (on of off, I've never had it trip). Could it be
defective?
--
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 Wed, 14 Feb 2007 05:30:27 +0000 (UTC), (Dave
Martindale) wrote:

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.


What I was trying to say before, is that I'd expect the vast majority
of overloads to be momentary.

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
--
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

Thanks to you and Dave for this. I know we've veered way off topic
and much of this is so technical it can be a little hard to digest
unless you happen to have some background in this area, but you've
both done a pretty good job of explaining it in a way those of us less
knowledgeable, like myself, might understand (and that can't be an
easy task).

One of the things that continues to amaze me about this forum (and
others like it) is the amount of knowledge out there and, moreover,
the willingness to help others. I've certainly benefited from this
myself, many times over, both in practical, everyday matters and some
of these more theoretical concerns as well.

Cheers,
Paul

On 14 Feb 2007 07:37:49 -0800, "dpb" wrote:

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.

Hi Mark,

Good question. I wonder if any of these burners are lower in wattage,
even though they're of the same physical size. Is there anything
stamped on the burner itself (e.g., at the prong ends) that might
indicate their wattage or do you happen to have the owner's manual?

At the outer edge, thirty amps at 240-volts provides us with a maximum
of 7,200 watts and the 80 per cent rule drops us down to just 5,760
watts. Your breaker might tolerate some minor, short-term overloading
(I honestly don't know), but four burners on high must be pushing that
30-amp circuit pretty hard, especially if your supply voltage should
fall much below 240.

Cheers,
Paul

On Wed, 14 Feb 2007 12:24:24 -0600, Mark Lloyd
wrote:

What I have here is built-in, where the oven and cooktop are separate
units. Each unit is on a separate 30A breaker.

The coils on the cooktop are the same size you mentioned (2 6 inch and
2 8 inch). That would seem to mean that would all require 35A. I have
had all 4 on high and it hasn't tripped the breaker.

That breaker (Square D) has a red trip indicator that is visible no
matter what (on of off, I've never had it trip). Could it be
defective?

On Feb 14, 12:12 pm, (Dave Martindale) wrote:
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.

--

Yes, Paul has already confirmed that is the case -- the thermostat
describes it's operation in terms of the "on" fraction of time
required to maintain the setpoint. It does seem to use the technology
in its promotion as being a little more than it really is (surprise,
surprise... ).

I'll respond to your other related response to my previous posting
here as well to (hopefully) bring this to a close in one location.

I agree w/ your definition of undistorted, certainly from the utility
supply side definition once I understand your intent. And also, as
you correctly point out, the effect of the switching transient goes
away very quickly and I, in fact, even though I had specifically
looked at the product brochure of Paul's thermostat had let his
question lead me down a (mostly) false path of considering the
switching transient only instead of the overall time-average effect.

So, now again mostly for Paul, I'll amplify this slightly and again
hopefully further clarify rather than confuse/obfuscate.

So, while it is true there will be wave distortion and all it's myriad
effects _at the moment of switching_, it's also true that for these
thermostats the fastest they operate is on a 15-20 sec cycle time so
that the switching transient lasts for only 2/[60*(15 to 20)] ~ 0.2%
of the time. Consequently, in the larger scheme of things, even for
the output there's effectively no distortion, and certainly not for
any primarily resistive load. It would take something _very_
sensitive to input power or radiated distortion for it to be a
problem.

So, overall, we're in agreement and thanks for pointing out where I
kinda' let myself go off into the weeds thinking of the details as
opposed to the bigger picture...

OK. Brace yourself, because I'm going to confirm what I understand
you to have said and the results could be a tad ugly. :-0

This electronic thermostat is simply cycling power on and off more
frequently than its mechanical counterpart, and any power disturbances
are rather trivial and occur only at the time the load is dropped and
then again as it is subsequently picked back up. It's really no
different from a conventional bimetal thermostat in this regard,
except that these minor transients [insert appropriate terminology
here] happen perhaps every 5 to 10 seconds, as opposed to once every
three to four minutes.

Hmm... no screaming, no gnashing of teeth, no hair pulling... I'm
taking that as a good sign....

Cheers,
Paul

On Feb 14, 2:34 pm, Paul M. Eldridge
wrote:
OK. Brace yourself, because I'm going to confirm what I understand
you to have said and the results could be a tad ugly. :-0

This electronic thermostat is simply cycling power on and off more
frequently than its mechanical counterpart, and any power disturbances
are rather trivial and occur only at the time the load is dropped and
then again as it is subsequently picked back up. It's really no
different from a conventional bimetal thermostat in this regard,
except that these minor transients [insert appropriate terminology
here] happen perhaps every 5 to 10 seconds, as opposed to once every
three to four minutes.

Hmm... no screaming, no gnashing of teeth, no hair pulling... I'm
taking that as a good sign....



Basically true -- only real difference is use of solid state switching
and the zero-crossing switching, but it still is basically "just a
switch". These both have advantages in reliability, but in reality
the frequency of control is probably overkill for the application but
it doesn't cost any more once go to the electronics anyway, so why
not? is one way to look at it. Of course, if they use the fancy words
and "high technology" to justify a high initial cost, that's another
thing, but it didn't seem all that far out of line...

I grew up in a house built in the early 1970s that had a four burner
General Electric cooktop with a push button control mounted on the wall
behind it. Each burner had eight or so buttons to regulate its heat
output. When I visited last Thanksgiving, it was still in service.

"wff_ng_7" wrote in
news:kDsAh.3688$Aa5.1057@trnddc01:
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.

"Peter A" wrote in message
...

-snip-

Microwave ovens work the same way, although I have some vague
recollection that some fancy models have variable power.

I recently purchased a new microwave that has variable output power (labeled
as "inverter technology"). It was sub $100 US so I wouldn't call it
"fancy". It has 4 power ranges and uses duty cycle control between the
power ranges to regulate the 10 available power settings. It is FAR
superior at the lower power settings. When defrosting or cooking on low, my
old microwave would singe and pause repeatedly. Cycling 1/4 power more
often yields MUCH better results.

The new microwave also behaves much better on my small backup generator -
conventional microwaves have really poor power factor AND significant even
order harmonics, both not appreciated by generator voltage regulators. The
even order harmonics are from the voltage doubler magnetron circuit. On one
half of the line waveform, the diode charges the cap up. On the other half
cycle, it fires the magnetron with the cap in series with the line voltage.
Since the back EMF when charging the cap is not equal to the fire voltage of
the magnetron minus the cap charge, the waveform is highly asymmetrical.