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Martin Whybrow
 
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Default RPC run capacitors


"Ignoramus19023" wrote in message
.. .
I am considering adding some run capacitors to my self starting RPC. I
am reading Jim Hanrahan's article at

http://www.metalwebnews.com/howto/ph-conv/ph-conv.html

and I am confused by something. I understand how self starting
RPCwould start with one cap between one leg 1 and leg 3 (the generated
one). That's how mine is wired. Jim makes a point that it works, but
makes unbalanced voltage.

But why would it start is capacitors are connected between 1-3 AND
2-3, like in this pictu

http://www.metalwebnews.com/howto/ph-conv/fig1.html

I cannot see how it would create assymmetric fields needed to spin up
the motor. Is that because capacitances across legs 1-3 are greater
than capacitance between leg 2-3?

I could try to use run caps at run time and start caps at start
time. In fact, I won a time delay relay for $9 on ebay yesterday, so I
could set the RPC to start on start cap (both caps between leg 1-3)
and then reconnect the same caps to become run caps, one between 1-3
and another between 2-3. Same TDR could, then, turn out output current
aftet time delay, allowing the RPC to spin up and switch to the run
mode.

As you can see, I am quite confused, but am willing to experiment. I
have 4 unused Furnas 75 A contactors that I can wire, with the time
delay relay, to do just about anything.

Idler: 10 HP

Capacitors: 92 mF each, 535 VAC rated, oil filled. I have 5 total, and
use 2 for the starting leg, so three are unused.

i

You are correct saying that it's because the capacitance between 1 + 3 is
higher than between 2 + 3.
My RPC, 10HP 440V 50Hz motor, has 40uF between 2 + 3, 60uF between 1 + 3 and
500uF switched by a start circuit between 1 + 3. The 500uF is a bit too
much, it starts very quickly (around 1/3 second) so I could probably drop
that to around 200uF.
Martin

--
martindot herewhybrowat herentlworlddot herecom


  #2   Report Post  
Robert Swinney
 
Posts: n/a
Default


"Ignoramus19023" wrote in message
.. .
I am considering adding some run capacitors to my self starting RPC. I
am reading Jim Hanrahan's article at

http://www.metalwebnews.com/howto/ph-conv/ph-conv.html

and I am confused by something. I understand how self starting
RPCwould start with one cap between one leg 1 and leg 3 (the generated
one). That's how mine is wired. Jim makes a point that it works, but
makes unbalanced voltage.

But why would it start is capacitors are connected between 1-3 AND
2-3, like in this pictu

http://www.metalwebnews.com/howto/ph-conv/fig1.html

I cannot see how it would create assymmetric fields needed to spin up
the motor. Is that because capacitances across legs 1-3 are greater
than capacitance between leg 2-3?


A 3-phase motor will start on single-phase current because the (3rd leg,
middle leg, whatever) has phase-shifted current applied to it. That leg in
a 3-phase motor operated on single-phase, has phase-shifted current applied
via the start cap. That current is "sufficiently out of phase" with the
current in the main winding to provide a starting field, if you will, for
the main field to operate against, thus causing start-up. Remember, we are
talking about a single-phase motor. After the starting interval, the motor
will continue to operate as a single phase machine.

The start cap, if left permanently in place, forms a "self starting" RPC. A
rather large amount of capacitance is required for starting any motor, be it
single-phase or 3-phase, when starting on single-phase current. Therefore,
a 3-phase motor running with an over-large starting capacitance, permanently
in place, is likely to exhibit severly unbalanced running voltages. This is
why serious RPC builders always seperate the starting and running functions.

Assymetric fields are necessary for starting, as above. The "running
current" flow paths in a RPC are quite complex and are also assynmetric even
in a tuned, voltage balanced RPC because of direction of rotation, among
other things. Suffice it to say, the current flow in a RPC and its load
requires "steerage" (think series resonance) in order for the fundamental
single-phase running current to be guided into paths that emulate 3-phase
conditions. Remember, you are still dealing with single-phase power. A RPC
does not "generate" 3-phase power - it merely performs adjustments in a
fundamentally single-phase scenario which emulates 3-phase.


I could try to use run caps at run time and start caps at start
time. In fact, I won a time delay relay for $9 on ebay yesterday, so I
could set the RPC to start on start cap (both caps between leg 1-3)
and then reconnect the same caps to become run caps, one between 1-3
and another between 2-3. Same TDR could, then, turn out output current
aftet time delay, allowing the RPC to spin up and switch to the run
mode.


By all means do "use run caps at run time and start caps at start time".
Forget about reconnecting start caps and using them as run caps. Either use
a simple push-button switch to temporarily connect the start caps or a NC
potential relay that senses 3rd leg voltage to open up the start circuit.
Then leave the start circuit alone. Period.


As you can see, I am quite confused, but am willing to experiment. I
have 4 unused Furnas 75 A contactors that I can wire, with the time
delay relay, to do just about anything.

Idler: 10 HP

Capacitors: 92 mF each, 535 VAC rated, oil filled. I have 5 total, and
use 2 for the starting leg, so three are unused.

It occurs, you are attempting to use components that may not be appropriate,
or the best way, just becasue you got them on the cheap. No amount of
aimless, and possibly dangerous experimentation, with the wrong things can
necessarily force success.

Bob Swinney


i



  #3   Report Post  
 
Posts: n/a
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On Fri, 05 Aug 2005 17:29:48 GMT, Ignoramus19023
wrote:

On Fri, 05 Aug 2005 17:16:59 GMT, Martin Whybrow wrote:

"Ignoramus19023" wrote in message
.. .
I am considering adding some run capacitors to my self starting RPC. I
am reading Jim Hanrahan's article at

http://www.metalwebnews.com/howto/ph-conv/ph-conv.html

and I am confused by something. I understand how self starting
RPCwould start with one cap between one leg 1 and leg 3 (the generated
one). That's how mine is wired. Jim makes a point that it works, but
makes unbalanced voltage.

But why would it start is capacitors are connected between 1-3 AND
2-3, like in this pictu

http://www.metalwebnews.com/howto/ph-conv/fig1.html

I cannot see how it would create assymmetric fields needed to spin up
the motor. Is that because capacitances across legs 1-3 are greater
than capacitance between leg 2-3?

I could try to use run caps at run time and start caps at start
time. In fact, I won a time delay relay for $9 on ebay yesterday, so I
could set the RPC to start on start cap (both caps between leg 1-3)
and then reconnect the same caps to become run caps, one between 1-3
and another between 2-3. Same TDR could, then, turn out output current
aftet time delay, allowing the RPC to spin up and switch to the run
mode.

As you can see, I am quite confused, but am willing to experiment. I
have 4 unused Furnas 75 A contactors that I can wire, with the time
delay relay, to do just about anything.

Idler: 10 HP

Capacitors: 92 mF each, 535 VAC rated, oil filled. I have 5 total, and
use 2 for the starting leg, so three are unused.

i

You are correct saying that it's because the capacitance between 1 + 3 is
higher than between 2 + 3.
My RPC, 10HP 440V 50Hz motor, has 40uF between 2 + 3, 60uF between 1 + 3 and
500uF switched by a start circuit between 1 + 3. The 500uF is a bit too
much, it starts very quickly (around 1/3 second) so I could probably drop
that to around 200uF.


Thank you Martin.

I have five 92 mF caps total.

What I have now is 2 capacitors connected between leg 1 and 3.

It seems that about the only thing that I need, then, wire it as follows:

1. 1 cap (92 mF) between legs 1 and 2, permanently connected.

2. 1 cap (92 mF) between 1 and 3, permanently connected.

3. 2-3 more caps (180-270 mF) between legs 1 and 3, connected
at start time and switched off by the time delay relay (TDR),
say 4 seconds after startup.

4. (optional) add another contactor on output side that would only
switch output on when the TDR actuates and switches off the start
caps.

Theat would get me an RPC that is:

- properly balanced across the range of output HP
- starts quickly
- only turns on when balanced power is output and full rotational
speed is achieved, that is, when good quality 3 phase power is available.

Does it make sense?

i


With five equal value oil filled capacitors your options are fairly
limited but you can still finish up wth a perfectly good converter.

One thing to remember is that capacitors of this type have extremely
low internal series resistance (milliohms)- that's why you get a
sizable bang if you short circuit a charged one!
The same thing happens when you connect a charged capacitor to an
uncharged one - VERY high peak currents flow as the charge voltage
equalises on the two capacitors.
The capacitors are not greatly bothered by this treatment but it's
very unkind to the switch contacts or relay contacts used to parallel
connect two capacitors if there is a substantial voltage difference at
the instant of connection.

Brute force oversizing of the switching contacts can give reasonable
contact life but it's much neater to avoid the problem by using
separate start and run capacitors that are never parallel connected.
This is easily done with your capacitor collection.

Use three parallel connected as your start capacitor. Only in circuit
for the few seconds needed for the idler to run up to speed.

You now have a bit unbalanced but perfectly usable converter system.
Ideally the load motor should not be switched in until the idler is up
to speed. However, if the load motor is initially running light, this
start capacitance is probably enough for a simultaneous idler and load
start.

Your START switching should be a changeover contact which EITHER
connects the start capacitor OR the run capacitor across L1 and L2.

For the run capacitor, try one capacitor or two capacitors series
connected to halve the effective value - whichever gives the best
voltage balance with full load on the load motor.

Do not place a capacitor across L2 L3. With the values you have
available this would do more harm than good.

Jim


  #4   Report Post  
Jerry Martes
 
Posts: n/a
Default


"Ignoramus19023" wrote in message
.. .
On Fri, 05 Aug 2005 22:20:26 +0100,
wrote:
On Fri, 05 Aug 2005 17:29:48 GMT, Ignoramus19023
wrote:

On Fri, 05 Aug 2005 17:16:59 GMT, Martin Whybrow
wrote:

"Ignoramus19023" wrote in message
.. .
I am considering adding some run capacitors to my self starting RPC. I
am reading Jim Hanrahan's article at

http://www.metalwebnews.com/howto/ph-conv/ph-conv.html

and I am confused by something. I understand how self starting
RPCwould start with one cap between one leg 1 and leg 3 (the generated
one). That's how mine is wired. Jim makes a point that it works, but
makes unbalanced voltage.

But why would it start is capacitors are connected between 1-3 AND
2-3, like in this pictu

http://www.metalwebnews.com/howto/ph-conv/fig1.html

I cannot see how it would create assymmetric fields needed to spin up
the motor. Is that because capacitances across legs 1-3 are greater
than capacitance between leg 2-3?

I could try to use run caps at run time and start caps at start
time. In fact, I won a time delay relay for $9 on ebay yesterday, so I
could set the RPC to start on start cap (both caps between leg 1-3)
and then reconnect the same caps to become run caps, one between 1-3
and another between 2-3. Same TDR could, then, turn out output current
aftet time delay, allowing the RPC to spin up and switch to the run
mode.

As you can see, I am quite confused, but am willing to experiment. I
have 4 unused Furnas 75 A contactors that I can wire, with the time
delay relay, to do just about anything.

Idler: 10 HP

Capacitors: 92 mF each, 535 VAC rated, oil filled. I have 5 total, and
use 2 for the starting leg, so three are unused.

i

You are correct saying that it's because the capacitance between 1 + 3
is
higher than between 2 + 3.
My RPC, 10HP 440V 50Hz motor, has 40uF between 2 + 3, 60uF between 1 +
3 and
500uF switched by a start circuit between 1 + 3. The 500uF is a bit too
much, it starts very quickly (around 1/3 second) so I could probably
drop
that to around 200uF.

Thank you Martin.

I have five 92 mF caps total.

What I have now is 2 capacitors connected between leg 1 and 3.

It seems that about the only thing that I need, then, wire it as follows:

1. 1 cap (92 mF) between legs 1 and 2, permanently connected.

2. 1 cap (92 mF) between 1 and 3, permanently connected.

3. 2-3 more caps (180-270 mF) between legs 1 and 3, connected
at start time and switched off by the time delay relay (TDR),
say 4 seconds after startup.

4. (optional) add another contactor on output side that would only
switch output on when the TDR actuates and switches off the start
caps.

Theat would get me an RPC that is:

- properly balanced across the range of output HP
- starts quickly
- only turns on when balanced power is output and full rotational
speed is achieved, that is, when good quality 3 phase power is available.

Does it make sense?

i


With five equal value oil filled capacitors your options are fairly
limited but you can still finish up wth a perfectly good converter.

One thing to remember is that capacitors of this type have extremely
low internal series resistance (milliohms)- that's why you get a
sizable bang if you short circuit a charged one!
The same thing happens when you connect a charged capacitor to an
uncharged one - VERY high peak currents flow as the charge voltage
equalises on the two capacitors.


An excellent point.

The capacitors are not greatly bothered by this treatment but it's
very unkind to the switch contacts or relay contacts used to parallel
connect two capacitors if there is a substantial voltage difference at
the instant of connection.


Yep, makes full sense.

Now, based on common sense, unlike CONNECTING capacitors,
DISCONNECTING them should not produce any big sparks.

I have a plan for a start circuit with a separate contactor. The
sequence of events is as follows.

1. I turh the ON switch.
2. The contactor that connects starting caps engages.
3. When it engages, it actuates the SECOND contactor that supplies
primary 240V voltage to the electric motor.
4. Everything starts spinning.
5. After a few seconds, a time delay relay that I won on ebay today
(NO and NC), will open a pair of contacts and DISCONNECT the starting
caps.

I think that I know how to wire this properly so that all of this
happens, and yet everything shuts down when I turn the on/off switch
to off. I have a mental picture. Basically both contactors would have
a neutral wired directly to neutral, but the 110V line for signal
would go through the switch, then to 3rd poles of second contactor,
and to the 3rd pole of the first contactor but through a time delay
relay. then from the switched side of both contactors to the hot
switch terminal of the second contactor.

6. When the NO contacts on the time delay relay close, it would
actuate the third contactor that would allow power to be output.

Sorry if I am speaking in strange language, I am a computer
programmer. It should work, logically.

read on...

Brute force oversizing of the switching contacts can give reasonable
contact life but it's much neater to avoid the problem by using
separate start and run capacitors that are never parallel connected.
This is easily done with your capacitor collection.

Use three parallel connected as your start capacitor. Only in circuit
for the few seconds needed for the idler to run up to speed.


Right.

You now have a bit unbalanced but perfectly usable converter system.


right. it's much nicer to have something working and add stuff to it,
than build a "dream system" without testing to only realize that
something went amiss. That's how I do my programming too.

Ideally the load motor should not be switched in until the idler is up
to speed. However, if the load motor is initially running light, this
start capacitance is probably enough for a simultaneous idler and load
start.

Your START switching should be a changeover contact which EITHER
connects the start capacitor OR the run capacitor across L1 and L2.

For the run capacitor, try one capacitor or two capacitors series
connected to halve the effective value - whichever gives the best
voltage balance with full load on the load motor.


Looks like one run cap per side should be good, based on Jim
Hanrahan's writings. I'll see.

Do not place a capacitor across L2 L3. With the values you have
available this would do more harm than good.


Hm, why is that so?

Anyway, my main question is: if I do all this (as I outlined in steps
1-6), will I get a Mercedes Benz of phase converters? Or am I wasting
my time for load up to and under 5 HP?

i


I

It would be rude for me to agree that you'd be wasting your time by
sophisticating this RPC. I can tell you that nothing you are likely to
*ever* use in your house will be able to operate any better because of
*anything you do beyond just spinning up that 10 HP idler.

Jerry


  #5   Report Post  
 
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On Sat, 06 Aug 2005 00:46:19 GMT, Ignoramus19023
wrote:

SNIP

For the run capacitor, try one capacitor or two capacitors series
connected to halve the effective value - whichever gives the best
voltage balance with full load on the load motor.



Looks like one run cap per side should be good, based on Jim
Hanrahan's writings. I'll see.


Do not place a capacitor across L2 L3. With the values you have
available this would do more harm than good.



Hm, why is that so?


Anyway, my main question is: if I do all this (as I outlined in steps
1-6), will I get a Mercedes Benz of phase converters? Or am I wasting
my time for load up to and under 5 HP?

i



This is an extract from an earlier post which goes some way to
explaining the peculiarities of "balancing" capacitors.

Snip

A converter of this type is basically a capacitor/inductor phase
shift system which produces an open vee 3 phase system. This phase
shifter is a series resonant circuit and when it is set up to give the
60 deg phase shift it is working a long way below its natural resonant
frequency. 60 deg is of course the correct phase angle between the
two legs of an open vee system.

The motor(s) is the inductor in the system and unfortunately the
apparent inductance of the motor changes with rotor speed. For any
particular rotor speed greater than about 90% of synchronous speed
(the lower limit varies a bit with motor type) it is possible to
choose a capacitor combination which produces a pretty close
approximation to balanced 3 phase at the motor terminals.

For near the full load rated speed of the motor, large run
capacitance is needed with most or all of it as a single capacitor
feeding the phantom phase from supply live. At light load the speed of
the rotor rises and if the capacitor value is chosen to achieve the
right phase angle the phantom phase voltage will be excessive. This
could be corrected by feeding the capacitor from a lower voltage
single phase source but this would mean feeding it from an auto
transformer across the supply.

It is much simpler (and of course everybody does this) to use
two capacitors arranged as a voltage divider to simultaneously achieve
the correct phase angle and phase voltage.

The effective capacitanceof the two capacitors connected in
series across the supply is the sum of the capacitances because the
source impedance of the supply is zero and this effectively parallels
the two capacitors.

Because the they also act as a voltage divider, this sum
capacitance is effectively fed from a voltage of supply voltage times
C1/(C1+C2) where C1 is the top capacitor and C2 is connected phantom
phase to neutral.

Because it looks nicely symmetrical there seems to be a
tendency to believe that C1 and C2 should be equal and any inequality
in their optimum value must result from some strange second order
effect. This is NOT true. There is nothing magic about equal C1 and
C2. It simply results in a capacitor of value C1+C2 fed from half the
supply voltage. At this low effective supply voltage it is only
possible to get close to balanced operation at no load or light loads
which enable the rotor to operate close to synchronous speed.
As the load increases with consequent slowing of the rotor speed the
total capacitance needs to increase with both more in C1 and less in
C2. By the time full load is reached the optimum value for C2 is
usually zero.

These effects are very noticeable if you're using a single
motor on a variable load up to near rated full load power and some
compromise necessary. The saving grace is that industrial motors are
surprisingly tolerant of reasonable overvoltage when operating at
light loads so the trick is to size the capacitors for at or near full
loads and to accept some overvoltaqe at light loads. This increases
the motor losses at light load but the total motor losses still remain
below the losses at rated full load so temperature rise is acceptable.

SNIP

In your case, while a small additional capacitor across L2 L3
might yield better phase balance for some load conditions, the 92uF
you have available is far too large and would do more harm than good.

I fully support Jerry Martes comment to the effect that fine
tuning rarely achieves significant practical benefit.

If you've a nice big idler and have solved the starting problem,
careful choice of balancing capacitors may give you a nice warm
feeling but you're unlikely to notice much practical difference in the
overall performance!

Jim


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