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Idler Motor

backyard

Plastic
Joined
May 22, 2006
Location
Hartsville, South Carolina
I have a 5hp rotary phase converter that i put together in 06. This RPC i got off the enternet, it'S by David O'Neill. The Idler motor is running very bad and i cant use it at all. I dont know if its the idler motor or its the run or start caps, or is it the potential relay ? I have taken the Idler motor apart and cleaned it. How do i find out whats wrong?
 
idler question

You might try switching motor leads on the idler, if it has a internal wiring fault it may act differently with another winding on the mfg phase. Not a definitive test by any means but if bad habits change with lead change idler probably bad, if no change your still where you started.
 
Here is a a suggestion that might help.

Make sure you have a diagram for the capacitor connections first.

Now disconnect all the capacitors

Spin the idler with a rope then turn on the power. If the idler comes up to speed and run normally you have determined the idler is still good. If not, the idler is bad.

IF the idler is good, Now reconect the start caps and potential relay

Start the RPC and see how the idler performs. If it is running OK then you know the start circuit is OK. If it does not run properly then you know the problem is in the start circuit.

If it ran Ok with the start circuit you can reconnect the largest run cap and try again. If it is still OK then connect in the smaller run cap and try again. You should have the problem isolated.
 
This is a related, but more general question. Many threads write about spinning the idler up with either: (1) a pony motor (a smaller 1PH motor used to bring the idler up to speed before applying the two 230V 1PH legs) or (2) a starter rope.

This might be a dumb question, but how do you know which direction to spin the idler before connecting the two 230V 1PH legs?

More importantly, what happens if you guessed wrong?
 
"This might be a dumb question, but how do you know which direction to spin the idler before connecting the two 230V 1PH legs?"

It doesn't matter in which direction the idler is rotating.


"More importantly, what happens if you guessed wrong?"

There are, however, considerations whenever using the unbalanced capacitor method. The start capacitor, if used, should be placed across the larger of the Cab or Ccb capacitor.
 
idler direction

Would the direction the idler is started determine the phase rotation of your 3 phase thus the direction motors powered by it would turn?
 
The phase sequence ... A, B, C or A, C, B ... is determined by the rotation of the idler.

If you use balancing caps, you want the balance to be somewhat out of balance ... 60/40 is a good start ... and for the machine to rotate towards the higher capacitance.

As a polyphase machine will not start except either: 1) by being mechanically started (pony motor or rope), or 2) by being electrically started (potential relay and a large value capacitor); you can ensure that the required imbalance is achieved by placing the starting capacitor across the larger of the two balancing capacitors.

If you do not use balancing capacitors, you may still use a starting capacitor, in which case it makes no difference in which direction the machine is started.

With balancing capacitors, the machine has its best dynamic performance if started in the direction stated: rotating towards the higher capacitance.

The purpose of the balancing capacitors is to stabilize the B-phase voltage over a wider range of loads.
 
"Rotate towards the higher capacitance."

I guess I've never been able to wrap my mind around this.
How this established?

For example, say I define the idler rotation via pony
motor starting, like in my setup.

Now if I wanted to add run capacitors, they will go
from manufactured leg to utility-supplied leg(s).

One of those utility-supplied wires is "special" because
it wants a larger capacitor. Can I figure out in
advance which one that is, or do I have to simply add
equal to both, and then experimentally try increasing
one, while looking at the manufactured leg's voltage?

Jim
 
"I guess I've never been able to wrap my mind around this. How this established?"

By measuring the dynamic performance of the tool motors.

It was observed that a balanced RPC ... Cab = Ccb ... while good, and much better than no capacitors at all, was not the ideal ... the ideal being as close to utility power as is possible.

If you have no balancing capacitors at all, then it matters not in which direction the idler is started.

Still, if you have no balancing capacitors at all, then it matters not whether you mechanically or electrically start the idler.

However, if you do have balancing capacitors, it matters not in which direction the idler is started only for the case where Cab = Ccb.

It was proved by objective testing (best time to speed, best plug reversing, etcetera) that the dynamic performance was best if the idler was slightly imbalanced, and 60/40 is a good rule of thumb. The absolutely best performance may be slightly higher or lower than 60/40, say, 63/37 or 57/43, depending upon the idler, the tool, or both.

But, in an unbalanced system in which direction do we select to rotate the idler?

Well, rotating towards the larger capacitance assists (improves) the dynamic performance, whereas rotating away from the larger capacitance does just the opposite.

And, particularly if you are electrically starting your idler, then the best way to ensure that it is started in that direction which provides the best performance is to place the starting capacitor across the larger of the two balancing capacitors.

If you are mechanically starting your idler, then I do not have any advice to give as this is not my preferred method of starting.

At the end of the day, if your RPC system is delivering the performance you expect, and require, than that's all that matters, and there is no need to agonize over "would-a, could-a, should-a".
 
" place the starting capacitor across the larger of the two balancing capacitors."

Good simple rule.

So it could be done by rigging a temporary start
capacitor, so the idler starts in the same direction as
the pony motor does it.

Then that side gets the larger run capacitor.

Thanks!

Jim
 
Jim,

This is part philosophical, and part objective.

I have no particular bias against mechanically starting an idler, but I far prefer electrically starting an idler as it is exquisitely easy to do for idlers equal to or less than 5 HP (Steveco 90-66, only), and it is only slightly more complicated for much larger idlers, larger or very much larger than 5 HP (Steveco 90-66 for sensing and control, plus a definite purpose contactor).

A polyphase motor, an idler in this case, will not start without first achieving a significant imbalance in its magnetic circuit, after which the rotor will continue to rotate without further intervention. It is this imbalance which effects the starting.

A rope or pony motor starting means, both of which are a mechanical starting means, can achieve the required imbalance.

Now, an imbalanced idler of the Fitch Williams type, which is usually imbalanced to about 60/40, still requires a "significant imbalance" to start, except that it is already significantly imbalanced by the balancing caps, so it takes much more effort to further significantly imbalance it in order to effect a good, clean start.

My "rule of thumb" is somewhat imprecisely, yet very practically stated as "1, 2, 3 and 30".

Which means, that for each quarter HP of idler capacity:

1) 1 µF for power factor correction, Cac,

2) 2 µF for Ccb phase-shifting,

3) 3 µF for Cab phase-shifting, and

4) 30 µF for Cs momentary phase-shifting and starting (in parallel with Cab).

There can be no ambiguity if 30 + 3 = 33 µF is placed between A and B, whereas 2 µF is placed between C and B.

33 µF >> 2 µF, therefore the idler starts quickly and positively, usually in 1-2 seconds, after which the 30 µF is removed and the idler thereafter operates with a 3 µF/2 µF ratio, which is the same as 60/40 percent.

About the opposite case, where the 30 µF is placed across the 2 µF capacitor, 30 + 2 = 32 µf and 32 µF is >> 3 µF, so the idler will still start quickly and positively, but it is rotating in the "wrong" direction.

I offer without proof that it is better to rotate as in the case first-mentioned, above.

Objective tests have proved this to be true.

Yet, both achieve the required "significant imbalance" for starting, whereas only one of these is that configuration which aids dynamic performance in actual operation, that is, after starting has been effected.

Peter
 
Need some advice - been following this thead, lots of good advice. I'm building a 3hp RPC and am a little stuck on the balancing - I've got the potential relay and start capacitor working fine - now taking the three wires out of the motor and using it to balance it without a load - following the FRW approach.

So, based on FRW's outstanding article, I'm trying to make the incoming voltage (i.e. L1 and L2 as represented by "a" = L1 and "b" = L2) Vab 8% to 10% higher than the generated leg "c" as measured on Vac and Vbc.

3 hp motor, Service Factor 1.15, heavy duty cast motor body with fins in the casting.

ab = incoming lines
c = generated leg
Cp = Capacitor between ac, the same legs as the start capacitor and potential rely circuit
Cs = Capacitor between cb, the legs that do not have a start capacitor
Cpf = power factor capcitor, between ab – the two incoming lines

First Run
Cp Cs Vab Vac Vbc
0 0 240 224 206
30 0 241 239 221
35 0 243 243 223
35 20 243 240 230
40 20 243 242 231

Second Run
Cp Cs Vab Vac Vbc
20 0 242 234 215
40 0 241 243 223
45 0 241 245 224


Third Run
Cp Cs Vab Vac Vbc
50 30 244 246 242

Problem that I'm having is that using the 1,2,3,30 rule of thumb, suggests that the Cp (the run capacitor on the same legs as the start capacitor) should be 3 X 4 (1/4 hp / hp) X 3 hp = 36 uF. If you look at my tables above, I've gone as high as 50uF on Cp and Vac is still not 8% = 10% higher than Vab (the incoming L1/L2 voltage).

Any suggestions? I'm going to have to order some more capacitors to balance this all out, but I need to know what I'll need in inventory to acomplish the task.

Also, if the Cp is exceeding the rule of thumb (i.e. I'm now at 50uF versus the 36uF prediction), what should I be thinking about in terms of the Cpf - the power factor capacitor which the formula suggests would be 1 X 4 X 3 = 12 uF.

Most importantly, I think the Rule of Thumb works great - I'm just trying to get mine dialed in a little tighter.

Finally, the amperage on the legs coming out of the motor, but before going into the capacitor banks are as follows:

a = 4.87 amps
b = 2.67 amps
c = 6.63 amps

But, I don't know if this is meaningful because I'm measuring it coming off the RPC, but before the lines go into the capacitor banks.

thanks
 
Your capacitor sizes are too small. I have built several 3 hp RPC and it takes about 35 mfd per hp. you would need about 105 mfd total with 60% of it on the phase where the start capacitor is loacated. This is not true for all motors as I have found some that require less and some that require more.

Try increasing the Cp by 20 mfd at a time till you see some changes in the voltage. I suspect you can run it with 60 mfd on Cp and 40 mfd on Cs. Be sure to check the current in the generated leg to be sure it does not exceed the FLA rating of the idler. If the current gets too high you will have to reduce the capacitance some.
 
"This is not true for all motors as I have found some that require less and some that require more"

Quite true.

Motors may be rather roughly divided into these groups, each having somewhat different characteristics and requirements as an idler:

1) pre-NEMA (generally, pre-1952),

2) early NEMA,

3) mid-NEMA, and

4) late-NEMA/High-Efficiency (generally, 1999 and later).

The 1, 2, 3 and 30 R.O.T. which I developed considered early and mid-NEMA motors, as that's what I had to work with.

Pre- and late-NEMA motors, and possibly some early- and mid-NEMA motors, too, may require more or less capacitance in each of the four positions.

(The deviations from any R.O.T. become exaggerated as the idler HP becomes much smaller than 1, IOW "fractional HP", or much larger than 5, IOW "large integral HP").

Generally, the 60/40 R.O.T. (after F. Williams) is a good start for good dynamic performance, but optimum could be greater or lesser (63/37 or 57/43, for example).

Williams' 60/40 R.O.T. is implicitly included in my 1, 2, 3, 30 R.O.T.

Peter
 
Peter,

If I've got to up the overall capacitance, to say 60uF on ac and 40uf on bc would the ROT suggest 20uf as Cpf. Restated, if 1-2-3 (or 3-2-1 as I've described it above) suggest that the power factor capacitor would be 20uF or 1/3 of the 60uF that I will probably settle on for Cp?

I don't mind ordering in some extra run capacitors, we're building at least 2 RPCs (one for me, one for a buddy) and when we're done, I'll probably sell the remaining run capacitors on one of the popular internet auction sites to the next guy who needs a good selection to balance a motor.

Let me know if you think 20uF would work for a Cpf assuming Cp = 60uF.

Thanks,

Bob
 
toolnut,

Thanks for sharing the practical experience, I'll order in a few more caps and get this balanced out. I never went high enough for Vac to significantly exceed Vab. It looked like I needed more on Cp, but I actually ran out of caps - except for (4) 1.25uF units that wouldn't have made the difference against the hassle of wiring them in parallel.
 
The issue is a pre-NEMA motor may require less capacitance than the R.O.T. suggests (possible much less than is suggested), whereas a late-NEMA motor may require more capacitance than the R.O.T. suggests (possible much more than is suggested).

I would have expected the B-phase (what you are calling the C-phase) voltage to go much higher, but it hasn't, possibly because your idler is much more efficient (lower per-unit impedance) than a typical motor. In which case, continue to add capacitance, but do monitor the current and don't exceed the FLA of the motor.

I always start out with lots of extra caps, and I buy these as new or "pulls" from Burden Sales Co Surplus Center in Lincoln, NE.

This link ...

http://www.surpluscenter.com/sort.asp?numrec=62&sort=1&search=370 run capacitor&catname=electric

... will give you a listing of all available 370 VAC capacitors. The duals are listed with the singles. I use singles, for the most part.

Burden's prices are quite reasonable compared to Grainger.
 








 
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