RPC pull start issue

[crashexpert]

I just picked up a 15 HP motor to build a RPC. I wired single phase wires in and then wired all three out put legs to a terminal block to test before hooking anything up to it and I found some weird issue going on. The 3 legs are really imbalanced.

L1-L2= 220
L2-L3= 208
L1-L3= 240

Then I noticed it will only run if I pull start it clockwise looking at the shaft. If I pull it counterclockwise looking at the shaft the breaker trips in the single phase box when I throw the incoming power on. Is this a bad winding? I don't understand why it will run one direction but not the other. Does it matter what 2 legs on the motor the incoming power is on? Should I swap it around? I wouldn't think that would matter but I'm lost at this point.

 

[jim rozen]

What sort of voltmeter? DVM or an analog (old school, simpson) meter? There are no capacitors of any kind, correct?

 

[CJ5Dave]

I have one like that. Only starts cw with rope and no start caps. Only starts ccw with start caps in line. And with no load volts are everywhere.

 

[johansen]

240/220/208 is normal.

My 5hp (93% efficient 91% power factor) was 240/216/212 iirc

I have tried to make sense of it and what i think is that the nominal 7fold fla amps at startup is usually half resistive half inductance. Which makes the inductance of the motor to be 1/14th or 7% at full load amps.

However, because the generated leg has to come from the rotor, which is driven by the primary leg, you have two 1/14th voltage drops in series, which makes 1/7th. Approximately the voltage drop found by millions. However, the amps drawn by the primary coil are only 57% of nominal full load amps at no load so the 1/7th voltage drop should be more like 10%. This however neglects the resistance losses in the motor.

Anyhow some capacitors across your 208 leg will pull it up. Motor run caps are pretty cheap.

 

[rons]

Then I noticed it will only run if I pull start it clockwise looking at the shaft. If I pull it counterclockwise looking at the shaft the breaker trips in the single phase box when I throw the incoming power on. Is this a bad winding? I don't understand why it will run one direction but not the other. Does it matter what 2 legs on the motor the incoming power is on? Should I swap it around? I wouldn't think that would matter but I'm lost at this point.

What is the current rating on the breaker?
L1, L2 are the input side. T1, T2, T3 are output side.
Swap and you should see a CW to CCW reversal in behavior. Doesn't make a difference.
The numbers are typical and will vary more when a load is attached. Time for capacitors.

 

[crashexpert]

What sort of voltmeter? DVM or an analog (old school, simpson) meter? There are no capacitors of any kind, correct?

Fluke. Can't remember the model.
No capacitors at all. Just a 3 phase motor wired to single phase

 

[crashexpert]

What is the current rating on the breaker?
L1, L2 are the input side. T1, T2, T3 are output side.
Swap and you should see a CW to CCW reversal in behavior. Doesn't make a difference.
The numbers are typical and will vary more when a load is attached. Time for capacitors.

60 amp breaker. Idle amps are 23 per leg. Not sure what loaded amps will look like yet.

I see all kinds of info but can't make since of some of it. How do I know what type of cap to use? I saw something that said "X amount" of mfd per volt needed increased. I would think I would want to hook up and run my lathe first and calculate balance from the normal load not the idle Imbalance correct? I was scared to hook my lathe up to such an old voltage but it seems like that's normal and won't hurt anything according to these replies

 

[CJ5Dave]

An old book I have somewhere says 40 to 70 mfd of run capacitor per idler hp. Adjust to somewhere in between.

 

[johansen]

Here ya go

50 MFD 370 Volt AC Run Capacitor CDE SFT37T50H291D-F | Motor Run Capacitors | Capacitors | Electrical | www.surpluscenter.com

50 MFD 370 Volt AC Run Capacitor CDE SFT37T50H291D-F, Motor Run Capacitors, 50 MFD 370 VAC RUN CAPACITOR New, CORNELL DUBILIER

www.surpluscenter.com

I doubt you need a full 10 of them, i would start with 4, 3 across the 208 leg and 1 across the 220 leg.

For every amp flowing through the run cap, it will pull 1 amp off the idle line amps at the supply. Tis relationship doesnt hold forever but your idle amps of 26 suggest you need about 250uF (its about 1 amp per 10uF at 250vac)

 

[rons]

60 amp breaker. Idle amps are 23 per leg. Not sure what loaded amps will look like yet.

I see all kinds of info but can't make since of some of it. How do I know what type of cap to use? I saw something that said "X amount" of mfd per volt needed increased. I would think I would want to hook up and run my lathe first and calculate balance from the normal load not the idle Imbalance correct? I was scared to hook my lathe up to such an old voltage but it seems like that's normal and won't hurt anything according to these replies

I noticed a relationship between run capacitors.
The capacitors between L1 to L3 (C13) and L2 to L3 (C23). L1 and L2 are incoming line.
C13 divided by C23 equals 1.8. This figure holds for two different horsepower idler motors and the same target 1Hp motor.
In my case a 5Hp RPC, C13 = 90 and C23 = 50. A 3Hp RPC, C13 =50 and C23 = 27.5.

I wouldn't be surprised if I had to swap C13 and C23 when I change the rotation of the 5Hp idler.
For 15Hp idler I would use those 100uF aluminum cans. Probably 3 100uF cans for C13 and a 100uF plus a 50uf for C23.
I don't like those black plastic case start capacitors. They eventually fail. Oil filled is a peaceful existence.
A rope start is not very elegant in this push-button age.

 

[BT Fabrication]

No capacitors at all. Just a 3 phase motor wired to single phase

There is your problem..... wont work like that. just dead heads the motor and heats the windings and trips the breaker.

buy a panel with everything in it.

 

[Nmbmxer]

It's hard (impossible?) to balance the line voltages at both idle and a full load. Mine at full load is really close, I don't recall the exact voltages. Idle has the wild leg different by maybe 10v but the single phase legs are close. This is a 25hp idler that I used with a 10hp CNC lathe. I used a spreadsheet or table that was posted here to get the cap values and then tweaked it from there. I got a box of various sizes from Surplus Center for cheap.

 

[JST]

There is your problem..... wont work like that. just dead heads the motor and heats the windings and trips the breaker.

buy a panel with everything in it.

It can work perfectly well, if pull, or pony started. It won't start without help, of course.

The 3 phase happens regardless of any capacitors, etc, once it is started.

The issue with capacitors is that the output voltage has to be (by physics) lower than the line voltage. The capacitors are a relatively crude and load-dependent way to raise the voltage.

As for stable voltages.... The idler cannot supply as much current as the power company does directly. But the larger the idler the more stable the voltage is, because the total impedance in series in the generated leg is less.

A better and somewhat more stable way to adjust the generated leg voltage is with a transformer. That is less load-dependent than the use of capacitors.

 

[jim rozen]

The real question nobody's adressed yet is why the 1,3 and the 2,3 voltages are different. (1,2 is probably the incoming line)

My first suggestion is to use an old-school analog meter to measure those voltages, the Fluke DVM might be fooled in some way. I would put NO capacitors on the motor until the difference between the manufactured leg and the two lines is close. Right now they're far apart.

 

[JST]

We do need to know which is the incoming. I'd expect the 240V is the input, that makes the most sense.

The 220 and 208 are close enough to believe for a motor running as an idler. The voltages from the manufactured leg to the incoming lines are normally lower than across the incoming without capacitors.

 

[jim rozen]

The 220 and 208 are pretty far apart for an idler with no compensation capacitors. They're also oddly near aliquot values which suggests something other than a good voltmeter are measuring (wigger?). Again suggest an analog meter for the test.

 

[JST]

The difference is only 5%. There is a good case for some errors in measurement, possibly due to a meter, possibly due to them being obviously taken at different times, (one meter, three measurements).

Analog meters are good for some things. For AC voltage, however, they typically only measure average, and are corrected by a fixed ratio to get approximate rms readings. There will be errors in the presence of any significant distortion of the waveform. Non-true-rms "digital" meters also only measure average, and apply the same correction.

The generated voltage will be a few percent under the incoming, to allow current to flow in the motor, since the generated voltage is just the back EMF. And, while phase is normally good, it may not be absolutely perfect, which can also cause a voltage difference.

Lots of sources for small voltage differences.

 

[DaveKamp]

Crash, like others have said, the voltages you see, that's entirely normal. You can hook your lathe to it, and run it with no serious concerns. If you have a mill AND a lathe hooked to it, start one, and just let it spin... and operate the lathe. This makes the mill a second idler, which will help equalize the voltages. When running the mill, let the lathe idle. The fact that you have more rotating mass, more windings, more inductance flying around, flux circulating in iron laminations, the 'stiffer' your converter will be.

You can add capacitors, and it will equalize the indicated voltages a bit, or you could become a purist, adjust their values 'till you go crazy, and then, when you turn on a load, the voltages will change. As JST noted, you could put a transformer in there, but in applied result, the effect is, for most cases, functionally insignificant. The voltage you see, WILL CHANGE with respect to change in load... capacitors or not, it will change. IF you feel the variation is affecting your machine's abilities, you'll need to step up to a different solution... VFDs, or 3-phase service.