Rotary Phase Converter Starting problems...but only sometimes.

Hi all,

I have a fairly unique starting problem with a rotary phase converter I'm helping a friend build. The motor we're using is a 10HP 1760 RPM off of an old air compressor. It works great on 3-phase power and even starts quite well as a Fitch Williams rotary phase converter...MOST of the time.

The problem we're running into is that there seem to be three positions the shaft can be in (~120 degrees apart) where the starting circuit, even with 675uF of starting capacitors all in, fails to spin the motor up but leaves the motor growling and frozen in place. If the converter is started with the shaft in ANY other position (we're talking less than 5 degrees rotation here), it spins up so quickly that it jumps a little on the cart.

To test the capacitors and starting circuit, we hooked up two other motors (7.5HP and 15HP), one at a time, and both started quickly every time (which seems to indicate that the capacitors are good and also of a sufficient quantity for the 10HP). We've confirmed that the 10HP motor is wired correctly for 240v (and not 480v) and that all connections are tight.

This problem has been constant from 400uF worth of start capacitors all the way up to the 675uF currently in place. Changes were made in about 50uF increments.

Testing is being done with a 50A circuit supplying power an appropriately sized magnetic contactor switching the motor power on/off and another appropriately sized contactor switching the start capacitors in/out of the circuit.

We're using the Fitch Williams diagram (found here: https://www.practicalmachinist.com/FitchWConverter.pdf) and are trying to get the basic operation going. No run capacitors or power factor capacitors have been added yet.

Does anyone have any thoughts on this? We're stumped, especially where the only variable causing issues seems to be the motor itself, but only in those three small ranges. Any help would be greatly appreciated!

I had a friend who recently bough a new RPC from a small company. Seems like they use surplus motors so every one may be a little different. Anyway he noticed the exact same thing. Started instantly 95% of the time, but randomly it would sit there growling and fail to start. Looking back at this I bet it was related to shaft position same as yours. He contacted the supplier and they send him a bigger start capacitor and it worked fine ever since.

Yeah bigger caps might do it Or blow it up

Could the motor have weak windings in one phase? Being so it works with the other motors fine ???


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mmurray70 said:

I had a friend who recently bough a new RPC from a small company. Seems like they use surplus motors so every one may be a little different. Anyway he noticed the exact same thing. Started instantly 95% of the time, but randomly it would sit there growling and fail to start. Looking back at this I bet it was related to shaft position same as yours. He contacted the supplier and they send him a bigger start capacitor and it worked fine ever since.

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Interesting idea--I had read that starting caps should be about 60uF per motor HP in one place, and 70-100 in another. I'll have to try adding more. All I have right now are run caps, so that's slow adding, but I'm pretty sure I'd be safe up to the high end of 1000uF. Might trip the breaker, but worth a shot--thanks!

Turbowerks said:

Could the motor have weak windings in one phase? Being so it works with the other motors fine ???


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It's certainly possible--how would we test for that? Is there a simple way with a basic multimeter?

Homebrewblob said:

Yeah bigger caps might do it Or blow it up

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Time will tell, I suppose!

i am thinking you have a broken rotor bar, or several.

Hi condition 001, You said that all connections are tight.
A connection has to be clean to make a good conductive electrical connection.

Jim

Check that the resistance through each winding is very close to equal. For a 10HP 230V motor, you're going to need a meter that is pretty accurate at low values - expect an ohm or so.

Try rotating all the phase connections, i.e. A becomes B, B becomes C, C becomes A.

There's a lot of variation in how one can build a 10HP induction motor, of varying starting torque, current, efficiency, power factor, slip etc. One size does not fit all, although if the dead band is about the same size (i.e. not 25 degrees at 400uF, 5 degrees at 675uF), the caps are probably fine.

A failed rotor bar does sound possible. I'd be curious to see if it started on proper 3~.

Jim Kennedy said:

Hi condition 001, You said that all connections are tight.
A connection has to be clean to make a good conductive electrical connection.

Jim

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All of mine are clean--I'll double-check the ones in the motor box!

johansen said:

i am thinking you have a broken rotor bar, or several.

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Is there a way to test for that?

A quick search pulled up the following, but I don't fully understand it:

nuclearfungus said:

It may have a broken rotor bar. You could try connecting it to 120v single phase on two leads with the motor wired for 480v and see if it "cogs" when you turn it by hand. This means with an ammeter the current varies as you turn the rotor. This is kinda more advanced motor troubleshooting, but, it could save you money on bearings. I've worked as a field engineer and a millwright and when aligning motors and pumps even motors of several hundred hp can be turned pretty easy. So if its hard to turn its the bearings if its not get a new motor.

Robert

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Would a "cog" mean a spike in current? Or a dip? Or?

Thanks!

SomeoneSomewhere said:

Check that the resistance through each winding is very close to equal. For a 10HP 230V motor, you're going to need a meter that is pretty accurate at low values - expect an ohm or so.

Try rotating all the phase connections, i.e. A becomes B, B becomes C, C becomes A.

There's a lot of variation in how one can build a 10HP induction motor, of varying starting torque, current, efficiency, power factor, slip etc. One size does not fit all, although if the dead band is about the same size (i.e. not 25 degrees at 400uF, 5 degrees at 675uF), the caps are probably fine.

A failed rotor bar does sound possible. I'd be curious to see if it started on proper 3~.

Click to expand...

Thanks for the input!

The problem stuck when swapping A and C phases. I'll rotate as suggested and check the resistance as well.

If it were a failed rotor bar, would it still get stuck in the same places on proper three phase?

I had the same issue for years and years. I didn't experiment with more start capacitance but recognized that the orientation of stator to rotator had a dead zone.
I switched to another couple of motors. Funny thing is that the current motor does not exhibit that dead zone problem. It's a new old stock motor and not a used
surplus motor. Maybe this isn't funny anymore ...

johansen said:

i am thinking you have a broken rotor bar, or several.

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Ahha! I had a phase o matic thad did that pita. I had the rotor out several times and rewelded the aluminum that was blown out. Worked great for awile until it did it again. It was a Lincoln motor 20 hp.
They machined it at phas o matic to change resistance for better starting, proprietary stuff they said. I say job security. I threw that in the garbage bought a panel from Phoenix phase converter and used a toshiba motor. The unit is rock solid. Panel could easily be built uses normal stuff. Have to add up the capacitor for start and see where its at for starting for fun.


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Machining for resistance is a "thing". And yes, it can make starting easier. But it sounds as if they overdid it "just a bit".

Normally the only part accessible is the ends, the part that shorts between the bars. All that can be done is to cut that, or part of it, thinner. If they do that wrong, cutting it too thin, it will raise the resistance alright.... high enough for the very heavy currents to melt it maybe. They cannot change the main part of the bars that are inside the rotor, so there is a temptation to cut too much off the end to reach a desired resistance.

Arco has done a similar thing, but rather than modifying, they specified theirs with a double cage rotor.... a much better idea, done at the factory, no machining, it is a design choice. And it allows optimizing both for starting and for running, separately.

Yes thats what they did was machine it on the ends, could have been windings loosing some insulation over time also. It would blow the aluminum out on the end of the rotor , would get plastered on the housing must have been a hell of a arc when it did it.
The last time I welded it up there were cracks around the ring along with a few 1/4 inch wide gaps. I welded the gaps and cracks and then welded a 1/8 inch ring around it. Got me by for 6 months till i got my new one up and running.
Funny story though and nobody could explain it Some mornings I would start it and it would run machines backwards. On my bridgeport the spindle turned the wrong way! Lucky i never started my cnc’s or it could have been costly. Restart the phase converter, check rotation on mill the carry on if everything was spinning right. Witchcraft i blame it ,doesn’t seem possible on a rpc. You have 2 legs from the power company and 1 manufactured leg , how the hell can it reverse direction on a motor?


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If it stops in just the right place, the probable unbalance of rotor resistance can give an off-center torque that can kick the rotor to go the wrong way. Not too surprising actually.

There are ways of machining the rotor to give a number of special purpose motors. You can even hack an induction motor to make it become essentially synchronous!

Maybe just substitute one of the other motors and be done with it.

Use a pony motor and only apply power to the idler when it's spinning?