Why?

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mcclary's electrical

Senior Member
Location
VA
Today, I worked on removing a 40 hp diesel motor and converting it into a 30 hp electric motor. 240 volt single phase. Feeding a rotary phase converter (40 hp converter).




At the motor I mounted controls and wired starter. 240 coil on starter. I used control voltage from b&c phase, and tried to run it. When the starter pulled in, it was short cycling very quickly, chattering. If I determed the motor leads from the bottom of starter, the starter would engage fine, hook leads back up to motor, and the starter chatters.
After verifying there were no loose control connections, I changed my control voltage taps to a&b phase. The starter performs perfectly.
I'm trying to understand why the coil doesn't perform off of b&c phase, but it performs perfectly on a&b phase. I read 240 volts at the coil either way.

What is it about the rotary phase, that the coil does not like?? What does such a voltage look like on an oscilloscope?
 

templdl

Senior Member
Location
Wisconsin
Personally I would check the control circuit. If you are not maintaining consistant voltage across the coil that pull in the starter, either not a hgh enough voltage of that the control circuit itself is pulsating the coil will cycle.
This is an excellent way to burn up a set of contacts and weld them together in the closed position.
 

mcclary's electrical

Senior Member
Location
VA
Personally I would check the control circuit. If you are not maintaining consistant voltage across the coil that pull in the starter, either not a hgh enough voltage of that the control circuit itself is pulsating the coil will cycle.
This is an excellent way to burn up a set of contacts and weld them together in the closed position.

It's not in the control circuit. I covered that completely. It's coming from the "created" c phase.
 

mcclary's electrical

Senior Member
Location
VA
Been a while since I connected a rotary phase converter but I believe the directions stated to not use the generated phase for anything but motor....which you very well know by now.

cool, some directions would have been nice here. This stuff was purchased years ago and stored for years, waitning for the day the diesel died. No documents for anything.

What does the sive wave look like?
 

Rick Christopherson

Senior Member
As was mentioned, you never want to use the generated phase for any control circuitry--including the self-sustaining coil on the relay circuit. The reason for this is because the voltage of the generated leg will not be stable as the load changes. In your case, you have seen how the generated leg will drop in voltage as the load motor ramps up to speed. The voltage dropped below the pull-in threshold of the coil.

The rapid cycling was due to a "buzzer effect" in that as soon as the relay energized, the voltage dropped, causing the relay to drop out, which in turn removed the cause for the low voltage, allowing the relay to re-engage.
 

Rick Christopherson

Senior Member
I'm still wondering what this type of voltage source looks like on a scope?.
Instead of a scope, I think a phasor-like diagram might be more informative. A scope is still going to show a reasonable sine wave, but the phase shift and voltage fluctuations won't be as easy to notice at a glance.

I have an old article on Phase Converters, and one of these days will finish the re-write to update it. In the mean time, here is one of the newer graphics from it (actually, I think this graphic was created specifically for one person's converter, which is why I show the specific voltages).

converterpower.jpg
 

BJ Conner

Senior Member
Location
97006
Check voltage drop on starting

Check voltage drop on starting

The voltage drop on starting may be enough to drop out the coil, the motor stops, the voltage rises, the coil picks up again the motor tries to start again.
Repeat Above.
 
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