Another Rotary Phase Converter

this may sound a little crazy or irrelevant, but is not meant to be disrespectful-

i think you could reduce the start capacitance and try to run a smaller motor first on the system (without balancing caps either)
to test your system before hooking up a 15hp motor.
 
this may sound a little crazy or irrelevant, but is not meant to be disrespectful-

i think you could reduce the start capacitance and try to run a smaller motor first on the system (without balancing caps either)
to test your system before hooking up a 15hp motor.

Mike, your comments are always welcome. :)

But...There is no start capacitance. The system is spun up to operating speed by a pony motor to reduce the start load. Upon reaching operating speed, the pony motor switches out and simultaneously the idler contactors pull in. The lights didn't even flicker when I fired it off. It ran on the idler for about 15 seconds before the breaker tripped. It sounded a little growly but I assumed that was because there were no balancing caps switched in. I do have a small 3 phase motor I could lash up and see how it reacts with that.

If you have any other ideas I could use some suggestions. Maybe I'm missing something.
 
Hi Jim,
that's right, you are using a pony motor to start the idler, my mistake.
if the pony motor is up to speed and then single phase power is applied to the idler- it should run if everything is copacetic with the motor.

3450 rpm RPC's take a second longer to spin up
 
Hi Jim,
that's right, you are using a pony motor to start the idler, my mistake.
if the pony motor is up to speed and then single phase power is applied to the idler- it should run if everything is copacetic with the motor.

3450 rpm RPC's take a second longer to spin up

That's what I was thinking.

My RPC works is a little different than most I have seen. The pony motor is powered by a small VFD. The accel is set to 10 seconds. Once it reaches operating speed, 3450 RPM, the output relay in the VFD is programmed to energize and that energizes a control relay which kills the run command to the VFD and simultaneously pulls in the idler contactors. The VFD is programmed to coast to stop, so the output section just shuts off and allows the pony motor to freewheel.
 
got it!
yes sir, the design is a bit different- but i like the thought put into the creation. very nice engineering.

i'm sure it will work, we just gotta pin down the problem.
the low resistance on the 15hp motor winding sounds like the real problem

you should only get a grumble from the system when it runs right before balancing is switched in
the idler motor is already spinning so the inrush should be low and momentary.

something else to consider
make sure that the idler and pony motors are running the same relative rotation.
i'd hate to think of what would happen if the pony spun the motor in one direction when the idler would want to run in the other direction.
 
something else to consider
make sure that the idler and pony motors are running the same relative rotation.
i'd hate to think of what would happen if the pony spun the motor in one direction when the idler would want to run in the other direction.

That would be a major problem. o_O

But let's see if I understand something correctly. Since the idler is already spinning at speed and direction, shouldn't want to continue spinning in the same direction when the single phase power is applied? Does the phase relationship apply to the single phase input? Now I'm realy confused. :confused 3:
 
anytime you switch the single phase supply legs, you reverse rotation
the same is true in 3 phase supply

something to consider anyway
 
anytime you switch the single phase supply legs, you reverse rotation
the same is true in 3 phase supply

something to consider anyway

Well that's interesting. I'll try reversing the pony motor after I check the resistance of the individual windings. I didn't go into it that deep yesterday.
 
"anytime you switch the single phase supply legs, you reverse rotation"
is that really true? It isn't for a single phase motor IIRC
Mark
 
i'd hate to think of what would happen if the pony spun the motor in one direction when the idler would want to run in the other direction.

In this case it seems to make no difference. Same result, about 15 seconds and the 50 amp breaker trips. I tried all possible combinations of rotation and connections with no difference. The load seems to be about 15KW, or about 62 amps. In checking the individual windings, I got 0.1 ohm across the board. I'm thinking a bad motor.

I managed to get a screen capture of the operation, the red line is the shop power, the blue line is total property power. Power in KW is vertical. The first spike was me hitting the E-stop before it was fully energized. The next two spikes are the time before breaker trip. When I had it wired as the original test, and only reversed the rotation of the pony motor it ran a couple of seconds longer than the second test. But in the second test the breaker was already warm so that may have been the time difference.

1510515305326.png
 
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