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Building Rotary Phase Converter...Running into Issues

ParkerMcD

Plastic
Joined
Nov 23, 2019
Hello All!

New to the forums and looking for a little help on my rotary phase converter project. I'm hoping someone can review my schematic (specifically the potential relay/start capacitor portion) and see if there are any obvious issues with it. I'll explain all the necessary details below:

I recently built a 10 HP phase converter for my dad so he could run his giant 5 HP bandsaw out in the shop. I came up with my design watching YouTube videos and reading through the Fitch Williams design (link below to my schematic and reference designs).

The design is pretty simple: 1) contactor for turning power on/off to idler motor/system 2) capacitors for balancing AB/BC phases 3) decoupling caps to reduce input current 4) motor start capacitors to lag current in AC winding and 5) potential relay monitoring AC phase to disconnect start capacitors once motor is spinning.

Managed to wire the system up according to schematic and had everything working. Idler motor fired up when start button was pressed, and shut down when stop was pressed. Great! I went through and balanced each phase and finally wired the bandsaw up and had it running without a hitch. This is where the trouble started:

The potential relays I purchased (APR-5 by Supco) have an adjustable pickup voltage and 1 second auto-open safety feature that in theory makes it impossible to destroy the start capacitors. Up till this point I had been relying on the 1 second safety feature to remove the start capacitors from the circuits. This actually worked really well, but there was some extra hesitation during turn-on I was hoping to get rid of by tuning the potential relays. So following the user manual I calibrated the potential relays according to the manufacturer, stepping the pick-up voltage down in 30V increments and each time seeing if the motor would start up without the 1 second safety features kicking on. Well, I stepped all the way down from the max 370 to min 130 and never found a good value. Each time the safety feature kicked in or as in the case I set the pick-up to 130, the relay immediately opened up and the motor never started.

Issues with design:

1) Can't find a good pick-up voltage. Either the safety feature kicks in, or the start capacitors drop out so quick the motor doesn't start.
2) The potential relays chatter a lot as the motor is ramping down. I thought the relays would only click one time...when the pick-up voltage was crossed during ramp down.

If anyone has any ideas as to why the APR-5 isn't working in this configuration I would much appreciate it. Thanks!

Cheers,
Parker

Picture of System:
https://drive.google.com/open?id=1hEHcPGZSzBU6hymc9i9mA9i1yQ9OuB-a

System Schematic:
https://drive.google.com/open?id=1sfoMThZ3OLVQ1Zz9z5p2bhmJE-RRM-ZO

APR-5 Calibration Manual:
https://drive.google.com/open?id=1-I8VkzNmz2uRfdg2zBtPAFmQ0SsgOOqT

Fitch Williams design:
https://drive.google.com/open?id=1o7hpcLfr2U04sfJqV8vwtAw9Pypk8SaJ
 
Ok I may have figured out what the problem is!

I have my potential relay coil and start capacitors connected across phase AC. According to the APR-5's datasheet the coil and start capacitor are not connected across the same phases. That could explain why I was unable to sense a drop-out voltage.

I believe the correct wiring is to hook the coil across BC and the start capacitor across AC. This way I'll be monitoring the back EMF of the motor on leg C, without the start capacitor effecting the voltage I'm trying to sense.

Updated Schematic:
https://drive.google.com/open?id=1dlj8imMR5xkzQNaQu0x_ZbBh3Q6sCu4V
 
Ok I may have figured out what the problem is!

I have my potential relay coil and start capacitors connected across phase AC. According to the APR-5's datasheet the coil and start capacitor are not connected across the same phases. That could explain why I was unable to sense a drop-out voltage.

I believe the correct wiring is to hook the coil across BC and the start capacitor across AC. This way I'll be monitoring the back EMF of the motor on leg C, without the start capacitor effecting the voltage I'm trying to sense.

Updated Schematic:
https://drive.google.com/open?id=1dlj8imMR5xkzQNaQu0x_ZbBh3Q6sCu4V

I'm in the process of building my own 10hp rotary converter. Curious if the change you made has led to success.

Thanks,

Jeff
 








 
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