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10 HP rotary phase converter issues

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Esmith41

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#1
Hello everyone, first post. I built a rotary phase converter and all seems well except when the converter is started up the 10 hp motor shaft locks up I can’t turn it by hand until I turn off the converter. I tried moving the 3 phase wires around on the motor leads with no luck. I wired the motor for low voltage per the attached diagram. 73A0F858-B804-4407-8B97-D9CBBC8B5CCD.jpeg 1AE501F1-A6F3-451B-B061-2C0A15A10AEE.jpeg 27DFBCD8-A4E7-4399-8E84-EF68478F2A22.jpeg 50E982E0-4292-4CB8-9096-AB3C474CDBBA.jpeg
I have not been able to balance the voltage yet due to the motor not generating the 3rd leg. The motor is brand new out of the crate ( see photos) 60 amp breaker at supply panel.
Any help will be greatly appreciated.
 

markba633csi

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#2
If it's humming loudly when it locks up then something must be mis-wired I would think- check your connections again- does the motor spin freely
when powered off? No problem with bearings? I'm also wondering if maybe you don't have enough starter cap?
Mark
 

dlane

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#3
Alls not well if it locks up, sounds like the rpc motor to me. , Ied vfd it if doable
 

Esmith41

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#4
The start capacitor is around 500 micro ferands. Just to be clear this is the “pony” motor used to generate the three phase power that I am having trouble with. It spins freely when not energized. I will look at the motor wiring again.
 

Canus

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#5
Think your start capacitor is too small. Try a 750-850 mfd 400 volt. Usually a good rule of thumb is 75+ mfd per horsepower.
 

JimDawson

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#6
I'm having a little trouble following your wiring in the panel, could you provide a drawing of the circuit?
 

Esmith41

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#7
Here you go. I am by no means an electrician or electrical engineer but here are “my schematics”. Probably not much better than the picture of the RPC. 90D89C36-195B-4B77-8499-6C680127D1BD.jpeg
 

markba633csi

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#8
I'm not 100% sure but I think your pushbutton might be miswired-
 

JimDawson

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#9
The attached PDF might help you. As @markba633csi said, I think the double throw switch is wired incorrectly. In the attached file, you can ignore the control power transformer and use full voltage coils in the contactors. The main idea is to temporarily connect the start cap between one of the input legs and the manufactured leg. Cpf in the drawing is not required, but does make the system more efficient.
 

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Esmith41

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#10
I highlighted the circuit for the start capacitor. I think I have it wired correctly. The only thing I see I could do differently is move the the supply power for the capacitor from the load side of the contactor to the line side. This would give power to the capacitor before switch is pushed and the contactor is pulled in. I have of the the momentary switch throws filed down so the capacitor is energized a split second after the contactor is energized.
Thanks for your suggestions everyone, Just talking about it and getting a different set of eyes on it make me think a little different and hopefully gets me closer to a solution.
Eric
 

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markba633csi

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#11
Your switch symbol is not clear, it looks like the contacts connect top to bottom? Tip: use standard schematic symbols
A la:
2psw1a.jpeg Mark
 

JimDawson

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After looking at your drawing, I think I agree that it's wired correctly. If your switch is wired correctly it looks like it should start the motor when the switch connects the start cap to the circuit. I think the start cap needs to connect before the contactor closes, then disconnect when the motor is spun up.
 

Esmith41

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#14
I found a loose spade end on one of the connections at the start capacitor. I repaired the end and the motor spun instead of locking up. It would not get up to speed so I’m thinking it will need another 300 mfd start capacitor to get it up to speed on the start. It may be awhile before I can get to it but I will let you know how it turns out.
Thanks again for your help.
Eric
 

markba633csi

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#15
It's always some little gotcha that gets ya
 

Esmith41

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#16
hey folks. A little update. After moving the start capacitor to the line side and making it so the the start capacitor engages before the contactor, the motor finally turned instead of locking up. Now that it was finally turning, the motor did not come up to speed. I added another 200 mfd start capacitor and this did not help either. I checked my voltage on the line side and the load side on start up and found the voltage droppping down to 175v. I am using 10 awg wire from a 60 amp breaker for approximately 6’. According to the voltage drop calculator I should be just fine, as it is only loading about .17% of the volts. Not being sure if upsizing the wire would work I hooked a 5 hp 3 phase motor and it ran perfect. I balanced the three lines and my Lathe is finally up and running. But, it still bothers me that the 10hp will not come up to speed. Do you think if I upsize the conductors to say, 8awg it will make a difference?
Thanks,
Eric finally going to make some chips for the first time Smith
 
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Keith Foor

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#17
A few things to do.
First isolate the run capacitors while the motor is starting.
Second, the motor power contactor needs to be closed while spinning you the motor.
Start time should be 5 to 6 seconds max on a motor that small. If it's not coming up, something ain't right.


I would be curious about the current draw during startup when you are seeing 175 volts across the 240 line.
I would advise you to read my other RPC posts and then redesign accordingly.

From what I am seeing you are using a panel mount pushbutton switch to connect your start capacitor.
That is a disaster waiting to happen. Those switches are rated for 3 to 5 amps. 10HP 3 phase motor, being forced to start with a start cap and ont 3 phase power, the current draw is an easy 40 to 60 amps. The push button is just not designed for that. You need a 30 amp contactor or relay for that power level.
 
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