Rotary Converter - Idler Hot

M

martik777

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After 1/2 hour, my idler is running approx 140F but the lathe motor is only at 95F. Is it normal for the idler to run hotter? It's rated at 50C temp rise so no issue there.

Both at 220, 3/4HP idler, 1/2HP lathe motor, no caps just a rope pull for now.

Also, my lathe motor only has 3 wires and NO Ground so I ran a separate ground to the frame. Is this ok?
 
Usually you want the idler motor to be twice the size of the service motor. You could build your own cap box, or buy one from Phase Craft or one of the many other suppliers of rotary phase converter control boxes. Jim has plans available to build them if you search here. The ground wire is fine IMO.

michael
 
you'll be ok grounding the motor as you did!

you can reduce the idler's heat by adding run capacitors between phases.
a small motor may only need around 10uf between phases to balance.
 
you may wish to use oil filled capacitors for longer life.
the small caps in the link may work for a period of time.
they are generally used on very small motors, but may work ok for balancing-
i just can't say for certain what length of service life you'll get from the capacitors.
 
Put your capacitors in a metal cabinet. There will be occasions that the capacitors can fail, and when they fail they pop. This can be a bit un-nerving if they are in open air blowing crap all over the place. A NEMA box will contain the caps and add a factor of safety.
 
The idler runs cooler if the load motor is running. If you are going to spend a lot of time between cuts, best to shut the RPC down. I'm installing remote switches at my SB 13" and BP J to turn the RPC on and off. (I have a contactor to start the RPC, so this is easy.)

John100_8404.JPG
 
Good idea, I was thinking, what's the point of a contactor circuit when you still have to go thru 2 steps to start the lathe motor. You will need 2 remote switches, for start and stop? It would be nice if the whole process was controlled by the lathe drum switch.
 
OK, seeing the RPC I can tell you why it's getting hot. You have one capacitor in there and I would be willing to bet that it's the start cap for the RPC. Every RPC I have ever built or worked on had a start capacitor or bank of capacitors and a set of run capacitors that were in between leg 1 and 3 and 2 and 3 with the single phase power being fed on legs 1 and 2. Having no run capacitors in between the first 2 fed legs and the 3rd leg makes the idler motor run as if has "lost a leg". That's the term used if one leg of a 3 phase feed fails. It's really rough on the motor. The third leg will float wildly in voltage with no capacitors or load. When you turn on the second motor it loads the 3rd leg and the motor cools off some but I am betting that you are loosing torque capacity on the load motor. The way a rpc works is that you are creating a tuned circuit of sorts with the capacitors and the winding of the 3rd leg. The tuned circuit is resonant at or about 60 Hz and it "rings" if you will creating the third leg voltage. The capacitors also feed some voltage in from the other 2 legs bumping the resonant circuit so it keeps ringing. Without the capacitors the 3rd leg winding just builds a magnetic field and then has no where to dump it since the capacitors are not there. Because of this no load condition, the magnetic field will claps quickly and the resulting voltage is VERY high. If you check your 1 to 3 and 2 to 3 leg voltages you will find them Much higher than the 1 to 2 voltage which should be about 240 volts. In an ideal situation you want the 2 to 3 and 1 to 3 voltages to closely match the 1 to 2 voltage. the only way to accomplish this is with run capacitors across the legs as discussed above. A number of converters I have built used the power from the building lighting circuit for the control relays, so the lights had to be on for the converter to start and run. You turn on the lights to work, when you needed to converter, you hit the start button and it did it's thing. At the end of teh day when the lights were turned off the converter would power down. That way it wasn't running all night.
 
"OK, seeing the RPC I can tell you why it's getting hot. You have one capacitor in there and I would be willing to bet that it's the start cap for the RPC. Every RPC I have ever built or worked on had a start capacitor or bank of capacitors and a set of run capacitors that were in between leg 1 and 3 and 2 and 3 with the single phase power being fed on legs 1 and 2. Having no run capacitors in between the first 2 fed legs and the 3rd leg makes the idler motor run as if has "lost a leg"."

That's MY RPC, not the OP's. It runs fine and is the standard Ronk design with disconnect and starter added. If you read the Ronk instructions they talk about temp rise and current running idle and loaded. They suggest other caps for adding additional load. Not needed now.

John
 
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