Rotary phase converter question

Is it possible to start a 20 hp lathe with a 7.5 hp RPC? I'm not expecting to get anywhere near full power but I just want to test the lathe and see it everything works correctly. I just don't want to hurt the converter or the lathe by doing so. The lathe in question would be a Monarch model 62 dyna shift.

dgibson66 said:

Is it possible to start a 20 hp lathe with a 7.5 hp RPC? I'm not expecting to get anywhere near full power but I just want to test the lathe and see it everything works correctly. I just don't want to hurt the converter or the lathe by doing so. The lathe in question would be a Monarch model 62 dyna shift.

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We agree the lathe is 'unloaded' and all that.. which means you could 'slow ramp' it into motation with a VFD of comparable undersize (approx 1/3 the need).

RPC don't have the luxury of 'ramping' to that extent. Worse, if you look up what commercial providers of RPC publish, starting can be tough and may require far larger idlers than load motors.

What you MIGHT consider is some means of using the 'pony motor' approach to spin-up the lathe before triggering the RPC.

Major downside is that if the load and RPC phases are not pre-matched pretty well, there can be breaker-tripping arguments when you cut-in the RPC.

Alternative approach...

What is your local cost to rent a 3-Phase capable Diesel gen set for a day or so? 30 kW about the minimum, more is better, ELSE you still have significant stress.

Bill

Since you only want to make it run unloaded, try starting it with capacitors, in effect a static converter. That is the way an RPC is started. Just leave off the idler motor and start it directly, then disconnect enough capacity after it is running to make it run without making weird noises. Excess capacity makes a loud, high pitched whine.

Bill

1000 Mfd starts my 15 snappily, so say 1300 for a 20. It lets you know in no uncertain terms if you don't drop these out fast enough

My thread on it

http://www.practicalmachinist.com/v...-and-vfd/how-make-old-15hp-3-phase-go-158528/

9100 said:

Since you only want to make it run unloaded, try starting it with capacitors, in effect a static converter. That is the way an RPC is started. Just leave off the idler motor and start it directly, then disconnect enough capacity after it is running to make it run without making weird noises. Excess capacity makes a loud, high pitched whine.

Bill

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After reading johnoders link and the suggestion of making a static converter I'm leaning towards buying a capacitor or two and trying it that way. I'm guessing the breaker size would be the same requirement for a static and rotary at the same given HP. I'll get on the RPC VFD forum and do more research. Thank you to all that shared

dgibson66 said:

After reading johnoders link and the suggestion of making a static converter I'm leaning towards buying a capacitor or two and trying it that way. I'm guessing the breaker size would be the same requirement for a static and rotary at the same given HP. I'll get on the RPC VFD forum and do more research. Thank you to all that shared

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Basically 'for testing purposes', you will be 'borrowing' the lathe's 20 HP for use as an PRC. Do your calculations as if that were the case and it HAD NO load.

Once motating, you can add the 7.5 HP as booster idler. That should then offset the - I'd guess approx 5 HP - it will take to carry lube pumping, gear and clutch drag, testing carriage & cross power traverse and even taking fair-decent cuts.

Bill