soft start

My Supermax has landed. This knee mill has both horizontal and vertical spindle motors. They are 5 hp. The vertical spindle will start with my 3 phase converter system, but repeated start/stop/start/stop will trip the breaker. The horizontal spindle trips the breaker unless the drive belts are removed. And then it "just" starts. Running load is just a few amps and I know I'll never go to full horsepower cutting.

I have a 30 amp breaker on #10 wire going to a homemade "Fitch" RPC. Then #12 wire throughout the shop. I can't read the nameplate on the RPC motor - I think its a 3 hp.

I'm looking for suggestions to be able to start this mill without a complete re-wire job. Some kind of soft start?? larger RPC pony motor?? I'll consider a VFD only as a last resort.

Otherwise, this is one serious mill. I can now look down my nose with disdain for those folks using a Bridgeport.


Reply to
Karl Townsend
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Larger idler motor on the phase convertor would probably do it, as would starting the vertical spindle before trying to start the horizontal so the vertical motor acts as an additional idler. The VFD is certainly the best way to go, and you could with appropriate safety interlocks multitask a single VFD to run either motor (one at a time).

Reply to
Pete C.

Karl, I once owned a hydraulic shift Cincinati knee mill with dual spindle motors as well. Lovely machine. Depending on the motors and the number of wires available, you could use a two way start switch where you start the motors in a "Y" configuration and then, when up to speed, switch them to delta for running under load, but you need 12 wires not 6.. Steve

Reply to
Steve Lusardi

There are two easy and obvious answers:

1) Use a bigger idler (easiest and cheapest by far)

2) Use a VFD (whether it is easy depends on the control system).

Reply to

How about an idler pulley/simple style "clutch"?

Reply to
Brian Lawson

Since starting is marginal, extra capacitors across the motor leads may help. You could try just one capacitor across the same two leads as the main capacitor in the RPC and/or equal capacitors across all leads to help the start power factor a bit. The capacitors should not have to be disconnected after starting.

Don Young

Reply to
Don Young

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