CNC rotary Converter

[racear*2865]

I built a rotary phase convert 15hp using an older all copper motor(probably today's 20hp). I have a 7 1/2hp spindle 3phase CNC mill. Because of the imbalance on the wild leg, having problems starting it up. Have 5 other 3 phase machines all with different hp motors, but none give me problems. Have a 50 mfd capacitor (run) across t generated leg to balance but still doesnt help the CNC mill. Any suggeestions??? Please
racear

[Geof]

On most CNC the three phase is rectified for a DC supply that is used for the servos and spindle drive; these do not drive directly off the incoming supply.

The internal electronics are run off one of the legs of the incoming supply and this is often through a transformer down to 120V. If you have it connected so the wild leg is the one being used by the internal electronics that could cause problems ; perhaps you can just switch the incoming lines to try all the possible combinations to see if this helps.

It is possible you will not solve the problem because some CNC machines want a closely balanced three phase input which is not really easy to get from a simpl rpc.

[racear*2865]

I had already made sure that the wild leg was not run thru the electronics. Only the spindle is off wild leg. But the machine apparently looks for balance of the legs. Is there any thing that I can do (rework or add too) to my RPC that will help it. Is there a controller or other means to get my RPC to be closer.
again thanks for inputs
racear

[Nastyzen]

I had to have a custom multitap transformer built for my lathe to keep it from crashing when I turned the cnc mill spindle on.
It seems that when you apply a load on the rotary phase converter the artificial line collapses a fair bit for a moment.My fanuc control trips out but my Dynapath Delta control never quits.
New tech can't take the fluctuations.
Good luck

[MattTheNoob]

How wild is your wild leg? Are you equipped to determine how much its dropping?

If it is the simplest mode of detection, there might be a "phase monitor" relay that is simply connected to all three incoming legs, and provides a contact as a signal to the rest of the controls. These phase monitors almost always have a sensitivity adjustment. They also usually detect phase order, so if you swap two of your incoming phases it might drop out as well.

Let us know what you find.

[TEntpeg]

I built a rotary phase convert 15hp using an older all copper motor(probably today's 20hp). I have a 7 1/2hp spindle 3phase CNC mill. Because of the imbalance on the wild leg, having problems starting it up. Have 5 other 3 phase machines all with different hp motors, but none give me problems. Have a 50 mfd capacitor (run) across t generated leg to balance but still doesnt help the CNC mill. Any suggeestions??? Please
racear

racear*2865,

There are a couple of things that you might want to check:

Have you measured the voltages betwen T1, T2 and T3?
If one voltage is out by greater than 10%, consider swaping the motor leads with T1, T2 and T3 until you get a better balance.
That is :

Motor leads:

1-7----------------2-8----------------3-9

T1-----voltage-----T2-----voltage-----T3-----voltage to T1

T1-----voltage-----T3-----voltage-----T2-----voltage to T1

T2-----voltage-----T3-----voltage-----T1-----voltage to T2

(note: make sure that you id the new wild leg wire)

If that doesnt work, google the Fitch Williams RPC on usenet and
try adding additional capacitors as outlined in the post(s) to balance
the voltages. Be careful, charged caps can bite even when removed from
current for a long time.

If the machine is over 50 feet away from the RPC, consider increasing the
feed wires to a size or two larger. That is, if you are running #6, go to a #4.

Finally, a last resort would be to add an additional 10hp idler or consider a voltage stabilizer transformer.

be safe.

regards,

Jim