[split63]

I built a Rotary Phase converter some time ago. The 2HP idler motor starts and runs great. The 2HP mill motor starts fine when the idler motor is running. If the idler motor is not running, there is only single phase and the mill motor will not start. Great.

Here is the bizarre thing, I discovered that when all is running there is no current in the the third phase. All the current is in L1 and L2, L3 has no current. Clearly the L3 circuit is working, as there must be power in L3 to start the Mill motor. So what the heck, why is there no current in L3 after the mill motor is running?
My sense is that the two motors are in phase and hence no current flows in L3, if so, I'm suddenly confused about how this can function...
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Thanks,

[Al_The_Man]

I think you have made a fairly correct assumption, the fact that an RPC is a rotary transformer, Once running, the fed motor is in parallel to the RPC motor and is performing the same function, if the produced voltage on the mill motor is in phase and same amplitude as the rpc then the resultant current should be zero.
It would be like Putting two identical power sources in parallel. no current will flow between them.
What about when you load the mill motor though?
Al.

[split63]

Well the problem is that the Mill motor is passing through a overload breaker. As the mill runs, more power than usual is flowing throug L1 and L2, so eventually the breaker heats and it pops. This causes the spindle to shutoff but the table keeps moving......disaster.

The load of mill is light so if there is current, its very small.
I'm not really seeing an answer to this problem....any ideas?

[Al_The_Man]

Obviously, regardless, the 1 phase ultimately is going to provide all the power as this is the only source.
The capacitor tuning makes a difference by improving power factor etc.
But what I would recommend is look for a deal on a VFD, I converted over from RPC to Mitsubishi Vector types with 1 ph in 3 ph out, and the added benefits are well worth it.
Al.

[split63]

I would recommend is look for a deal on a VFD,
Al.

Its been a while, but as I recall a VFD was going to be a problem because I not only had the motor to drive, but the BOSS 6.0 power system. I remember folks commenting that the VFD would not work in this application??

[Al_The_Man]

I am not that familiar with the BP, you may want to post in that forum and confirm, but If the spindle motor is the only thing 3 ph then you may be able to swing the Servo's and definately the controller, over to single phase, this way the VFD is used only for the sp. motor.
Al.