Large Rotary Phase converter with smaller pony motor

[phy6]

I have the opposite problem of a lot of people: I installed/rebuilt a large 40HP rotary phase converter in my shop. At the time I was planning on powering a handful of 3 phase 230v CNC machines. Now I only want to power one old CNC machine, which I think only has a 3 phase 230v 2 or 3 horsepower motor.

The capacitor bank for the phase converter is sized for a 40HP motor. I imagine I can't just hook up a spare 7.5HP pony motor in it's place (or can I?).
Basically with the 40HP motor, the pony motor draws 30A just at idle with no load.

Alternative would be to use a ~7.5hp VFD and hope the old Allen Bradley CNC electronics don't care.

 

[phy6]

This is my little mill

 

[American Locomotive]

Something to keep in mind is that just because your clamp meter says its drawing 30A of current, doesn't actually mean it's using a lot of power. The reactive power factor on an unloaded phase converter is very bad. That means it draws a large gulp of current, doesn't do much with it, and then returns it back to the grid. If this is on a residential power supply - you aren't billed on power factor. You are only billed for the "real" power you use. Larger commercial and industrial power supplies do usually bill for power factor, and they will charge you for having things like big motors idling along doing nothing.

However, It's still somewhat wasteful to have a bigger RPC than you need idling. You cannot just stick a 7.5HP motor in its place. The capacitor sizing will be all wrong.

You also cannot use a VFD to power the mill the directly. The VFD very specifically needs to drive motor loads . They look for feedback from a motor. They cannot be used to drive general purpose loads, like electronics.

That being said, there likely is a solution for you based on these observations:
- Your mill appears to be using DC servos
- Your mill appears to have a manual speed control handwheel (variable pulley setup), with a single speed motor

If the above observations are correct, you can likely just ditch the whole RPC setup. Most, if not all old-school DC servo drives I've seen are actually single-phase input. They usually just pick two of the phases to draw from. The motor is just connected straight through to the 3-phase mains (if it has a manual switch up on the head), or through a contactor (if the machine can turn the motor on/off).

So the only thing you need to do is get a small VFD (just big enough for the motor on the mill) and do a little re-wiring. Disconnect the motor from the machines internal power, wire it directly to a VFD, and then re-configure all of the servo drives to draw power from one phase. If the machine has a contactor to turn the spindle motor on, you can run the VFD's run/stop wires into that contactor. If the machine has a manual switch to turn the spindle motor on, you will need to re-wire that into the drive's run/stop lines.

 

[phy6]

Something to keep in mind is that just because your clamp meter says its drawing 30A of current, doesn't actually mean it's using a lot of power. The reactive power factor on an unloaded phase converter is very bad. That means it draws a large gulp of current, doesn't do much with it, and then returns it back to the grid. If this is on a residential power supply - you aren't billed on power factor. You are only billed for the "real" power you use. Larger commercial and industrial power supplies do usually bill for power factor, and they will charge you for having things like big motors idling along doing nothing.

However, It's still somewhat wasteful to have a bigger RPC than you need idling. You cannot just stick a 7.5HP motor in its place. The capacitor sizing will be all wrong.

You also cannot use a VFD to power the mill the directly. The VFD very specifically needs to drive motor loads . They look for feedback from a motor. They cannot be used to drive general purpose loads, like electronics.

That being said, there likely is a solution for you based on these observations:
- Your mill appears to be using DC servos
- Your mill appears to have a manual speed control handwheel (variable pulley setup), with a single speed motor

If the above observations are correct, you can likely just ditch the whole RPC setup. Most, if not all old-school DC servo drives I've seen are actually single-phase input. They usually just pick two of the phases to draw from. The motor is just connected straight through to the 3-phase mains (if it has a manual switch up on the head), or through a contactor (if the machine can turn the motor on/off).

So the only thing you need to do is get a small VFD (just big enough for the motor on the mill) and do a little re-wiring. Disconnect the motor from the machines internal power, wire it directly to a VFD, and then re-configure all of the servo drives to draw power from one phase. If the machine has a contactor to turn the spindle motor on, you can run the VFD's run/stop wires into that contactor. If the machine has a manual switch to turn the spindle motor on, you will need to re-wire that into the drive's run/stop lines.

Hey, thanks for all the amazing info, that really gets me thinking about what options I have. I didn't even consider what kind of servos I had.

You are correct in that I have manual speed control and a single speed motor. It uses a tapered pulley belt system, like a CVT.

It would be really neat (I think) to eventually convert this to LinuxCNC or Mach4 etc and have it control the spindle speed via VFD.