Is special "CNC phase Converter" needed?

I purchased a 3-axis CNC Milltronics Partner 2 knee mill circa 1989, retrofitted with a 5hp VFD spindle motor. Is the special CNC RPC needed? Or can I use a general purpose RPC?

In the manual for the VFD, it is stated that the voltage needs to be balanced within 2% between legs. The machine was previously powered by standard 3 phase power from the power company...which I understand is not nearly that balanced.

Any advice is greatly appreciated.

LoveMachine said:

In the manual for the VFD, it is stated that the voltage needs to be balanced within 2% between legs.

Click to expand...

That's a mighty tight spec. I thought 5% was a more typical value.

But in any case, you need to meet the requirements given in the manual.

- Leigh

I dunno if that's even possible from an RPC!

RPCs are not going to be well balanced over a wide range of loads just because of the way they work. 2% is a TIGHT tolerance for a phase converter. I don't think it's really necessary UNLESS the VFD has a way to monitor the voltages, and trips on a 2% imbalance. The 5% is the defacto standard for imbalance on a CNC type VFD, and I don't even think the power company holds that tight of a tolerance.

The motor probably doesn't care much but the controls do. Chances ate the control loads - the electronics and servo drivers - are single phase possibly distributed among the legs of the three phase. If you wish to run the machcine from single phase you need a phase converter of course but you'll have to look into the electrics and bring out all the control loads and connect them to the line legs leaving the generated leg of the converter to be connected to the motor contactor.

+/- 2% balance for the VFD? Every VFD I've seen was promoted by the manual and sales info to be the immune to all electrical faults. All NEMA rated motors are warranted to operate at full load from line voltage varying +/- 10% from the nameplate. VFD's are commonly rated for voltage excursions wider thatn that. Very rarely you come across a VFD furnished with phase-out protection but generally there's a parameter that can be set to ignore the state. If you plant to run the machine at full HP at full RPM you may need to replace the existing VFD for one that's either single phse rated or one de-rated for running from single phase. A 5 HP motor will require at least a 7 1/2 HP VFD for full load full RPM operation.

Or you can finesse the whole business by plunking down a pile of cash and get a "Phase Perfect" electronic phase converter that makes well regulated, well balanced three phase from single phase with solid state electronics. Here's a link: http://www.phaseperfect.com/. They do have the goods but they aint cheap. OTH it saves a major hassle.

I did some looking, and it looks like the CNC rated RPC will be the way to go.

That 5% figure you mention does pop up everywhere I look.

I saw American Rotary mentioned in another thread, which offers all types of phase converters, including the Phase Perfect. Sounds pretty sweet, but at $2800 I will pass. They have a 5 hp CNC rated RPC for $425.

Does anyone use an American Rotarty RPC? Are you satisfied with it?

You should call American Rotary to size your RPC. You will not be able to run the 5 hp motor with a 5 hp RPC, at least a 10 hp unit should be required.

From CFL-CNC:
"You should call American Rotary to size your RPC. You will not be able to run the 5 hp motor with a 5 hp RPC, at least a 10 hp unit should be required. "

CFL, Its funny you should mention that because i just got off the phone with American Rotary. That's the exact same thing they told me. In addition he stated that the reason being is the 5% balance is guaranteed from 0% to 60% load on the phase converter.

I also asked them about the 2% voltage balance spec on the FVD. He said that since the power companies don't usually guarantee anything better than 10% balance, and it has been running on that power for years, it shouldn't be a problem.

So my new 10 HP RPC is on the way! It set me back $746 with shipping + another $20 for the little rubber feet.

Thanks

Thanks to all who replied. I have been reading this forum for some time, but just now made my first post. I have always been pleased with the people here and thier willingness to help others.