rotary phase converter VS VFD

[mturner]

I'm going to go out on a limb here and risk exposing my ignorance.

A VFD is designed to power a motor. There are many 3 phase applications that don't involve a motor, where a VFD will not work.

You can use the VFD to drive the motors on the sinker, but you cannot use it to supply power to the control system. VFD's use a bunch of "tricks" to do their magic. Motors don't mind the tricks, but many other systems will have a problem with it. In other words, a VFD should not be used as a general 3 phase power supply. It's possible that if you left the VFD locked at 60hz, it might work, but it's still converting AC to DC and then using something similar to pulse width modulation to create the 3 phase supply. Most VFD's are "smart" these days, and it will probably figure out that it's not connected to a motor and might just throw alarms.

If you need a 3 phase power supply for the entire machine, a rotary phase converter is the way to go.

That's exactly why we used a (CNC qualified) rotary phase converter for our CNC back in Texas. Our simple rotary phase converters were not good enough, but they worked great for the manual machines. The one for the CNC was a 25HP with a ton of support electronics to clean things up a bit.

Michael

 

[brickeyee]

If you need three phases at the correct angles you need either a VFD or an actual motor-alternator set.

VFDs have gotten better with new devices used to construct their output waveforms.
But the voltage and current waveforms are still not all that 'clean.'

The voltage on the 'generated' phase is also NOT all that well regulated.

The phase angles on a rotary type setup are not 120 degrees apart.
The most common is 180 degrees (from the two single phase legs) and a generated leg between them from the rotary phase converter.

The angles are close enough to get a 3-phase motor up and running though.

Since actual rated power from the machine's motor is rarely actually required (or used) the slight loss from the wrong phase angles is not all that important.

Especially once everything is up to speed and moving.
It can result in slightly less cut smoothness at low speeds and heavy cuts.

Like large low TPI cuts with a large bite.
There may not be enough stored momentum in the moving object to adequately smooth out the less than optimum phase angles driving the motor.

Use capacitors to filter and clean typical 3-phase voltage levels is a nightmare.
You must use 'non-polarized' capacitors, or polarized capacitors in series with each other to 'create' a non-polarized capacitor.
At a LARGE penalty in the capacitance. And the need in many cases for resistors (and even diodes) to make sure the voltage
on each capacitor stays in the correct allowed range.

The 'Farad' range capacitors have VERY low voltage ratings.
And stacking them for more voltage becomes ridiculous.

 

[Rubicon Prec.]

I'm going to go out on a limb here and risk exposing my ignorance.

A VFD is designed to power a motor. There are many 3 phase applications that don't involve a motor, where a VFD will not work.

You can use the VFD to drive the motors on the sinker, but you cannot use it to supply power to the control system. VFD's use a bunch of "tricks" to do their magic. Motors don't mind the tricks, but many other systems will have a problem with it. In other words, a VFD should not be used as a general 3 phase power supply. It's possible that if you left the VFD locked at 60hz, it might work, but it's still converting AC to DC and then using something similar to pulse width modulation to create the 3 phase supply. Most VFD's are "smart" these days, and it will probably figure out that it's not connected to a motor and might just throw alarms.

If you need a 3 phase power supply for the entire machine, a rotary phase converter is the way to go.

I think this is the correct answer.

Even using a fairly high-end name brand VFD on my knee mill, it wouldn't work because I didn't realize there was a couple small fans to cool the spindle motor. A VFD us used for a single motor, not a full machine. I use a 40hp RPC for my turning center and would use a RPC for an EDM.