600V 3-phase motor (milling machine) --> 240V operation

[Jester]

Making progress........

Managed to get the mill home in my wife's mini-van, disassembled it into three pieces. Loading was easy, four "He-man" at the pick up location man handled it into the van , and then I pulled it up and out the side door with a chain lift. Then built a rolling base to get it to its new home location. I'm not sure how much it weighs but must be in the ballpark of 450-500lbs.  I have a used 1.5HP, "Inverter Duty" 240V, 3-phase motor with the same frame size as the original on order.

I also picked up a used but supposedly good Altivar 31 VFD which is now wired (less the motor side). The VFD has three-phase input, however it's rated for 3HP, so hopefully the DC bus will not have excessive ripple. This VFD detects loss of phase, however this alarm can be disabled. I will try it with the alarm enabled initially to get a sense for how hard I have to push the machine before the VFD has excessive ripple.

I would like to perform a preliminary function test of the VFD, test: forward, reverse, speed etc. If I were to try powering the original 600V motor (no load) with the 240V, VFD do you think it will spin with the reduced voltage?

[james_s]

Most likely. With no load there is quite a lot of wiggle room.

[johansen]

I also picked up a used but supposedly good Altivar 31 VFD which is now wired (less the motor side). The VFD has three-phase input, however it's rated for 3HP, so hopefully the DC bus will not have excessive ripple. This VFD detects loss of phase, however this alarm can be disabled. I will try it with the alarm enabled initially to get a sense for how hard I have to push the machine before the VFD has excessive ripple.

I would like to perform a preliminary function test of the VFD, test: forward, reverse, speed etc. If I were to try powering the original 600V motor (no load) with the 240V, VFD do you think it will spin with the reduced voltage?

you may be able to fool the vfd by simply wiring L2 and L3 together.

anyhow, yes you can run the motor, you can get up to about 16% of nameplate torque out of it at 40% of its nameplate volts per hz.

if you program the vfd for 240v at 24 hz you can get full nameplate torque up to 40% of the motor's nameplate rpm. (neglecting resistance losses in the motor, which could cost you 10-20%)

anyhow, if the motor won't even start up at the vfd default settings of 240v 60hz (presuming this is what you set it to), then i would not be surprised. the spindle bearings and belts can suck up a good bit of static torque.

[Jester]

I paralleled the 2nd and 3rd phase on the input and hooked it up to the original 600V motor for a reality test. Unloaded it works just fine, forward, reverse and from very slow to > 1800RPM.

VFD was a great suggestion, thanks to all.


I would like to learn more about these VFD's. With 240V in, I measured the DC bus at 340Vdc. If I now think about a 240Vrms sinewave (680Vp-p).

Am I correct to assume they have a DC-DC converter to produce a higher or bipolar supply for the output transistors?

Are IGBT's typically used for the output devices?

[james_s]

Thought you'd like it, I love VFDs, used to be nobody wanted 3 phase motors but now they're generally preferable to single phase.

Power supply design varies from unit to unit, most do use IGBTs on the output.

[johansen]

I would like to learn more about these VFD's. With 240V in, I measured the DC bus at 340Vdc. If I now think about a 240Vrms sinewave (680Vp-p).

Am I correct to assume they have a DC-DC converter to produce a higher or bipolar supply for the output transistors?

Are IGBT's typically used for the output devices?

a 240vac waveform is 680v peak to peak, but you only have 340vdc at the igbts.. there is no dc-dc converter. the output 3 phase ac actually has 340volts ac at the pwm frequency in the form of common mode voltage. it is this voltage at the pwm frequency that causes bearing damage in vfd driven motors.

in addition, if you filter each phase separately with an LC filter, you won't get a sine wave. they add the third harmonic back to the output, otherwise the output voltage would be limited to about 80% of the input voltage.

but the phase to phase voltage (which is what the motor sees) is pretty close to a sine wave. its just bouncing up and down at the carrier frequency of the pwm.

if you were to make the front end of a vfd a voltage doubler, then you could run a 680volt dc bus and you could generate a cleaner sine wave with no common mode voltage if you had 12 igbt's instead of 6. this is how much larger vfds are wired.