[Scott Kelley]

I was going to buy a transformer to use for a 1500 watt DC power supply for driving servo motors, then had the thought that one could rectify the output of a VFD to serve the same purpose - would cost about the same (using a 2HP Teco 50 at $145), would require less capacitance to achieve the same ripple (because of higher frequency AND 3 phase), and it seems that it would be more efficient . . .

Now tell me why it won't work . . .

Scott Kelley

[Al_The_Man]

Just strip the DC power supply out of the VFD, 3 phase bridge and caps.
Why go to the trouble of trying to rectify again.
IF it would work, you are not going to get any more capacity than the DC supply can deliver anyway.
Al.

[Scott Kelley]

My thought was that If the DC supply in the VFD is fixed voltage, it would not likely be the voltage that I'm after (~140). On the other hand, if the controller is changing the output voltage by regulating the DC supply, it might work... there may be some kind of protection circuitry in the controller that would sense that things aren't right...

But back to the basic premise, any reason to think that it would not work?

[Mike Nash]

Modern VFDs don't typically regulate the DC supply. The output transistors regulate the AC output voltage via pulse width modulation. The waveform is a series of variable width pulses switched between zero volts and full bus voltage. Older inverters (VVI) did regulate the DC bus voltage. You could use a DC drive, but I would pick one that already has an output filter capacitor.

[Al_The_Man]

My thought was that If the DC supply in the VFD is fixed voltage, it would not likely be the voltage that I'm after (~140). On the other hand, if the controller is changing the output voltage by regulating the DC supply, it might work..

Not so much whether it will work as will it be practical, you are taking an unregulated DC supply, and has been mentioned, you are turning it into a 3 phase supply which consists of a high frequency chopped pseudo sinewave, which I would assume you figured to run at 60hz.
You would in all likely hood require high speed diodes for the bridge.
And the weakest link is most likely still going to be the input bridge supply.
Also I can see the output amplitude rising and falling with current variation.
Also remember the output is not isolated from the incoming AC service supply.
Al.

[Scott Kelley]

You could use a DC drive, but I would pick one that already has an output filter capacitor.

DC Drive???

The only thing I'm familiar with along the lines of DC motor drives are servo drivers (which require a DC supply).

[Al_The_Man]

There are basically two types of DC drives, mains fired SCR bridge, both 3ph and 1ph, these have no DC capacitive supply whatsoever, the other type is the PWM which are built very similar to a VFD but provide a Pulse width Modulated output, generally the only DC is the power supply same as the VFD.
Al.

[Scott Kelley]

"which I would assume you figured to run at 60hz."
Would probably run at Fmax to minimize ripple.

"You would in all likely hood require high speed diodes for the bridge."
There are some big-ass chokes in there that were used with the servo drives - could use one of them on the output - they may be Too big tho...

"And the weakest link is most likely still going to be the input bridge supply."
Wouldn't it be plenty strong enough if the VFD is properly rated?

"Also I can see the output amplitude rising and falling with current variation."
Not a problem - that will be the case with an un-regulated DC supply anyway.

"Also remember the output is not isolated from the incoming AC service supply."
I didn't think about that... Not sure about the effect of that... The rectifiers need to tie to ground... I would think that all 3 phases would be at the same level relative to ground, and that there would be no output when the VFD is Off... Hmmmm

[Scott Kelley]

There are basically two types of DC drives, mains fired SCR bridge, both 3ph and 1ph, these have no DC capacitive supply whatsoever, the other type is the PWM which are built very similar to a VFD but provide a Pulse width Modulated output, generally the only DC is the power supply same as the VFD.
Al.

Expensive? Where are they used? Who supplies?

[Al_The_Man]

The rectifiers need to tie to ground... I would think that all 3 phases would be at the same level relative to ground, and that there would be no output when the VFD is Off... Hmmmm

If no isolation, there will always be a reference to ground, you will blow any bridge output if it is also connected to ground, or if on the VFD output, it will shut down completely.
Al.

[Al_The_Man]

Expensive? Where are they used? Who supplies?

DC motor SCR drives are usually the cheapest, Baldor, KB etc, but PWM generally more costly than a VFD with 3ph motor.
Al.