Re: [CAD_CAM_EDM_DRO] Transformers & frequency....

doug98105 wrote:

> Group,
>
> What's the story on transformer hertz ratings? I have a couple of
> transformers connected in an open delta configuration to boost my
> 220V three phase to 440V for my motors that are not dual voltage. If
> I were to input these transformers from a VFD, how would the variable
> frequency (up to at least 120hz) affect them?

You can't, according to the VFD makers. Run the 440 to a 440 V
VFD. The problem is that the transformers will not handle the low
frequencies when the motor is starting up, and will also have very
high core and copper losses from the high-frequency PWM pulses
that the VFDs use to simulate a sine wave.

> I'm asking this question because I notice all transformers seem to
> have a hertz rating.

Thansformers will accept higher frequencies (within reason) without
much trouble, but they can't handle lower frequencies at all (at full
voltage) as the iron core will saturate. Most transformers have very
little headroom on the V-T (voltage x time) spec. You can run a
60 Hz 240 V transformer from 120 V at 30 Hz (the V-T product is
the same, time being the inverse of frequency). But, this only applies
to sine waves, and modern VFDs do NOT produce sine waves, they
produce 400 V sharp pulses on a 220 V VFD, or 800 V pulses
for a 440 V VFD. These average out (to the motor) to look like a
sine wave, mostly.

Jon