On 2007-10-07, BobH wrote:
Ignoramus12852 wrote:
I am thinking about something. 480 VAC drives can be bought for next
to nothing. What they have is an inverter circuit and control
circuit.

They probably would not run on 240v because the internal transformer
that would supply voltage to the control circuit would supply only 1/2
voltage. They also might have an indervoltage sensor. I cannot think
of more things that depend on 480 volts being there.

So, if a hack could be done and a 1:2 transformer inserted between
480v inputs and the control circuit, then the drive would become a
240v drive with the same amp rating (and half HP).

What am I missing?

The control logic power supply is one thing. It would probably be worth
opening up the box to see if there are jumpers on the control power
primary that would allow the control electronics to run from 240V. You
already mentioned the undervoltage sense.

A few other things to consider would be the high voltage rectification
to DC and the output driver biasing.

If the high voltage rectification is power factor corrected which I
think is fairly likely, all bets are off. The power factor correction is
a boost converter that syncronously rectifies the incoming AC and
regulates the resulting DC to a voltage above the peak to peak AC input
voltage. This would have to be modified to boost to a different voltage.

If the high voltage recifier is a simple bridge, then the filter caps on
the high voltage DC supply and the recifier will still be sized for half
the current.

The output driver transistor biasing might or might not be OK at the
lower voltage. I assume that VFD's use IGBT outputs, so there is no
gate/base current to speak of like a bipolar transistor would require.

I would not bet even one beer that this would be as simple as just
getting the control electronics to run.


Bob, I will try to find a cheap 480 v VFD and will play with it just
for the kicks. For maybe $15 or so, it will be a valuable educational
experience. I will try to find a 2 HP 480v VFD and will try to modify
it to run a 1 HP 240v motor. Worst case is, I will waste time and
money.

i