Re: [CAD_CAM_EDM_DRO] Inverters/phase converters

ptengin@... wrote:

> Doug,
>
> I was told by the EMS/ Yaskawa people that a 3 phase drive powered
> by single phase will work if the output of the unit is derated. You
> don't want to overheat the 3 phase bridge rectifier. I can't recall
> what the actual percent value was but I think 20 or 30 % was what they
> quoted. If you can keep the rectifiers cool, I think you could drive
> them really hard. Most VFD's come with temp sensors and fans, some
> don't Perhaps a fan retrofit would help the device live if it came
> with out one.

I looked into this with my 3-phase in, 3-phase out inverter, and got the
data sheets for
the power block they use. The rectifiers are rated for 2 X the current
rating of
the transistors. Since I didn't expect to use full power very often at
all on a
Bridgeport mill, I figured that I could get away with running on single
phase input.
I have been using it almost a year with no trouble at all. The rated
output for a
3-phase, 240 V 1 Hp motor is 3.3 A. The transistors are rated for 10 A,
the
rectifiers are rated for 20 A RMS continuous.

The heat sinks on mine run almost completely cold - maybe a 5 C temp
rise
at the worst. A fan would not be much use with the unit running that
cool.

> One other idea would be to step up voltage to the device so current is
> lower through the rectifiers? I'm not sure if this would work. Many
> VFD's will accept 440 volts?

These units do not convert voltage. You run 240 V motors with a 240 V
input,
and a 240 V rated inverter. You run 440 V motors with a 440 V input,
and a
440 V inverter. In theory, you could run a 240 V motor from a 440 V
inverter
with 440 V input, and program the motor for 240 V at 60 Hz. But, you'd
need a 2 Hp inverter to run a 1 Hp motor, as the current doubles when
the
rated voltage drops to half. Just look at the rated current on the
nameplate
of a dual voltage motor. Also, the 240 V motor has 1/4th the inductance
of
a 440 V motor, so the PWM peak current will not come out right at all
for
a 440 V inverter running a 240 V motor, and you would likely get false
overcurrent trips. So, you really get no benefit, and will pay extra,
to be
back where you started from.

Jon