Re: [CAD_CAM_EDM_DRO] Sizing a power supply

Ron Ginger wrote:

> "Carey L. Culpepper" wrote:
> >
> ..... Just for grins I decided to hook my digital meter in
> > series with the power supply to the driver.I am running the six wire
>
> > motors in series with the center wire unused. While running, the
> > motor/driver drew about3.8A. But when holding(not turning) it drew
> 10A.
>
> no good using a digial meter here. The current is a chopped current,
> not
> a steady DC. What the meter will show is a kind of average value, but
> not one of any use- think about the current going on and off- at the
> chopper rate while the motor is at rest, then further switched by the
> step pulses while its moving. The digital meter keeps sampling the
> current, at a completely different frequency determined by the meter,
> sometimes it reads when the current is high, sometimes low. but in any
>
> case the reading is useless.
>
> The only way to measure the current is to use a scope, then you have
> to
> do an integration to get the average current since the current is
> chopped. Not easy.

Maybe. He was measuring the INPUT current to the driver. If the driver
has
absolutely no filtering at its input, you would probably be right. It
depends on
the type of averaging built into the meter. If the driver had
sufficient filtering
at the input so that the current varied less than 10% or so, then the
reading
should be meaningful. This is for the INPUT current, the motor winding
current
(while moving) would be VERY hard to interpret without a scope. Again,
the motor winding current, while not moving, would have large voltage
steps,
but not large current changes due to chopping action (which his driver
may
not have) and should be able to be read with a DVM, but the chopping
action
could cause inaccurate readings.

Anyway, the input to a chopper drive generally HAS to be filtered, or
the
inductance of the wires from the power supply will cause enough
transient spikes to blow transistors very quickly.

Jon