Re: [CAD_CAM_EDM_DRO] Sizing a power supply

"Carey L. Culpepper" wrote:

> I am in the middle of that myself. I was just reading the section on
> power supplies on the gecko site when your e-mail came in. I saw some
> information there that was very confusing to me. On my Bridgeport
> conversion I have 6 wire nema 42 Compumoters. They are supposed to be
> rated at 5A/phase. I am (for now) using the Oatley kit drivers rated
> at
> 100V and 14A. 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.
> You have the 5A/phase times two phases ,I guess.

I take it the Oatley drivers are not the chopping type? Are there
series resistors to
limit motor current? What power supply voltage are you giving it?

This is the expected behavior of the R-L type drivers, which use a huge
power resistor
in series with the power input to the driver (or motor) to limit the
current when idling.
When the motor is moving, the inductance of the windings limits current
(and speed).

A chopping-type driver limits current by turning the input power off
when the current
limit is reached. This has the advantage of not dissipating the
difference between
winding and supply voltage, but accomplishing this by varying the duty
cycle.
This has the effect of lowering input current by the factor of the duty
cycle.
Assuming just one winding for now, if it has a rated current of 5 amps,
and the
resistance of the winding plus the driver transistor is such that it
drops 5 volts,
but you have a 50 Volt power supply, then the duty cycle would be 10% to

accomplish the voltage drop. But, the current drops to 10%, also! So,
the
current draw from the power supply would be 0.5 A, while 5 amps
recirculates
through the motor winding!
The second advantage is that without the power resistor in series, the
winding current
can be increased faster.

Finally, the current draw on most chopping-type drivers INCREASES when
the
motor is moving, as energy is thrown away when the magnetic field in
each
winding has to be dumped as the motor is stepped.

Jon