Re: [CAD_CAM_EDM_DRO] Power supplies sizing

Carol & Jerry Jankura wrote:

> If these are stepper motors, the winding current is drawn whether or not the
> motors are actually turning. You may find that some controllers reduce the
> winding current when the motors are not turning, but none (to my knowledge)
> reduce it to zero. So, unless you know that all of the motors will not be
> turning at the same time and that the controllers reduce current for static
> motors, you should have a power supply rated for AT LEAST the full fifteen
> amps. I'd probably get a power supply with some reserve capacity - say
> twenty to twenty-five amps.

Not at all! This would be true for R-L drivers, but nobody in their right
mind uses them anymore, except in existing equipment, where the power
supply should already be correctly sized.

All chopper-type stepper drivers will draw LESS current than the motor
winding current at idle. How much less depends on the design and the
ratio of motor voltage at rated current to supply voltage. In the case
where supply voltage = 2 x motor voltage, then the idling current draw
of the driver could equal rated motor current, since there are 2 windings,
both drawing rated motor current in full step mode. Usually, you run
the power supply many times motor voltage, so current draw at the
driver is LESS than rated motor current. But, this is only true at standstill.
Current draw at the driver increases as motor speed increases. Where
it peaks and how high the current is in relation to motor rated current is
a very complicated matter, having to do with motor inductance, mostly.
But, the peak current is generally a transient phenomenon, and making people
buy a 25 A power supply to supply an avarage current draw of 5 - 7 Amps
is wasting money, and breaking people's backs. Do you know how heavy
a 25 A 75 V power supply is? This is 1875 Watts, not counting any
losses in the supply. If you really loaded it fully, you couldn't run it off
a 110 V outlet! And, since you won't be loading it fully, you can try, but it
may blow the breaker every time you turn it on, as it will draw a huge
surge charging its capacitors.

Jon