Re: Power Supply for 4-axis CNC stepper driver

> BUT, it you did extract full
> mechanical power from the motor, you would find that it would be
> drawing the rated phase currents at near the power supply voltage,
> not at the voltage implied by resistance times current. This would be
> due to the back EMF and the inductance effects of the motor windings.
> Remember that power is RPM times torque, so a stepper motor at rest
> is producting no mechanical power. To get full mechanical power you
> must be spinning the motor fast and loading it at high torque,

But the whole idea of a chopper is that the motor gets slices of
power, PWM and all that.

so if a motor is 2.4 volts and 6 amps, then powered by a chopper, it
will get 60 volts at a pulse. as soon at it gets it's 14.4 watts, the
chopper cuts back or chops the power. this is repeated constantly.

Even if you double the ampacity (I just like that word) of the power
supply, the motor can only take so much. all the rest is wasted. Not
debating if some magical 37.5296 % is used because no one ever runs
all 4 axes at full, but at some much lower value. Assume full power
for this discussion.

Double it for the second phase or double it for emf or double it for
iron losses or whatever, the total required current appears to be much
lower than what might otherwise be supplied.

Even if we consider that each pulse will deliver 50% more current than
the motor rating, that is still only 10% of the amps regardless of
voltage being supplied.

What I'm missing is why if we run a 2.4 volt motor at 12 volts, we use
the nameplate 6 amps at 12 volts, or if we run it at 60 volts, why do
we need to supply so much more current ?

The limitation is in the motor. The chopper will only ever deliver
2.4 volts at 6 amps. If the 60 volts is delivered, then 6 amps is
reached 10 times faster and the chopper does it's thing and only has
to PWM 1/10 the time (or some such) so, the average current is equal
to the motor rating of 2.4 volts and 6 amps.

If you consider that each phase is separate and at speed, the back emf
on one phase is really power for the other phase, the power supply
does not need to be oversized.

I still think I'm missing something in all this, so I could be dead on
my chance or dead wrong and not know it. However, my use of power
supplies has imperically be working. (namepalate V x nameplate A X 2
phases X # of motors)

Dave