RE: [CAD_CAM_EDM_DRO] Re: Power Supply for 4-axis CNC stepper driver

Hi Steve,

Great way to describe that. For the folks not familiar with DC motors I
might add that it's the back emf that limits the speed on a DC motor.
Increase the voltage and the motor speeds up until the back emf again equals
applied voltage. This is grossly simplified but as a general rule on how to
get ones head around it.

The steppers suffer from the same issue -- as Steve put so well. Once the
current through a winding is changed, it takes time for the current to
increase thereby increasing the magnetic field which causes the armature to
rotate. Remove some (or all) of the current and the magnetic field
collapses which generates the back emf. This takes time, especially with
higher inductance motors.

If the microstepper driver wants one winding to go from 5.9 amps to 5.64
amps and it takes 20mS to do this then the armature will have finished it's
movement 20mS later. BTW, that's a rotational speed of 50 steps per second
which is very slow.

Most of the simple drivers just leave the applied voltage off and change the
comparator reference voltage. Once the current through the sensing resistor
drops below this new reference voltage, the transistors are turned on again
and the chopping cycle begins once again. So the time is totally dependant
on how fast the power supply can absorb the back emf and the inductance and
resistance of the windings.

I won't even begin to try and analyze how the inductance changes as the
armature moves and all the other really complex motor dynamics. Not sure I
could even do that anymore.

John Dammeyer

>
> Dave,
>
> The function of the chopper is to limit the Current, not the Power,
> flowing into the motor. When it turns on it will indeed apply the
> full supply voltage to the winding until the current rises to the set
> point of the chopper. This is not always going to result in a PWM
> duty cycle that would produce the equivalent of the rated current
> times the rated resistance. The increased power comes from the fact
> that if the motor is moving, it has a higher back EMF and it will
> take longer for the applied voltage to cause the current to increase
> to the chopping setpoint during each cycle. In theory if the motor
> has enough back EMF (or winding inductance, though that is another
> issue) the chopper may never switch the current off. The current and
> resistance ratings of the motor only describe the power that is lost
> as heat due to the winding resistance. The mechanical output power
> has to come from somewhere. Many people describe stepper motors as
> constant power devices and explain that as mechanical power increased
> the resistive losses decrease. This is only true if the driver is not
> able to force the rated current into the motor despite the fact that
> it is turning and has back EMF. With the 25x overvoltage factor
> favored by many on this list, the driver will do a fairly good job of
> forcing full rated current through the motor windings at reasonable
> motor speeds.
>
> Regards,
> Steve Stallings
> www.PMDX.com
>
>