Re: [CAD_CAM_EDM_DRO] Stepper motor voltage VS amps.

Miroslav Pejic wrote:

>Hello Scott,
>There is a light:
>The driver regulates motor current in pulse manner. Every moment when
>current reaches 3 A, (actually, some more then 3 A) driver shut the
>current off. Then, after little pause current get to zero. In this
>moment driver supply the current again, and the process repeats, with
>frequency of 15 KHz or more. So, the average current is never bigger
>then 3 A, and this happens on 40-50 Volts.
>Real power consumption is 3A * 40 V = 120 Watts.
>
>

This is not correct. Real power consumption (for one winding) would be
3 A * 40 V * fraction of time current is ON. Assuming the winding draws
3 A at 2 V, then the duty cycle would be roughly 2/40 = 0.05, and so
the power in would be 3 * 2 * .05 = 0.3 Watts. (Transistor and core
losses in the motor would increase this somewhat.) This solution
only holds for a stationary motor, and is the MINIMUM power draw
when the motor is idle. Power consumption will rise dramatically
as the motor speeds up.

Also, note that PWM drivers will not let the current fall all the way to
zero before turning on again. That would cause excessive heating, as well
as reduce average winding current to 1/2 the rated value, reducing torque.
Most drivers turn the charging transistor on and off with a 10 - 15%
proportional current band around the setpoint. Some others use the
'bang-bang' control algorithm, such that every sampling cycle, if current
is below the setpoint, charging is on for the full cycle, if current is
above
the setpoint, then charging is off for the full cycle. But, all of these
follow the previous paragraph.

Jon