Re: Confused over My own Calculations!

Bill,

The answer to your question is in what happens after the current
reaches its desired value.

When the motor is stopped or turning slowly, the pulse-width
modulation is at a near 50% duty cycle. This means it draws current
from the supply equal to both the motor's phase currents, then turns
around and returns the same current back to the supply 20,000 times a
second.

Let's say your motor is a 7A motor, the PWM draws current for 30 uS,
then returns that same current to the supply for 20 uS. Your power
supply responds to the average current drawn, which would be (30 -
20 / 30 + 20) times 14A. That would reduce to 1 / 5 times 14A or 2.8A.

Now let's say your motor is turning at a high speed. The PWM goes to
a 100% duty cycle, meaning the motor current does not reach the
required value of 7A before it is time to switch to the next step.
The power supply will be delivering current for the entire step
period and the value of that current will be much higher than when
the motor is stopped.

Mariss

> Jon, or anyone else
>
> This is just to satisfy my curiosity as I'm running servos and

really don't

> ever plan on using steppers again. The servo performance is so much

better.

>
> Anyway, we always talk about raising the voltage on a stepper

system to

> overcome the inductance in a stepper motor's windings. It would

seem that at

> a high step rate the voltage will not be building up to the max

that the

> power supply can deliver, and since the resistance of the windings

will not

> change, why wouldn't the current be more when the motor was not

turning.

> At idle the voltage will have had plenty of time to raise to the

max level

> and and hence, using ohms law the full current that can be

delivered by the

> drive should be flowing. But at speed the voltage will be lower and

it seems

> as though the current should also be lower. What am I missing here
>
> I know a few years back while running a stepper system with a

chopper drive

> the motors would get hot as hell while standing still but cool down
> considerably when running. Obviously there was no current reduction

on that

> drive.
>
> Just curious
> Bill