Re: [CAD_CAM_EDM_DRO] Re: Confused over My own Calculations!

William Scalione wrote:

> On Sunday 18 August 2002 01:59 pm, Jon wrote:
>
> >
> > Now, this is ONLY at idle, and will vary due to the design of the motors
> > and drivers. When the motor is producing substantial torque at some
> > speed, the power drawn will increase dramatically, and therefore the
> > current mist also rise. But, the peak power may occur only during
> > acceleration, and therefore be transient. So, you should not need a power
> > supply rated for continuous output of 8 A per motor in the above example.
> > Probably 4 A/motor would work in most cases, except where you know all
> > motors would be accelerating simultaneously.
> >
> > Jon
> >
>
> 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.

Because the stepper driver shuts off the voltage when the current rises to
the current limit setpoint. If it didn't do that, the motor would burn up.

>
> 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

That the driver adjusts the on-time of the transistors to limit the current.
When the transistors are on for 100% of the time the motor phase should
be polarized that direction, then the current and torque start to drop.

> 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.

That drive probably had a slow on-off cycle, thereby causing a lot of Delta-I
(current change, min to max) and a low DC power supply voltage, so current
dropped after the motor reached a modest speed.

Jon