Re: [CAD_CAM_EDM_DRO] Re: stepper driver help

Sebastian Kuzminsky wrote:

>John Dammeyer wrote:
>
>
>>It's easiest to think of things happening in stages.
>>
>>The device turns on and puts say 50V across the winding which draws no
>>current initially, so there's no current through the driver and no power
>>turned into heat.
>>
>>Next the current flows over time into the motor until it reaches the
>>current rating of the motor.
>>
>>
>
>Stephen Wille Padnos <spadnos@...> wrote:
>
>
>>Just so people understand this, the current increases until it hits the
>>limit of the driver - there's no other magic that limits current to the
>>rating of the motor. If you set it wrong, you may burn up the motor.
>>
>>
>Wait, that doesn't sound right. All I know about Brushed DC motors I
>learned from the Pittman motor people, via this excellent document:
>
>

I'm reasonably sure we were talking about steppers :) They are motors
though, and have many charisteristics that are the same as DC motors.

> <http://www.pennmotion.com/pdf/220000ALL.pdf>
>
>
>They model a DC motor electrically as an inductor, a resistor, and a
>voltage source, all in series, between the motor leads.
>
>

Yep, I can agree with that.

>Here's a quote from that file:
>
> When a step voltage is applied to a motor at rest there is an initial
> inrush current limited only by the circuit impedance since the back
> emf voltage is zero. This inrush current produces a large torque
> which begins to accelerate the motor and the connected load.
>
> As the angular velocity increases, the back emf voltage increases
> and begins to limit the motor current. The steady state speed of
> the motor will be that speed at which the generated back emf voltage
> limits the current to a value that produces a torque equal to the sum
> of the load and internal motor torques.
>
>
>If I understand that right, everything's ok as long as the power supply
>and the driver are rated higher than the motor.
>
>

Actually, everything is fine as long as the power supply is rated for
the correct voltage, and at least the rated current. The difference is
that steppers are usually run with voltages higher than their nameplate
voltage rating, at least with chopper drives. The reason is that unlike
a DC motor, a stepper has inrush every full step. You energize a coil,
then you shut that one off and energize the next, and continue ad
infinitum. You can half-step and microstep, etc, but the fact remains
that for each full step, current must be started up in one coil and shut
down in another. It's the inrush scenario every step. So, you use a
high voltage supply with a current-limiting chopper drive. This gets
you a fast build-up to full current, which allows you to get full torque
at a higher step rate.

>Let's say we have a motor rated for 24 V and 2 A stall current, with a
>winding resistance of 12 Ohm (let's neglect the reactive impedance for
>this example). Let's say our driver and power supply are capable of
>providing 100 A @ 24V.
>
>When we apply power, the inrush current will be I = V/R = 24/12 = 2 A.
>As the motor spins up, the back emf reduces the current from there.
>
>No smoke.
>
>

Now use a supply that's 20x the rated voltage, or 480V. That will
result in smoke, and that's the scenario I was talking about :)

>Am I misunderstanding something here?
>
>

I believe so :)
The key is that running a stepper motor is like starting a DC motor for
every step. To get around the speed problems inherent with steppers,
you have to use higher voltages than the motor is rated for. If you use
a higher voltage than nameplate, then you need a driver that will limit
current, because the motor won't do it for you in that case. (well, it
will, but probably not before letting out the smoke)

- Steve