Re: [CAD_CAM_EDM_DRO] Re: stepper driver help

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:

<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.


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.

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.

Am I misunderstanding something here?


--
Sebastian Kuzminsky