Electronic – How to model a DC motor in LTSpice if only its nominal voltage and max. current is known

dcltspicemotor

Is there a simple way to model a DC motor in LTSpice when controlling it with a PWM signal? Lets say the motor is 9V/2A.

I was thinking the safest way to measure the stall current and use the nominal voltage but I think there is more to handle this.

edit: Or should I say, can we model a DC motor as a resistance if we know at a particular rpm and voltage it draws known amount of current?

Best Answer

What are you modeling it for?

Noise immunity of nearby circuitry?
PID control loop characteristics, to allow loop tuning?
Short circuit, temperature and stall current protection?
Overvoltage protection, including driving the shaft without applied power? Or basic startup and speed control in the driver?

All of these require different simple models, or one really comprehensive model, which would be unnecessarily complicated in most scenarios..

For a simple model for a toy motor, you can ignore mechanical aspects like the inertia of the load.

I suggest running several models of increasing complexity or severity.

  1. A simple resistance = rated voltage/rated current. Tests basic operation. Look at power dissipation in the driver, compare with your heatsink.
  2. A simple resistance = measured winding resistance. Tests that the driver can survive a stalled motor. It's OK if it shuts down or consumes enough current to reliably blow a fuse (say >= 2x the fuse rating). Or Look at power dissipation in the driver in this worst case, compare with your heatsink.
  3. A more complex model including back-EMF (speed-dependent voltage source) and a realistic inductance in series with the stall resistance. This tests the driver can supply enough current to start it, and overvoltage conditions from the motor's inductance. Drive the voltage source with a ramp from 0 to rated speed over a few seconds.
  4. If it is possible to rotate the shaft while the driver is unpowered, teh motor will act as a generator. Model (3) can be used to test that the driver will survive this scenario.
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