Wouter van Ooijen wrote:
> A DC motor question: I have a DC motor for 24V nominal. How does the
> current drawn depend on the voltage, and does that dependency depend on
> the load? I guess in a stall condition the motor behaves as simple a
> resistive load, but is this also the case under a 'nomral' load? And
> unloaded? I guess these questions show my lack of basic motor
> knowledge....

At first approximation, think of a motor as a resistor in series with a
voltage source.  The voltage source is proportional to how fast the motor is
spinning, and it apposes the applied voltage when spinning in the direction
the applied voltage drives the motor.  So the rough idea of current is your
applied voltage minus some factor times the motor speed, divided by the
resistance.  You can get the resistance from the stall specs, because the
internal voltage source is 0 at stall.  You can get some idea of the voltage
source coefficient (V/Hz) from some specs or from measurement.  The no load
speed of a motor will usually produce a back voltage of 80-90% or so of the
applied voltage, of course varying with the motor.

What I described most directly fits a brushed DC motor, but is mostly how
motors work in general.  Other issues like phase, slip, and driving
frequency crop up with various types of AC motors.


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