At 07:49 AM 11/10/2006, you 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....
>
>Wouter van Ooijen

For a permanent magnet DC motor, to a first approximation, the
steady state armature current is proportional to the product of the torque
at the output shaft and the input voltage. The steady-state shaft speed is
proportional to the input voltage with a given torque at the output shaft.

You can model the PM motor as a resistor in series with a generator. The
generator produces a voltage linearly proportional to the shaft RPM. If
you spin the shaft at a certain RPM, say 5,000, the voltage the generator
produces will be the same as the input voltage (say 24VDC) and there will be
zero input current. If you change the speed, then current will flow into
the motor or out of it. Stop it entirely and you have your stall condition,
spin it at double the speed (or suddenly reverse the voltage with the
shaft spinning at full speed) and you've got the "stall" current coming
out of the motor.

Because of inertia (of the motor and the load) the speed does not change
instantly. For example, if you apply full voltage with the shaft sitting
still and a flywheel attached, the current will initially be just the
same as the stall current (and the torque will also be the same as the
stall torque). As the speed increases and approaches the steady-state
speed, the torque falls off, like a capacitor charging from a resistor.
The equations are easily derived with first-year calculus.

That's for a PM motor.. there are other types, such as those with field
coils, BLDC etc.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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