John Gilmer said:
in advance.
Oh, come on.
12 volts, 10 amps? DC motor (not ac/dc)
Just a WAG but a 1.2 ohm resistance would have that motor running at about
1/2 speed. That's ASSuming it's providing the same torque.
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Recheck this. As you have done it correctly for your fan example, I ASSume
this is a slip.
I did start to give an example similar to what you have done but then I
realised that the type of DC motor and the speed- torque characteristics of
the load can throw such estimates out the window. In addition the range of
control wanted would be handy to know for choosing a rheostat.
Overcautious- maybe.
Given that -the assumptions that you have made are the most likely but
without further information one can't be sure.
Assuming that the field is constant (permanent magnet or connected on the
supply side of the resistor) at 1.2 ohms- the "no load" speed would be
nearly the same as it was without the resistor and at rated current, the
speed would be 0 as the rotor voltage would be 0. This is not half the
original speed at this current (and torque). At 5A the speed would be a bit
less than half the speed without the resistor (at the same torque). At 2.5A
the speed would be 75% of the original. This does not take into account the
load's speed torque characteristic as you have done below.
If it is a series motor the results would generally be the same -half speed
at 5A, 75% speed at 2.5A and lets avoid no load.
If the motor has a wound shunt field and is such that some expertise would
be needed to separate field and armature connections, then use of an
external rheostat would not be effective (i.e. at 5A the speed would be near
normal but the torque would be halved)- hence the desire for more knowledge
of the motor and application.
As for what you have said below- no problems -assuming the motor is as you
assume.