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Armature control for shunt DC motors

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savvej

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In lab,when we conducted experiment on DC shunt motor speed control using armature control,
The motor had no load connected to it and the field was kept constant.
Also the motor was started using three point starter
The observations are after the motor was in running condition.
what we observed is this:

1)As Va decreases,(i.e. by increasing the Ra connected in series with the armature),
N decrease.
2)Also what we found was Ia also decreased as Va was decreased

Now Ia,depends on the load connected to the motor,right?
There is no external load,also the field is constant,so should'nt the Ia have been constant too?as T=KtxφxIa ,as T is constant Ia should have been constant..rather than graduallly decreasing...
I forgot to note to take the name plate readings of the motor but I will send them by tomorrow.Have attached the readings.
 

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Now Ia,depends on the load connected to the motor,right?

Motors always have internal resistance and mechanical load. And that load changes with speed.
 
by load I mean external torque.As far as motors own weight is concerned,it is balanced by force by the bearings and this force doesnt create any torque on the motor about its axis.
Internal resistance -by this do you mean armature resistance variation with temperature?Coz here again,I can include it with R connected in series with motor and that is what I am varying.Also this cant cause any torque on the motor.
could you explain?
 
By mechanical load I mean friction in the bearings and air resistance (drag) of the spinning rotor. The drag will increase with speed.
The electrical resistance in the commutator can also change with speed.
 
In a crude speed control Va = Vm - IaRa where Va is the armature voltage, Vm is the measured armature voltage, Ia is the armature current and Ra is the armature winding resistance.

Sometimes this may be called IR compensation as a speed control technique. Voltage is proportional to speed and I is proportional to torque. When there is no I, when a motor is used as a tach the IR term drops out of the equation.
 
thanks @ misterT .

@KeepItSimpleStupid could you explain "When there is no I, when a motor is used as a tach the IR term drops out of the equation"
As in do you mean as using motor as a generator with no load?
 
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