motor coil

[lucas123]

The motor coil has a resistance of 10 Ω and is powered by a voltage of 120 V. When the motor is operating at rated speed, its emf is 70 V. Thus, it is questioned:

a) What is the coil current when the motor is started?
b) What is the coil current at rated speed?

I am having doubts as to which equation should I use to resolve these issues.
Thank you very much in advance

 

[IRstuff]

When the motor is operating at rated speed, its emf is 70 V. Thus, it is questioned:

Does calling it "back"emf make it clearer why there are two questions?

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[waross]

Is this homework?
First, you should be concerned with impedance, not resistance.
The impedance of the rotor increases as the speed increases.
The resistance of the stator and of the rotor is unchanged regardless of speed.
70 Volts back EMF seems unrealistically low for a 120 Volt motor.
In the real world, you may use either the motor curves or a rule of thumb rather than an equation.


Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[IRstuff]

The homework question, as posed, is likely to be an overly simplified EE101 kind of problem. As such, one might expect to use Ohm's Law to solve the problem, unless the OP left off critical information that makes it a more complicated problem.

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[waross]

I may have been at fault assuming an AC motor given the 120 Volts and coil resistance rather than armature resistance.
I should have considered a DC motor.
I apologize.
The difference between the applied voltage and the back EMF drives the current through the winding or armature.
Industrial motors of reasonable efficiency normally have a back EMF of over 95% of the supply voltage.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!