# error in calculating amp draw in motor

Discussion in 'General Electronics Chat' started by firesmithdan, Mar 1, 2014.

1. ### firesmithdan Thread Starter New Member

Mar 1, 2014
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I need help figuring out what is wrong with my understanding of ohms law and how it applies to motors.

I was looking at a new motor. I tested the resistance in the windings and got a reading of 12 ohms. It is a 120V motor. I thought that this would give me an amp draw of 10 amps (according to my limited understanding and application of ohms law). The motor plate states that the motor only has a 1.6 amp rating so there's a big discrepancy between my calculations and what the motor states. I figure I must be applying ohms law in a way that I can't apply it, but I'd like some help in figuring out why I'm wrong.

Jul 18, 2013
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You didn't say if it is a DC or AC motor, but assuming this is an AC motor, what you are basing your calculations on is simple measured or DC resistance, this does not take into account the inductive reactance (Ohms) that limits the current.
When an induction motor is up to speed and unloaded, it acts as mainly as a large inductor.
DC motor are similar but current is mainly limited by generated BEMF.
Max.

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3. ### firesmithdan Thread Starter New Member

Mar 1, 2014
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0
You are correct, it is an AC motor. I guess I can't use that to help me troubleshoot bad motors. Thank you for your help.

4. ### crutschow Expert

Mar 14, 2008
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4,316
The dc resistance will tell you what the approximate startup current will be at the instant the power is applied, but as soon as the motor shaft starts to turn it generates back emf which increases the apparent impedance and reduces the motor current. Once it reaches its normal speed the motor current will consist of a small amount of inductive current that magnetizes the windings and an in-phase (real) component that is proportional to the power the motor is providing to the load.

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5. ### alfacliff Well-Known Member

Dec 13, 2013
2,449
429
dc resistance can be used to locate open and shorted windings, it just dosnt have much to do with the morot current. smaller motors have higher resistance windings, and three phase or two phase motor windings should have similar resistance windings. another use for a meter is to measure ac voltage across windings while rotating the motor, motors will generate a small value of ac if the windings are good, a shorted winding will not.