Motor Readings

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jmellc

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Durham, NC
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Facility Maintenance Tech. Licensed Electrician
Correct me if I'm wrong but isn't E over I/R the correct table for motor readings? I had 2 motors recently, both new, 208/240, 2.1 amp. Ohm reading should be somewhere about 99 to 114. One was 40 something, the other 60 something. Same labels on both. What am I missing here?

Also, is a motor listed for 208/240 OK on 277? Some of my coworkers say yes, some no. These are hand dryer motors and we have some dryers on 277. I haven't had a chance to replace one of them and pull the motor to verify.

Thanks for any feedback.
 
More than I know and super simplified.

As the magnetic field expands around a wire (coil) there is also a collapsing field from the previous cycle. Counter Electro Motive Force, CEMF. This limits the current flow of the motor or coil.

from Wikipedia:
Counter-electromotive force (counter EMF, CEMF, back EMF), is the electromotive force (EMF) manifesting as a voltage that opposes the change in current which induced it. CEMF is the EMF caused by electromagnetic induction.
 
jmellc said:
Correct me if I'm wrong but isn't E over I/R the correct table for motor readings? I had 2 motors recently, both new, 208/240, 2.1 amp. Ohm reading should be somewhere about 99 to 114. One was 40 something, the other 60 something. Same labels on both. What am I missing here?

Also, is a motor listed for 208/240 OK on 277? Some of my coworkers say yes, some no. These are hand dryer motors and we have some dryers on 277. I haven't had a chance to replace one of them and pull the motor to verify.

Thanks for any feedback.
Click to expand...
Something what?
 
230727-1259 EDT

jmellc:

What is E over I/R ?

The equation for a linear invariant resistance is V = I * R .

A motor of most kinds is not the equivalent of an invariant resistance.

A shunt wound DC motor with a constant field excitation or permanent magnetic field is approximately "V source voltage" = (" I load" * "rotor resistance") + counter EMF. The Counter EMF is the induced rotor voltage that is a function of motor RPM, and of opposite polarity to the applied motor voltage. AC induction motors are more difficult to study.

. .
 
gar said:
230727-1259 EDT

jmellc:

What is E over I/R ?

The equation for a linear invariant resistance is V = I * R .

A motor of most kinds is not the equivalent of an invariant resistance.

A shunt wound DC motor with a constant field excitation or permanent magnetic field is approximately "V source voltage" = (" I load" * "rotor resistance") + counter EMF. The Counter EMF is the induced rotor voltage that is a function of motor RPM, and of opposite polarity to the applied motor voltage. AC induction motors are more difficult to study.

. .
Click to expand...
Same equation. I was just taught it with E, electromotive force for voltage. From high school in the 70's. Another example of nomenclature changing through the years. Ohms into voltage should give approximate amperage or Intensity. Amperage into voltage should give approximate ohms. I would think 2 identical motors should give close to same ohms. Then again, I haven't done a lot of specific motor work over the years so I don't claim specific expertise there.
 
jmellc said:
Same equation. I was just taught it with E, electromotive force for voltage. From high school in the 70's. Another example of nomenclature changing through the years. Ohms into voltage should give approximate amperage or Intensity. Amperage into voltage should give approximate ohms. I would think 2 identical motors should give close to same ohms. Then again, I haven't done a lot of specific motor work over the years so I don't claim specific expertise there.
Click to expand...

It’s not the same equation. You stated “E over I/R”. That’s not even an equation, there’s no equal sign. And “I/R” means division. It should be multiplied (Gar’s example), not divided.
 
jmellc said:
Same equation. I was just taught it with E, electromotive force for voltage. From high school in the 70's. Another example of nomenclature changing through the years.
Click to expand...
The proper nomenclature has not changed, although the slang way it is used has.
Ohm's law has never changed it is E = I x R (V =I x Z is commonly used for AC circuits).
Many instructors use different techniques and presentations, probably in an attempt to simplify the math.

However the issue you are experiencing involves trying to measure the cold DC resistance of a motor's windings and then trying to relate that to the AC current drawn when the motor is warm and running. The motor current is actually based on a complex impedance of which your measured resistance is only a small component.
 
I didn’t have all the characters on keyboard. Here it is in writing. We’re saying the same thing, just not quite the same way.

Sorry I didn’t say it to your satisfaction.

Oh no! Pic is upside down. You can rotate it or your head to read it.
 

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