why so much variation in amperage

C

cg 2005

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I have four ac electric motors and none of them are close to the w/v=a formula, even allowing for 81% efficiency.

examples from the motor data plates, all at 115v

  1. 1/3hp @ 5a
  2. 1/3hp @ 9a
  3. 3/4hp @ 10a
  4. 1hp @ 12a

using the power formula and adding for 81% efficiency they should be, respectively

  1. 2.68a
  2. 2.68a
  3. 6.05a
  4. 8.05a

why such variation from the theoretical and difference between the two 1/3 hp motors?
 
Are they under load? A motor will draw considerably less current when just idling.
 
No, these are the specifications as indicated on the motor I.D. plate.

I am aware of the idle amperage being lower since not all the power is in use at idle. I believe the spec voltage is full load, correct?
 
yes, but that can vary +- 10% or so. if you have high voltage, your current will be lower also.
 
Your formula does not take power factor into consideration. Explaining power factor is not simple. In a DC circuit, volts X amps = watts; in an AC circuit the current reverses direction 120 times per second. The peak amperage and the peak voltage do not occur at the same time, which complicates the calculation quite a bit.

An analogy is a clock pendulum; mass swings back and forth but the actual work done can not be calculated with just the pendulum mass and distance traveled.
 
Here is some really technical info on the PF .... Complex indeed. There is also a calc link on the page but I did not try it yet

http://www.rapidtables.com/electric/Power_Factor.htm
Good post and neat theory! Thanks all.
CG.

den-den said:
Your formula does not take power factor into consideration. Explaining power factor is not simple. In a DC circuit, volts X amps = watts; in an AC circuit the current reverses direction 120 times per second. The peak amperage and the peak voltage do not occur at the same time, which complicates the calculation quite a bit.

An analogy is a clock pendulum; mass swings back and forth but the actual work done can not be calculated with just the pendulum mass and distance traveled.
Click to expand...
 
It appears that the power factor, PF, you reference is primarily a measure of efficiency since it is a ratio between power applied by the motor to power available in the circuit. In other words power output to power input.

Getting back to my original post. The specs as stated would then suggest that the 1/3 HP motor rated at 5A is more efficient than the 1/3 HP motor rated at 9A. The 9A motor is more than two times larger than the 5A motor in all dimensions. I suspect the 9A motor has much greater torque rating. I am going to research that a bit.

All of this was brought to light because I just bought a new Enco 5x6 band saw, model 505-6840, to replace my ten year old HF 4 1/2x 6.

The Enco has a 1/3 HP, 9A motor that is larger than the HF 3/4 HP, 10A motor.
 
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Power factor and efficiency are not the same thing. Another issue here is the motor ratings. "Horsepower" is well defined but people have been playing games with HP ratings since before electric motors were invented.
 
on efficiency...

Personally I would have an interest in the efficiency of a refrigerator or
anything that runs continuously. On a band saw with intermittant use I
wouldn't give it a second thought.
 
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