A.C. Motor Current Draw Question
 

Can't seem to get a consistent answer on what I believe is a basic question,
and would appreciate some feedback. It concerns a used A.C. induction
motor of the type used for boat lifts. The question is posted here because
this motor lives in a sea air environment with all that says about possible
deterioration of windings and so forth.

The motor of interest can be wired for either 115V or 230V. It's 3/4 HP,
1725 RPM, capacitor start, 115V @ 10.8Amps / 230V @ 5.4Amps, 60Hz..

On a no-load bench test @115v, the motor draws approx 11Amps, essentially
the labeled rating. Two motor shop guys assert that modern motors draw close
to their rated Amps no-load and that the draw doesn't change much under load
if the motor is good. The motor in question draws around the same Amps, load
or no-load, with no more than a 1Amp difference. Another motor shop guy
asserts that this motor should draw at most 6-7Amps, no-load, and feels the
motor is bad. In other words, he says the labeled Amp draw is under-load,
while the no-load Amp draw should be much lower.

Who's right?

Len


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A.C. Motor Current Draw Question
 

Len Krauss wrote:
Can't seem to get a consistent answer on what I believe is a basic question,
and would appreciate some feedback. It concerns a used A.C. induction
motor of the type used for boat lifts. The question is posted here because
this motor lives in a sea air environment with all that says about possible
deterioration of windings and so forth.

The motor of interest can be wired for either 115V or 230V. It's 3/4 HP,
1725 RPM, capacitor start, 115V @ 10.8Amps / 230V @ 5.4Amps, 60Hz..

On a no-load bench test @115v, the motor draws approx 11Amps, essentially
the labeled rating. Two motor shop guys assert that modern motors draw close
to their rated Amps no-load and that the draw doesn't change much under load
if the motor is good. The motor in question draws around the same Amps, load
or no-load, with no more than a 1Amp difference. Another motor shop guy
asserts that this motor should draw at most 6-7Amps, no-load, and feels the
motor is bad. In other words, he says the labeled Amp draw is under-load,
while the no-load Amp draw should be much lower.

Who's right?

Len



Hi Len,

My understanding is that the no-load
current is usually close to the
full-load current for a single phase
induction motor.

There is no simple way to calculate
no-load current from the full-load
nameplate current rating.

Hopefully, others will confirm this.

Other than shorts and leakage, which
would may not show up in current
measurements, motor case temperature
rise may be the simplest indication of a
problem with the motor.

Chuck

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