Motor current in relation to motor temperature

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philly

Senior Member
Does the current drawn by a motor increase, for an increased temperature in a motor. For example if the motor gets hotter will it draw more current.

I would think it would actually decrease some becuase the copper has a positive temp coefficient so an increased temperature would produce an increased resistance in the stator windings and reduce current some.

I guess as an example if I have a motor with a load of 100A on it will a motor in a hot enviorment draw more than 100 compared to the same motor and load in a cool enviornment?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I think that a motor that get's that hot is going to overheat, because an increase in resistance would reduce the magentic field, causing more slip, which increases heat. Snowball city.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
All other things being equal, the hotter the motor is, the less efficient it will be.

If the mechanical load remains the same, then the motor will consume more real power. If it consumes more real power and the supply voltage remains the same, then it will consume more real current.

The increased winding resistance may decrease magnetizing (reactive) current flow, so it is possible but unlikely that _total_ current flow would go down.

-Jon
 

Mule

Senior Member
Location
Oklahoma
Most motors are rated at 40c above ambient....take a reading of the ambient surroundings, and subtract it from the hottest spot on the motor, which is right on top of the motor winding, or the one of the bearings. When trending motor temps, always consider ambient temperature. Them turkeys will get really hot in weather like this if they are loaded up. ........hot enough to discolor paint sometimes...

So if the motor current is within nameplate FLA and SF ratings, and the motor does not exceed 40c above ambient, and the bearings are not too hot...your alright...
 
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Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
...
I would think it would actually decrease some becuase the copper has a positive temp coefficient so an increased temperature would produce an increased resistance in the stator windings and reduce current some.
...

Most motors have a positive coefficient of temperature but some smaller ones (with aluminum windings) have a negative. But either way, the NET effect is relatively negligible. First you must recognize that motor winding resistance, while not insignificant, is only a fraction of motor power consumption, representing (IIRC) 10-12% of total. That means that right off the bat, any change in motor winding resistance is only going to account for a fraction of a percentage of total power consumption.

As the temperature increases, the stator winding resistance increases roughly 4% for every 10deg C rise. But at the same time the rotor is going to heat up and it too increases resistance in the rotor bars so the flux doesn't penetrate as much, you lose torque and that causes an increase in slip, which increases current again in the stator windings. So overall the net change in current ends up being relatively meaningless.

Up until the insulation starts to burn off... :wink:
 

Besoeker

Senior Member
Location
UK
Does the current drawn by a motor increase, for an increased temperature in a motor. For example if the motor gets hotter will it draw more current.
Interesting question.
I routinely have to carry out calculations on induction motor performance from equivalent circuit values so I thought I'd check what happens. I picked a motor for which I had both calculated and verified data.
I increased both stator and rotor resistance elements by 30% to simulate a significant temperature increase.

For a constant torque load it increased the current by just over 0.5% and for a centrifugal load (fans, pumps) it decreased the current by about 0.4%.

So the answer, in this case, is that there isn't much change.
 
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