Operating Induction Motor at Reduced Speed 1

[gcaudill]

Anyone care to guess, or have some tool, to estimate the temperature rise that will result from operating a non-inverter rated induction motor at less than base speed from an inverter supply?

I have:
5HP motor, 1750RPM, 184T frame, 230V/3/60, 87.5% Eff (at full load), NEMA design B, open drip proof enclosure.
Operating at:
.3HP, 175RPM, 100F. ambient, sensorless vector mode inverter supply.

Please include any assumptions in your guess/estimation.

Thanks in advance.

 

[jbartos]

Suggestion: With the manufacturer's tech support (that could potentially provide Power Factor for the motor), an electrical equivalent circuit for the motor might be obtained. Then, RI**2 might be used to calculate the amount of heat developed via inverter supplied voltage and current waveforms to the motor terminals. The amount of heat developed can be related to the sought temperature differential, increase or rise.

 

[UKpete]

electricpete - in the circumstances with little or no forced air i.e. natural convection, the largest temperature drop (and the most difficult to predict, hence the concentration on it in that article) will be between the frame and ambient. The temperature gradients will still be there inside the motor but if anything they will be reduced from those in the rated condition, because the losses are reduced therefore the heat flow is reduced. Typically the winding might be 40-60°C above the frame temperature.

There are two key interfaces within the machine where the temperature drops are large - in the slot wall (and in the slot itself if the slot fill is poor), and between the laminations and the frame. For instance there is GE data that suggests that the surface roughness of the lamination o/d and the bore of the frame is critical i.e. ideally in small machines with high power density they should be ground surfaces, and a good interference fit, otherwise temperature drops are excessive.

 

[electricpete]

Thanks UKPete. I learn something new here every day.

 

[mdonner]

1. Do not try it if your load is of the constant torque nature. Copper loses can not be handled without external cooling.

2. Motor should have thermal protection/detection to prevent burnout.

 

[jbartos]

Suggestion: Visit

http://www.engineeringtalk.com/news/mop/mop100.html

for Motor-CAD software that can be used for the motor themal analysis