Minimum RPM for TEFC motor

[grefzap]

We are using a VFD to control a 3/4 HP, TEFC, 1800 RPM, 1.15 SF motor with rolled steel frame. The Application is constant torque (positive displacement pump). The actual HP (at full speed) is only about 1/4 HP. Is there any way to estimate the minimum RPM for the motor without overheating. We want to operate at 12Hz. If we test this using trial and error, what should the max. frame temperature be? Are there other issues besides temperature that we should be concerned about?

 

[acmotorengineer]

If the rolled steel frame is in direct contact with the stator core, the top dead center temperature measurement (out of air flow) should be approx 10C less than average coil temp. However the hot spot of TEFC motors is usually the end turns - add another 10C for hot spot. A round about way of saying add 20C.

I would think you should have no temperature problem at 12HZ and 33% load.

Caution: if the motor has overbolts, sometimes they complete a circuit and overheat. If so, Change to non-magnetic SS bolts.

 

[gcaudill]

Another idea. Check with the manufacturer to ensure that you are operating in the listed speed range of the motor. For example, 10:1, 100:1, 1000:1.

Manufacturer lists speed range at which motor may operate at full torque without overheating and committing thermal suicide.

1800RPM/360RPM = 5:1 speed range. Should be okay if motor is rated for inverter duty.

I esitmated you are operating at 360RPM with the assumption the motor is rated is for 60Hz operation, and your listed 1800 RPM specification.

 

[wired1]

Your question can be most adequately answered by the motor mfr. In general, TEFC is not the best choice for running slow, since proper cooing depends on the fan speed. I would anticipate a need for supplemental cooling at lower than 40hz (assuming the mtr is vector duty). You could help yourself by changing to a TENV (vector duty).