Operation Below and Above Nominal Speed in Speed Control Applications

It is possible to obtain constant torque from the motors below the rated speed, and constant power can be obtained above the rated speed, but the field weakening above about 85 Hz increases the losses and this causes the power to decrease. 

Operating Below Rated Speed 

If the supply voltage is reduced proportionally to the frequency when the frequency is decreased, the motor whose magnetic flux remains constant can be loaded at constant torque. If the torque is kept constant, the current and power coefficient does not change. For the torque to not depreciate at low frequencies, the voltage must be larger than proportional to the frequency. Thus, the voltage drop across the stator resistor is compensated. In load applications requiring variable torque, such as centrifugal pumps and fans, where the torque decreases with speed when choosing a frequency converter, it should be taken into account that the voltage should be lower than proportional to the frequency.
At low speeds, the cooling air produced by the motor propeller decreases in proportion to the speed. In applications that require constant torque, the heat generated in the motor does not change as the magnetic flux remains constant when the speed drops, so the cooling air of the propeller is not sufficient. The heat generated in the motor cannot be completely removed, and the reduction of iron losses at low speeds cannot fully compensate for the negativities caused by insufficient cooling. In this case, it may be necessary to reduce the output power of the motor or to provide for forced (independent) cooling.
 

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