Certain steps are standard when selecting an electric motor16 for a screw compressor. The motor must accommodate the design load imposed by the compressor on a continuous basis. Even a potential motor overload (when the suction pressure rises above the design value, for example) can be handled by the automatic unloading of the compressor using the slide valve.
In selecting the motor to drive a screw compressor, the power and torque demanded of the motor during start-up must be evaluated carefully. Even though the slide valve is to be in its fully unloaded position, the moments of inertia of the rotors are relatively high. The torque delivered by the motor should be 20% higher than required by the compressor throughout the entire speed range experienced during startup. The draw of current during startup may be five to seven times that occurring during full-speed operation, depending upon the type of starter. Since the heating of the windings is proportional to the square of the electrical current, the rate of motor heating is 25- to 50-times that of normal operation. The duration of this high-current period must be kept short by choosing a drive motor with adequate starting torque.