Characteristics of Polyphase Induction Motor

Author: E.E. Kimberly

The polyphase induction motor is the simplest, cheapest, sturdiest, and generally most reliable industrial motor in general use in sizes greater than 1/2 hp. The squirrel-cage rotor is extremely simple and has no electrical connections to any other part of the motor or to the power supply. With proper design proportions the polyphase motor may be satisfactorily started by connecting it directly to the power supply through a manually operated or automatic starting switch of simple construction and low cost. However, it does have certain disadvantages, such as low efficiency and low power factor at less than half-load; and, being essentially a constant-speed motor, it cannot be adapted to variable-speed duty without serious loss in efficiency. In the variable-speed form of polyphase motor, the rotor must be provided with collector rings to allow connection to an external resistor, and this form is not so simple and rugged as the constant-speed form.