Starting of Polyphase Induction Motors

Author: E.E. Kimberly

When a polyphase squirrel-cage induction motor with a low-resistance rotor is connected directly to a power line of its rated voltage and frequency, it draws from that line 5 to 7 times its rated full-load current. Such starting is called full-voltage starting. If the impedance of the power line is small, and the voltage regulation and frequency regulation of the generator are small and the rapid acceleration of the load is not objectionable, then full-voltage starting is satisfactory. If the voltage regulation and frequency regulation of the power system are not unusually good, the starting current of a motor with a low-resistance rotor that develops 7 1/2 or more hp causes intolerable line disturbances and may seriously affect the performance of other apparatus on the same line. For these reasons such a motor should be started by a reduced-voltage starter. Voltage reduction is usually accomplished by "step-down" auto-transformers, by series line resistors, or by smoothly-variable voltage regulators.

The auto-transformer type of starter, one of which is shown in Fig. 18-16, is usually arranged for a choice of two or three starting voltages ranging from 50 to 80 per cent of full line voltage. The choice of starting voltage is determined by the starting torque required by the load.

Fig. 18-16. Auto-Transformer Type of Induction-Motor Starter

Motors with line-start rotors, high-torque high-resistance rotors, or phase-wound rotors may be started on full voltage. Such starting requires merely a starting switch that provides overload and low-voltage protection. A starter of this type is called a "line-starter." The only advantage of full-voltage starting over reduced-voltage starting lies in the difference in cost of starting equipment. A full-voltage automatic line-starter is shown in Fig. 18-17. The National Electric Code¹ requires that a safety switch like that shown in Fig. 18-18 be installed between the starter and the line to insure safety from shock when the starter is being repaired or adjusted. A three-phase motor with all six phase leads available may be started on reduced voltage in another manner, if it is designed for delta connection. The phase leads are connected to a multi-blade doublethrow switch, by which the phases may be connected in Y for starting and in delta for running. This arrangement providestimes the normal phase voltage for starting and is satisfactory when the starting duty is not severe.

Fig. 18-17. Automatic FullVoltage Induction-Motor Starter

Fig. 18-18. Safety Switch

1	National Electric Code Handbook, by A. L. Abbott. McGraw-Hill Book Co.