Application of Low Frequency to Industrial Control Problems

The performance of an induction motor on low-frequency voltages is readily predictable by means of the relations shown in Appendix I. We find from these relations that full torque is available from the motor providing the impressed frequency is at least equal to the slip frequency at which this torque would normally occur. The change in speed from full load to no load under the foregoing conditions is approximately the same as under normal operating conditions. Small high-speed machines can be effectively employed in generating the voltages needed for low-frequency operation. Although the generating system which we have described is best suited to supplying one frequency, a selection of frequencies and speeds can be obtained by simultaneously switching the relatively low current exciting circuits to provide the necessary change in circuit constants for different frequencies. Speed control in the manner described is particularly useful in applications requiring several induction motors to be started either individually or simultaneously without immediately accelerating to a high speed. This feature is required in printing press and other drives where a number of individual units are flexibly interconnected. The ease with which these units can be arranged in various combinations is greatly enhanced if they can be individually driven. Ordinary wound rotor induction motor control does not provide the required slow threading speeds and the use of individual gear shifting or other mechanical speed changers complicates the mechanical system. The use of a low-frequency power source is one solution to this type of problem.