Variable Speed Coupling

In this method, a variable speed drive is installed between the motor and the driven load. This drive may be an eddy current clutch or a fluid clutch (sometimes called a fluid coupling). Use of such a drive allows the motor to accelerate without accelerating the driven load. After the motor is up to speed, the variable speed drive is activated and the load brought up to operating speed. In this process, the losses that would have otherwise occurred in the motor rotor now occur in the drive, and must be dissipated from the drive. Usually, the drive has more effective cooling than the motor, and also the rate of acceleration of the load can be controlled so that the rate of heat generation in the drive can be controlled.

Variable Speed Drive Starting

Starting Characteristics:

  1. Allows higher starting inertias.
  2. Allows very low starting current using reduced voltage starter.


  1. Usually used with high inertia conditions where the motor is incapable of absorbing the heating.
  2. Also used where very low inrush is required.
  3. Also used where variable load speed is required.