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Automatic Rotor Controller


    Slip ring motors require a high starting torque, and require additional starters. This is achieved by connecting additional starting resistances while starting the rotor circuit. As a result, we require starters that operate on full motor voltage and high starting torque. External resistance is added through slip ring of the motor which restrict the starting current & boost torque to drive various applications.
    We have two models in Slip ring starters :

    • 1.Automatic Rotor Controller
    • 2.Automatic Slip Regulators

    Both models are used for different applications, but work on the principle of varying the rotor frequency with the magnetically amplified resistance.

    Automatic Slip Regulators are preferred for Sugar Industry applications such as Fibrizor, Crusher, Chopper, Cutters and Levellers.

    Automatic Rotor Controllers on the other hand are preferred for applications like Pumps (Centrifugal, Vertical turbine, Mixed flow, Oil hydraulic), Compressors (Centrifugal, Reciprocating, Process Compressor, Refrigeration, Screw, Air-conditioning) and heavy equipment (Ball Mill, Crusher, Fans) etc.


    Automatic Rotor Controller


    Automatic rotor controller starters work on the principle of a magnetically amplified resistance which is directly proportional to the rotor frequency. They are designed with a lossy core and help lower the starting current by automatically reducing resistance proportional to motor speed. As per the Slip ring motor theory the rotor frequency of the motor is proportional to the Slip, and Torque is proportional to the active component of the rotor current. These are represented as
    FCMA soft starters reduce starting current, accelerates motor to full speed and then is bypassed when the motor reaches full speed Thus ensuring that the motor continues to run regularly on full voltage. The circuit diagram below illustrates the working of the soft starter.

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Rotor power factor is unity due to the large external starting resistance of the Rotor Controller module. Thus, to maintain a constant torque the rotor current has to be maintained constant. The Rotor voltage of the slip ring motor is governed by the following equation.

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As the motor starts accelerating rotor voltage decreases. Thus, if the rotor current is to be maintained constant for constant torque, the starting resistance has to be decreased as the motor accelerates. The rotor controller allows for step-less, jerk-free smooth motor starting. After the motor reaches full speed, the rotor starter will be out of the circuit with the help of an inbuilt bypass contactor or circuit breaker.
Summary Features of Rotor Controller:

  • • Allows for step-less smooth starting of motor
  • • Very low maintenance as stepless cutting of resistance while starting
  • • High starting torque allows motors to reach full speed easily


Rotor controller starter is bypassed soon after starting with optimal switching device Bypass Contactor or Circuit Breaker

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Torque speed curve comparison for slipring motor starting systems

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In the curve shown above, we can see that the Rotor controller produces a step-less response as opposed to the Grid Resistance Starter. The resistance of the rotor controller is speed-dependent as compared to the LRS system, where the resistance cutting is time-dependent. As a result, after the specified time, we observe a kick in the motor due to the lowering of the resistance.