automation4me - Drive Tech

The power supply to the induction motor is through the stator winding terminal. The speed control of the induction motor is possible at the stator winding terminal, by appropriately changing the electrical supply voltage, frequency or the internal winding. The rotor circuit windings available in a slipring induction motor, allows an additional means to control the speed. This method of varying the motor speed by adding resistance in the rotor circuit is known as rotor resistance control ( RRC ). The operating principle of RRC is explained as follows:

In the rotor resistance control method, the speed variation in a motor can be achieved by altering the slip the motor can operate. This method is applicable for slipring induction motors, as it involves addition of the external resistance in the rotor circuit of the motor (as shown in fig ).The principle employed in the rotor resistance control is changing the internal motor circuit parameters, by adding external rotor resistance. This in turn changes the torque-speed characteristics of the motor.

With increasing resistance, the slope of the motor curve decreases, shifting the stable operating point for the given load curve to a point with higher slip. Thus the speed control is achieved in the rotor resistance control.( This is represented in the fig. below)

Figure- Torque vs Slip at various rotor resistances

The above graph shows the variations of the torque with slip, the other factors remaining constant. The change in slip is attained by changing the value of rotor resistances. In the graph, the curves A, B, C & D have rotor resistances Ra, Rb, Rc & Rd respectively. The relative values of resistance's is as follows: Rd>Rc>Rb>Ra. It is observed that a significant amount of input power has to be dissipated in the external resistors. This power lost due to the increase in slip is called as slip power. The ratio of slip power to total power input changes with speed.

Rotor Resistance Controller (RRC) : Rotor Resistance Controller (RRC) is a method of speed control applicable to the slip-ring induction motor only.

Advantages of RRC:

No harmonic generation: Unlike AC drives & SPRS, RRC has no adverse effect, such as harmonics generation which affects the distribution network.

Ambient conditions: RRC has no electronic components like that of other electronic variable speed drives. Hence, they can be installed in even adverse environments.

Disadvantages of RRC:

External cooling: A portion of the input power has to be dissipated in the external rotor resistors. These resistors require cooling fans to dissipate the heat generated by them. The cooling fans form an additional load.

Speed adjustment: In this methods the speed adjustment is in steps or with very poor regulations

Maintenance: This method of control has lot of contractors & orther moving parts, which requires regular maintenance.