Rotor resistance starter

But where accurate speed control is the process requirement, static controllers, termed slip recovery systems (Section 6.16.3) are recommended, which in addition to exercising extremely accurate speed control, also conserve slip losses. Static drives are discussed in Chapter 6. Below we will describe a procedure to determine the value of resistance, its steps and switching and control schemes for these steps for a rotor resistance starter. 

Figure 3.32 Rotor resistance starter for a wound rotor machine.

The.se arc similar to stator resistance starters, as discussed in Section 4.2..3 and can be used in the rotor circuit to control the rotor side resistance. Figure 5.10 shows the smooth variation of resistance by electrolytic vaporization compared to a conventional metallic resistance variation. I hc self-variable resistance of electrolyte is equivalent to almost three or four steps of a metallic resistance and makes such starters economical. Normally one step is sufficient for motors up to 160 h.p. (For spccd-torquc... 

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The basic control circuit consists of a full-voltage starter and a balanced, adjustable three-phase resistor, wye-connected in the rotor circuit. Speed can be established for a given load by adjusting rotor resistance once set, speed will vary with load conditions. 

When the motor is first switched on the externai rotor resistance is at a maximum. As the motor speed increases the resistance is reduced untii at fuii speed the externai resistance is cut out and the machine continues to run as a cage induction motor. The starter is provided with overioad and no-voit protection and an interiock to prevent the machine from being switched on with no rotor resistance connected. These are not shown in Fig. 3.32 because the purpose of the diagram is to show the principie of operation. 

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