1) In a slip ring induction motor ( SRIM) , a three phase variable resistor $R_2$ can be inserted in the rotor circuit as shown in Fig 12.37 (a) . By varying the rotor circuit resistance $R_2$. the motor torque can be controlled as shown in fig 12.37 ( b)

2) The starting torque and starting current can also be varied by controlling the rotor circuit resistance, Fig 12.37 ( b) and ( c )

3) The disadvantages of this method of speed control are

• Reduced efficiency at low speeds
• Speed changes very widely with load variation
• Unbalances in voltages and currents if rotor circuit resistances are not equal.

4) In spite of these, this method of speed control is used when speed drop is required for short time, as for example in overhead cranes, in load equalization.

5) In chopper method of speed control for SRIM , the slip power is dissipated in the external resistance and it leads to poor efficiency of the drive.

6) However, instead of wasting the slip power in the rotor circuit resistance , it can be conveniently converted by various schemes for the speed control of SRIM.

7) Two important slip power recover schemes are as follows:

• Static Kramer drive
• Static scherbius drive

8) Concept of slip power

Rotor Input Power=Mechanical Power+ Rotor copper loss

$\therefore P_R=P_M+P_C \\ P_M= Mechanical \ \ \ Power \\ \hspace{2 cm}P_C=Rotor \ \ copper \ \ loss \\ P_R=W_ST \\ P_M=WT \\ \therefore P_C=P_R-P_M=(W_s-W)T \\ But \ \ slip s=\frac{W_s-W}{W_s} \\ \hspace{2 cm} =W_s-W=SW_s \\ \therefore P_c=W_sT=P_R-P_C \\ \; \\ P_M=P_R-SP_R \\ P_M=(1-s)P_R \\ \; \\ η=\frac{P_M}{P_R} \\ η=1-S \\ $ If Rotor input Power remains constant then from (1)

$P_c=SP_R \\ \therefore P_c \alpha S$

And $P_c$ is known as Slip Power

Now, in rotor resistance control, the extracted power from the rotor (slip power ) is dissipated in an external resistor.

9) Instead , if it is reutilized either by feeding it back to supply or used to supply an additional motor which is mechanically coupled to main motor , the efficiency of the system will increase to great extent.

10) This type of drive system is known as slip power recovery systems.