Cascade Control

Cascade Control AU : May-04,05,07,08,11,12,13,16,18, Dec.-04,07,ll,16

This method is also called concatenation or tandom operation of the induction motors.

In this method, two induction motors are mounted on the same shaft. One of the two motors must be of slip ring type which is called main motor. The second motor is called auxiliary motor. The arrangement is shown in the Fig. 7.10.1. The auxiliary motor can be slip ring type or squirrel cage type.

The stator of the main motor is connected to the three phase supply. While the supply of the auxiliary motor is derived at a slip frequency from the slip rings of the main motor. This is called cascading of the motors. If the torques produced by both act in the same direction, cascading is called cumulative cascading. If torques produced are in opposite direction, cascading is called differential cascading.

Now let, P_A = Number of poles of main motor

P_B = Number of poles of auxiliary motor

f = Supply frequency

N_SA = 120 f / P_A

N = Speed of the set

The speed N is same for both the motors as motors are mounted on the same shaft.

S_A = N_SA – N / N_SA

Now  f_A = Frequency of rotor induced e.m.f. of motor A

f_A = s_Af          ... As f_r = s f

The supply to motor B is at frequency fA, i.e. f_B = f_A

Now on no load, the speed of the rotor B i.e. N is almost equal to its synchronous speed N_SB.

Key Point Thus the speed N of the set is decided by the total number of poles equal to P_A+P_B. This is possible for cumulatively cascaded motors.

If by interchanging any two terminals of motor B, the reversal of direction of rotating magnetic field of B is achieved then the set runs as differentially cascaded set. And in such a case effective number of poles are P_A – P_B

Thus in cascade control, four different speeds are possible as,

a. With respect to synchronous speed of A independently,

N_S = 120f / P_A

b. With respect to synchronous speed of B independently with main motor is disconnected and B is directly connected to supply,

N_S = 120f / P_B

c. Running set as cumulatively cascaded with,

N_S = 120f / P_A + P_B

d. Running set as differentially cascaded with,

N = 120f / P_A - P_B

This method is also rarely used due to following disadvantages :

1. It requires two motors which makes the set expensive.

2. Smooth speed control is not possible.

3. Operation is complicated.

4. The starting torque is not sufficient to start the set.

5. Set cannot be operated if P_A = P_B.

Example 7.10.1 A 4-pole induction motor and a 6-pole induction motor are connected in cumulative cascade arrangement. The supply frequency is 50 Hz while the frequency in rotor circuit of 6-pole motor is 1 Hz. Determine the slip of each machine and combined speed of the set.

Solution :

The arrangement is shown in the Fig. 7.10.2.

Let f_rl = Rotor frequency of motor A

f_r2 = Rotor frequency of motor B = 1 Hz

P_A = Poles of A = 4

P_B = Poles of B = 6

The set is cumulatively coupled.

Hence the synchronous speed of the set can be obtained as,

This is the combined speed of the set

The s_A and s_B are the slip values of machine A and B respectively.

Example 7.10.2 Two induction motor are used for the cascade control. The main motor has 4 poles while the auxiliary motor has 6 poles. Determine the various synchronous speeds possible for the set. Assume supply frequency 50 Hz.

Solution :

P_A = 4, P_B = 6 and f = 50 Hz.

Though P_A – P_B is negative, in practice the effective poles are 2 only. The negative sign indicates the direction of torque.

Review Questions

1. Explain the principle of cascade connection of three phase induction motors. AU : May-13, Marks 8

2. Explain the different speed control methods of induction motor from stator side. AU : May-04, 05, Marks 8

3. Explain the different speed control methods of induction motor from rotor side. AU : Dec.-04, Marks 8

4. Explain the different speed control methods of three phase induction motors. AU : May-07, 16,18, Dec.-07, 16, Marks 16

5. Write short notes on : Cascade operation of three phase induction motors. AU : May-08, 11, 12, Dec.-ll, Marks 8