Saturday, September 24, 2011

Cacade Control

       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. 1. The auxiliary motor can be slip ring type or squirrel cage type.
Fig. 1 Cascade control pf two induction motor
       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 torque 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,     PA = Number of poles of main motor
                         PB = Number of poles of auxiliary motor
                         f = Supply frequency
                         NSA = 120f / PA
                         N = Speed of the set
       The speed N is same for both the motors as motors are mounted on the same shaft.
                         sA = ( NSA - N)/NSA
       Now          fA = Frequency of rotor induced  e.m.f. of motor A
                         fA = sA f  .... as fr = s f
       The supply to motor B is at frequency fA, i.e. fB = fA
       Now on no load, the speed of the rotor B i.e. N is almost equal to its synchronous speed NSB.
       Key Point : Thus the speed N of the set is decided by the total number of poles equal to PA- PB. 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 PA- PB.
       Thus in  cascade control, four different speeds are possible as,
a. With respect to synchronous speed of A independently,
       Ns = 120f/PA
b. With respect to synchronous speed of B independently with main motor is disconnected and B is directly connected to supply,
       Ns = 120f/PB
c. Running set as cumulatively cascaded with,
       N = 120f / (PA + PB)
d. Running set as differentially cascaded with,
       N = 120f / (PA - PB )
       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 can not be operated if  PA = PB.

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