This method has two motors mounted on same shaft called in tandem or cascade operation. The motor “A" which is connected to the mains is called as the main or the master motor. This motor has slip rings mounted on its rotor shaft from which the motor “B" gets its supply from is called as auxiliary or the slave motor. It is to be noted that both the motors are mounted on same shaft. Thus it is evident that either the motors must run at same speed or it may have some gear arrangements.
The main motor is necessarily a slip ring induction motor but the auxiliary motor can be slip ring or squirrel cage induction motor. For satisfactory operation, motor “A" must be phase wound/ slip ring type with the stator to rotor winding ratio of 1:1, so that in addition to cascade operation, they can also run from supply mains separately. Since the supply for the slave motor is from the slip rings of the master motor, and it is forming a chain of sequential operation, the system is called as “Tandem or Cascade or Concatenation" operation. Three or four different combinations are possible for attaining different speeds.
1. Main motor may be alone on the mains, where Na = 120f/Pa, where Pa is the number of poles in motor “A."
2. Auxiliary or the slave motor running alone on the mains, where Nb = 120f/Pb, where Pb is the number of poles in motor “B."
3. The combination may be in cascade operation. In this operation, the important point is that the phase rotation of the stator fields of the motors “A" and “B" must be in same direction. Thus the synchronous speed of this cascaded motor set is given by Nc = 120f/ (Pa + Pb).
In my next article, we will discuss on the cascade set starting phenomena and speed control by injecting e.m.f in the rotor circuit.