21. FACTORS GOVERNING PERFORMANCE OF INDUCTION MOTORS
The operating or performance characteristics of an induction motor are governed by the following factors/parameters:
- Rotor resistance;
- Air gap length;
- Shapes of both stator and rotor slots and teeth.
1. Rotor resistance:
- The rotor resistance is the most important parameter and is determined primarily by the material used in construction and the area of cross-section of the end rings in case of a squirrel case induction-motor, The commonly used materials are copper and aluminium.
- An increase in rotor resistance affects the performance ofthe motor as mentioned below:
- With the increase in starting resistance the starting torque increases until it-attains the maximum value, after which it diminishes.
- An increase in rotor resistance reduces the starting current and operating efficiency and causes increase in per cent regulation. The maximum torque remains unaffected because it depends upon the rotor reactance only.
2. Air gap length:
- The air gap is one of the two main sources of the low power factor at which the induction motor operates.
- Too much reduction of air-gap may cause
- increase in noise, pulsation losses and magnetic pull ;
- reduction in over-load capacity.
3. Shapes of both stator and rotor slots and teeth:
- The shapes of rotor teeth and slots affect the reactance, which in turn, affects the starting current and maximum torque. The induction motors with deeper slots have more leakage reactance and. so less value of starting current, maximum torque and slip corresponding to maximum torque.
- The slots in the rotor are not always, made parallel to the shaft, but are given a slight twist or skew. The motor noise ant vibration, cagging and synchronous cups can be reduced or even eliminated by shewing either the stator or rotor slots (in India, practice is to skew the rotor slots).
Harmonic torques can be reduced by :
- integral slot winding;
- increasing air-gap length;
- skewing of rotor slots.
Note. Space harmonic fields are developed by the windings, slotting, magnetic saturation, gap length irregularity. These harmonic fields induce e.m.fs. and circulate harmonic currents in the rotor windings, These harmonic currents in the rotor interact with the harmonic fields and develop harmonic torques, vibration