- With reference to a solid state (inverter) starter for a 3 phase induction motor:
(a) explain the starting characteristics of this system;4)
(b) list the advantages and disadvantages when compared to conventional starting methods.(6)
(a) Starting characteristics of a solid-state (inverter) starter (4 marks)
- The inverter starter controls both voltage and frequency supplied to the motor during start.
- By maintaining a constant V/f ratio, the motor develops high starting torque without excessive inrush current.
- Starting current is limited to near rated current (compared with 6–8 times rated current in direct-on-line starting).
- Acceleration is smooth and adjustable, minimising mechanical and electrical stress.
(b) Advantages and disadvantages compared to conventional starting methods (6 marks)
Advantages:
- Reduced starting current – avoids high inrush current associated with DOL starting.
- High starting torque – unlike star-delta or autotransformer methods, inverter control gives torque close to rated from standstill.
- Smooth acceleration – prevents mechanical shock to shafts, gears, and couplings.
- Variable speed capability – inverter allows precise control of motor speed during and after starting.
Disadvantages:
- Higher cost – more expensive than traditional starters (DOL, star-delta, autotransformer).
- Complexity and maintenance – requires skilled personnel and more sophisticated maintenance compared to simple electromechanical systems.