Introduction to Induction Motor Starting

An induction motor is part of a system comprising the driven load, the induction motor, the starter and the supply. The best starting conditions can only be met if all components of the system are correctly engineered as a group. The driven load requires torque to accelerate to full speed. If insufficient torque is applied to the driven load, it can not reach full speed. The Induction motor converts current into torque to accelerate the motor. If there is insufficient start current available, the motor can not develop enough torque, and the load can not reach full speed.

To engineer the system, it is important to firstly establish the starting torque requirements of the driven load. Next the starting characteristics of the induction motor should be analyzed in order to establish the start current required by the motor to develop the required starting torque. A starter can now be designed/selected to meet the start current requirement, and an appropriate supply connected.

Induction motors exhibit a very low impedance at speeds less than their rated speed. This results in a very high start current when Direct On Line started. The Direct On Line starting current is independent of the motor load and is dependent only on the motor design, rotor speed and the applied voltage. Variations in motor loading will affect the start duration only. Typically, the Direct On Line starting current falls somewhere between 550% Full Load Current and 900% Full Load Current. The actual start current of a given design is determined primarily by the design of the rotor. Shallow bar rotor designs are generally referred to as Design 'A' rotors and are characterized by a high start current (650% - 900%) and a low starting torque (60% - 150%). Design 'B' rotors are deeper bar rotors and typically exhibit a starting current of (550% - 650%) and a starting torque of (150% - 300%).

In many installations, the maximum starting torque is not required, and the very high starting current places stress on the supply causes voltage disturbances and interference to other users on the supply. Reduced voltage starting is a means of reducing the start current, however a reduction in the start voltage will also reduce the starting torque.
In order to achieve a useful start at a reduced starting current, it is important that the motor is able to develop sufficient torque at all speeds up to full speed to exceed the load torque at those speeds. If the reduced torque developed by the motor is less than the load torque at any speed, the motor will not accelerate to full speed. Stepping the starter to full voltage at less than full speed will result in a high current and little if any advantage over using a Direct On Line starter. The selection of a start voltage that is too low will result in an inferior start characteristic.

Star/Delta (Wye/Delta) starters are open transition. When the transition is made from the reduced voltage to full voltage, there is a period of time when the motor is effectively open circuited from the supply. During this period, the motor is effectively acting as a generator at a frequency proportional to it's actual shaft speed. When the starter reconnects the motor to the supply in Delta, there is a very high transient current and resulting transient torque which is much more severe and damaging than the Direct On Line starting conditions.

Other reduced voltage starters commonly employed are the Autotransformer Starter and the Solid State Soft Starter.