Question

The direct on line start of squirrel cage motor is used for most electrical drives on a.c. powered ships.

Describe with sketches as necessary one method of overcoming each of the following problems:

(a) High starting current;

(b) Low starting torque.

(a) The three sets of stator windings have end connections which are brought out to a starter box.

Changeover contacts in the starter enable the six ends to be star-connected for starting (Figure ) and then, as the rotor comes up to speed, to be reconnected in delta.

Star starting has the effect of reducing the voltage per phase to 57.7% of the line voltage.

Starting current and torque are only a third of what they would be with direct on-line starting.

The low-current start is obtained at the expense of torque and star-delta motors can only be used with light starting loads.

Automatic switching to the delta running condition is preferred to manual changeover which may be made too soon or too slowly and cause a current surge.

In the delta running condition, phase voltage is equal to line voltage and the motor behaves as a straightforward squirrel-cage type.

Built-in interlocks or double-throw switches prevent star and delta contacts from being closed together.

The starter is also designed so that star contacts have to be made before it is possible to change to the run position.

Star-delta starter

b) Starting torque of a simple squirrel cage motor is low in relation to maximum possible operating torque.

Starting torque can be improved by increasing resistance of the rotor conductors.

However, high resistance in the current path results in high starting torque but poor performance at speed, unless the resistance can be reduced as the speed builds up.

Induction motors with wound rotor and double squirrel cage motors are designed to start on load with resistance but to run at normal speed with the resistance removed or compensated for.

Wound rotor motor