Question

Direct on line starting for large induction motors such as those for bow thrust units, may not be viable,

(a) State the reasons for this.

(b) Sketch a starling system that may be useful for such motors.

(c) Describe the starting method sketched hi (a, b ).

Most induction motors are DOL started because of being inexpensive and simple to operate and maintain. When very large motor such as bow thrusters unit motor is DOL started it causes a significant disturbance of voltage called voltage dip on supply lines due to the large starting current surge. This voltage disturbance may result in malfunction of other electrical equipment connected to the supply example lighting defects and flickering effects.

Also high starting current surge may cause serious heating damage to the motor if the motor is repeatedly started and stopped in short time period. The current surge would be as high as 7 times the full load current

DOL method is used usually for motor up to 5KW rating.

A DOL start squirrel cage induction motor has 3 drawback

High starting current

Low starting torque

Single speed operation

To limit the starting current some large motor are started at reduced voltage and then have the full supply voltage reconnected, when they have accelerated close to their rated speed.

Two method of reduced voltage starting are

Star delta starting

Auto transformer starting

Also some times an electronic soft starting can be used.

Direct on-line starter circuits

Typical characteristic curves for a direct on-line start squirrel-cage motor

Star-delta starter