Question

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

(a) State the reasons for this. (4)

(b) Sketch a starting system that may be used for such motors. (8)

(c) Describe the starting method sketched in Q. 1(b) (4)

Answer 1

Part (a)

High starting current: on DOL starting the motor draws a starting (locked rotor) current of 5 to 8 times full-load current. For a large motor this can be several thousand amperes.
Voltage dip: this large inrush current causes a heavy voltage drop in the generator and cable impedance. The busbar voltage may fall below the acceptable limit (typically not below 85 to 90% of rated), which can cause contactors to drop out, other motors to stall, and sensitive electronic equipment to malfunction.
Generator overload: the sudden load may exceed the capacity of the running generators, causing the prime mover to slow, frequency to fall, and possibly the overload protection to trip the generator.
Mechanical shock: DOL starting applies full torque suddenly, causing high mechanical stress on the motor shaft, coupling, gearbox and the thruster unit, and can cause excessive wear.
Thermal stress: the high starting current produces high I squared R heating in the windings; frequent DOL starts can overheat the motor.
Starting torque may be too high for the driven load, causing damage to the propeller or thrust unit.
The supply system (cables, switchgear, fuses) must be rated for the starting current, which is uneconomic for a large motor.

Part (b)

A main contactor (line contactor) connecting the supply to the motor.
A star contactor which connects the three motor phase windings in star during starting.
A delta contactor which connects the windings in delta for running.
A timer (time delay relay) which changes over from star to delta after the motor has accelerated.
Overload relay, fuses or MCCB, start and stop push buttons, and an indicating lamp.
The motor has six terminals (U1 V1 W1 and U2 V2 W2) brought out to the starter.
During starting the star contactor closes so each winding receives line voltage divided by root 3 (phase voltage = line/1.732), reducing starting current to about one third of the DOL value. After a set time the star contactor opens and the delta contactor closes, reconnecting the windings in delta so full line voltage is applied for running.

Part (c)

On pressing start, the line and star contactors close. The motor windings are connected in star, so each phase winding is subjected to phase voltage (line voltage / root 3). The starting current is therefore reduced to approximately one third of the DOL starting current, and the starting torque is reduced to about one third of the DOL torque.
The motor accelerates under reduced voltage. After a preset time (set by the timer, typically a few seconds, allowing the motor to reach near rated speed), the star contactor opens and the delta contactor closes.
The windings are now connected in delta, so each winding receives full line voltage and the motor runs at its normal rated condition.
The changeover is timed so that the motor has accelerated sufficiently to avoid a large current surge when delta is applied. The overload relay protects the motor against sustained overload, and the timer prevents too early or too late a changeover.

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