Question

(a) Explain the construction and working principle of a three-phase induction motor used on ships. How is the direction of rotation of the motor reversed? (8)

(b) Discuss the maintenance procedures for an electric propulsion motor on a ship. What common faults can occur in the motor, and how would you diagnose and repair them? (8)

Answer 1

Part (a)

Construction and working principle of a three-phase induction motor, and reversal of direction:

Construction: the motor has a stator and a rotor. The stator is a laminated iron core with slots carrying a three-phase distributed winding. The rotor is either a squirrel-cage (bars short-circuited by end rings) or a wound rotor (slip-ring) with a three-phase winding. The rotor is mounted on bearings inside the stator, with a small air gap between them.
Working principle: when a three-phase supply is applied to the stator winding, it produces a rotating magnetic field (the three-phase currents are 120 degrees apart in time, producing a field that rotates at synchronous speed Ns = 120 f/P). This rotating field cuts the rotor conductors and induces an e.m.f. in them (by Faraday's law). Since the rotor conductors are short-circuited, currents flow in them. The interaction between the rotor currents and the rotating field produces a torque (by the motor action F = B I l), which drives the rotor in the direction of the rotating field. The rotor always runs slightly slower than the synchronous field; the difference is the slip.
Reversal of direction: the direction of rotation of the rotating field (and hence the motor) is reversed by interchanging any two of the three supply phases to the stator. This reverses the direction of the rotating magnetic field, so the motor runs in the opposite direction. This is done by a reversing contactor or by changing over two supply leads.

Part (b)

Maintenance procedures for an electric propulsion motor, common faults, diagnosis and repair:

Maintenance: regularly inspect the windings for signs of overheating, moisture, or contamination; measure the insulation resistance (megger test) of the windings; check and tighten all connections; inspect the bearings (lubrication, temperature, vibration); check the air gap; clean the motor and ventilation system; check the cooling system; inspect the slip rings/brushes (for wound rotor or synchronous motors); check for vibration and unusual noise; periodically test the protection devices.
Common faults and diagnosis:
Overheating: caused by overload, poor ventilation, blocked cooling, low voltage, or insulation deterioration. Diagnose by checking load, cooling, voltage, and insulation resistance.
Insulation failure / earth fault: caused by moisture, contamination, or ageing. Diagnose by megger test and visual inspection; repair by drying, cleaning, or rewinding.
Bearing failure: caused by wear, misalignment, or lack of lubrication. Diagnose by vibration, noise, and temperature; repair by replacing the bearing.
Vibration: caused by unbalance, misalignment, or a bent shaft. Diagnose by vibration analysis; repair by balancing or realignment.
Unbalanced currents / single-phasing: caused by a faulty supply or a broken connection. Diagnose by measuring the phase currents; repair the connection or supply.
Rotor faults (broken bars): cause vibration and torque pulsation. Diagnose by current signature analysis; repair by replacing the rotor.
Repair: isolate and lock off the supply, discharge any stored energy, and follow the manufacturer's procedures. Repairs may include drying and varnishing the windings, replacing bearings, rebalancing, or rewinding. After repair, test the insulation resistance and run the motor on no load before returning it to service.

✓ Verified Model Answer (Text Solution)

Construction and working principle of a three-phase induction motor, and reversal of direction:

Maintenance procedures for an electric propulsion motor, common faults, diagnosis and repair: