Marine Engineering Study Materials

MEO Orals on Marine Electro Technology Function 5- Part 6

What is induction motor ?

• It is a motor which run at a speed below that of the rotating magnetic field.
• Types are squirrel cage induction motor and slip ring induction motor.

Squirrel cage induction motor

• Most widely used of all types of AC motors, due to simplicity, strength of construction and ease of maintenance
• Made up of two main parts, rotor and stator and no direct electrical connection between them.
• No wire winding or slip rings.
• Rotor has a series of plain bars (copper or Al) running in slots the length of the iron core
• Each end of the bar is brazed into 2 heavy copper rings, one at each end. Those bars form a cage, that looks like squirrel cage
• Stator has 3 separate windings supplied from a 3 phase AC supply
• Phase difference 120 and number of poles 2, 4, 6 or more, depending on speed required

Difference between Synchronous Motor and Induction Motor ?

• Synchronous motor is almost exactly the same as an alternator
• Induction motor cannot run normally at synchronous speed. It has slip.

What are AC motor overloads protections ?

1. Magnetic overload relay
2. Thermal overload relay
3. Built-in protective device

Causes of motor overheat ?

• Motor running at overload
• One phase failure (single phasing)
• Defective cooling fan blades
• Bearing failure
• Poor ventilation 
• Misalignment 

When phase failure occur, A.C motor run or not ?

• When motor is running, the motor keep on running but over load and over heat will occur.
• When motor is starting, the motor cannot run.

What is moving iron instruments ?

• It is an instrument suitable for measuring A.C voltage or current as well as D.C voltage or current.

What is potentiometer ?

• It is an instrument which is used to measure or adjust the potential difference in the circuit.

 What is moving coil instruments ?

• It is an instrument used for D.C only and as voltmeter or ammeter.

 What is Static Electricity ?

• Electricity at rest instead of in motion
• Electric charges may be induced by friction or atmosphere effect

How to prevent Static Electricity hazards in oil tanker ?

1. Earthing device, earth bond across flanges on pipeline
2. Inert gas

What are the causes and effects of Single phasing in motors ?

1. Single phasing occurs when one of the 3 phase circuits is opened, hence the remaining circuits carry excess current.
2. One phase of the circuits becomes open, due to blown fuse, faulty contactor, or broken wire.
3. It prevents a motor from starting, but a running motor may continue to run with this fault.
4. For a running motor, it can be detected by overloaded device in supply line, or overheating.
5.  For an idle motor, it cannot be started.
6. Due to single phasing, overheating in a stalled or running motor will cause, burnt out overloaded coil.

What is residual magnetism ?

1. Magnetism remaining in the fields of a generator, after exciting current is cut off.
2. Residual magnetism is essential for initial generation of current, necessary for further build up of shunt field strength.
3. Generator may fail to excite, if there is loss or reversal of residual magnetism of the pole.

What action, when generator fails to excite ?

1. Pass a current through shunt field coil in correct direction
2. Correct direction means the current will re-magnetise the iron core in the right way
3. Current for restoration can be obtained from another DC generator or from a battery.
4. If battery is used fro re-magnetising: A 12 V battery is connected [exclusively] across shunt field coil with the machine stopped
5. Current flow in right direction, for a few seconds, only will establish the field.
6. During this time faulty generator must be in stopped position.

What are windlass safety devices ?

• Emergency stop
• Overload trip
• Over speed trip
• Mechanical break
• Slipping clutch for over load, to prevent any undesirable damage such as hull damage due to anchor and rope brake out.

What is Slipping Clutch in windlass ?

1. Slipping clutch is commonly fitted between prime mover and gearing
2. It is incorporated with motor, magnetic brake and drive shaft
3. Set to slip at approximately 133% of full load torque
4. Letting go or dropping speed is controlled by friction brake
5. Hauling speed is 0.15 m/sec

Why Slipping Clutch is fitted in windlass ?

1. In windlass, undue stresses must not be applied to chain cable and machinery
2. Without slipping cultch, excessive stresses could be applied to cable, by armature momentum, by sudden obstruction when heaving, or when bringing the anchor into hawsepipe. 
3. Fitted also to avoid inertia of prime mover being transmitted to windlass machinery in the event of shock loading on cable, when anchor is being housed
4. When ship is riding at anchor, bow stopper prevents the strain for windlass

Explain electric breaking system in marine machinery ?

1. When electric deck motor is used for lowering or lifting load, electric brake system is used. All brakes are failsafe types.
2. In the event of power failure, brake automatically applied, thus preventing the load running back. A number of brake pads [free to move] are located in carrier, which is keyed to motor shaft.
3. Armature plate applies pressure to brake pads, by means of a number of springs, to force against friction surface of back plate, so that it prevents the motor shaft from turning.
4. When energized, armature plate is attached to magnet, thus releasing the thrust pressure and allowing the shaft to rotate

Reference:

Oral Guide by – MIN ZAR TAR

MEO Orals on Marine Electro Technology Function 5- Part 6