MEO Orals on Marine Electro Technology Function 5- Part 6
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 ?
- Magnetic overload relay
- Thermal overload relay
- 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 ?
- Earthing device, earth bond across flanges on pipeline
- Inert gas
What are the causes and effects of Single phasing in motors ?
- Single phasing occurs when one of the 3 phase circuits is opened, hence the remaining circuits carry excess current.
- One phase of the circuits becomes open, due to blown fuse, faulty contactor, or broken wire.
- It prevents a motor from starting, but a running motor may continue to run with this fault.
- For a running motor, it can be detected by overloaded device in supply line, or overheating.
- For an idle motor, it cannot be started.
- Due to single phasing, overheating in a stalled or running motor will cause, burnt out overloaded coil.
What is residual magnetism ?
- Magnetism remaining in the fields of a generator, after exciting current is cut off.
- Residual magnetism is essential for initial generation of current, necessary for further build up of shunt field strength.
- Generator may fail to excite, if there is loss or reversal of residual magnetism of the pole.
What action, when generator fails to excite ?
- Pass a current through shunt field coil in correct direction
- Correct direction means the current will re-magnetise the iron core in the right way
- Current for restoration can be obtained from another DC generator or from a battery.
- If battery is used fro re-magnetising: A 12 V battery is connected [exclusively] across shunt field coil with the machine stopped
- Current flow in right direction, for a few seconds, only will establish the field.
- 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 ?
- Slipping clutch is commonly fitted between prime mover and gearing
- It is incorporated with motor, magnetic brake and drive shaft
- Set to slip at approximately 133% of full load torque
- Letting go or dropping speed is controlled by friction brake
- Hauling speed is 0.15 m/sec
Why Slipping Clutch is fitted in windlass ?
- In windlass, undue stresses must not be applied to chain cable and machinery
- 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.
- 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
- When ship is riding at anchor, bow stopper prevents the strain for windlass
Explain electric breaking system in marine machinery ?
- When electric deck motor is used for lowering or lifting load, electric brake system is used. All brakes are failsafe types.
- 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.
- 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.
- 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