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Electro-technology:

Supplementary:

Common faults in DC Generators and Motors:

1. 2. 3.

1. 2. 3. 4. 5. 6.

1. 2.

1. 2.

1. 2. 3. 4.

1. 2.

1. 2. 3. 4.

Sparking at brushes.

OverheatingFailure to excite.

Sparking at Brushes:

Wrong brush position.

Dirty commutators.

Brushes not properly bedded.

Incorrect spring pressure on brushes.

Wrong grade of brush.

Overloading.

Overheating:

Overloading

Blocking up of ventilation passages with dirt.

Why Megger Reading is taken?

To verify insulation resistance.

To detect insulation fault.

How Insulation Resistance of a motor is tested?

Switch-off at Main Switchboard.Disconnect the connection from starter.

Test with Megger.

Insulation Resistance is tested while at hot condition because it is minimum at that

time.

Test forOver Current Trip:

By increasing current injectors; (e.g. by using Welding Machine.)Test forReverse Power Trip:

By reducing Frequency.Excitation Loss:

Energize with Battery.

Tap with hammer tofield coil core of Excitation Motor.

Rotary Converter:

ConvertAC to DC.

Synchronous motorandgeneratorcombined unit.

Field and armature coils are similar to DC generator, except that slip rings are

located on the end of the shaft opposite to commutator.

AC turns the converter (as synchronous motor) and DC is taken from commutator

brush.

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Carbon Pile Regulator: (AVR)

1. A resistance from a carbon pile (stack), which is varied bypressure changes,controls the current flow through exciter shunt field.

2. Pressure is appliedby springs andrelievedby magnetic field strength of

electromagnetic coil.3. Current forelectromagnetic coil is supplied fromAlternator outputto switchboard.4. AVR is designed such that variations inAlternator Voltage, due to load change will

effect strength ofelectromagnetic coil and hence alters carbon pile resistance.

5. WhenAlternator voltage is low, spring exerts greater pressure andcarbon pileresistance becomes low, so more currents flow through exciter shunt fieldand then

increase the output voltage.

6. WhenAlternator voltage is high, electromagnetic coil relievespressure on carbonpile andresistance becomes high. Less current flows through exciter shunt fieldand

decreases the voltage.

(Strength ofElectromagnetic coil relieves spring pressure on carbon pile.)

Fuse, to order:

1. 2. 3.

1. 2. 3. 4.

1. 2.

1. 2. 3. 4. 5.

Amperage of the circuit. (AC/DC)

Type of fuse wire (Tin or Lead wire).

Standard Wire Gauge (SWG)

12 (0.104"dia.),14,16,18 or 20.

Battery, to order:

Voltage

Ampere/hour

Size

Type ( Lead Acid or Alkaline )

Static Electricity in oil tank, Prevention;

Earthing device, earth bondacrossflanges on pipeline.

Inert gas

Ward Leonard System:

Used for fine control ofshunt motor speedfrom zero to full in either direction.Able to give the motor a robust torque characteristic.

Can be used for motors ofelectric steering gears of ships with DC power.

Used today on ships with AC power for deck machinery such as windlass, mooring

winch etc.

Working motor, which powers the steering gear, windlass or other equipment is a

DC machine, because speed control of these is easy.

Method:

1. 2.

A DC generator is driven by AC squirrel-cage induction motor (AC powered ship).

Output voltage of DC generator is applied aspower supply to armature of workingmotor.

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3. 4.

Speed and direction of working motor varies with magnitude and direction of

applied voltage.

Output voltage of DC generator is increased or decreased by Potentiometer, as

magneticfield strength is altered by changing thefield currenttofield windings of

the generator.

5. 6. As output voltage of the generator varies, speedof the working motor also varies.Change ofcurrent flow direction, also by Potentiometer, through thefield poles of

the generator will cause the change in direction ofgenerated current, supplied to

the working motor and thereby also the running direction of the motor.

How to change speed?

Through Potentiometer by changing the strength of field currenttofield winding of

DC generatoroutput voltage of DC generator changedworking motorspeedis also changed.

How to change direction?

Through control handle movement of Potentiometerby changing the directionoffieldcurrent through field poles of DC generator

direction ofgenerated currentis changed

running direction of working motor is changed.Surveyor approvedIR(m)

1 m =

1. 2. 3. 4. 5.

1. 2. 3. 4. 5. 6.

a)

b)

c)

d)

e)

10

Switchboard Survey, what documents to give to Surveyor?

Voltmeter, Wattmeter, Ammetercalibration test result.

ACB test result (Survey result).

Control Circuit Safeguard test results (Safety trips and Alarm test results).

Maintenance reports.

IR test results ofeach terminal: all 440V and 220V main circuits.

Data to be given to Surveyor forElectrical Survey:

Voltmeter, Wattmeter, Ammetercalibration test result.

ACB test result (Survey result).

Control Circuit Safeguard test results (Safety trips and Alarm test results).

Maintenance reports.

IR test results ofeach terminal: all 440V and 220V main circuits.

IR test results of all motors & theirsafety factors:

Test date, time, place

Voyage No.

Ambient temperature

Weather condition, hot or humidMachine is hot or cold.

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Navigation lights:

1. 2. 3. 4. 5.

1. 2. 3. 4.

1. 2.

Fore Mast (No. 1&2 or Up, Down).

Main or Aft Mast (No. 1&2 or Up, Down).

Stern light (No. 1&2 or Up, Down).

Port light (No. 1&2 or Up, Down).Starboard light (No. 1&2 or Up, Down).

Emergency Lighting:

Engine room lighting

Bridge lighting

Passage way lighting

Embarkation light

How will you check thefrequency ofshore power supply?

Shore Power Supply Connection Box shows Phase Sequence of shore powergenerator with bright and dark lights.

Frequency can be check atMain Switchboard, after shore supply is on through

link switch orcircuit breaker.