Electrical oral questions class 4 exams

 Q)What is special about steering gear safety.

Ans.)It has got overload alarm at 150% and short circuit trip.

Q)Maintenance of alternator what all to do.

Ans.) (a) Clean the felt/ air filters.

(b) Remove dust/ dirt from the rotor.

(c) Clean the connections and put some petroleum jelly.

(d) Check the air gap with the plastic feeler gauge.

(e) Check the lub oil level in bearing.

(f) Insulation resistance to take with megger.

(g) Check condition of winding.

(h) Carry out continuity test.

Q)Purpose of air gap and how checked.

Ans.)To avoid the mechanical contact between the stator & rotor and to determine the condition of the bearing. Measured with the help of feeler gauge.

Q) what are the conditions for paralleling alternators?

Ans.)  Frequency of incoimg generator should be slightly more than that of bus bar

(b) Phase angle of incoming and bus bar voltage should be same

(c) Voltage of incoming and bus bar should be same

Q)How to parallel alternators

Ans.)There are three methods of Paralleling alternators:-

(i) Dark lamp method

(ii) Bright lamp Method

(iii) Synchorscope

 

Q)Suppose a motor is not starting what checks u will make

Ans.
Electrical Checks:

1. Check the connections.
2. Check, if has tripped, then reset and try to start.
3. Check supply is coming from MSB or not.
4. Check single-phasing has not occurred.

Mechanical :

Check the coupling between the motor and driven part is not too tight.

Q)Suppose a motor is running hot what checks.

Ans.)Single-phasing, Overload is occurring, Check condition of bearing.

Q) Why insulation resistance checked and how?

Ans.)A measurement of the insulation resistance gives the best guide to the state of health of the electrical equipment.

1. Disconnect the electrical supply.
2. Refer to the electrical circuit & manufacturer’s instruction before testing.
3. Measure the insulation of the windings to the earth and as soon as possible when the machine is just hot after it has stopped. The minimum value should be 1 MW.

 

Q) Why motor insulation deteriorates?

Ans.)1.Moisture – results in decrease in the measured value of insulation resistance.

2.Dust & dirt deposits – reduces insulation resistance & failure of insulation due to oxidation.

3.Oil & Grease – prevents heat dissipation & causes dirt and dust to settle on the insulation.

4.Ageing – over the period of time the insulation deteriorates due to it getting affected by temperature variation, mechanical stresses, vibrations, moisture and some varnish tend to become hard with age & crack during the operation.

5.Temperature – Excessive temperature dehydrates and oxidizes the insulation, making it brittle & disintegrate under vibration & shock.

Q)What are portable electrical testing instruments used in ER ?

Ans.)1.Clampmeter

2.Megger

3.Multimeter

4.Tester

Q)Why preferential trip provided Purpose?

Ans.)Preferential trip is provided to safe guard the machineries which are important for safety of ship & safety of personnel onboard.

Preferential trips are designed to disconnect the non-essential services i.e., Breakers controlling Air Conditioning, Galley power, blowers, refrigeration) in the event of partial overload or partial failure of the supply, with the aim of preventing operation of the main breaker trip & loss of power to essential services.