LATEST ELECTRICAL TECHNOLOGY MMD ORAL Q & A PART-4

0
1131

Q Explain emergency battery charging system. Show a typical wiring diagram for two batteries working on a charge-discharge

Emergency batteries charging system

  • A simple arrangement for lead-acid batteries is shown in figure. Load switch(L), charging switch(C)
  • At loss of main power, emergency power provided by automatic connecting of battery
  • This type of secondary cell loses charge gradually over period of time.
  • Rate of loss kept to minimum by maintaining cells in clean and dry state by using trickle charge
  • In normal condition, batteries are on standby (L) open and (C) closed.
  • This position of switches is held by the electromagnetic coil against spring pressure
  • Loss of main power cause of de-energizing coil, spring pressure moving operating rod.
  • Batteries disconnected from mains as switch (C) opens, and (L) close connect to emergency load
  • Loses charge is recovery by trickle charge using continuous constant current. Replace for losses, no result of external load
  • When batteries discharged on load, trickle current is insufficient to recharge them.
  • Full charge restore by switching in quick charge. Afterwards batteries put back on trickle charge.

Charge – Discharge cycle

  • figure shown is typical wiring diagram for two batteries working on a charge discharge cycle.
  • Essential power (e.g. for radio equipment, fire detection, general alarm circuit) supplied from two batteries worked on a regular charge/discharge cycle.
  • One battery is discharge to load other is charge from ship’s mains.
  • In case of main failure, feedback from the battery on charge is prevented by a blocking rectifier in the circuit.
  • Each change-over switch has an “Off” position and operated independently so that both batteries put in parallel to the load during change-over operation.
  • Continuous electrical supply to the load at all times. “off ” positions are essential to avoid excessive overcharging.

Write short notes for the followings with the aid of simplified circuit diagram.

(a) Battery charging from d.c main.

(b) Battery charging from a.c main

(c) Function test of relays

(d) Function test of contactors

(a) Charging from d.c main

  • Circuit consist resistance connected in series, to reduce current flow from higher mains voltage.
  • At main failure, feedback from battery on charge is prevented by relay (which is de-energized) and spring arranged to automatically disconnect battery.
  • Contacts are spring operated; gravity opening is not acceptable for marine installations

(b) Charging from a.c main

  • Main a.c supply is transformed and rectified, supply DC for battery charging.
  • Supply current change from 230 V to 30 volts for charging 24 volt battery.

(c) Function test of relays

  • Relay coil tested by measuring its resistance.
  • Disconnect all power sources, relay is removed from circuit and checked with analog or digital meter.

For relay coil

  • Relay coil have a measurable resistance (not open or not shorted).
  • Resistance vary design and operating voltage.
  • When coil is open, meter reading is infinite Ω. When coil is short, meter reading is O Ω

For contacts

  • The contacts should ohm as either open or shorted because they are a switch. Never have a measurable resistance.
  • When coil de-energized, normally open (NO) contacts read infinite Ω (OL), normally closed (NC) contacts read 0 Ω.
  • When coil energized, readings reverse: normally open (NO) contacts read 0 Ω, normally closed (NC) contacts read infinite Ω (OL).

(d) Function test of contactors

  • After servicing the contacts, operation checked with an ohmmeter. Ensure the circuit is de-energized.
  • Disconnect at least one of leads to contact surfaces to ensure other parallel circuits are not read by ohmmeter.
  • Connect one lead of ohmmeter to one side of contacts and another lead to the other side of the contact.
  • Ohmmeter readings while contact physically open and close
  • when contacts open, read infinite resistance, when contacts closed, Ohmmeter read zero resistance

(a) Describe changes which take place within a lead-acid battery during discharge and when charging is taking place?

(b) Explain

(1) Why the rate of charge will effect gassing?

(2) The risk associated with gassing and safeguards in battery compartments?

(3) The reason for distilled water topped up?

(4) The remedy for spillage of electrolyte on the skin?

(a)

During discharge

  • At positive plates, hydrogen ions (H+) remove oxygen from lead peroxide (PbO2) and combine with it to form water (H2O).
  • Loss of oxygen from the lead peroxide reduces it to grey lead (Pb).
  • At negative side of cell, sulphate ions (SO–) combine with pure lead of (-) plates to form a layer of white lead sulphate (PbSO4).

When charging,

  • Sulphate goes back into solution as sulphate ions (SO4-), leaving the plates as pure lead.
  • In electrolytes water breaks down, returning hydrogen ions (H+) to solution, allows oxygen to recombine with lead of (+) plate and form lead peroxide (PbO2).

(b) (1) Gassing

  • Charge progress, gassing begins when cell voltage reaches 2.30 ~ 2.35 V per cell and increases. This process cannot be prevented entirely.
  • To reduce amount of gassing, charging voltages about 2.30 V per cell minimized.
  • Rate should not be too high to avoid violent gassing.

(2) Gas emission

  • When almost full-charged and during overcharge-
  • hydrogen and oxygen are generated from lead acid batteries through cell vent-caps into battery compartment.
  • If hydrogen is allowed to accumulate, there is an explosion risk
  • Regulations require good ventilation to remove gas and safeguards against naked lights or sparks in battery compartments.

(3) Topping up

  • Gassing and normal evaporation cause water loss,
  • All rechargeable batteries (Other than the sealed type) topped up distilled water periodically.
  • Exposure of cell plates to air rapidly reduce battery-life.

(4) Remedy for spillage of electrolyte on the skin

1. An alkaline cell uses an electrolyte of potassium hydroxide and a lead-acid cell uses Sulphuric acid.

2. Both are diluted with distilled water and electrolyte cause skin burns.

3. When electrolyte splashed on skin, first aid treatment give immediately.

4. Rapidly wash eyes and skin with plenty of fresh water.

5. For Potassium hydroxide splash, wash out with a solution of boric acid powder.

6. For Sulphuric acid splash, wash out with saline solution

7. In battery compartment, both types of first aid equipment.

(a) State the reason for carry out insulation tests on electrical machinery?

(b) How will you test the insulation resistance on electrical motor?

(c) What is the minimum requirement for insulation result?

(d) Describe the procedure for drying out an electric motor which has been immersed in sea water.

(e) State the routine maintenance which should be carried out an A.C motor.

(a) State reasons for insulation test

  • Measurement of the insulation test gives state of health of electrical equipment.
  • Measure resistance between insulated conductors and earth, and between conductors

(b) Insulation resistance test on electric motor

  • Before applying the test, approve tester, test equipment disconnected from live power supply
  • Megger tester use to check circuit live or dead. If circuit is dead, insulation resistance measured.
  • For phase-to-phase insulation resistance, 3 readings measure U-V, V-W, and W-U for between conductor
  • For phase-to earth insulation resistance as U-E, V-E and W-E for conductor to earth
  • Insulation resistance decreases when temperature increase
  • Record ER temp, Equipment temp.
  • Insulation tester is a high reading resistance meter, using a high-test voltage 500 V DC.
  • Test voltage produced an internal hand-driven generator battery, electronic voltage charger.
  • Test voltage of 500V DC use for ships’ equipment rated at 440 V AC.
  • Test voltages of 1000 V and 5000 V used for high voltage (HV) systems

(c) Minimum requirement for insulation result

Insulation resistance at least 1MΩ, maintain equipment in a clean condition to prevent tracking

(d) Procedure for drying out electric motor

  • Insulation resistance of the stator windings restored to a high value. This is achieved in three stages Cleaning, drying & re-varnishing
  • Motor dismantled and check all parts.
  • Salt contamination removed by washing with clean, fresh water.
  • Any grease or oil on the windings removed using electro cleaner
  • Dry the stator windings with low-power electric heaters or lamps
  • Windings heated by current injection from a welding set or from a special injection transformer.
  • Keep current injected level below motor’s full load rating.
  • If windings clean and dry, restore insulation resistance to high value, apply good quality air-drying insulating varnish

(e) Routine maintenance for AC motors

(1) Keep motor clean and free from dirt and oil.

(2) Check for dampness around the motor or inside the motor

  • This can reduce insulation strength of motor winding.
  • keep motor dry internally as well as externally
  • If not in use for long time, run the motor for few hours to dry out moisture.

(3) Periodic inspection of motor for accurate shaft alignment

  • For directly coupled motor – check alignment between motor shaft and load shaft
  • For belt-type system – check belt condition and belt tension.

(4) Check bearing condition on a regular basis.

  • Bearing lubricated with prescribed lubricant in proper quantity.
  • Excess as well as lesser quantity can do harm.

(5) Check for any abnormal noise or excess vibration from the motor or coupling. Do vibration analysis if necessary.

(6) Check motor heating – If motor heats up quickly, check and clean air filters to get adequate air flow.

Motor Maintenance

  • Maintenance required for cage rotor induction motors are.
  • Keep insulation resistance high and contact resistance low
  • Lubricate correctly and maintain uniform air gap
  • Ensure both interior and exterior always clean and dry

(a) State with reasons, the most common locations of earth faults aboard ship.

(b) Explain how earth faults are detected on a three-phase insulated AC system.

(a) Common location of earth faults onboard ship

The most common locations of earth fault onboard ships are

1. Lamp fitting on open decks exposed to weather.

2. Washing machine and galley equipments which works in humid condition.

3. Loose strands, all strands must be used in machine wiring. Unconnected one or two strands can touch grounded conductor. Care to be taken when connection point in solenoid valve, limit switch, pressure switch. etc.

4. Conductor can be touch with metal case of components

5. Insulation tape may not be wrapped properly.

6. Aging destroys its insulation properties.

7. Dripping water over electrical machines.

8. Insulation affected by condensate due to machine not use for long time.

(b) Earth fault detection

  • In a 3-Phase A.C, there have no earth faults, the lamps glow with equal half brilliance.
  • If an earth fault occurs on one line, the lamp connected to that line dim or extinguished.
  • The other lamps will glow brighter than before (due to increased voltage).
  • This method is commonly used for many years due to inexpensive installation and easy- to -understand.
  • Disadvantage is not very sensitive and fail to indicate a high impedance earth fault.
  • Instrument-type earth fault monitor connects a small DC voltage.
  • Any resulting DC current is a measure of the insulation resistance of the system.
  • The injection-type instrument limits the maximum earth fault monitoring current to only 1 Ma (compared with about 60 mA for earth lamps)