28 July 2016

FREQUENTLY ASKED QUESTIONS AND ANSWERS-ETO


FREQUENTLY ASKED QUESTIONS AND ANSWERS-ETO

Q.  What is a phase?

      Phase  in A.C system is the fraction of time period which has elapsed since current/voltage last passed through zero position of reference.  The unit is in secs or radians.

Q.  What is the difference between shock received due to DC and that due to AC?

 Current from a steady DC source in passing through the skin, will tend to cause muscular contraction at the initial contact and when the contact is broken. AC produces a continuing spasm in the muscles through which current passes, with its change from forward to reverse flow either at 50 Hz or 60 Hz rate.  AC has the ability to stimulate nerves directly.

Q. How will you check a diode with multi meter?

      Remove or disconnect the diode from the circuit.  ( If the power is turned off instead of disconnecting the diode, discharge all capacitors and remove all fuses in the circuit).  Multi meter drives a small DC current (a few mA) through the diode P-N junction, and acts as a voltmeter to measure the voltage across it.  Connect the two probes across the diode.  Read the voltage drop, it should be 0.5V – 0.8V  (500 mV to 800 mV) for a healthy silicon diode.  Reverse the probe connections and the display should indicate ‘over range’.  If the display indicates over range in both directions, means the diode is open circuited.  If the display indicates less than 1V in both directions, diode may be short circuited.

Q.  Why does a motor rotate?

      DC Motor:  when field is excited and armature is supplied with current from supply mains, they experience a force tending to rotate armature (direction given by Fleming’s left hand rule).  Each conductor experiences a force and all these forces collectively produce a driving torque which sets armature rotating.

      Induction motor:  Rotor does not receive electric power from supply, but by induction from stator.  When a 3 phase stator windings are fed by a 3 phase supply, a magnetic flux of constant magnitude, rotating at synchronous speed is set up.  The flux passes through the air gap, sweeps past the rotor surface and so cuts the rotor conductors which are as yet stationary.  Due to relative speed between the rotating flux and the stationary conductors an emf is induced in rotor.  The frequency of this induced emf is the same as supply frequency.   Magnitude is proportional to the relative velocity between flux and conductors (Fleming’s right  hand rule).  Due to this emf rotor current is produced (rotor conductors form a closed circuit), whose direction (Lenz’s slaw) is such as to oppose the very cause producing it.  In this case the cause which produces the rotor current is the relative velocity between the rotating flux of the stator and the stationary rotor conductors.  Hence to reduce the relative velocity, rotor starts running in the same direction as that of flux and tries to catch up with the rotating flux.

Q.  What are the functions of choke in the tube light circuit ? 

To provide starting voltage  and to maintain the p.d across the tube  ( when main supply is AC)

When the supply is DC, ballast resistor is used.

Q.  What is an electric shock?

Electric shock is due to flow of current through the body.  This is often from hand to hand or hand to foot.  A shock current of 15mA  AC or DC may be fatal.

Q.  What are the conditions that affect the electric shock?

The size of electric shock current is related to following factors:  1.  Applied voltage  (2)  Body resistance.  The size of human body resistance depends on other factors also, like state of health, degree of contact with live wires, perspiration or dampness of skin.  Typical dry full contact body resistance is about 5000Ω at 25V, falling to about 2000Ω at 250V.  This indicates that human body resistance goes down as the applied voltage goes up, giving rise to the increased current at high voltages.

Q.  What are the types of switch boards available ?

Switch boards may be open type in which all essential switch gear is exposed on the front of the panel, or ‘dead front’ type in which all the live parts are concealed behind sheet steel panels.  Only the operating handles and instruments appear on the front.  While DC switch boards may employ open type panels, ‘dead front’ type is mandatory on AC system.

Q.  Which system of supply is more lethal, AC or DC ?

AC supply is more lethal than DC supply at similar voltages.  On no account should work of even the simplest nature be undertaken on live AC apparatus without the operator being fully protected.

Q.  What are the precautions to be observed before commencing work on a switch board?

Before any work is done on the electrical equipment, follow the following safety procedures:

1.there should be an approved rubber matting on the floor, in front of the switchboard, for safety.
2.  Rubber gloves should be specially made for the purpose and comply with regulation requirements.
3.  portable hand lamps if used should be fully insulated without metal guards so that there is no risk of shock or short circuits.
4.  Precautions must  be observed to isolate and lock off the apparatus.  Where fuses are provided, these should also be removed.
5.  Where pilot lights, control circuits are concerned, these may still have power from separate source, even though main power circuit may be isolated.
6.  If the draw-out type circuit breaker is being overhauled, it will become dead, when it is pulled out fully.
7.  Before commencing work, it should be verified that the apparatus is actually dead using a live line tester.  This test should be conducted between phases, and also between phase and earth.
8.  A warning sign to be posted indicating the work is progressing in the apparatus.
9.  A switch board can not be considered dead unless all generators connected to it are off and all other supplies disconnected.

Q.  What do you understand by the term ‘dead switch board’?

A dead switch board is the one in which there is no supply of power available, whatsoever.

Q.  What is the setting of reverse power relay?

10% of generator [power rating with 5 secs time delay for diesel driven alternator.
2 to 5% of generator power rating with 5 secs time delay for turbo alternator.

Q.  What is understood by the term ‘Insulation Resistance’?

Insulation resistance is a measure of opposition offered to the current by the insulating materials

QWhat are the factors that affect insulation resistance ?

Insulation is affected by many factors such as:

1  Moisture – results in a decrease in the measured value of insulation resistance.
2. Dust and dirt deposits  -  reduces insulation resistance and failure of insulation due to oxidation.
3. Oil and grease  -  prevents heat dissipation and cause dust and dirt to settle on the insulation, thereby reduces the insulation resistance and leads to failure.
4. Ageing -  over a period of time the insulation deteriorates due to it getting affected  by temperature variation, mechanical stresses, vibrations, moisture, dirt deposits, chemical contacts etc  Some varnishes tend to become hard with age and crack during the operation.
5. Temperature  -  excessive temperature dehydrates and oxidizes the insulation, making it brittle and disintegrate under vibration and shock.

Q.  What is understood by the term ‘permissible temperature rise’ in case of insulation?

Permissible temperature rise is the difference between the ambient temperature and maximum permitted temperature for the class of insulation in use.

Q.  What is the relationship between the insulation and the temperature?

The insulation resistance falls with temperature rise.  Life of insulation will be halved for every 10ºC above the permitted temperature.  For example a machine designed for continuous operation at 70ºC will have its useful life cut in half when operating at 80ºC.

Q.  Why the insulation tests are carried out on electrical equipments?

A measurement of the insulation resistance gives the best guide to the state of health of the electrical equipment.  The marine environment is particularly arduous for electrical equipment, due to damp salt laden atmospheres, extreme temperatures and vibrations.  The continuous operation of equipment on board ship demands high operating efficiency and optimum economy in order to keep down costs.

Q.  What is the most preferred condition for measuring insulation resistance ?

Insulation resistance becomes more leaky at high temperatures.  So insulation test should be done while the machine is hot just after it has stopped.  This will give the realistic reading at near working temperature.

Q.  What is ‘megger’ ?

The instrument used for measuring insulation resistance in Mega Ohms.

Q.  What is armouring in a cable ?

Extra mechanical protection provided by armouring  with woven wire braid made of galvanized steel or tinned phosphor bronze.  Steel armouring is not preferred for single core cables carrying heavy alternating currents  because of magnetic properties.

Q.  What is meant by current rating of a cable ?

The passage of current through a cable conductor results in rise of temperature due to resistance of the conductor.  Therefore maximum current rating is based upon maximum operating temperature of the insulation and sheathing.

Q.  What is the short circuit rating ?

Under short circuit condition there is rapid rise in the conductor temperature, affecting cable insulation.  Mechanical forces due to magnetic effect and longitudinal expansion of conductors due to heat have also to be considered.  It is therefore necessary to match the cable size with the short circuit protection equipment (fuses or circuit breakers) which may take certain time (a fraction of a second to several seconds ) to trip.

Q.  What are the factors that are to be complied by the installed cables ?

The cables installed must comply with Current ratings and Voltage drop limitations.  Passage of current results in voltage drop along the length of the cables,.  Hence voltage drop becomes a problem in very long cables.  Excessive voltage drop can seriously affect the proper operation of electronic equipment.  Motors can give starting problem if voltage drop at the starting current is excessive.  The voltage drop in cables from the main switch board to the appliance must not exceed 6%   (practice it is about 2%)

Q.  Comment on the requirements of conduits in regard to cable installation

    1. Cable outlets from conduits should be bushed to prevent abrasion at the ends.
    2. Total cross sectional area of cables should be less than 40% of the cross section area on conduit.
    3. Ventilation of conduits should provided, and it should be ensured that water does not collect in the conduit.
    4. The conduit should be earthed.

Q.  How the cables are grouped or ‘bunched’ ?

1.  When several cables are run close together or are touching, the heat from one cable affects others, and thus requires lowering rating.  For example, if more than 6 cables are running bunched together, it is usual to reduce the current ratings to 85%. 
2.  Where possible, cables intended for different operating temperatures should not be bunched in common clips, glands, conduits or pipes.

Q.  What are the sources of electrical power on ship?

  1. Main alternator
  2. Emergency generator
  3. Emergency batteries
  4. Shore power

Q.  What are the functions of circuit breakers, fuses relays ?

The circuit breakers and switches are the means of controlling the flow of electric current.  The fuses and the relays protect the distribution system from large fault currents.

Q.  What is understood by ‘essential loads’ ?

Essential loads or services are those required for the safety of personnel and for safe navigation and propulsion of ship.


Q.  Why emergency source of electrical power is provided?

Emergency source of power supply is provided so that in the event of an total power failure, a supply will still be available for emergency lighting, alarms, communications, water tight doors and services necessary to maintain safety.

Q.  Comment on the sequence of events following a mains failure ?

Control and monitoring equipment of emergency generator on sensing a large drop in voltage or frequency sends a starting signal to emergency generator.  Bus tie breaker between the main switch board and emergency switch board opens.  In 45 secs the emergency generator starts and comes on load.  During this time the supply for emergency lighting, alarm etc are maintained by the emergency batteries.  In case emergency generator fails to come on load, these batteries can supply power to selected lighting, communication and alarm circuits for minimum period of half an hour.

Q..  How the emergency power supply is tested ?

Correct functioning of the auto start equipments is obviously vital to the production of emergency power.  Regular, weekly testing of the emergency generator should include simulation of the loss of normal power.  The start up equipment may provide a push button to interrupt the normal voltage supply to the panel which then triggers the start sequence.  Loss of main power supply can easily be simulated by pulling a fuse in the auto start panel which supplies the under voltage or under frequency relay.

Emergency generators should be regularly checked and run up for short test runs to comply with safety regulations.  These no load running checks should, when practicable, be supplemented occasionally by a proper load test.  This requires the disconnection of normal mains power while the emergency generator is loaded up to near its rated value.  Only a proper load test will prove the performance of generator and its prime mover, together with the circuit breaker operation.

Q.  What is the reason for using insulated neutral system in ships ?

The priority requirement on board ship is to maintain continuity of the electrical supply to equipment in the event of a single earth fault occurring.

Q.  What are the causes for short circuit ?

Short circuit may occur because of breakdown of insulation of a equipment due to overheating or ingress of water.  Sometimes terminal connections become loose, causing two power supply cables to make electrical contact.  Short circuit may also occur in the windings of a transformer because of insulation breakdown due to overheat.  Electrical cables exposed to fire, mechanical damage, cuts  etc. can cause short circuit.

Q.  Why the equipments are earthed on board the ship?

In order to protect against the dangers of electric shock and fire that may result from earth faults, the metal enclosures and other non current carrying metal parts of electrical equipment must be earthed.  The earthing conductor connects the metal enclosures to earth (ship’s hull) and prevents such metal parts from attaining dangerous voltage with respect to earth in the event of an earth fault occurring.

Q.  What are the common locations of earth faults ?

Earth faults are found at lamp fittings and terminals etc., where moisture has found its way in.
1. Lamp fittings on open deck, exposed to rough weather  are prone to ingress of moisture.

2. In accommodation, washing machines drenched with water area another source of earth faults.

3. In galley, the ovens, hot plates etc., are also exposed to earth faults.

4. Dripping water over electrical machinery.

5. Motors standing idle for long time - their insulation gets affected by condensed moisture.

6. Overheated insulations of motor and cables cause drop in insulation resistance.

7. Dirty electrical apparatus with surface tracking  (leaking current)

8. Old electrical apparatus whose insuslation has aged has become brittle.

9. Terminal wire which has become loose from the terminal and is touching the metal casing of electrical machinery.

Q.  How the earth faults are treated?

Earth faults should be eliminated when located.  Damaged conductor insulation must be repaired the method of repairing depends on the cause of earth fault and this is determined by visual examination.  A lamp fitting that is damaged must be replaced.  Dampness or moisture in insulation must be dried out by gentle heat and then precautions taken to prevent future ingress of moisture.  Insulation that has been mechanically damaged or damaged by overheating must be made good again.  If surface dirt is the cause, a thorough cleaning with electro cleaner solvent will probably cure the fault.

Q.  What is the rating of a fuse ?

Fuse rating is the current fuse will carry continuously and it is important that rating is correct for the normal current flowing in the circuit it protects.

Q. What are the reasons for installing fault protection in electrical distribution system?

1.  To disconnect and isolate faulty equipment in order to maintain the power supply to the remaining healthy circuits in the system.
2.  To prevent damage to equipment from thermal and magnetic forces that occur during short circuit and overload faults.
3.  To protect personnel from electric shock.

Q.  How is the strength of circuit breaker or fuse is specified ?

By its breaking capacity, which is the maximum fault current it can safely interrupt.

Q.  Why HRC fuses are normally preferred for Electrical installations on board ship?

    1. Time/current fusing characteristics are very consistent.
    2. HRC fuses are reliable and non deteriorating in service
    3. Fuses have very high breaking capacity.
    4. They also have very high speed of operation at high short circuit fault, faster than a circuit breaker.

Q.  How a fuse is selected for circuit protection ?

  1. Rating of fuse for LIGHTING or HEATING circuit :  it is the current which a fuse will carry continuously.  For a 50A circuit, a 50A fuse would be appropriate, unless it were a motor circuit.
  2. Rating of fuse for MOTOR circuit  :  In motor circuits, fuse should be large enough to carry starting current for the time necessary to start the motor plus necessary margin.  For motor, fuses are typically rated at 2-3 times mot full load.

Q.  How the overload protection device is tested and set?

Overload relays and circuit breakers should be tested periodically.  Bimetal elements in thermal overload relays become insensitive to heat from metal fatigue and fail to operate under overload condition.  These relays should be checked for corrosion, accumulation of dirt and other foreign matter that block free movement of tripping element.

Over current relays and circuit breakers may be tested by passing a specified over current through the heater or overload coil and observing the time required for it to trip.  The actual tripping time should then be compared with the manufacturer’s current time characteristics.

Test an overload rely, it should be disconnected from the power line and connected to low voltage, high current power source like variable autotransformer.  Current is adjusted to test value by connecting to one of the bimetal heaters via heavy test leads.  A stop watch is used to determine tripping time.

Magnetic overload trips and circuit breakers may be tested in similar manner.

Q.  What is under voltage protection for Generator and Motor?

An under voltage release is fitted to all generator’s circuit breaker.  Its main function is to trip the circuit breaker when severe voltage dip occurs.  The under voltage release on a generator circuit breaker also prevents it being closed when the generator voltage is very low or absent.  The under voltage relay prevents the closure of the circuit breaker of the dead generator.

Under voltage protection is also required for motor starters.  The starter contactor normally provides this protection as it drops out when the supply voltage is lost or is drastically reduced.  The starter circuit will not normally allow the motor to restart when the voltage supply is restored, except when special automatic restarting facilities are provided.  This ensures that when the supply returns after a black out, all the motors do not start together, there by tripping the generator circuit breaker again..

Essential motors may be arranged to start automatically by sequential start with staggered time delays.

Q.  How the under voltage relay is checked and calibrated ?

Under voltage protection can be electronic or electromagnetic .  checking and calibration of generators’ under voltage relays can only be done accurately by voltage injection.   A known variable voltage is directly applied to the under voltage relay to check :  1.  the voltage at which relay pulls in  (2)  the voltage at which relay drops out.  Generator under voltage relays have time delay to prevent false tripping during transient voltage dips (typically 15%), caused by large motor starting currents.

Q.  How does a current flow in the shaft of an generator ?

Machines with a very strong magnetic field often generates a voltage in the shaft that causes a current flow through bearings.

Q.  What are the effects of shaft current in a generator?
Flow of electric current through the bearings causes electrolytic action, which will cause bearing failure in due course and possible sludging of lubricating oil.

Q.  How the shaft currents are prevented ?

To eliminate shaft currents either (a) outboard bearing shell is insulated in the housing, or (b) outboard bearing pedestal is insulated from the foundation.

All lub oil pipe connections that make connection with the shaft or the particular bearing must also be insulated.

Q.  Why both bearings are not insulated?

Insulating both bearings would permit the build up of electrical charges (static electricity).  The resulting high voltage will be dangerous to operating personnel.

Q.  How the insulation of pedestal bearing checked?

1. The pedestal insulation of a dismantled generator may be tested with a megger.  Insulation of 20 kΩ and above is considered satisfactory

2. Condition of insulation can also be checked while machine is running by measuring milli-volts between the shaft and the bedplate.  Two readings are taken, one with the jumper connecting shaft to pedestal and one without the jumper.  If insulation is good, both readings will be alike.  If the insulation is defective, the reading in (1) will be higher than in (2).

Q.  How is the insulation resistance of alternator checked?

Disconnect any electronic circuit components which may be damaged by a 500V insulation test.  Consult the wiring diagrams and manufacturer’s instructions before testing.  Measure the insulation of the stator winding and the rotor windings to earth, and between stator phases.  A minimum value is 1MΩ, but a lower value may be acceptable to surveyor based on 1kΩ/volt, e.g. 450 kΩ for a 450 volt generator.


Q.  What is the purpose of shading coil in a contactor?

To reduce vibration, chattering and  noise.

Q.  What is the purpose of ICCP?   (impressed current cathodic protection)

To prevent corrosion of the hull, propeller, rudder and line shafting


Q.  What is the supply given to shipboard general alarm?

24V  DC.

Q.  When a component is to be removed from a PCB, what precaution will be taken?

Before desoldering the component a tightly knit metal braid wire will be used.

Q.  Armature coils are tested with a low voltage source.  What will be the indication for short circuited coils?

For short circuited coils, the voltmeter reading will show low or zero, other coils will have higher readings.

Q.  What do you understand by the term ‘varnished cambric’?

This is an insulating material made of cotton cloth dipped (or coated) with insulating varnish.

Q.  What is understood by ‘dead front’ switchboard?

Dead front switchboard is one with insulated switches and no exposed terminals.


 Q.  What is the unity power factor?

The voltage and current developed in an AC circuit reach their peak values at the same time.
The voltage and current are said to be in phase when power factor is unity.

Q.Under what circumstances would semi-conductor (or high speed fuses) be used on a soft starter application? What purpose do they serve?

Semiconductor fuses are an energy limiting fuse using specially shaped silver elements in a silicon sand environment. Because of their energy limiting characteristics, they are able to protect semiconductor devices from excess energy let through under fault current conditions.
A soft starter uses reverse parallel connected SCRs or thyristors in series with the supply to the motor. If a short circuit occurs between the starter and the motor, or in the motor itself, there is no limiting impedance to restrict the current flow and so the current is essentially the short circuit current of the supply. In most cases, this is enough to damage or destroy the SCRs. It is not practical to turn an SCR off once current has begun to flow, so electronic protection is not practical. Current will stop at the next zero crossing which could be half a cycle later. The semiconductor fuse is able to interupt the current flow in less than half a cycle, and the rupture time is dependant on the total energy flow.
SCRs have a maximum short term energy rating that is usually called I squared t (Current squared times time) and provided the maximum let through I2t (or total clearing I2t) of the fuse at the operating voltage of the supply, is less than the I2t of the SCRs, then the fuse should fail before the SCRs.
Semiconductor fuses can be a bit of a problem at times because their pre arcing I2t is well below their total clearing I2t and they can be stressed by the normal operation of the starter and fail prematurely. It is important to compare the overload time current curve of the fuse with the operation of the starter.



 Q.  What are the guidelines for understanding the specific rating of the DG Sets ?



STANDBY POWER RATING:

Applicable for supplying emergency power for the duration of the utility power outage. No overload capacity is available for this rating. Under no condition is an engine allowed to operate in parallel with the public utility at the Stanby Power Rating.This rating should be applied where reliable utility power is available. A Standby rated engine should be sized for a maximum of an 80% average load factor and 200 hours of operation per year. This includes less than 25 hours per year at the Standby Power Rating. Standby Ratings should never be applied except in true emergency power outage. Negotiated power outages contracted with a utility company are not considered an emergency.

PRIME POWER RATING:
Applicable for supplying electric power in lieu of commercially purchased power. Prime Power Applications must be in the form of one of the following two categories:

   UNLIMITED TIME RUNNING PRIME POWER:
     b_bullet Prime power is available for an unlimited number of hours per year in a variable load application. 

    b_bullet Variable load should not exceed a 70% average of the Prime Power rating during any operation period of 250 hours. 
    b_bullet The total operating time at 100% Prime Power shall not exceed 500 hours per year. 
    b_bullet A 10% overload capability is available for a period of 1 hour within a 12 hour period of operation. 
    b_bullet Total operating time at the 10% overload power shall not exceed 25 hours per year. 


   LIMITED TIME RUNNING PRIME POWER:

    b_bullet Limited time Prime Power is available for a limited number of hours in a non-variable load application. It is intended for use in situation where power outages       are contracted, such as utility power curtailment. 
    b_bullet Engines may be operated in parallel to the public utility up to 750 hours per year at power levels never to exceed the Prime Power Rating. 
    b_bullet The customer should be aware, however, that the life of any engine will be reduced by this constant high load operation. Any operation exceeding 750 hours        per year at the Limited time Prime Power rating should use the Continuous Power Rating. 


CONTINUOUS POWER RATING:Applicable for supplying utility power at a constant 100% load for an unlimited number of hours per year. No overload capability is available for this rating.


SPACE HEATER
Small resistance heater units mounted in a motor,  that are energized, during motor shutdown, to prevent condensation of moisture on the motor windings. 

 INDUCTION MOTORS:

Q.  What is slip?

SLIP
The difference between the speed of the rotating magnetic field (which is always synchronous) and the rotor in a non-synchronous induction motor is known as slip and is expressed as a percentage of a synchronous speed. Slip generally increases with an increase in torque. 




Q.  What happens if a motor is operated in Star instead of Delta?

If the motor is designed to be operated in delta on your local supply voltage, then operating continuously in star can cause damage to the motor. When a delta motor is connected in star, the voltage across the windings is reduced by the square root of three resulting in reduced flux in the iron. This will reduce the magnetising current, and will also reduce the torque capacity of the motor. If you operate at light loads, there will be no problem, however if you operate at high loads, the slip of the motor will be increased dramatically and it may stall. The increased slip will result in a dramatic increase in the power dissipated in the rotor. If the motor begins to stall, the stator will also suffer excess heating causing a motor failure.

Q.  Can I run a 60Hz rated motor on 50Hz?
Induction motors use an iron core and require flux in the iron to operate. In order to achieve the commercial goals of smallest size and lowest price at best efficiency, induction motors are designed to operate at a high level of flux in the iron. The flux is determined by the turns, voltage and frequency. In a modern motor, if the flux is increased by a small amount, the iron losses increase and the iron tends towards saturation. At saturation, the inductance begins to fall and the current increases further. To reduce the flux at a given voltage and frequency, the turns on the stator are increased. This reduces the Iron loss, but a longer length of thinner wire is used and the copper loss increases. Design becomes a balancing act between copper loss and iron loss and so the design is optimised for a given voltage and frequency.
If the voltage applied to the motor is held constant and the frequency is increased, the inductive reactance increases and so the flux reduces. This effectively reduces the maximum torque capacity of the motor and so the motor power rating at the higher frequency remains the same.
If the voltage applied to the motor is held constant and the frequency is reduced, the current will increase and in theory, the torque will also increase. The motor should be able to deliver the same power also, BUT the flux in the iron is now too high resulting in excessive iron loss, and the motor will fail prematurely. Above a very low frequency, (5 - 10Hz) the impedance of the magentising circuit of the motor is primarily inductive and so in order to keep the flux within limits, it is important to keep a linear V/F ratio (Voltage to Frequency ratio). If the frequency is reduced by 10%, the voltage must also be reduced by 10%. Because the flux in the iron remains the same, the torque capacity remains the same and so the power rating of the motor also drops by 10%.
Provided the voltage is dropped by the same proportion as the frequency, it is OK to run a 60Hz motor on 50Hz. The speed will be reduced by the reduction in frequency and the power capacity will also reduce by the reduction in frequency.
60 Hz Volts
50 Hz Volts
480
400
460
383
440
367
230
191



Q.  How do you set up a thermal over load? Do you set it for the current drawn or for the motor rating?

Strictly speaking, you would normally set the overload protection to the rating of the motor, but if the motor always operates below its rating, then setting the overload closer to the actual operating current will afford a higher level of protection, provided that the motor is still able to start without tripping the overload.
The method that normally advocated for setting a thermal overload is to operate the motor at maximum load for an extended period of time, then slowly adjust the overload down until it trips, then set the relay a small margin higher. The problem with thermal overloads is that the calibration is coarse to say the least, and this method ensures that you have protection against a change in the load characteristics of motor condition. If there are occasional nuisance trips, then the setting can be increased, but should not exceed the motor rating.

Q.  Under what circumstances would semi-conductor (or high speed fuses) be used on a soft starter application? What purpose do they serve?

Semiconductor fuses are an energy limiting fuse using specially shaped silver elements in a silicon sand environment. Because of their energy limiting characteristics, they are able to protect semiconductor devices from excess energy let through under fault current conditions. 
A soft starter uses reverse parallel connected SCRs or thyristors in series with the supply to the motor. If a short circuit occurs between the starter and the motor, or in the motor itself, there is no limiting impedance to restrict the current flow and so the current is essentially the short circuit current of the supply. In most cases, this is enough to damage or destroy the SCRs. It is not practical to turn an SCR off once current has begun to flow, so electronic protection is not practical. Current will stop at the next zero crossing which could be half a cycle later. The semiconductor fuse is able to interupt the current flow in less than half a cycle, and the rupture time is dependant on the total energy flow.


SCRs have a maximum short term energy rating that is usually called I squared t (Current squared times time) and provided the maximum let through I2t (or total clearing I2t) of the fuse at the operating voltage of the supply, is less than the I2t of the SCRs, then the fuse should fail before the SCRs.


Semiconductor fuses can be a bit of a problem at times because their pre arcing I2t is well below their total clearing I2t and they can be stressed by the normal operation of the starter and fail prematurely. It is important to compare the overload time current curve of the fuse with the operation of the starter.


Q.  Can I Bridge out the slip rings and use a soft starter on a slip ring motor?

A slip ring motor uses resistors in the rotor circuit to modify the starting characteristics of the slip ring motor. Increasing the resistance in the rotor circuit has two effects:
1. It reduces the start current
2. It increases the slip at which maximum torque occurs.

If the slip ring motor has been employed to provide a very high starting torque across the entire speed range during start, then the slip ring or secondary resistance starter can not be replaced. In this case, the first stage of the resistors would be selected to provide a high torque at 100% slip (zero speed) and a number of stages are then employed, each with reducing resistance to move the Slip point in steps from 100% towards 0%. The effect of this is to provide maximum torque at all speeds and at a reduced start current. (typically 200 - 300%)
Shorting out the slip rings and attempting any form of reduced voltage start in the stator supply, will result in a much reduced start torque at a much higher start current. Effectively, the motor could exhibit a Locked Rotor Current in excess of 1000% and a Locked Rotor Current less than 100%. If we reduce the start current down to say 400%, then the start torque would be less than 100 x (400/1000) x (400/1000) or less than 16%!
If the driven load does not require a high start torque, then the slip ring motor can be set up to emulate a standard cage motor by applying rotor resistance that will cause a full voltage start current of about 550%. A reduced voltage starter can now be applied, and the rings should be shorted out once the machine reaches full speed. If you do not short the rings at full speed, the slip will be higher than ideal and the motor efficiency will be reduced. There will be a high power dissipation in the resistors

Q.   What is understood by service factor?

SERVICE  FACTOR

1. When used on a motor nameplate, a number which indicates how much above the nameplate rating a motor can be loaded without causing serious degradation, (i.e., a 1.15 S-F can produce 15% greater torque than the 1.0 S-F rating of the same motor).

2. When used in applying motors or gear motors, a figure of merit which is used to "adjust" measured loads in an attempt to compensate for conditions which are difficult to measure or define. Typically, measured loads are multiplied by service factors (experience factors) and the result in an "equivalent required torque" rating of a motor or gear motor. 



Q.  What is skew?

SKEW
Arrangement of laminations on a rotor or armature to provide a slight angular pattern of their slots with respect to the shaft axis. This pattern helps to eliminate low speed cogging effects in an armature and minimize induced vibration in a rotor as well as reduce associated noise. Also can help to increase starting torque.


Q.  What is a splash proof motor?

SPLASH-PROOF MOTOR
An open motor in which the ventilating openings are so constructed that drops of liquid or solid particles falling on it or coming toward it in a straight line at any angle not greater than 100 degrees from the vertical, cannot enter either directly or by striking and running along a surface of the motor. 



Q.  What is understood by split phase start?

SPLIT PHASE START
Motor which employs a main winding and an auxiliary winding, which is called the starting winding. The windings are unlike and thereby "split" the single phase of the power supply by causing a phase displacement between the currents of the two windings thus producing a rotating field. After the motor has attained approximately 75% of rated speed, the starting winding is automatically disconnected by means of a centrifugal switch or by a relay. The motor then continues to run on a single oscillating field, which in conjunction with the rotation of the rotor, results in a rotating field effect. Since there is no rotating field, after the starting winding is de-energized, the rotation cannot be changed until the motor has come to rest or at least slowed down to the speed at which the automatic switch closes. Special starting switches are available as well as special reversing switches which have a means for shunting the open contacts of the automatic switch while the motor is running and thus permits the split phase motor to be reversed while rotating. This type of starting is found typically on single phase fractional motors.


Q.  What are the different types of enclosures available for induction motors?

1.Totally enclosed, Non ventilated type :  Such motors have solid frames and end shields, but no openings for ventilation.

2.Splash proof type:  In the frames of such motors, the ventilating openings are so constructed that the liquid drops or dust particles falling on the motor will not be able to enter the motor when the incident angle is not greater than 100º.

 3.Totally enclosed, Fan cooled type

4. Protected type :  end covers have perforated covers.

5.Drip proof motors :  the frames are so constructed that liquid drops or dust particles, falling on the machine at any angle greater than 15º from the vertical, cannot enter the motor, either directly or by striking and running along a horizontal or inwardly inclined smooth surface.

6.Self ventilated type :  consists of enclosed shields with provision for pipe connection on both the shields.  The motor fan circulates sufficient air through pipes which are of ample section.

 7.Separately (forced) ventilated type.

Q.  How do changes in supply voltage and frequency of the performance of induction motor?

High voltage decreases both power factor and slip, but increases torque.  Low voltage has the opposite effect.  Increase in frequency increases the power factor, but decreases the torque. The slip remains unchanged.  Decrease in frequency has the opposite effect.

Q  What is in brief, the basis of operation of a 3 phase induction motor ?

The revolving magnetic field which is produced when a 3-phase stator winding is fed
from  a 3-phase supply.

Q.  What factors determine the direction of rotation of the motor ?

The phase sequence of the supply lines and the order in which these lines are connected to the
stator winding.

Q.  How can the direction of rotation of the motor be reversed ?

By transposing or changing over any two line leads.

Q.  Why are induction motors called asynchronous ?

Because their rotors can never run with the synchronous speed.

Q.  How does the slip vary with load ?

Greater the load, greater is the slip or slower is the rotor speed.

Q.  What modifications would be necessary if a motor is required to operate on voltage different from that for which it was originally designed ?

The number of conductors per slot will have to be changed in the same ratio as the
change in voltage. If the voltage is doubled, the number of conductors per slot will have
to be doubled.

Q.  Enumerate the possible reasons if a 3-phase motor fails to start.

Any one of the following reasons could be responsible :
1.  one or more fuses may be blown.
2.  voltage may be too low.
3.  the starting load may be too heavy.
4.  worn bearings due to which ihe armature may be touching field laminae, thus
    introducing excessive friction.

Q.  A motor stops after starting i.e. it fails to carry load. What could be the causes ?

Any one of the following:
1.  hot bearings, which increase the load by excessive friction.
2.  excessive tension on belt, which causes the bearings lo heat.                    :
3.  failure of short cut-out switch.
4.  single-phasing on the running position of the starter.

Q.  Which is the usual cause of blow-outs in induction, motors ?

The commonest cause is single-phasing.

Q.  What is meant by 'single-phasing' and what are its causes ?

Single-phasing is meant the opening of one wire (or leg) of a three-phase circuit
thereupon the remaining leg at once becomes single-phase. When a three-phase circuit
functions normally, there are three distinct currents flowing in the circuit. As is known,
any two of these currents use the third wire as the return path i.e. one of the three phases
acts as a return path for the other two. Obviously, an open circuit in one leg kills two of
the phases and there will be only one current or phase working, even though two
wires are left intact. The remaining phase attempts to carry all the load. The usual cause
of single-phasing is, what is generally referred to as running fuse, which is a fuse whose
current carrying capacity is equal lo the full-load current of the motor connected in the
circuit. This fuse will blow-out whenever there is overload (either momentary or sustained
on the motor.

Q.  What happens if single-phasing occurs when the motor is running ? And when it is stationary?

1.  If already running and carrying half load or less, the motor will continue running as
single-phase motor on the remaining single-phase supply, without damage because half
loads do not blow normal fuses.

2.  If motor is very heavily loaded, then it will stop under single-phasing
since it can neither restart nor blow out the remaining fuses, the burn-out is
very prompt.

A stationary motor will not start with one line broken. In fact, due to heavy standstill
current, it is likely to burn-out quickly unless immediately disconnected.

Q.  How can the motors be protected against single-phasing ?

(i) By incorporating a combined overload" and single-phasing relay in the controlgear.
(ii) by incorporating a phase-failure relay in the control gear. The relay may be
voltage or current-operated.

Q.  Can a 3-phase motor be run on a single-phase line ?

Yes, it can be. But a phase-splitter is essential.

Q.  What is a meant by a phase-splitter ?

It is a device consisting of a number of capacitors so connected in, the motor circuit
it produces, from a single input wave, three output waves which differ in phase from
each other.

Q.  What is the standard direction of rotation of an induction motor ?

Counterclockwise, when looking from the front end i.e. non-driving end of the motor.

Q.  Can a wound-motor be reversed by transposing any two leads-from the slip-ring

No. There is only one way of doing so i.e. by transposing any two line leads.

Q.  What is jogging ?
    
It means inching a motor i.e. make it move a little at a time by constant starting and
stopping.

Q.  What is meant by plugging ?

It means stopping a motor by instantaneously reversing it till it stops.

Q.  What are the indications of winding faults in an induction motor ?

Some of the indications are as under:
i.   excessive and unbalanced starting currents
ii.  some peculiar noises and (iii) overheating.

Q.  What is ‘Crawling’ of Induction motor?

Squirrel cage motors sometimes exhibit a tendency stably at speeds as low as 1/7th of their synchronous speed.  This is known as crawling.

Q.  What is ‘Cogging’?

This is also known as magnetic locking.  When the voltage is low, the motor does not start.  This happens when the number of stator teeth is equal to the number of rotor teeth and due to the magnetic locking, the motor does not start. This can be overcome by making the number of rotor slots prime to the number of stator slots.


"If you are not willing to risk the usual you will have to settle for the ordinary." --Jim Rohn




SHARE

DON'T MISS

FREQUENTLY ASKED QUESTIONS AND ANSWERS-ETO
4/ 5
Oleh

Subscribe via email

Get Free Study Materials? Add your email address to subscribe.