Showing posts with label Electrical Interview Questions. Show all posts
Showing posts with label Electrical Interview Questions. Show all posts

Tuesday, 15 December 2015

Manthan | 01:33 | |

 Hello Friends. How are you ?

  We shared a lot of interview questions on electrical machines, power system and power electronics. Today we are sharing interview questions on transformer. These transformer interview questions is collected from various sources. lets start transformer interview questions with answer.

Q.1. How is magnetic leakage reduced to a minimum in commerical transformers ?
Ans.By interleaving the primary and secondary windings.

Q.2. Mention the factors on which hysteresis loss depends ?
Ans.(i) Quality and amount of iron in the core
       (ii) Flux density and
       (iii) Frequency.
Must Read :
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  2. Construction Of DC Machine
Q.3. How can eddy current loss be minimised ?
Ans.By laminating the core.

Q.4. In practice, what determines the thickness of the laminae or stampings ?
Ans.Frequency.

Q.5. Does the transformer draw any current when its secondary is open ?
Ans.Yes, no-load primary current.

Q.6. Why ?
Ans.For supplying no-load iron and copper losses in primary.

Q.7. Is Cu loss affected by power factor ?
Ans.Yes, Cu loss varies inversely with power factor.

Q.8. Why ?
Ans.Cu loss depends on current in the primary and secondary windings. It is well-known that current required is higher when power factor is lower.

Q.9. What effects are produced by change in voltage ?
Ans.1. Iron loss.........varies approximately as V2.
       2. Cu loss.........it also varies as V2 but decreases with an increase in voltage if constant kVA output is assumed
       3. Efficiency.........for distribution transformers, efficiency at fractional loads decreases with increase in voltage while at full load or overload it increases with increase in voltage and viceversa.

       4. Regulation.........it varies as V2  but decreases with increase in voltage if constant kVA output is assumed.
     5. Heating.........for constant kVA output, iron temperatures increase whereas Cu temperatures decrease with increase in voltages and vice-versa
.
Q. 10.How does change in frequency affect the operation of a given transformer ?
Ans. 1. Iron loss .........increases with a decrease in frequency. A 60-Hz transformer will have nearly 11%      higher losses when worked on 50Hz instead of 60 Hz. However, when a 25-Hz transformer is worked on 60 Hz, iron losses are reduced by 25%.

2. Cu loss.........in distribution transformers, it is independent of frequecy.
3. Efficiency.........since Cu loss is unaffected by change in frequency, a given transformer efficiency is less at a lower frequency than at a higher one.
4. Regulation.........regulation at unity power factor is not affected because IR drop is independent of frequency. Since reactive drop is affected, regulation at low power factors decreases with a decrease in frequency and vice-versa.

For example, the regulation of a 25-Hz transformer when operated at 50-Hz and low power factor is much poorer.

5. Heating.........since total loss is greater at a lower frequency, the temperature is increased with decrease in frequency.

If you face any problem then comment below

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Monday, 18 May 2015

Manthan | 04:46 | | |

Q:Why syn. generators are used for the production of electricity?
A:synchronous machines have capability to work on different power factor(or say different imaginary power varying the field emf. Hence syn. generators are used for the production of electricity.

Q:What is the difference between synchronous generator & asynchronous generator?
A:In simple, synchronous generator supply's both active and reactive power but asynchronous generator(induction generator) supply's only active power and observe reactive power for magnetizing.This type of generators are used in windmills.

Q:1 ton is equal to how many watts?
A:1 ton = 12000 BTU/hr and to convert BTU/hr to horsepower, 12,000 * 0.0003929 = 4.715 hp therefore 1 ton = 4.715*.746 = 3.5 KW.

Q:Enlist types of dc generator?
A:D.C.Generators are classified into two types 1)separatly exicted d.c.generator 2)self exicted d.c.generator, which is further classified into;1)series 2)shunt and 3)compound(which is further classified into cumulative and differential).

Q:What is Automatic Voltage regulator(AVR)?
A:AVR is an abbreviation for Automatic Voltage Regulator.It is important part in Synchronous Generators, it controls theoutput voltage of the generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator.

Q:What is an exciter and how does it work?
A:There are two types of exciters, static exciter and rotory exciter.purpose of excitor is to supply the excitation dc voltage to the fixed poles of generator.Rotory excitor is an additional small generator mounted on the shaft of main generator. if it is dc generator, it will supply dc to the rotory poles through slip ring and brushes( conventional alternator). if it is an ac excitor, out put of ac excitor is rectified by rotating diodes and supply dc to main fixed poles.ac excitor is the ac generator whose field winding are stationary and armature rotates. initial voltage is built up by residual magnetism.It gives the starting torque to the generator. 

Q:Difference between a four point starter and three point starter?
A:The shunt connection in four point stater is provided separately form the line where as in three point stater it is connected with line which is the drawback in three point stater

Q:Why use the VCB at High Transmission System ? Why can't use ACB?
A:Actually the thing is vacuum has high arc queching property compare to air because in VCB ,the die electric strengths equal to 8 times of air . That y always vaccum used as inHT breaker and air used as in LT .
Q:What is the difference between surge arrestor and lightning arrestor?
A:LA is installed outside and the effect of lightning is grounded,where as surge arrestor installed inside panels comprising of resistors which consumes the energy and nullify the effect of surge.

Q:What will happen when power factor is leading in distribution of power?
A:If their is high power factor, i.e if the power factor is close to one:
1.losses in form of heat will be reduced,
2.cable becomes less bulky and easy to carry, and very cheap to afford, &
3. it also reduces over heating of tranformers.

Q:whats the one main difference between UPS & inverter ? And electrical engineering & electronics engineering ?
A:uninterrupt power supply is mainly use for short time . means according to ups VA it gives backup. ups is also two types : on line and offline . online ups having high volt and amp for long time backup with with high dc voltage.but ups start with 12v dc with 7 amp. but inverter is startwith 12v,24,dc to 36v dc and 120amp to 180amp battery with long time backup.


Q:What is 2 phase motor?
A:A two phase motor is a motor with the the starting winding and the running winding have a phase split. e.g;ac servo motor.where the auxiliary winding and the control winding have a phase split of 90 degree.

Q:Advantages of vvvf drives over non vvvf drives for EOT cranes?
A:1.smooth start and stop.
2.no jerking of load.
3.exact posiitoning
4.better protection for motor.
5.high/low speed selection.
6.reliability of break shoe.
7.programmable break control.
8.easy circutry
9.reduction in controls
10.increases motor life

Q:What is the significance of vector grouping in Power Transformers?
A:Every power transformer has a vector group listed by its manufacturer. Fundamentally it tells you the information about how the windings are connected (delta or wye) and the phace difference betweent the current and voltage. EG. DYN11 means Delta primary, Wye Secondry and the current is at 11 o clock reffered to the voltage.

Q:Which type of A.C motor is used in the fan (ceiling fan, exhaust fan, padestal fan, bracket fan etc) which are find in the houses ?
A:Its Single Phase induction motor which mostly squirrel cage rotor and are capacitor start capacitor run

Manthan | 04:36 | | |

Q:How tube light circuit is connected and how it works?
A:A choke is connected in one end of the tube light and a starter is in series with the circuit. When supply is provided ,the starter will interrupt the supply cycle of AC. Due to the sudden change of supply the chock will generate around 1000volts . This volt will capable of to break the electrons inside the tube to make electron flow. once the current passes through the tube the starter circuit will be out of part. now there is no change of supply causes choke voltage normalized and act as minimize the current.

Q:whats is MARX CIRCUIT?
A:It is used with generators for charging a number of capacitor in parallel and discharging them in series.It is used when voltage required for testing is higher than the available.

Q:What is encoder, how it function?
A:An encoder is a device used to change a signal (such as a bitstream) or data into a code. The code may serve any of a number of purposes such as compressing information for transmission or storage, encrypting or adding redundancies to the input code, or translating from one code to another. This is usually done by means of a programmed algorithm,especially if any part is digital, while most analog encoding is done with analog circuitry.

Q:What are the advantages of speed control using thyristor?
A:Advantages :
1. Fast Switching Characterstics than Mosfet, BJT, IGBT 2. Low cost 3. Higher Accurate.

Q:Why Human body feel Electric shock ?? n in an Electric train during running , We did nt feel any Shock ? why?
A:Unfortunately our body is a pretty good conductor of electricity, The golden rule is Current takes the lowest resistant path if you have insulation to our feet as the circuit is not complete (wearing rubber footwear which doing some repairs is advisable as our footwear is a high resistance path not much current flows through our body).The electric train is well insulated from its electrical system.

Q:what is the principle of motor?
A:Whenever a current carrying conductor is placed in an magnetic field it produce turning or twisting movemnt is called as torque.It is based on farady's laws of electromagnet induction.

Q:What is electric traction?
A:Electric traction means using the electric power for traction system (i.e. for railways,trams, trolleys etc). Electric traction means use of the electricity for all the above machines. Now a days, magnetic traction is also used for bullet trains.and basically dc motors are used for electric traction systems.

Q:How can you start-up the 40w tube lite with 230v AC/DC without using any choke/Coil?
A:It's possible by means of Electronic chokes,otherwise it's not possible to ionize the particles in tube light with normal voltage.

Q:What is "pu" in electrical engineering?
A:Pu stands for per unit and this will be used in single line diagram of power distribution and it is like a huge electrical circuit with no of components (generators, transformers, loads) with different ratings (in MVA and KV). To bring all the ratings into common platform we use pu concept in which, in general largest MVA and KV ratings of the component is considered as base values, then all other component ratings will get back into this basis.Those values are called as pu values. (p.u=actual value/base value).

Q:Operation carried out in Thermal power stations?
A:The water is obtained in the boiler and the coal is burnt so that steam is obtained this steam is allowed to hit the turbine , the turbine which is coupled with the generator generates the electricity.

Q:Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?
A:Link is provided at a Neutral common point in the circuit from which various connection are taken for the individual control circuit and so it is given in a link form to withstand high Amps. But in the case of Fuse in the Phase of AC circuit it is designed such that the fuse rating is calculated for the particular circuit (i.e load) only.So if any malfunction happen the fuse connected in the particular control circuit alone will blow off.

Q: What is the difference between electronic regulator and ordinary electrical rheostat  regulator for fans?
A:The difference between the electronic and ordinary electrical  regulator is that in electronic regulator power losses are less because as we decrease the speed the electronic regulator gives the power needed for that particular speed but in case of ordinary rheostat type regulator, the power wastage is same for every speed and no power is saved.In electronic regulator, triac is employed for speed control by varying the firing angle speed and it is controlled but in rheostatic ,control resistance is decreased by steps to achieve speed control.

Q:Why, when birds sit on transmission lines or current wires doesn't get shock? 
A:Its true that if birds touch the single one line (phase or neutral) they don't get electrical shock... if birds touch 2 lines than the circuit is closed and they get electrical shock.. so if a human touch single one line(phase) then he doesn't get shock if he is in the air (not touching - standing on the ground if he is standing on the ground then touching the line (phase) he will get a shock because the ground on what we standing is like line (ground bed - like neutral)। and in the most of electric lines the neutral is grounded..so that means that human who touch the line closes the circuit between phase and neutral.

Q:what is meant by armature reaction?
A:The effect of armature flu to main flux is called armature reaction. The armature flux may support main flux or opposes main flux.

Q:what happen if we give 220 volts dc supply to d bulb r tube light?
A:Bulbs [devices] for AC are designed to operate such that it offers high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current through lamp would be so high that it may damage the bulb element.

Q:Which motor has high Starting Torque and Staring current DC motor, Induction motor or Synchronous motor?
A:DC Series motor has high starting torque. We can not start the Induction motor and Synchronous motors on load, but can not start the DC series motor without load.

Q:what is ACSR cable and where we use it?
A:ACSR means Aluminium conductor steel reinforced, this conductor is used in transmission & distribution. 

Q:What is vaccum currcuit breaker.define with cause and where be use it Device?
A:A breaker is normally used to break a ciruit. while breaking the circuit, the contact terminals will be separated. At the time of seperation an air gap is formed in between the terminals. Due to existing current flow the air in the gap is ionised and results in the arc. various mediums are used to quench this arc in respective CB's. but in VCB the medium is vaccum gas. since the air in the CB is having vaccum pressure the arc formation is interrupted. VCB's can be used upto 11kv.

Tuesday, 24 March 2015

Manthan | 10:41 | | | | |

Why star delta starter is preferred with induction motor?

Star delta starter is preferred with induction motor due to following reasons:
• Starting current is reduced 3-4 times of the direct current due to which voltage drops and hence it causes less losses.
• Star delta starter circuit comes in circuit first during starting of motor, which reduces voltage 3 times, that is why current also reduces up to 3 times and hence less motor burning is caused.
• In addition, starting torque is increased and it prevents the damage of motor winding.

State the difference between generator and alternator

Generator and alternator are two devices, which converts mechanical energy into electrical energy. Both have the same principle of electromagnetic induction, the only difference is that their construction. Generator persists stationary magnetic field and rotating conductor which rolls on the armature with slip rings and brushes riding against each other, hence it converts the induced emf into dc current for external load whereas an alternator has a stationary armature and rotating magnetic field for high voltages but for low voltage output rotating armature and stationary magnetic field is used.

Why AC systems are preferred over DC systems?

Due to following reasons, AC systems are preferred over DC systems:
a. It is easy to maintain and change the voltage of AC electricity for transmission and distribution.
b. Plant cost for AC transmission (circuit breakers, transformers etc) is much lower than the equivalent DC transmission
c. From power stations, AC is produced so it is better to use AC then DC instead of converting it.
d. When a large fault occurs in a network, it is easier to interrupt in an AC system, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.

How can you relate power engineering with electrical engineering?

Power engineering is a sub division of electrical engineering. It deals with generation, transmission and distribution of energy in electrical form. Design of all power equipments also comes under power engineering. Power engineers may work on the design and maintenance of the power grid i.e. called on grid systems and they might work on off grid systems that are not connected to the system.

What are the various kind of cables used for transmission?

Cables, which are used for transmitting power, can be categorized in three forms:
• Low-tension cables, which can transmit voltage upto 1000 volts.
• High-tension cables can transmit voltage upto 23000 volts.
• Super tension cables can transmit voltage 66 kV to 132 kV.

Why back emf used for a dc motor? highlight its significance.

The induced emf developed when the rotating conductors of the armature between the poles of magnet, in a DC motor, cut the magnetic flux, opposes the current flowing through the conductor, when the armature rotates, is called back emf. Its value depends upon the speed of rotation of the armature conductors. In starting, the value of back emf is zero.

What is slip in an induction motor?

Slip can be defined as the difference between the flux speed (Ns) and the rotor speed (N). Speed of the rotor of an induction motor is always less than its synchronous speed. It is usually expressed as a percentage of synchronous speed (Ns) and represented by the symbol ‘S’.

Explain the application of storage batteries.

Storage batteries are used for various purposes, some of the applications are mentioned below:
• For the operation of protective devices and for emergency lighting at generating stations and substations.
• For starting, ignition and lighting of automobiles, aircrafts etc.
• For lighting on steam and diesel railways trains.
• As a supply power source in telephone exchange, laboratories and broad casting stations.
• For emergency lighting at hospitals, banks, rural areas where electricity supplies are not possible.

Monday, 23 March 2015

Manthan | 19:27 | |

1.  State the characteristic features of synchronous motor.
Ans:   a.  the motor is not inherently self starting
          b.   The speed of operation is always in synchronous with the supply frequency irrespective of load   conditions
            c.   The motor is capable of operating at any power factor.

2.  In what way synchronous motor is different from other motors?
All dc and ac motors work on the same principle. Synchronous motor operates due to magnetic locking taking place between stator and rotor magnetic fields.
3.  Name any two methods of starting a synchronous motors
•    By an extra 3 phase cage induction motor
•    By providing damper winding in pole phases
•    By operating the pilot excitor as a dc motor
4.  What is the effect on speed if the load is increased on a 3 phase synchronous motor?
The speed of operation remains constant from no load to maximum load in the motor operating at constant frequency bus bars.
5.  Why a synchronous motor is a constant speed motor
 Synchronous motor work on the principle of force developed due to the magnetic attraction established between the rotating magnetic field and the main pole feed. Since the speed of rotating magnetic field is directly proportional to frequency the motor operates at constant speed.
 6. What is the phasor relation between induced emf and terminal voltage of a 3 phase  synchronous motor?The rotating magnetic field is initially established by the prime source of supply V. The main field then causes an emf e to get induced in the 3 phase winding. Hence when the machine operates as a synchronous motor the emf phasor always lags the terminal voltage phasor by the load1torque angle   .
7. At what load angle is power developed in a synchronous motor becomes its maximum value ?
When its load angle   is equal to the impedance angle   .

8. What are V and inverted V curves of synchronous motor ?
The variation of magnitude of line current with respect to the field current is called V curve . The variation of power factor with respect to the field current is called inverted V curve.

9. What happens when the field current of a synchronous motor is increased beyond the normal value at constant input?
Increase in emf causes the motor to have reactive current in the leading direction. The additional leading reactive current causes the magnitude of line current, accompanied by the decrease in power factor.

10.Distinguish between synchronous phase modifier and synchronous condenser

A synchronous motor used to change the power factor or power factor in the supply lines is called synchronous phase modifier.

A synchronous motor operated at no load with over excitation condition to draw large leading reactive current and power is called a synchronous condenser

Manthan | 12:18 | |

Q1: Where do we require single-phase induction motors?
Ans:Single-phase induction motors are required where
(i) 3 phase supply is not available 
(ii) efficiency is of lesser importance
(iii) Rating is less than one H.P. 
(iv) Equipment is portable

Q2: Why is the power factor of a single-phase induction motor low?
Ans:It is due to the large magnetizing current which ranges from 60% to 70% of full-load current. As a result, even at no-load, these motors reach temperatures close to the full-load temperature.

Q3: What is the function of centrifugal starting switch in a single-phase induction motor?
Ans:The centrifugal switch is connected in series with the starting winding. The primary function of the centrifugal switch is to produce rotating flux in conjunction with main winding at the time of starting. When the motor has started and reaches nearly 75% of synchronous speed, it produces its own rotating field from the cross field effect. The starting winding now has no function to perform and is removed from the circuit by a centrifugally operated switch.

Q4: What happens when the centrifugal starting switch fails to open?
Ans:If the starting switch fails to open when needed, then the starting winding will overheat and burn out and motor will not start next time.

Q5: What happens when the centrifugal switch fails to close when needed?
Ans:If the centrifugal starting switch fails to close, the motor will overheat the main winding without any failure of the main winding.

Q6: Why are resistance split-phase inductions motors most popular?
Ans:These motors are most popular due to their low cost. They are used where moderate starting torque is required and where the starting periods are not frequent. They drive fans, pumps, washing machines, small machine tools etc. They have power ranting between 60 watts and 250 watts.

Q7: What is the draw back of the resistance split-phase induction motor?
Ans:The starting winding has a relatively small number of turns of fine wire and its resistance is higher than that of the main winding. Therefore the current density is high and the winding heats up quickly. If the starting period lasts for more than 5 seconds, the winding begins to smoke and may burn out unless the motor is protected by a built-in-thermal relay.

Q8: Why is the starting torque of a resistance split-phase induction motor not high?
Ans:The starting torque is given as, Ts = K Im Is Sin Ф
Where
            K = constant whose magnitude depends upon the design of the motor
(i) The angle between Is and Im is small (approximately 25 degree) in a resistance split-phase induction motor, so the starting torque is small.
(ii) Since currents Is and Im are not equal in magnitude, the rotating magnetic field is not uniform and the starting torque produced is small.
Q9: Why is the starting torque of a capacitor start induction motor high?
Ans:The capacitor C in the starting winding is so chosen that Is leads Im by 75 degree. Since the starting torque is directly proportional to Sin Ф, and it is quite high in capacitor-start induction motor.

Q10: Why do we use capacitor-start induction motors in applications requiring high starting torque in preference to repulsion induction motors?
Ans:Capacitors are easily available, cheaper and reliable. Repulsion-induction motors posses a special commutator and brushes that require maintenance. Most manufacturers have stopped making them.

Q11: What is the principle of operation of shaded-pole induction motor?
Ans:A shaded-pole motor is basically a small single-phase squirrel cage motor in which the starting winding is composed of short-circuited copper ring (called shading coil) surrounding one-third of each pole. The effect of the shading coil is to cause a flux to sweep across the pole faces, from unshaded to shaded portion of the pole, producing a weak rotating magnetic field. As a result, the rotor is set in motion due to induction principle.
Q12: Which type of torque is developed in single phase motors?
Ans:Pulsating torque is produced.

Q13: If a single phase motor is driven in any direction by any means, it starts running in that direction. Explain why?
Ans:Actually a pulsating torque has two components which are equal in magnitude and rotate in opposite direction with synchronous speed at unity slip. Now if the motor rotates in any direction, the slip decreases and the torque component in this direction increases than the other component and hence motor runs in that direction.

Q14: What is a fractional H.P. motor?
Ans:A small motor having H.P. less than unit is called fractional H.P. motor.

Q15: Which type of rotor is used in single phase motors?
Ans:Squirrel cage type

Q16: How the starting winding produce rotation in a single phase resistance start induction motor?
Ans:The starting winding is highly resistive and the main winding is inductive. So the phase difference between the two currents becomes nearly 90 degree and hence the motor start as two phase motor.    
    
Q17: How the starting winding is made resistive?
Ans:It consists of only few turns of smaller diameter.

Q18: How the speed of rotation of a split phase induction motor is reversed?
Ans: The terminal connections of the starting windings are reversed with respect to main running windings.

Q19: What will happen if the centrifugal switch fails to open the starting winding?
Ans:Excessive heat will be produced due to high resistance of the starting winding due to which stator temperature will rise and eventually both windings will burn.

Q20: How speed control is made in single phase motors?
Ans:It is usually controlled by applying a variable voltage from tapped transformers, variacs, potentio meters, and tapped reactors.

Q21: Is there any relation between the capacitances of two capacitors used in two value capacitor motor?
Ans: Starting capacitor has about 10 – 15 times high capacity than the value of running capacitor.

Q22:  What is size of shaded-pole motor?
Ans: These are usually built in small fractional H.P, not exceed 1/4 H.P.

Q23: Why shaded-pole single phase induction motor does not need any special starting technique like capacitors and auxiliary winding etc.
Ans:Because it is inherently self started motor. The construction of the poles is such that they give a sweep to the magnetic flux and motor starts rotating.

Q24: How can a universal motor be reversed?
Ans: By reversing either the field leads or armature leads but not both.

Q25: What are applications of Stepper motors?
Ans:(i) Paper feed motors in typewriters and printers 
(ii) Positioning of print heads 
(iii) Pens in XY-plotters 
(iv) Recording heads in computer disc drives etc.

Q26: Why do we use capacitor-start induction motors in applications requiring high starting torque in preference to repulsion induction motors?
Ans: Capacitors are easily available, cheaper and reliable. Repulsion-induction motors posses a special commutator and brushes that require maintenance. Most manufacturers have stopped making them.

Q27: If a single phase motor is driven in any direction by any means, it starts running in that direction. Explain why?
Ans:Actually a pulsating torque has two components which are equal in magnitude and rotate in opposite direction with synchronous speed at unity slip. Now if the motor rotates in any direction, the slip decreases and the torque component in this direction increases than the other component and hence motor runs in that direction.

Q28: What is a fractional H.P. motor?
Ans: A small motor having H.P. less than unit is called fractional H.P. motor.

Q29: Which type of rotor is used in single phase motors?
Ans:Squirrel cage type

Q30: How the starting winding produce rotation in a single phase resistance start induction motor?
Ans:The starting winding is highly resistive and the main winding is inductive. So the phase difference between the two currents becomes nearly 90 degree and hence the motor start as two phase motor.  
      
Q31: How the starting winding is made resistive?
Ans:It consists of only few turns of smaller diameter.

Q32: How the speed of rotation of a split phase induction motor is reversed?
Ans:The terminal connections of the starting windings are reversed with respect to main running windings.

Wednesday, 11 March 2015

Manthan | 01:51 | | | |

Consider the rotating magnetic field as equivalent to physical rotation of two stator poles N1 and S1.
       Consider an instant when two poles are at such a position where stator magnetic axis is vertical, along A-B as shown in the Fig. 1(a).
       At this instant, rotor poles are arbitrarily positioned as shown in the Fig. 1.
       At this instant, rotor is stationary and unlike poles will try to attract each other. Due to this rotor will be subjected to an instantaneous torque in anticlockwise direction as shown in the Fig. 1(a).
       Now stator poles are rotating very fast i.e. at a speed Ns  r.p.m. Due to inertia, before rotor hardly rotates in the direction of  anticlockwise torque, to which it is subjected, the stator poles change their positions. Consider an instant half a period latter where stator poles are exactly reversed but due to inertia rotor is unable to rotate from its initial position. This is shown in the Fig. 1(b).


Why synchronous Motor Is Not Self Starting
Fig.  1
       At this instant, due to the unlike poles trying to attract each other, the rotor will be subjected to a torque in clockwise direction. This will tend to rotate rotor in the direction of rotating magnetic field.
       But before this happen, stator poles again change their position reversing the direction of the torque exerted on the rotor.
 Key Point : As a result, the average torque exerted on the rotor is zero. And hence the synchronous motor is not self starting.
Note : The question is obvious that will happen if by chance the rotor position is in such a way that the unlike rotor and stator poles are facing each other ? But owing to the large inertia of the rotor, the rotor fails to rotate along with the stator poles. Hence again the difference of position of magnetic axes gets created and rotor gets subjected to quickly reversing torque. This is because the speed with which rotating magnetic field is rotating is so high that it is unable to rotate the rotor from its initial position, due to the inertia of the rotor. So under any case, whatever may be the starting position of the rotor, synchronous motor is not self starting.

Tuesday, 10 March 2015

Manthan | 18:21 | | | | | |

Ques.[1] For torque transmission over a long distance with the help of electrical wires only, which of the following two synchros are used ?

(a)CX and CT
(b)CX and CR
(c)CX and CD
(d)CT and CD
Answer :B 

Ques.[2] Squirrel-cage induction motor is finding increasing application in high-power servo systems because new methods have been found to

(a) increase its rotor resistance
(b) control its torque
(c) decrease its intertia
(d) decouple its torque and flux. 
Answer:D 

Ques.[3] The control ........................ synchro has three- phase winding both on its stator and rotor.

(a) differential
(b) transformer
(c) receiver
(d) transmitter
Answer:A 

Ques.[4] A dc servomotor is similar to a regular d.c. motor except that its design is modified to cope with

(a) electronic switching
(b) slow speeds
(c) static conditions
(d) both (b) and (c). 
Answer:D 

Ques.[4] The low-torque synchros cannot be used for

(a) torque transmission
(b) error detection
(c) instrument servos
(d) robot arm positioning.
Answer:D 

Ques.[5].Which of the following synchros are used for error detection in a servo control system ?

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(a) control transmitter
(b) control transformer
(c) control receiver
(d) both (a) and (b).
Answer:D  

Ques.[6] The arrangement required for producing a rotation equal to the sum or difference of the rotation of two shafts consists of the following coupled synchros.

(a) control transmitter
(b) control receiver
(c) control differential transmitter
(d) all of the above.
Answer:D 

Ques.[7] Which of the following motor would suit applications where constant speed is absolutely essential to ensure a consistent product ?

(a) brushless dc motor
(b) disk motor
(c) permanent-magnet synchronous motor
(d) stepper motor.
Answer:C 

Ques.[8] A switched reluctance motor differs from a VR stepper motor in the sense that it

(a) has rotor poles of ferromagnetic material
(b) rotates continuously
(c) is designed for open-loop operation only
(d) has lower efficiency.
Answer:B 

Ques.[9] The electrical displacement between the two stator windings of a resolver is

(a) 120º
(b) 90º
(c) 60º
(d) 45º
Answer:B 

Ques.[10] Which of the following motor runs from a low dc supply and has permanently magnetized salient poles on its rotor ?

(a) permanent-magnet d.c. motor
(b) disk d.c. motor
(c) permanent-magnet synchronous motor
(d) brushless d.c. motor.
Answer:A 

Ques.[11] Regarding voltages induced in the three stator windings of a synchro, which statement is false ?

(a) they depend on rotor position.
(b) they are in phase.
(c) they differ in magnitude.
(d) they are polyphase voltages. 
Answer:D

Ques.[12] A dc servomotor is similar to a regular d.c. motor except that its design is modified to cope with

(a) electronic switching
(b) slow speeds
(c) static conditions
(d) both (b) and (c).
Answer:D 

Ques.[13] One of the basic requirements of a servomotor is that it must produce high torque at all

(a) loads
(b) frequencies
(c) speeds
(d) voltages.
Answer:C 

Ques.[14] The most common two-phase ac servomotor differs from the standard ac induction motor because it has

(a) higher rotor resistance
(b) higher power rating
(c) motor stator windings
(d) greater inertia.
Answer:A 

Ques.[15] Though structurally similar to a control transmitter, a control receiver differs from it in the following way :

(a) it has three-phase stator winding
(b) it has a rotor of dumbell construction
(c) it has a mechanical damper on its shaft
(d) it has single-phase rotor excitation.
Answer:C

Ques.[16] A switched reluctance motor differs from a VR stepper motor in the sense that it

(a) has rotor poles of ferromagnetic material
(b) rotates continuously
(c) is designed for open-loop operation only
(d) has lower efficiency. 
Answer:B 

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