Type of construction of CT's:

Type of construction of CT's: 

Ring type CT or Window type CT:

This is the simplest type of CT. The core has three types of popular shapes (1) Rectangular shape (2) Oval shape (3) Ring shape.

Ring Type CT

Bar primary current transformers:

✥The core is of a Nickel-Iron Alloy, or grain oriented sheet steel. 
✥The core is continuously wound type. Before applying secondary winding,
the core is insulated by means of end collars and circumferential wraps.

Bar Primary Current Transformer

✥Recently the continuously wound cores are available in encapsulated form. Synthetic resins are used as encapsulating material.
✥The material is applied by fluidized beds or electrostatic spraying.

(A)Core of A Ring Type CT

(B) Core is taped with insulting tape

(C)Secondary winding wound on core by toroidal winding machine

(D)Exterior taping is completed with circumferential insulating wraps

✥The secondary winding conductor is then wound as the insulated core in the form of toroidal winding by hand winding or toroidal winding machine. 
✥The secondary winding is then completely wrapped by external tape with or without exterior ring ends and circumferential insulating wraps.

Current transformers for high voltage installation : 

✥Separately mounted post type CT's They are suitable for out door service.
✥They are usually installed in the outdoor switchyard.
✥The primary conductor is at voltage with respect to earth. Hence it insulated by means of an insulator column filled with dielectric oil. Alternatively SF6 gas at a pressure of 2 or 3 atmospheres is now being used.

CT for high Voltage

Types of Resistance Welding

Types of Resistance Welding:(A)Butt welding:

❉ Rods, pipes and wires are welded by this method. As shown in Fig. (A) there is a transformer, across secondary of the transformer two pieces of metal to be welded are held. The two pieces are held in clamps. One clamp is fixed and other is movable

          (A) Butt welding

❉ The pieces of pipe are made to touch each other and force/pressure is applied through springs. When supply is given to transformer, current passes through the pieces of pipes and sufficient heat is developed at the joint causing welding at the joint.

(B)Flash butt welding:

❉ This is modification of butt welding. The ends of the pipe to be welded are held together with a little force/pressure applied to them. Arcing at the joint is allowed to take place. This removes any unevenness at the joint. A weld having a fin around the joint is obtained Fig.(B)

       (B) Flash butt welding

❉ The fin can be easily removed. Two dissimilar metals can be effectively welded with this method. This welding method is faster than other methods.

(C)Spot welding:

❉ In spot welding, the welding is done at certain points on metallic sheets. The pieces to be welded are held between the two electrodes as shown in Fig.(C)

            (C) Spot welding

❉ When current passes through the electrodes a spot weld is produced between the sheets.

(D)Projection welding :

❉ It is a modified form of spot welding. The electrodes used are flat and not pointed as in case of spot welding.

         (D) Projection welding :

❉ Projections are made on one sheet so that heat is concentrated in that region. When current is passed through the electrodes. The current density and pressure required is less hence electrode life is more. The welding automatically takes place only at the projections. 

(E) Seam welding:

❉ It is similar to sport welding. In seam welding, a series of spots is produced by roller electrodes. As shown in Fig. (E)

          (E) Seam welding

❉ Two sheets are passed under the two rolling electrodes. Depending upon the number of welding current pulses are second and speed of electrodes, a series of spot welds is obtained.

(F)Energy storage welding/percussion welding:

❉ Welding is obtained by discharging the stored energy in capacitor. As shown in Fig. (F) it consists of a bridge rectifier, capacitor, switching arrangement.

    (F) Energy storage welding

❉ The pieces to be welded one kept at a certain distance apart (1.5 mm). Switch 'S' is put on position 1. So capacitor gets charged through the bridge rectifier.

Type of Transmission for Drive

Type of Transmission for Drive:

☞The motor power output available at the shaft is the transmitted to respective load through a transmission, media like chain, belt, gear, rope etc.

☞The choice is governed by most economical speed of motor. The dimensions and hence the cost of motor of given output is inversely proportional to the speed.

1. Belt drive:

This is a simple and cheap drive. Following types of belts are used: Leather belt, nylon

belt, terylene belt, rubberized cotton ply belt, hair belt.

  The belt drives are of two types:

   (1) Falt belt drive

   (ii) V-belt drive

(i)Flat belt drive :

It is a long distance drive. The belt is generally made up of leather. Other material listed in above can also be used for belt. It is preferred in horizontal position. The slack side of the belt is kept at top so that natural sag increases the area of contact of the two pulleys.

Advantages:

1. Simple.

2. Greater flexibility.

3. Any desired speed ratio can be obtained.

4. When sudden load change occurs there is a tendency of belt to slip on pulley. This is better for both driving and driven member.

Disadvantages:

1. More space is required.

2. Maximum power it can transmit is 150 to 200 kW. So transmission capacity is limited.

3. It exerts side pull on bearing causing wear and tear of bearings. Following are the methods of transmitting mechanical power of motor to the load:

(a) Direct drive: The motor shaft (driving member) is directly connected to the driven member. E.g. motor-generator set in electrical lab.

☞A flexible coupling is used for such purpose. This can be used only where speed requirement of driven member matches with speed of driving member.

(ii) V-belt drive: It is suitable for motors with rating upto 450 kW. Because of wedge effect, there is increased friction between the belt and groove. This gives a better grip of about 3 times as that of flat belt.

Advantages:

 1. Less wear and tear of bearings.

 2.Short distances can be maintained.

 3. Less noise.

2. Rope drive :

It is used for power ranges beyond the limit of V belt drive. It is a long centre drive. Main advantages of rope drive are negligible slip and ability to take sudden loads.

3. Chain drive:

It has no slip and no initial tension is present hence bending stress are eliminated. It can be used for high speed ratio (7:1). It is particularly suitable for damp and dirty conditions. The chain must run concentrically on the sprocket and at right angles to the shaft.

Chain Drive

4. Gear drive:

☞It is a short centre positive rive. Proper alignment is very essential otherwise motor shaft may bend.

Gear Drive

☞Modern straight cut gears and particular variation found in worm reduction drive are common features in many machines such as cloth calendar.

A.C. Welding Sets and Methods of AC Welding

A.C. Welding Sets and Methods of AC Welding

Welding transformer:

Single phase or three phase step down transformers are used. The transformer is designed for high leakage reactance. The secondary voltage on open circuit is about 80-100 volt. Some control device is also used along with this transformer.

The secondary winding is air cooled and made up of copper strip. Oil cooled transformers are also used.

The air cooled sets are cheaper in cost and lighter in weight and easier to maintain. It can be easily shifted from one location to other.

Oil cooled sets are more compact but these are heavy so suitable for stationary application, e.g. in workshops.

The current control in welding transformer can be obtained by following ways:

(i) Tapped reactor method : 

 A series reactor is used with tapping on it fig.(a)

(a) Tapped Reactor Method

The output current can be varied by selecting appropriate tap. Different values of currents can be obtained by this method.

(ii) Moving Coil Method:

The transformer used in shell type with a movable primary winding as show in Fig.(b)

(b) Moving Coil Method

The movement of primary is possible by rotating the handle. If primary is moved near to secondary the current increases and if it is moved away current decreases.

(iii) Magnetic Shunt Method:

A magnetic shunt is used to divert the flux. The position of magnetic shunt can adjusted. Hence, value of flux in transformer core changes and thus current can be changed. fig.(c)

(c) Magnetic Shunt Method

Core Induction Furnace: Construction, Operation & Disadvantages

Core Induction Furnace: Construction, Operation & Disadvantages

Construction Core Induction Furnace

It consists of magnetic core having thin laminations of silicon steel. The primary is wound on one limb and a crucible type non-conducting ring is provided on the secondary. The crucible provides housing for the metal to be heated. 

The crucible is made up of ceramic material so that I can withstand high temperatures. The core of the transformer is designed such that it does not saturate even if high current is passed

 (a) Core Type Induction

Operation  Core Induction Furnace

First of all the charge in the form of different sizes is poured into the crucible. Due to these different sizes of the material there is a possibility that the secondary circuit may not complete, to avoid it, this more conductive material of powdered form is put on the pieces of material.

When A.C. supply is given to the primary, alternating flux is produced which links with secondary. So c.m.f. is induced in the charge itself. Heavy current is circulated through the top

powdered layer of conductive material and heat is produced in the top layer of charge which is transferred to bottom layer of charge.

Disadvantages Core Induction Furnace

(i)Due to irregular shape of the charge, magnetic coupling between primary and secondary is poor.

(ii) High leakage reactance due to poor magnetic coupling causes poor power factor.

(iii) To avoid low power factor low frequency supply is necessary. So low frequency generator is required to be installed. The overall cost of the furnace is increased. 

(iv) Pinch effect may occur which causes open circuit to secondary side.

(v) Crucible is of odd shape and it is difficult to manufacture. 

Pinch-off effect in core type induction furnace :

The magnetic coupling between primary and secondary is very poor, resulting in high leakage reactance and low power factor to overcome this difficulty the frequency is kept small (as low as 10 Hz).

(a) Pinch-off Effect

The electromagnetic forces produce turbulence of the molten metal. This effect is dependent on frequency and current density. The turbulence effect is severe at high values of frequency and current density.

(b) Pinch-off Effect

Hence, frequency is kept upto 10 Hz and current density is 5 amp/mm². The current flowing around the melt interacts with the alternating flux and produces constrictive forces on a section of the metal. This squeezes the melt and results in complete interruption of the section circuit. This effect is known as Pinch off effect.

Buchholz Relay: Operation, Advantages & Disadvantages

Buchholz Relay: Operation, Advantages & Disadvantages

1. When the inci pant faults are occured in the transformer, no relay will detect such

faults due to very small magnitude of fault current.

2. To detect such slow developing faults, Buchholz relay is used.

3. When the transformer is totally oil immersed and having kVA rating above 750 kVA; Buchholz relay will provide protection against such faults

4. Basically Buchholz relay is a gas-actuated relay installed in oil-immersed transformers for protection against all kind of fault.

5. When there is inci pant fault in  transformer Buchholz relay give an alarm. When there is severe fault in transformer, it disconnects the transformer from supply.

6. It is usually installed in the pipe  connecting the conservator to the main tank.

Location of Buchholz  Relay Construction

 Fig (b) shows the constructional details of Buchholz relay. It consists of a domed vessel and located between the main tank and the conservator.

Buchholz relay has two elements. The upper element consists of a mercury type switch attached to a float and used to close an alarm circuit during incipient fault.

The lower element contains a mercury switch mounted on a hinged type flap cate I the direct path of the flow of oil from the transformer to the conservator and is arranged to trip the circuit breaker in case of severe internal faults.

Buchholz Relay

Operation of Buchholz Relay

1.When an inci pant fault occurs inside the transformer, it generates the heat due to fault current.

2.This heat causes the decomposition of some transformer oil in the main tank.

3. Decomposition of transformer oil produces more than 70 % of hydrogen gas.

4.Due to lightweight of the hydrogen gas it tries to go into the conservator and in the process gets accumulated in the upper part of relay chamber. 

5. When a predetermined amount of gas get accumulated in the conservator it exert 

sufficient pressure on the float to cause it to tilt and close the contacts of mercury switch attached to it.

6.Closing of mercury switch completes the alarms circuit and gives an alarm. 

7. When a major fault occurs inside the transformer, an enormous amount of gas is generated in the main tank.

8. The oil in the main tank rushes towards the conservator via the Buchholz relay and

in doing so tilts the flap to close the contacts of mercury switch. 

9. This completes the trip circuit of the circuit breaker and circuit breaker disconnects the transformer by opening its contacts.

Advantages Of Buchholz Relay

➡️Inci pant faults are difficult to detect but with the help Buchholz relay it is possible to detect it at much earlier stage.

➡️Construction of Buchholz relay is much simpler.

Disadvantages Of Buchholz Relay

➡️Buchholz relays can only be used with oil-immersed transformer equipped with conservator tanks.

➡️Faults at connecting cable have to be provided with separate protection scheme since the device can detect only faults below oil level in the transformer.

Merits, Demerits and Applications of Three Phase Induction Motor

Merits, Demerits and Applications of Three Phase Induction Motor. 

Advantages Of Three Phase Induction Motor

• simple and rugged construction
• low initial as well as maintenance cost 
• nearly constant speed
• high overload capacity
• simple starting arrangement and
• high power factor. 

Because of low resistance of rotor in comparison to that of slip-ring induction motor, it has low rotor copper loss and, therefore, higher efficiency.

Disadvantages Of Three Phase Induction Motor:

1. large starting current 
2. very sensitive to fluctuations in supply voltage 
3. low power factor at light loads
4. speed control very difficult and 
5. very poor starting torque due to its low rotor resistance.

Application Of Three Phase Induction Motor:

These motors are suitable for industrial drives of small power where speed control is not required such as for printing machinery, flour mills and other shaft drives of small power.

The slip-ring induction motors in comparison to squirrel cage motors have low starting current and high starting torque but suffer due to low power factor and low efficiency. 

The motors are used for most industrial drives of high power a requiring high starting torque.

3- Point Starter For DC Shunt Motor

3- Point Starter For DC Shunt Motor

Working of 3 Point starter: 

The starter resistance R, is divided in different steps and are connected to brass-studs. When the arm 'A' is moved by the handle to stud number 1. then R. is connected to armature circuit. 

The field and the armature is connected to the supply and the motor starts with the safe current. The arm is then moved to the next studs successively and the starter resistance (Rs) is gradually cut-out and finally the arm is moved on the last stud.

The soft iron piece (S.I.P.) of the arm is attracted by the N.V.C. electro-magnet. The N.V.C. electro-magnet gets the supply voltage and energized. 

The attractive force of N.V.C. is more than the spring tension of the arm. Now if the supply fails, then the N.V.C. gets de magnetized and the handle is thrown back to off position due to the action of the spring. 

Now if the supply resumes then the operator has to move the arm and motor starts with R in the circuit. 

If N.V.C. and spring were not thereto then there would be a chance that motor may start directly when the supply resumes. Thus, spring and N.V.C. protects the motor from direct starting.

R = Resistance,

OLC = Overload coil,

FR = Field regulator

S = Spring,

NVC = No volt coil,

There is a second electro-magnet O.L.C. This is connected in the motor circuit and carries the motor current.T

The full current of the motor is say = 10 Amp. Then this will also pass through the coil of this electromagnet (O.L.C.) Due to this current, the OL.C. 

will try to attract the strip beneath it. But the strip is kept at the certain distance so that due to this force it is not attracted. But, in case of overloading (i.e. motor takes current more than, normal full load current say 20 Amp).

The overload current also passes through the O.L.C. and this creates more force of attraction. This force is now sufficient to attract the strip upwards. 

When the strip is attracted, then the N.V.C. is short circuited and N.V.C. is de magnetized.

Due to the action of the spring, the arm is thrown back to off position and hence motor stops. It is protected from overloading, O.L.C. comes in play under overload condition only and not in normal condition.

Thus, starter is the important device, which limits the starting current to the safe value. In addition, it protects the motor from direct starting and protects from overloading.

3-Point D.C. shunt motor starter

Comparison Between Synchronous and Induction Motor

Comparison Between Synchronous and Induction Motor.

Comparison Between Synchronous and Induction Motor.

Comparison Point 1:

Synchronous Motors.

✅ it has got no self starting torque and some external means is required for its starting

Induction Motors.

✅ It has got self starting torque and no special means is required for its starting.

Comparison Point 2 :

Synchronous Motors.

✅ Its average speed is constant and independent of load.

Induction Motors.

✅Its speed falls with the increase in load nd is always less than

Comparison Point 3 :

synchronous speed.

✅ it can be operated under a wide range of power factor, both lagging and leading

Induction Motors.

✅It operates at only lagging power factor, which becomes ven poor at light loads.

Comparison Point 4 :

Synchronous Motors.

✅ It requires dc excitation so it is a doubly excited machine.

Induction Motors.

✅ It requires no dc excitation so it is a singly excited machine. 

Comparison Point 5 :

Synchronous Motors.

✅ No speed control is possible.

Induction Motors.

✅Speed can be controlled but to small extent.

Comparison Point 6 :

Synchronous Motors.

✅ It is used for supplying mechanical load as well as for power factor improvement.

Induction Motors.

✅ It is used for supplying mechanical load only.

Comparison Point 7:

Synchronous Motors.

✅ Its torque is less sensitive to change in supply voltage.

Induction Motors.

✅Its torque is more sensitive to change in supply voltage.

Comparison Point 8:

Synchronous Motors.

✅ Breakdown torque is proportional to the supply voltage.

Induction Motors.

✅Breakdown torque is proportional to the square of the supply voltage.

Comparison Point 9:

Synchronous Motors.

✅ It is more complicated and costs more comparatively.

However, synchronous motors with speeds below 500 rpm and ratings exceeding about 40 kW or with medium speeds from 500 to 1,000 rpm and ratings exceeding about 500 kW cost less than induction motors.

Induction Motors.

✅It is more simple and costs less comparatively.

Thermal instruments.

Thermal Instruments.            

In thermal instruments the action depends upon the heating effect of the current under mainly of two types namely 

(i) hot wire instruments and 

(ii) thermocouple instructions measurement. 

Thermal instruments are ments. The hot wire instrument operates on the fact that when a current is passed through a wire the wire gets heated and so expands while thermocouple instrument operates on there are  fact that when the junction of 2 dissimilar metals is heated by passing current through it, an emf is developed. 

These instructions. ments are free from errors due to frequency, waveform and external magnetic fields when used on ac and so can be used for measurement of current at frequencies above the range of moving iron and dynamometer type instruments.

Hot wire ammeters were very popular in the last decade of the nineteenth century and in the first two decades of the twentieth century but because of their inherent drawbacks they have been largely superseded by thermocouple instruments and are not used extensively nowadays. 

However, owing to their good transfer characteristics at high frequencies, they are employed for determination of the differences of indications of other types of instruments on ac and de and in special measurement applications.

Hot wire instruments have the advantages of (i) no stray magnetic field effect (ii) same cable thermal instruments the action depends upon the heating effect of the current under mainly of two types namely (i) hot wire instruments and (ii) thermocouple instructions measurement. Thermal instruments are ments.

The hot wire instrument operates on the fact that when a current is passed through a wire the wire gets heated and so expands while thermocouple instrument operates on the fact that when the junction of two dissimilar metals is heated by passing current through it, an emf is developed. 

These instructions. ments are free from errors due to frequency, waveform and exter nal magnetic fields when used on ac and so can be used for measurement of current at frequencies above the range of moy. ing iron and dynamometer type instruments.

Hot wire ammeters were very popular in the last decade of the nineteenth century and in the first two decades of the twentieth century but because of their inherent drawbacks they have been largely superseded by thermocouple instruments and are not used extensively nowadays. 

However, owing to their good transfer characteristics at high frequencies, they are employed for determination of the differences of indications of other types of instruments on ac and de and in special measurement applications.

Hot wire instruments have the advantages of 

1. no stray magnetic field effect 
2. same calibration for dc as well as for ac
3. fair accuracy 
4. simple construction 
5. low cost 
6. negligible temperature error if suitably adjusted and 

suitability for measurement of currents at very high frequencies and disadvantages of 

1. delicate construction 
2. relatively higher power consumption
3. uneven scale 
4. incapability of taking overload
5. sluggish in action 
6. need of frequent adjustment of zero position due to temperature variations and 
7. different deflections for ascending and descending values.

Thermocouple instruments have the advantages of 

(i) instrument indications almost independent of frequency and waveform 

(ii) no stray magnetic field effect 

(iii) high sensitivity and 

(iv) utility as transfer instruments and

disadvantages of 

(i) considerable power losses and 

(ii) delicate construction Applications. These instruments are used for measurement of currents from power frequencies up to 100 MHz, the upper limit is determined by the skin effect and stray capacitance depends on whether the instrument is an ammeter or a voltmeter and its current rating ratio for dc as well as for ac 

(iii) fair accuracy 

(iv) simple construction 

(v) low cost 

(vi) negligible temperature error if suitably adjusted and 

(vii) suitability for measurement of currents at very high frequencies and 

disadvantages of 

(i) delicate construction 

(ii) relatively higher power consumption 

(iii) uneven scale 

(iv) incapability of taking overload 

(v) sluggish in action 

(vi) need of frequent adjustment of zero position due to temperature variations and

(vii) different deflections for ascending and descending values.

Thermocouple instruments have the advantages of 

(i) instrument indications almost independent of frequency and waveform 

(ii) no stray magnetic field effect 

(iii) high sensitivity and 

(iv) utility as transfer instruments and

disadvantages of 

(i) considerable power losses and 

(ii) delicate construction Applications. These instruments are used for measurement of currents from power frequencies up to 100 MHz, the upper limit is determined by the skin effect and stray capacitance depends on whether the instrument is an ammeter or a voltmeter and its current rating.

STANDARD TYPES OF 3-PHASE SQUIRREL CAGE INDUCTION MOTOR

STANDARD TYPES OF 3-PHASE SQUIRREL CAGE INDUCTION MOTOR.       

These motors are available in a range of standard ratings up to 150 kW at various standard frequencies, voltages and speeds in order to meet the usual requirements of industry.

According to this is  electrical characteristics, such motors are divided into six types as given below:

1. Class A Motors. 

These motors have low resistance and low

reactance and have low slip and high efficiency at full load. These is high starting current (6 to 9 times full-load current at rated voltage) is the main drawback. 

Such motors are used in small ratings for machine tools, centrifugal pumps, fans, blowers etc.

2. Class B Motors. 

These motors have high reactance. there design is common in the 5-150 kW range. 

Such motors have largely replaced class A motors for new installations as its running characteristics are quite similar to those of class A motor but have smaller starting current (around 5 times the full-load current at rated voltage).

3. Class C Motors. 

These motors are double cage motors and provide high starting torque with low starting current. 

Typical applications are in driving air compressors, conveyors, reciprocating pumps, crushers, mixers, large refrigerating machines etc.

4. Class D Motors. 

Such motors are provided with a high resistance squirrel cage rotors and, therefore, give high starting torque with low starting current.

 These motors have low operating efficiency and their use is limited to driving of intermittent loads involving high accelerating duty and to drive high-impact loads such as punch presses, shears, bulldozers, small hoists etc.

5. Class E Motors. 

This is  motors have a relatively low starting torque, normal starting current and have a relatively low slip at rated load.

6. Class F Motors. 

There motors have relatively low starting torque, low starting current and normal slip.

DC Machines Commutation.

DC Machines Commutation.

Improvement of Commutation. 

There are two methods of improving commutation i.e., of making the current in the short circuited coil attain its full value in the reverse direction by the end of the short circuit period. They are known as 

1. resistance commutation and 
2. emf commutation.

1. Resistance Commutation. 

The first method of obtaining spark less commutation is by use of high resistance carbon brushes. The carbon brushes have the advantages of 

(i) high contact resistance resulting in good commutation, 

(ii) lubricating and polishing the commutator as it rotates and 

(iii) in the event of sparking, reducing the damage to the commutator by the brushes.

The carbon brushes,have some  drawbacks also as enumerated below:

(i) Due to high contact resistance, a loss of about 2 Vis caused. Hence these are not much suitable for small machines where this voltage loss forms an appreciable The carbon brushes, however, have some

percentage loss.

(ii) Because of the above large loss, the commutator has to be made somewhat larger than with copper brushes so as to the dissipate heat efficiently without appreciable rise in temperature.

(iii) Because of lower current density, large brush holders are needed.

2. EMF Commutation. 

In this method of improving commutation an arrangement is made to neutralize the reactance voltage by producing a reversing emf in the short-circuited coil under commutation. 

This is accomplished by placing a small nd), pole, called the interpole or the commutating pole, over the short-circuited coil i.e., the location of commutating poles must be midway between the main poles. 

Commutating poles neutralize the reactance voltage automatically and make the commutation the spark less and neutralize the cross magnetizing effect of armature reaction.

Commutating poles are so effective in producing good and commutation that the majority of all dc machines built nowadays are provided with them. 

In non-commutating pole machines satisfactory and commutation is produced by shifting the brushes from the mechanical neutral so as to lie in the magnetic neutral plane. 

Since the amount of brush shift required varies with the load supplied by the machine, so brushes are required to be shifted with each change in load. Hence this method of reducing sparking at the brushes is satisfactory for the machines operating a constant loads only.

Comparison of Squirrel Cage and Slip- ring Induction motor.

Comparison of Squirrel Cage and Slip- ring Induction motor.                       

Squirrel Cage Rotor

                       .                                     

                                     Vs

Slip- Ring Rotor     

                                                        

Comparison Point 3 :

Squirrel cage induction Motor : 

✅Construction simple ane rugged

Slip-ring induction motor : 

✅Construction Needs Slip rings,brushes short- circuiting device etc.

Comparison Point 2 :

Squirrel cage induction Motor : 

✅overhang less 

Slip-ring induction motor : 

✅ overhang large 

Comparison Point 3 : 

Squirrel cage induction Motor : 

✅Space factor in slots better

Slip-ring induction motor : 

✅space factor in slots poor

Comparison Point 4 : 

Squirrel cage induction Motor : 

✅Cost (initial as well as maintenance) Less

Slip-ring induction motor : 

✅Cost (initial as well as maintenance) More

Comparison Point 5 : 

Squirrel cage induction Motor : 

✅Copper losses small

Slip-ring induction motor : 

✅Copper losses More

Comparison Point 6 : 

Squirrel cage induction Motor : 

✅ Efficiency High (only for machines, not designed for high starting torque)

Slip-ring induction motor : 

✅Efficiency Low 

Comparison Point 7: 

Squirrel cage induction Motor :

✅power factor better 

Slip-ring induction motor : 

✅power factor poor

Comparison Point 8: 

Squirrel cage induction Motor : 

✅ Cooling Cooling better because of its bare end rings and availability of more space for rotor fans

Slip-ring induction motor : 

✅Cooling Not quite efficient

Comparison Point 9 :

Squirrel cage induction Motor : 

✅ Speed regulation better 

Slip-ring induction motor : 

✅Speed regulation Poor when operated with external resistances in the rotor circuit

Comparison Point 10 : 

Squirrel cage induction Motor:

✅ Starting simple 

Slip-ring induction motor :

✅starting The motor needs slip rings,brush gear,short-circuiting devies and starting resistors etc.

Comparison Point 11 : 

Squirrel cage induction Motor:

✅ starting torque starting torque is  low with large starting current 

Slip-ring induction motor :

✅ strating torque Possibility of increasing starting torque by insertion of external resistances in the rotor circuit

Comparison Point 12 : 

Squirrel cage induction Motor:

✅power factor at start poor

Slip-ring induction motor :

✅power factor at start can be improved 

Comparison Point 13 : 

Squirrel cage induction Motor:

✅Speed control No Possibility 

Slip-ring induction motor :

✅speed control Possible by insertion of external resistor in the rotor circuit

Comparison Point 14 : 

Squirrel cage induction Motor:

✅Protection against explosion Explosion proof

Slip-ring induction motor :

✅Protection against  explosion Not explosion proof

MCQ 21- 40 TRANSFORMERS

21. A transformer may have negative voltage regulation if the loadp powerfactor (pf) is 

(a)Leading for some values of pf

(b) Unity pf

(c) lagging but not zero pf

(d) Only zero pf lag

Ans : (a)Leading for some values of pf

22.Positive voltage regulation is an indication of_______load

(a) inductive

(b) capacitive(

c) either inductive or capacitive

 (d) pure resistive

Ans : (c) either inductive or capacitive  

23. A transformer has a percentage resistance of 2% and percentage reactance of 4%. What are its regulations at power factor 0.8 lagging and 0.8 leading, respectively?

(a) 4% and -0.8%

(c) 1.6% and -3.2%

(b) 3.2% and -1.6%

(d) 4.8% and -0.6%

Ans : (a) 4% and -0.8%

24. The percentage resistance and reactance of a transformer are 2% and 4% respectively. The approximate regulation on full load at 0.8 pf lag is

(a) 12%

(b) 8%

(c) 6%

(d) 4%

Ans : (d) 4%

25. The voltage regulation of a transformer having 2% resistance and 5% reactance, at full load, 0.8 pf lagging is

(a) 4.6% 

(b) -4.6%

 (c) -1.4%

 (d) 6.4%

Ans : (a) 4.6%

26.The regulation of transformer in which ohmic loss is 1% of output and reactance drop is 5% of the voltage, when operating the at 0.8 power factor lagging is

(a) 3.8%

(b) 4.8%

(c) 5.2%

(d) 5.8%

Ans : (a) 3.8%

27. 10kVA, 400 V/200 V single phase transformer with a resistance of 3% and reactance of 6% is supplying a current of 50 A to a resistive load. The voltage across the load is

(a) 194 V

(b) 196 V

(c) 198 V

 (d) 390 V

Ans : (a) 194 V

28. A single-phase 100 kVA, 1000 V/100 V, 50 Hz transformer has a voltage drop of 5% across its series impedance at full load. O this, 3% is due to resistance. The percentage regulation of th transformer at full load with 0.8 lagging power factor is

(a) 4.8

(b) 6.8

(c) 8.8

(d) 10.8

Ans : (a) 4.8

29. The losses in a transformer are :

(I) Copper loss 

(II) Eddy current loss

(III) Hysteresis loss.

The constant power loss of a transformer is given by

(a) I only.

(b) I and II only.

(c) II and III only.

(d) I,II and III.

Ans : (c) II and III only.

30. On which of the following factors does hysteresis loss depend?

(1) Flux density.

(2) Frequency.

(3) Thickness of laminations. 

(4) Time.

(a) 2 and 3.

 (b) 1 and 2.

 (c) 3 and 4. 

(d) 1 and 4.

Ans : (b) 1 and 2.

31.In a power transformer iron losses remain, practically constant Form no load to full load. This is because

(a) core flux remains constant.

(b) leakage flux remains constant.

(c) both (a) and (b).

(d) neither (a) nor (b).

Ans : (a) core flux remains constant.

32. In a power transformer, if in place of sinusoidal wave, a peaked wave voltage is fed to the primary

(a) copper losses will be less 

(b) noise level will be reduced.

(c) iron losses will be more. 

(d) iron losses will be less.

Ans : (d) iron losses will be less.

33. The full-load copper loss and iron loss of a transformer are 6.400 W and 5,000 W respectively. The copper loss and iron

loss at half load will be, respectively

 (a) 3,200 W and 2.500 W.

 (b) 3,200 W and 5,200 W.

(c) 1,600 W and 1.250 W. 

(d)1,600 W and 5,000 W.

Ans : (d)1,600 W and 5,000 W.

34.If the frequency of input voltage of a transformer is increased keeping the magnitude of voltage unchanged, then

(a) both hysteresis loss and eddy current loss in the core will increase

(b) hysteresis loss will increase but eddy current loss will decrease.

(c) hysteresis loss will decrease but eddy current loss will increase.

(d) hysteresis loss will decrease but eddy current loss will remain unchanged

Ans : (d) hysteresis loss will decrease but eddy current loss will remain unchanged

35. Open-circuit test in a transformer is performed with

 (a) rated transformer voltage.

(b) rated transformer current

(c) direct current

(d) high frequency supply.

 Ans : (a) rated transformer voltage.

36. The open-circuit test on a transformer is usually performed by exciting the low voltage winding. This is because

 (a) it draws sufficiently large no-load current which can be conveniently measured,

 (b) The required power input is low.

(c) it is not advisable to work on high voltage side

 (d) the voltage required is low.

Ans : (a) it draws sufficiently large no-load current which can be

37. Consider the following statements: The open-circuit test in a transformer can be used to obtain

1. core losses.

2. magnitude of exciting current.

3. copper losses. 

4. equivalent series impedance.

Correct statements are

(a) 1, 2, 3 and 4.

(b) 1 and 3 only.

(c) 1 and 2 only.

(d) 2 and 4 only.

Ans : (c) 1 and 2 only.

38. Consider the following losses for short circuit test on a transformer

1. Copper loss.

2. Copper and iron losses.

3. Eddy current and hysteresis losses.

4. Friction and windage losses.

(a) 1 only.

(b) 2 only.

(c) 3 only.

(d) 2,3 and 4.

Ans : (a) 1 only.

39.In transformers, which of the following statements is valid?

(a) In an open-circuit test, copper losses are obtained while in short- circuit test,

core losses are obtained.

(b) In an open-circuit test, current is drawn at high power factor.

(c) In a short-circuit, current is drawn at zero power factor.

(d) In an open-circuit test, current is drawn at low power factor.

Ans : d) In an open-circuit test, current is drawn at low power factor.

40. Consider the following tests:

1. Load test

2. Short-circuit test

3.OC test

4. Retardation test 

Which of the above tests are to be conducted for the determination of voltage regulation of a transformer?

(a) 1 only.

(b) 2 only. 

(c) 2 and 3.

(d) 3 and 4.

Ans : (b) 2 only. 

MCQ 1-20 TRANSFORMERS

1. The basic function of a transformer is to change

 (a) the level of the voltage.

(b) the power level.

(c) the power factor.

(d) the frequency.

Ans : (a) the level of the voltage

2. The efficiency of a power transformer is around 

(a) 50%. (b) 60%. (c) 80%. (d) 95%.

Ans : (d) 95%

3. In a transformer, electrical power is transferred from primary to secondary

(a) through air.

(c) through insulating medium.

(b) by magnetic flux.

(d) none of these.

Ans : (b) by magnetic flux

4. The two windings of a transformer are

(a) conductively linked. 

(b) inductively linked.

(c) not linked at all.

(d) electrically linked.

Ans :(b) inductively linked.

5.Transformer action requires a

(a) constant magnetic flux. 

(b) increasing magnetic flux.

(c) alternating magnetic flux. 

(d) alternating electric flux.

Ans : (c) alternating magnetic flux. 

6. The flux created by the current flowing through the primary winding induces emf in

(a) primary winding only.

(b)secondary winding only. 

(c) transformer core only.

(d)both primary and secondary windings.

Ans: (d)both primary and secondary windings.

7. The primary and secondary windings of a power transformer always have 

(a) a common magnetic circuit.

(b) separate magnetic circuits.

(c) wire of same size.

(d) same number of turns.

Ans : (a) a commin magnetic circuit

8. The iron core in a transformer provides a path to the main flux.

(a) low reluctance

(b) high reluctance

(c) low resistance

(d) high conductivity

Ans : (a) low reluctance

9. If rated de voltage is applied instead of ac to the primary of a transformer

(a) secondary of transformer will burn.

 (b) primary of transformer will burn.

(c) secondary voltage will be excessively high. 

(d) there will be no secondary voltage.

Ans : (b) primary of transformer will burn.

10. A transformer transforms

(a) voltage

(b) current.

(c) voltage and current.

(d) frequency

Ans : (c) voltage and current.

11. Which of the following would refer to an ideal transformer?

1. Winding resistances are negligible. 

2. Leakage-fluxes are included.

3. Core-losses are negligible.

4. Magnetization characteristic is linear.

(a) 1,2 and 3 only.

(b) 1, 3 and 4 only.

(c) 1, 2 and 4 only.

(d) 2,3, and 4 only.

Ans : (b) 1,3 and 4 only.

12. The flux in transformer core.

(a) increases with load.

(b) decreases with load. 

(c) remains constant irrespective of load.

(d) none of the above.

Ans : (c) remains constant irrespective of load.

13. The primary ampere-turns are counter balanced by

(a) secondary ampere-turns.

 (b) primary flux

(c) increase in mutual flux.

(d) increase in secondary current.

Ans : (a) secondary ampere-turns.

14. The mutual flux in a loaded transformer can be varied by varying

(a) primary current.

(b) load impedance.

(c) secondary current.

(d) reluctance of the magnetic path.

Ans : (d) reluctance of the magnetic path.

15.  In a transformer flux____decreases with the increase in leakage flux.

(a) primary induced emf

(b) secondary induced emf

(c) secondary terminal voltage 

(d) none of the above.

 Ans : (c) secondary terminal voltage 

16. Power transformed from primary to secondary depends upon 

(a) number of primary turns.

(b) number of secondary turns.

(c) current transformation ratio.

(d) magnetic coupling between primary and secondary windings.

 Ans : (d) magnetic coupling between primary and secondary windings.

17.A transformer is supplying pure resistive (unity pf) load. The power factor on primary side will be

(a) about 0.95 (lead). 

(b) about 0.95 (lag).

(c) zero.

(d) unity.

 Ans : (b) about 0.95(lag).

18. In a transformer supplying inductive load 

(a) the secondary current results in equivalent primary current in phase opposition.  

(b) the secondary terminal voltage is less than the secondary induced emf. 

(c) the power factor on primary side will be lower than that of load.

(d) all of the above.

Ans : (d) all of the above.

19. The phasor diagram of a transformer on load can be drawn only if we know

(a) equivalent circuit parameters of the transformer.

(b) load current

(c) load pf.

 (d) all of the above.

Ans : (d) all of the above.

20. When a transformer is operating on no load, the primary applied voltage is approximately balanced by

(a) primary induced emf.

(b) secondary induced emf.

(c) terminal voltage across the secondary (d) voltage drop across the resistance and reactance.

Ans : (a) primary induced emf.