The magnetizing current in a transformer is rich in

1. Third harmonics
2. Fifth harmonics
3. Seventh harmonics
4. Thirteen harmonics

An auto-transformer having a transformation ratio of 0.8 supplies a load of 3kW. The power transferred conductively from primary to secondary is

1. 0.6 kW
2. 2.4 kW
3. 1.5 kW
4. 0.27 kW

While performing the open-circuit and short-circuit tests on a transformer to determine its parameters the status of the low voltage (L.V) and high voltage (H.V) will be such that

1. In Open circuit L.V is open and in short circuit H.V is shorted
2. In Open circuit H.V is open and in short circuit L.V is shorted
3. In Open circuit L.V is open and in short circuit L.V is shorted
4. In Open circuit H.V is open and in short circuit H.V is shorted

If a sinusoidal voltage source is connected to a power transformer, its no-load current would be

1. Sinusoidal and lagging the voltage by ninety degrees
2. Sinusoidal and lagging the voltage by less than ninety degrees
3. Rich in third harmonic and its fundamental would lag the voltage by ninety degrees
4. Rich in third harmonic and its fundamental would lag the voltage less than ninety degrees

The utility of delta-connected tertiary windings in star-star connected power transformer is that it

1. Makes supply available for small loads
2. Provides low reactance paths for zero sequence currents
3. Is used to suppress harmonic voltages
4. All of these

A 200/100V, 50 Hz transformer is to be excited at 40 Hz from the 100V side. For the exciting current to remain the same, the applied voltage should be

1. 150 V
2. 125 V
3. 100 V
4. 80 V

A 2-phase, 4 pole permanent magnet stepper motor has a step of

1. 90 degrees
2. 45 degrees
3. 30 degrees
4. 22.5 degrees

Supply to one terminal of delta-star three-phase core type transformer which is on no-load fails. Assuming magnetic circuit symmetry, voltages on the secondary side will be

1. 230, 230,115
2. 230,115, 115
3. 345, 115, 115
4. 345, 0, 345

A 220/440 V, 50Hz, 5kVA single phase transformer operates on 220V, 40 Hz supply with secondary winding. Then

1. The eddy current loss and hysteresis loss of the transformer decrease
2. The eddy current loss and hysteresis loss of the transformer increase
3. The hysteresis loss of the transformer increases while eddy current loss remains the same
4. The hysteresis loss remains the same whereas eddy current loss decreases

The voltage regulation of a large transformer is mainly influenced by

1. No-load current and load power factor
2. Winding resistances and load power factor
3. Leakage fluxes and load power factor
4. Winding resistances and core losses

A 2500/250V, 25kVA transformer is connected as an auto transformer to give 2500/2750 V. Its rating would be

A)     250 kVA; 225 kVA is transformed conductively

B)      250 kVA; 225 kVA is transformed inductively

C)      275 kVA; 250 kVA is transformed inductively

D)     275 kVA; 250 kVA is transformed conductively

A single phase transformer is to be switched to the supply to have minimum inrush current. The switch should be closed at

A)     Maximum supply voltage

B)      Zero supply voltage

C)      1/1.414 maximum supply voltage

D)     0.5  times maximum supply voltage

Auto transformer is used in transmission and distribution

A)     When operator is not available

B)      When iron losses are to be reduced

C)      When efficiency considerations can be ignored

D)     When the transformer ratio is small

Keeping in view the requirement of parallel operation, which of the 3-phase connections given below are possible?

A)     Delta-delta and delta-star

B)      Delta-delta and star-delta

C)      Star-star to delta-delta

D)     Delta-star to star-delta

A 2/1 ratio, two winding transformer is connected as an auto transformer. Its KVA rating as an auto transformer compared to a two-winding transformer is

A)     Same

B)      1.5 times

C)      2 times

D)     3 times

On the two sides of a star/delta transformer

A)     Voltages and currents are both in phase

B)      Voltages and currents both differ in phase by 30⁰

C)      Voltages differ in phase by 30⁰ but currents are in phase

D)     Currents differ in phase by 30⁰ but voltages are in phase

The efficiency of transformer at full-load 0.85 pf lag is 95%. Its efficiency at full-load 0.85 pf lead will be

A)     Less than 95%

B)      More than 95%

C)      95%

D)     100%

In a transformer the phase angle between primary and secondary terminal voltage is

A)     90⁰; primary voltage leading the secondary voltage

B)      90⁰; primary voltage lagging the secondary voltage

C)      A few degrees, primary voltage leading the secondary voltage

D)     A few degrees, primary voltage lagging the secondary voltage

Transformer excited from a sinusoidal voltage source will draw a no-load current which

A)     Is sinusoidal and of the same frequency as the voltage source

B)      Comprises the fundamental frequency (same as that of the voltage source) and the peaking third harmonic

C)      Comprises the fundamental frequency and the depressing third harmonic

D)     Is sinusoidal of frequency three times that of the voltage source

A transformer on no-load is switched on to a source of voltage. It will draw a current which

A)     The same as the steady-state magnetizing current

B)      Several times the steady-state magnetizing current depending upon the initial state of the residual flux in the transformer core

C)      Several times the steady-state magnetizing current depending upon the initial state of the residual flux in the transformer core

D)     Twice the steady-state magnetizing current provided the core has no residual flux

For a fixed primary voltage how does the secondary terminal voltage of a transformer change if the power factor of inductive load is raised from 0.7 to 0.9 with load current remaining fixed?

A)     No change in voltage

B)      Voltage decreases

C)      Voltage rises

D)     Voltage will rise, go through  maximum and then decrease

For a fixed primary voltage how does the secondary terminal voltage of a transformer change if the power factor of capacitive load rises from 0.7 to 0.9 with load current remaining fixed?

A.      No change in voltage

B.      Voltage decreases

C.      Voltage rises

D.      Voltage will rise, go through  maximum and then decrease

The applied voltage of a certain transformer is increases by 50% while the frequency reduced to 50%. The maximum core flux density will be

A)      Becomes three times

B)      Becomes 1.5 times

C)      Becomes half

D)     Remains the same

Which of the following tests must be performed on a transformer to determine its leakage reactance

A)     SC test only

B)      OC test only

C)       Both OC and SC tests

D)     Test by an impedance bridge

Power input to a transformer on no load at rated voltage comprises predominantly

A)     Copper loss

B)      Hysteresis loss

C)      Core loss

D)     Eddy current loss

Hysteresis losses are directly proportional to the

A)     Square of frequency and flux density to the power 1.6

B)      Frequency and flux density to the power 1.6

C)      The square of frequency and proportional to the square of flux density

D)     Frequency and proportional to flux density

A coil wound on magnetic core is excited from an AC voltage source. The source voltage and its frequency are both doubled. The eddy current loss in the core will become

A)     Half

B)      Remains same

C)      Double

D)     Four times

Building steel core out of stampings reduces eddy current loss because it

A)     Increases core resistivity

B)      Increases the effective length of eddy currents paths thereby increasing effective resistance to the flow of eddy currents

C)      Increases core permeability

D)     Reduces the effective length of eddy current path, thereby reducing effective resistance to the flow of eddy currents.

A 200/100 V, 50 Hz transformer is to be excited at 40 Hz from 100V side. For the exciting current to remain the same, the applied voltage will be

A)     150 V

B)      125 V

C)      100 V

D)     80V

Which three phase connection can be used in a transformer to introduce a phase difference of 30 degree between its output and corresponding input line voltages?

1. Star-Star
2. Star- Delta
3. Delta-Delta
4. Delta-zigzag

Answer : Star - Delta

A single phase transformer when supplied from 220V, 50 Hz has eddy current loss of 50 W. If the transformer is connected to a voltage of 330V, 50 Hz, the eddy current loss will be

A)     168.75 W

B)      112.5 W

C)      75 W

D)     50 W

The power factor of a synchronous motor

A)     Improves with increase in excitation and may even become leading at high excitations

B)      Decreases with increase in excitation

C)      Is independent of its excitation

D)     Increases with loading for a given excitation

The synchronous impedance method of finding the voltage regulation by a cylindrical rotor alternator is generally considered as

A)     A pessimistic method because saturation is not considered

B)      An optimistic method because saturation is not considered

C)      A fairly accurate method even if power factor is not taken into account while determining synchronous impedance

D)     A fairly accurate method when power factor is taken into account while determining synchronous impedance

Generally the no-load losses of an electrical machine is represented in its equivalent circuit by

A)     Parallel resistance with a low value

B)      Series resistance with a low value

C)      Parallel resistance with a high value

D)     Series resistance with a high value

A DC series motor is accidentally connected to a single phase AC supply. The torque produced will be

A)     Of zero average value

B)      Oscillating

C)      Steady and unidirectional

D)     Pulsating and unidirectional

In case of auto transformer, which of the following statements are correct?

I. An auto-transformer requires less copper as compared to a conventional 2-winding transformer of the same capacity

II. An auto-transformer provides isolation between the primary and secondary windings

III. An auto-transformer has less leakage reactance as compared to the conventional 2-winding transformer of same capacity

Select the correct answer using the codes given below

A)     1,2 and 3

B)      1 and 2

C)      1 and 3

D)     2 and 3

Switched reluctance motor means

A)     Salient pole synchronous motor without excitation winding

B)      A stepper motor with salient poles

C)      Synchronous motors with salient poles on stator and rotor

D)     A stepper motor with closed loop control and with rotor position sensor

A single-phase transformer when supplied from 220V, 50Hz has eddy current loss of 50W. if the transformer is connected to a voltage of 330V, 50Hz, the eddy current loss will be

A)     168.75W

B)      112.5W

C)      75W

D)     50W

The torque of a reluctance motor can be effectively increased by

A)     Increasing reluctance of the magnetic circuit along the direct axis

B)      Decreasing the reluctance of the magnetic circuit along the quadrature axis

C)      Increasing the ratio of the quadrature axis reluctance to direct axis reluctance

D)     Decreasing the ratio of quadrature axis reluctance to direct axis reluctance

The rotor power output of 3-phase induction motor is 15kW. The rotor copper losses at a slip of 4% will be

A)     600W

B)      625W

C)      650W

D)     700W

Skewing of the rotor in a three-phase squirrel-cage induction motor reduces

A)     Noise, parasitic torque, starting torque and pullout torque

B)      Noise and parasitic torque, but increases starting torque and pull out torque

C)      Noise and pullout torque, but increases parasitic torque and starting torque

D)     Noise, parasitic torque and starting torque, but increases pullout torque

In a shaded pole induction motor, the rotor runs from the

A)     Shaded portion to the unshaded portion of the pole while the flux in the former leads that of the latter

B)      Shaded portion to the unshaded portion of the pole while the flux in the former lags that in the latter

C)      Unshaded portion to the shaded portion while the flux in the former leads that it in the latter

D)     Unshaded portion to the shaded portion  while the flux in the former lags that in the latter

A DC shunt generator when driven without connecting field winding shows an open circuit terminal voltage of 12V. when field winding is connected and excited, the terminal voltage drops to zero because

A)     Field resistance is higher than critical resistance

B)      There is no residual magnetism in the field circuit

C)      The field winding is wrongly connected

D)     There is a fault in armature circuit

The advantage of the double squirrel-cage induction motor over single cage rotor is that its

A)     Efficiency is higher

B)      Power factor is higher

C)      Slip is higher

D)     Starting current is lower

An induction motor when started on load does not accelerate up to full speed but runs at 1/7 th of the rated speed. The motor is said to be

A)     Locking

B)      Plugging

C)      Crawling

D)     Cogging

Synchronous condenser means

A.      A synchronous motor with capacitor connected across stator terminals to improve power factor

B.      A synchronous motor operating at full load with leading power factor

C.      An over-excited synchronous motor partially supplying mechanical load, and also improving power factor of the system to which it is connected

D.      An over-excited synchronous motor operating at no-load with leading power factor used in large power stations for improvement of power factor

In cylindrical rotor synchronous machine, the phasor addition of stator and rotor mmfs is possible because

A)     Two mmfs are rotating in opposite direction

B)      Two mmfs are rotating in same direction at different speed

C)      Two mmfs are stationary with respect to each other

D)     One mmf is stationary and the other mmf is rotating

In an electromechanical energy conversion device, the developed torque depends upon

A)     Stator field strength and torque angle

B)      Stator field and rotor field strengths

C)      Stator field and rotor field strengths and the torque angle

D)     Stator field strength only

Consider the following statements: The use of interpoles in dc machines is to counteract the

   I. Reactance voltage
II. Demagnetizing effect of armature mmf in the commutating zone
III. Cross-magnetizing effect of armature mmf in the commutating zone
Which of these statement(s) is/are correct?

A)     1 &2

B)      2&3

C)      1&3

D)     3 alone

Superposition theorem is not applicable to networks containing

A)     Nonlinear elements

B)      Dependent voltage sources

C)      Dependent current sources

D)     Transformers

The fourier series expansion of a periodic function with half wave symmetry contains only

A)     sine terms

B)      cosine terms

C)      odd harmonics

D)     even harmonics

Voltage transfer function of a simple RC integrator has

A)     a finite zero and a pole at infinity

B)      a finite zero and a pole at the origin

C)      a zero at the origin and a finite pole

D)     a zero at infinity and a finite pole

The impulse response of an R-L circuit is a

A)     Rising exponential function

B)      Decaying exponential function

C)      Step function

D)     Parabolic function

A water boiler of home is switched on to the AC mains supplying power at 230V/50Hz. Th frequency of instantaneous power consumed by the boiler is

A)     0 Hz

B)      50 Hz

C)      100 Hz

D)     150 Hz

In a series RLC high Q circuit, the current peaks at a frequency

A)     Equal to the resonant frequency

B)      Greater than the resonant frequency

C)      Less than the resonant frequency

D)     None of these

A passive 2-port network is in a steady-state. Compared to its input, the steady state output can never offer

A)     Higher voltage

B)      Lower impedance

C)      Greater power

D)     Better regulation

Two two-port networks are connected in cascade. The combination is to be represented as a single two-port network. The parameters of the network are obtained by multiplying the individual

A)     Z-parameter matrix

B)      h-parameter matrix

C)      y-parameter matrix

D)     ABCD parameter matrix

If a capacitor is energised by a symmetrical square wave current source, then the steady state voltage across the capacitor will be

A)     Square wave

B)      Triangular wave

C)      Step wave

D)     Impulse wave

If the step response of an initially relaxed circuit is known, then the ramp response can be obtained by

A)     Integrating the step response

B)      Differentiating the step response

C)      Integrating the step response twice

D)     Differentiating the step response twice

According to lenz’s law, the direction of induced emf and hence current

A)     May be found by the right-hand rule

B)      Is always determined by the rate of cutting flux

C)      Always opposes the cause producing it

D)     Depends on whether the coil is wound with a right or left-hand spiral