This theorem has been introduced in the year of 1952 by Dutch Electrical Engineer Bernard D.H. Tellegen. This is a very useful theorem in network analysis. According to Tellegen theorem, the summation of instantaneous powers for the n number of branches in an electrical network is zero. Are you confused? Let's explain. Suppose n number of branches in an electrical network have i₁, i₂, i₃, .............i_n respective instantaneous currents through them. These currents satisfy Kirchhoff's current law. Again, suppose these branches have instantaneous voltages across them are v₁, v₂, v₃, ........... v_n respectively. If these voltages across these elements satisfy Kirchhoff Voltage law then,

v_k is the instantaneous voltage across the k^th branch and i_k is the instantaneous current flowing through this branch. Tellegen theorem is applicable mainly in general class of lumped networks that consist of linear, non-linear, active, passive, time variant and time variant elements. This theorem can easily be explained by the following example.

In the network shown, arbitrary reference directions have been selected for all of the branch currents, and the corresponding branch voltages have been indicated, with positive reference direction at the tail of the current arrow. For this network, we will assume a set of branch voltages satisfy the Kirchhoff voltage law and a set of branch current satisfy Kirchhoff current law at each node. We will then show that these arbitrary assumed voltages and currents satisfy the equation.

And it is the condition of Tellegen theorem.

In the network shown in the figure, let v₁, v₂ and v₃ be 7, 2 and 3 volts respectively. Applying Kirchhoff voltage law around loop ABCDEA. We see that v₄ = 2 volt is required. Around loop CDFC, v₅ is required to be 3 volt and around loop DFED, v₆ is required to be 2. We next apply Kirchhoff current law successively to nodes B, C and D.

At node B let i_i = 5 A, then it is required that i₂ = − 5 A. At node C let i₃ = 3 A and then i₅ is required to be − 8. At node D assume i₄ to be 4 then i₆ is required to be − 9. Carrying out the operation of equation,

we get, 7 X 5 + 2 X ( − 5) + 3 X 3 + 2 X 4 + 3 X ( − 8) + 2 X ( − 9) = 0 Hence Tellegen theorem is verified.

Objective Questions on Circuit Theory (MCQs)

Superposition Theorem

Thevenin Theorem and Thevenin Equivalent Voltage and Resistance

Norton Theorem | Norton Equivalent Current and Resistance

Maximum Power Transfer Theorem

Millman Theorem

Reciprocity Theorem

Substitution Theorem

Compensation Theorem

Please add your con

What is Reciprocal Property?

In many electrical networks it is found that if the positions of voltage source and ammeter are interchanged, the reading of ammeter remains the same. It is not clear to you. Let's explain it in details. Suppose a voltage source is connected to a passive network and an ammeter is connected to other part of the network to indicate the response. Now any one interchanges the positions of ammeter and voltage source that means he or she connects the voltage source at the part of the network where the ammeter was connected and connects ammeter to that part of the network where the voltage source was connected. The response of the ammeter means current through the ammeter would be the same in both the cases. This is where the property of reciprocity comes in the circuit. The particular circuit that has this reciprocal property, is called reciprocal circuit. This type of circuit perfectly obeys reciprocity theorem.

Explanation of Reciprocity Theorem

The voltage source and the ammeter used in this theorem must be ideal. That means the internal resistance of both the voltage source and ammeter must be zero. The reciprocal circuit may be a simple or complex network. But every complex reciprocal passive network can be simplified into a simple network. As per reciprocity theorem, in a linear passive network, supply voltage V and output current I are mutually transferable. The ratio of V and I is called the transfer resistance. The theorem can easily be understood by this following example.

substitution theorem

Statement

Substitution theorem states that "if an element in a network is replaced by a voltage source whose voltage at any instant of time is equals to the voltage across the element in the previous network then the initial condition in the rest of the network will be unaltered". Or alternately "if an element in a network is replaced by a current source whose current at any instant of time is equal to the current through the element in the previous network then the initial condition in the rest of the network will be unaltered".

Explanation

Let us take a circuit as shown in fig - a, Let, V is supplied voltage and Z₁, Z₂ & Z₃ is different circuit impedances. V₁, V₂ and V₃ are the voltages across Z₁, Z₂ & Z₃ impedance respectively and I is the supplied current whose I₁ part is flowing through the Z₁ impedance whereas I₂ part is flowing through the Z₂ and Z₃ impedance.

Now if we replace Z₃ impedance with V₃ voltage source as shown in fig-b or with I₂ current source as shown in fig-c then according to Substitution Theorem all initial condition through other impedances and source will remain unchanged. i.e. - current through source will be I, voltage across Z₁ impedance will be V₁, current through Z₂ will be I₂ etc.

Example of Substitution Theorem

For more efficient and clear understanding let us go through a simple practical example:

Let us take a circuit as shown in fig - d As per voltage division rule voltage across 3Ω and 2Ω resistance are If we replace the 3Ω resistance with a voltage source of 6V as shown in fig - e, then According to Ohm’s law the voltage across 2Ω resistance and current through the circuit is Alternately if we replace 3Ω resistance with a current source of 2A as shown in fig - f, then Voltage across 2Ω is V_2Ω = 10 - 3× 2 = 4 V and voltage across 2A current source is V_2A = 10 - 4 = 6 V We can see the voltage across 2Ω resistance and current through the circuit is unaltered i.e all initial condition of the circuit is intact.

Superposition Theorem

Thevenin Theorem and Thevenin Equivalent Voltage and Resistance

Norton Theorem | Norton Equivalent Current and Resistance

Maximum Power Transfer Theorem

Millman Theorem

Reciprocity Theorem

Maximum Power Transfer Theorem

Suppose we have a voltage source or battery that's internal resistance is R_i and a load resistance R_L is connected across this battery . Maximum power transfer theorem determines the value of resistance R_L for which, the maximum power will be transferred from source to it. Actually the maximum power, drawn from the source, depends upon the value of the load resistance. There may be some confusion let us clear it. Power delivered to the load resistance,

To find the maximum power, differentiate the above expression with respect to resistance R_L and equate it to zero. Thus,

Thus in this case, the maximum power will be transferred to the load when load resistance is just equal to internal resistance of the battery . Maximum power transfer theorem can be applicable in complex network as follows-

A resistive load in a resistive network will abstract maximum power when the load resistance is equal to the resistance viewed by the load as it looks back to the network. Actually this is nothing but the resistance presented to the output terminals of the network. This is actually Thevenin equivalent resistance as we explained in Thevenin's theorem if we consider the whole network as a voltage source. Similarly, if we consider the network as current source, this resistance will be Norton equivalent resistance as we explained in Norton theorem.

Thevenin Theorem

This theorem is very conceptual. If we think deeply about an electrical circuit, we can visualize the statements made in Thevenin theorem. Suppose we have to calculate the current through any particular branch in a circuit. This branch is connected with rest of the circuits at its two terminal. Due to active sources in the circuit, there is one electric potential difference between the points where the said branch is connected. The current through the said branch is caused by this electric potential difference that appears across the terminals. So rest of the circuit can be considered as a single voltage source, that's voltage is nothing but the open circuit voltage between the terminals where the said branch is connected and the internal resistance of the source is nothing but the equivalent resistance of the circuit looking back into the terminals where, the branch is connected. So the Thevenin theorem can be stated as follows,

1. When a particular branch is removed from a circuit, the open circuit voltage appears across the terminals of the circuit, is Thevenin equivalent voltage and,
2. The equivalent resistance of the circuit network looking back into the terminals, is Thevenin equivalent resistance.
3. If we replace the rest of the circuit network by a single voltage source , then the voltage of the source would be Thevenin equivalent voltage and internal resistance of the voltage source would be Thevenin equivalent resistance which would be connected in series with the source as shown in the figure below.

To make Thevenin theorem easy to understand, we have shown the circuit below, Here two resistors R₁ and R₂ are connected in series and this series combination is connected across one voltage source of emf E with internal resistance R_i as shown. One resistive branch of R_L is connected across the resistance R₂ as shown. Now we have to calculate the current through R_L. First, we have to remove the resistor R_L from the terminals A and B. Second, we have to calculate the open circuit voltage or Thevenin equivalent voltage V_T across the terminals A and B.

The current through resistance R₂, Hence voltage appears across the terminals A and B i.e. Third, for applying Thevenin theorem, we have to determine the Thevenin equivalent electrical resistance of the circuit, and for that; first we have to replace the voltage source from the circuit, leaving behind only its internal resistance R_i. Now view the circuit inwards from the open terminals A and B. It is found the circuits now consist of two parallel paths - one consisting of resistance R₂ only and the other consisting of resistance R₁ and R_i in series. Thus the Thevenin equivalent resistance R_T is viewed from the open terminals A and B is given as. As per Thevenin theorem, when resistance R_L is connected across terminals A and B, the network behaves as a source of voltage V_T and internal resistance R_T and this is called Thevenin equivalent circuit. The current through R_L is given as,

Thevenin Equivalent Circuit

CONCEPTS OF CONTROL SYSTEMS

BASICS OF CONTROL SYSTEMS

OBJECTIVE TYPE QUESTIONS
1. In an open loop control system
(a) Output is independent of control input
(b) Output is dependent on control input
(c) Only system parameters have effect on the control output
(d) None of the above
Ans: a
2. For open control system which of the following statements is incorrect ?
(a) Less expensive
(b) Recalibration is not required for maintaining the required quality of the output
(c) Construction is simple and maintenance easy
(d) Errors are caused by disturbances
Ans: b
3. A control system in which the control action is somehow dependent on the output is
known as
(a) Closed loop system
(b) Semiclosed loop system
(c) Open system
(d) None of the above
Ans: a
4. In closed loop control system, with positive value of feedback gain the overall gain of
the system will
(a) decrease
(b) increase
(c) be unaffected
(d) any of the above
Ans: a
5. Which of the following is an open loop control system ?
(a) Field controlled D.C. motor
(b) Ward leonard control
(c) Metadyne
(d) Stroboscope
Ans: a
6. Which of the following statements is not necessarily correct for open control system ?
(a) Input command is the sole factor responsible for providing the control action
(b) Presence of non-linearities causes malfunctioning
(c) Less expensive
(d) Generally free from problems of non-linearities
Ans: b
7. In open loop system
(a) the control action depends on the size of the system
(b) the control action depends on system variables
(c) the control action depends on the input signal
(d) the control action is independent of the output
Ans: d
8 has tendency to oscillate.
(a) Open loop system
(b) Closed loop system
(c) Both (a) and (b)
(d) Neither (a) nor (b)
Ans: b
9. A good control system has all the following features except
(a) good stability
(b) slow response
(c) good accuracy
(d) sufficient power handling capacity
Ans: b
10. A car is running at a constant speed of 50 km/h, which of the following is the feedback
element for the driver ?
(a) Clutch
(b) Eyes
(c) Needle of the speedometer
(d) Steering wheel
(e) None of the above
Ans: c
11. The initial response when the output is not equal to input is called
(a) Transient response
(b) Error response
(c) Dynamic response
(d) Either of the above
Ans: a
12. A control system working under unknown random actions is called
(a) computer control system
(b) digital data system
(c) stochastic control system
(d) adaptive control system
Ans: c
13. An automatic toaster is a ______ loop control system.
(a) open
(b) closed
(c) partially closed
(d) any of the above
Ans: a
14. Any externally introduced signal affecting the controlled output is called a
(a) feedback
(b) stimulus
(c) signal
(d) gain control
Ans: b
15. A closed loop system is distinguished from open loop system by which of the following
? (
a) Servomechanism
(b) Feedback
(c) Output pattern
(d) Input pattern
Ans: b
16 is a part of the human temperature control system.
(a) Digestive system
(b) Perspiration system
(c) Ear
(d) Leg movement
Ans: b
17. By which of the following the control action is determined when a man walks along a
path ?
(a) Brain
(b) Hands
(c) Legs
(d) Eyes
Ans: d
18 is a closed loop system.
(a) Auto-pilot for an aircraft
(6) Direct current generator
(c) Car starter
(d) Electric switch
Ans: a
19. Which of the following devices are commonly used as error detectors in instruments ?
(a) Vernistats
(b) Microsyns
(c) Resolvers
(d) Any of the above
Ans: d
20. Which of the following should be done to make an unstable system stable ?
(a) The gain of the system should be decreased
(b) The gain of the system should be increased
(c) The number of poles to the loop transfer function should be increased
(d) The number of zeros to the loop transfer function should be increased
Ans: b
21 increases the steady state accuracy.
(a) Integrator
(b) Differentiator
(c) Phase lead compensator
(d) Phase lag compensator
Ans: a
22. A.C. servomotor resembles
(a) two phase induction motor
(b) Three phase induction motor
(c) direct current series motor
(d) universal motor
Ans: a
23. As a result of introduction of negative feedback which of the following will not
decrease ?
(a) Band width
(b) Overall gain
(c) Distortion
(d) Instability
Ans: a
24. Regenerative feedback implies feedback with
(a) oscillations
(b) step input
(c) negative sign
(d) positive sign
Ans: d
25. The output of a feedback control system must be a function of
(a) reference and output
(b) reference and input
(e) input and feedback signal
(d) output and feedback signal
Ans: a
26 is an open loop control system.
(a) Ward Leonard control
(b) Field controlled D.C. motor
(c) Stroboscope
(d) Metadyne
Ans: b
27. A control system with excessive noise, is likely to suffer from
(a) saturation in amplifying stages
(b) loss of gain
(c) vibrations
(d) oscillations
Ans: a
28. Zero initial condition for a system means
(a) input reference signal is zero
(b) zero stored energy
(c) ne initial movement of moving parts
(d) system is at rest and no energy is stored in any of its components
Ans: d
29. Transfer function of a system is used to calculate which of the following ?
(a) The order of the system
(b) The time constant
(c) The output for any given input
(d) The steady state gain
Ans: c
30. The band width, in a feedback amplifier.
(a) remains unaffected
(b) decreases by the same amount as the gain increase
(c) increases by the sane saaaajajt as the gain decrease
(d) decreases by the same amount as the gain decrease
Ans: c
31. On which of the following factors does the sensitivity of a closed loop system to gain
changes and load disturbances depend ?
(a) Frequency
(b) Loop gain
(c)

BASICS OF DC MOTOR

1. No-load speed of which of the following motor will be highest ?

(a) Shunt motor

(b) Series motor

(c) Cumulative compound motor

(d) Differentiate compound motor

Ans: b

2. The direction of rotation of a D.C. series motor can be changed by

(a) interchanging supply terminals

(b) interchanging field terminals

(c) either of (a) and (b) above

(d) None of the above

Ans: b

3. Which of the following application requires high starting torque ?

(a) Lathe machine

(b) Centrifugal pump

(c) Locomotive

(d) Air blower

Ans: c

4. If a D.C. motor is to be selected for conveyors, which rriotor would be preferred ?

(a) Series motor

(b) Shunt motor

(c) Differentially compound motor

(d) Cumulative compound motor

Ans: a

5. Which D.C. motor will be preferred for machine tools ?

(a) Series motor

(b) Shunt motor

(c) Cumulative compound motor

(d) Differential compound motor

Ans: b

6. Differentially compound D.C. motors can find applications requiring

(a) high starting torque

(b) low starting torque

(c) variable speed

(d) frequent on-off cycles

Ans: b

7. Which D.C. motor is preferred for elevators ?

(a) Shunt motor

(b) Series motor

(c) Differential compound motor

(d) Cumulative compound motor

Ans: d

8. According to Fleming's left-hand rule, when the forefinger points in the direction of the

field or flux, the middle finger will point in the direction of

(a) current in the conductor aovtaat of conductor

(c) resultant force on conductor

(d) none of the above

Ans: a

9. If the field of a D.C. shunt motor gets opened while motor is running

(a) the speed of motor will be reduced %

(b) the armature current will reduce

(c) the motor will attain dangerously high speed 1

(d) the motor will continue to nuvat constant speed

Ans: c

10. Starters are used with D.C. motors because

(a) these motors have high starting torque

(b) these motors are not self-starting

(c) back e.m.f. of these motors is zero initially

(d) to restrict armature current as there is no back e.m.f. while starting

Ans: d

11. In D.C. shunt motors as load is reduced

(a) the speed will increase abruptly

(b) the speed will increase in proportion to reduction in load

(c) the speed will remain almost/constant

(d) the speed will reduce

Ans: c

12. A D.C. series motor is that which

(a) has its field winding consisting of thick wire and less turns

(b) has a poor torque

(c) can be started easily without load

(d) has almost constant speed

Ans: a

13. For starting a D.C. motor a starter is required because

(a) it limits the speed of the motor

(b) it limits the starting current to a safe value

(c) it starts the motor

(d) none of the above

Ans: b

14. The type of D.C. motor used for shears and punches is

(a) shunt motor

(b) series motor

(c) differential compoutid D.C. motor

(d) cumulative compound D.C. motor

Ans: d

15. If a D.C. motor is connected across the A.C. supply it will

(a) run at normal speed

(b) not run

(c) run at lower speed

(d) burn due to heat produced in the field winding by .eddy currents

Ans: d

16. To get the speed of D.C, motor below the normal without wastage of electrical energy

is used.

(a) Ward Leonard control

(b) rheostatic control

(c) any of the above method

(d) none of the above method

Ans: a

17. When two D.C. series motors are connected in parallel, the resultant speed is

(a) more than the normal speed

(b) loss than the normal speed

(c) normal speed

Ans: c

18. The speed of a D.C. shunt motor more than its full-load speed can be obtained by

(a) decreasing the field current

(b) increasing the field current

(c) decreasing the armature current

(d) increasing the armature current

Ans: a

19. In a D.C. shunt motor, speed is

(a) independent of armature current

(b) directly proportional to the armature current

(c) proportional to the square of the current

(d) inversely proportional to the armature current

Ans: a

20. A direct on line starter is used: for starting motors

(a) iip to 5 H.P.

(b) up to 10 H.P.

(c) up to 15 H.P.

(d) up to 20 H.P.

Ans: a

21. What will happen if the back e.m.f. of a D.C. motor vanishes suddenly?

(a) The motor will stop

(b) The motor will continue to run

(c) The armature may burn

(d) The motor will run noisy

Ans: c

22. In case of D.C. shunt motors the speed is dependent on back e.m.f. only because

(a) back e.m.f. is equal to armature drop

(b) armature drop is negligible

(c) flux is proportional to armature current

(d) flux is practically constant in D:C. shunt motors

Ans: d

23. In a D.C. shunt motor, under the conditions of maximum power, the current in the

armature will be

(a) almost negligible

(b) rated full-load current

(c) less than full-load current

(d) more than full-load current

Ans: d

24. These days D.C. motors are widely used in

(a) pumping sets

(b) air compressors

(c) electric traction

(d) machine shops

Ans: c

25. By looking at which part of the motor, it can be easily confirmed that a particular

motor is D.C. motor?

(a) Frame

(b) Shaft

(c) Commutator

(d) Stator

Ans: c

26. In which of the following applications D.C. series motor is invariably tried?

(a) Starter for a car

(b) Drive for a water pump

(c) Fan motor

(d) Motor operation in A.C. or D.C.

Ans: a

27. In D.C. machines fractional pitch winding is used

(a) to improve cooling

(b) to reduce copper losses

(c) to increase the generated e.m.f.

(d) to reduce the sparking

Ans: d

28. A three point starter is considered suitable for

(a) shunt motors

(b) shunt as well as compound motors

(c) shunt, compound and series motors

(d) all D.C. motors

Ans: b

29. In case-the conditions for maximum power for a D.C. motor are established, the

efficiency of the motor will be

(a) 100%

(b) around 90%

(d) less than 50%

Ans: d

30. The ratio of starting torque to full-load torque is least in case of

(a) series motors

(b) shunt motors

(c) compound motors

(d) none of the above

Ans: b

32. In D.C. motor which of the following can sustain the maximum temperature rise?

(a) Slip rings

(b) Commutator

(c) Field winding

(d) Armature winding

Ans: c

33. Which of the following law/rule can he used to determine the direction of rotation of

D.C. motor ?

(a) Lenz's law

(b) Faraday's law

(c) Coloumb's law

(d) Fleming's left-hand rule

Ans: d

34. Which of the following load normally needs starting torque more than the rated

torque?

(a) Blowers

(b) Conveyors

(c) Air compressors

(d) Centrifugal pumps

Ans: b

35. The starting resistance of a D.C. motor is generally

(a) low

(b) around 500 Q

(c) 1000 Q

(d) infinitely large

Ans: a

BASICS OF TRANSFORMER

MCQS ON TRANSFORMER

   
100. N atural air coo ling is generally restricted for transformers up to
(a) 1.5 MVA
(b) 5 MVA
(c) 15 MVA
(d) 50 MVA
Ans: a
101. A shell-type transformer has
(a) high eddy current losses
(b) reduced magnetic leakage
(c) negligibly hysteresis losses
(d) none of the above
Ans: b
102. A transformer can have regulation closer to zero
(a) on full-load
(b) on overload
(c) on leading power factor
(d) on zero power factor
Ans: c
103. A transformer transforms
(a) voltage
(b) current
(c) current and voltage
(d) power
Ans: d
104. Which of the following is not the standard voltage for power supply in India ?
(a) llkV
(b) 33kV
(c) 66 kV
(d) 122 kV
Ans: d

105. Reduction in core losses and increase in permeability are obtained with transformer
employing
(a) core built-up of laminations of cold rolled grain oriented steel
(b) core built-up of laminations of hot rolled sheet
(c) either of the above
(d) none of the above
Ans: a
106. In a power or distribution transformer about 10 per cent end turns are heavily
insulated
(a) to withstand the high voltage drop due to line surge produced by the shunting
capacitance of the end turns
(b) to absorb the line surge voltage and save the winding of transformer from damage
(c) to reflect the line surge and save the winding of a transformer from damage
(d) none of the above
Ans: a
107. For given applied voltage, with the increase in frequency of the applied voltage
(a) eddy current loss will decrease
(b) eddy current loss will increase
(c) eddy current loss will remain unchanged
(d) none of the above
Ans: c
108. Losses which occur in rotating electric machines and do not occur in trans formers
are
(a) friction and windage losses
(b) magnetic losses
(c) hysteresis and eddy current losses
(d) copper losses
Ans: a
109. In a given transformer for a given applied voltage, losses which remain constant
irrespective of load changes are
(a) hysteresis and eddy current losses
(b) friction and windage losses
(c) copper losses
(d) none of the above
Ans: a
110. Which of the following statements regarding an idel single-phase transformer having
a turn ratio of 1 : 2 and
drawing a current of 10 A from 200 V A.C. supply is incorrect ?
(a) Its secondary current is 5 A
(b) Its secondary voltage is 400 V
(c) Its rating is 2 kVA
(d) Its secondary current is 20 A
(e) It is a step-up transformer
Ans: d
111 The secondary of a current transformer is always short-circuited under operating
conditions because it
(a) avoids core saturation and high voltage induction
(b) is safe to human beings
(c) protects the primary circuit
(d) none of the above
Ans: a
112. In a transformer the resistance between its primary and secondary should be
(a) zero
(b) 10 Q
(c) 1000 Q
(d) infinity
Ans: d
113. A good voltage regulation of a transformer means
(a) output voltage fluctuation from no load to full load is least
(b) output voltage fluctuation with power factor is least
(c) difference between primary and secondary voltage is least
(d) difference between primary and secondary voltage is maximum
Ans: a
114. For a transformer, operating at constant load current, maximum efficiency will occur
at
(a) 0.8 leading power factor

(b) 0.8 lagging power factor
(c) zero power factor
(d) unity power factor
Ans: d
115. Which of the following protection is normally not provided on small distribution
transformers ?
(a) Overfluxing protection
(b) Buchholz relay
(c) Overcurrent protection
(d) All of the above
Ans: b
116. Which of the following acts as a protection against high voltage surges due to
lightning and switching ?
(a) Horn gaps
(b) Thermal overload relays
(c) Breather
(d) Conservator
Ans: a
117. The efficiency of two identical transformers under load conditions can be
determined by
(a) short-circuit test
(b) back-to-back test
(c) open circuit test
(d) any of the above
Ans: b
118. Which of the following insulating materials can withstand the highest temperature
safely ?
(a) Cellulose
(b) Asbestos
(c) Mica
(d) Glass fibre
Ans: c
119. Which of the following parts of a transformer is visible from outside ?

(a) Bushings
(b) Core
(c) Primary winding
(d) Secondary winding
Ans: a
120. The noise produced by a transformer is termed as
(a) zoom
(b) hum
(c) ringing
(d) buzz
Ans: b
121. Which of the following loss in a transformer is zero even at full load ?
(a) Core loss
(b) Friction loss
(c) Eddy current loss
(d) Hysteresis loss
Ans: b
122. Which of the following is the most likely source of harmonics in a transformer ?
(a) poor insulation
(b) Overload
(c) loose connections
(d) Core saturation
Ans: d
123. If a transformer is continuously operated the maximum temperature rise will occur
in
(a) core
(b) windings
(c) tank
(d) any of the above
Ans: b
124. The hum in a transformer is mainly attributed to
(a) load changes
(b) oil in the transformer

(c) magnetostriction
(d) mechanical vibrations
Ans: c
125. The maximum load that a power transformer can carry is limited by its
(a) temperature rise
(b) dielectric strength of oil
(c) voltage ratio
(d) copper loss
Ans: c
126. The efficiency of a transformer, under heavy loads, is comparatively low because
(a) copper loss becomes high in proportion to the output
(b) iron loss is increased considerably
(c) voltage drop both in primary and secondary becomes large
(d) secondary output is much less as compared to primary input
Ans: a
127. An open-circuit test on a transformer is conducted primarily to measure
(a) insulation resistance
(b) copper loss
(c) core loss
(d) total loss
(e) efficiency
(f) none of the above
Ans: c
128. A no-load test is performed on a transformer to determine
(a) core loss
(b) copper loss
(c) efficiency
(d) magnetising current
(e) magnetising current and loss
Ans: e
129. The voltage transformation ratio of a transformer is equal to the ratio of
(a) primary turns to secondary turns
(b) secondary current to primary current

(c) secondary induced e.m.f. to primary induced e.m.f.
(d) secondary terminal voltage to primary applied voltage
Ans: c
130. Part of the transformer which is most subject to damage from overheating is
(a) iron core
(b) copper winding
(c) winding insulation
(d) frame or case
(e) transformer tank
Ans: c