Transformer Multiple Choice Questions
1) A transformer
Explanation: A transformer is an electrical device used to step- up or step-down voltage with constant power. Therefore, a change in voltage level causes a change in current. It means, if the voltage is doubled, the current will be halved. i.e., V = 0.5 I
2) The primary winding of a transformer has a 120 V ac supply. What is the value of secondary voltage if the turn ratio is 10?
N2/N1 = 10
V1 = 120
As we know,
V2/V1 = N2/N1
V2/120 = 10
V2 = 120*10
V2 = 1200 V
3) Power transformers are designed to have maximum efficiency at
Explanation: Power transformers are usually used for transmission purposes as a step-up device. Power transformers are not directly linked to the consumer side; therefore, it causes very less load fluctuation. So, the power transformer can operate on full load.
4) If the supply frequency (f) of a transformer decreases, the effect of frequency on the transformer's secondary output voltage?
Explanation: The transformer is a static electromagnetic device that transforms the voltage from one side of its coil to the other side of the coil without a change in frequency. As we know, the working principle of transformer is based on Mutual induction, which happens at a constant frequency.
5) During the open circuit test of a transformer
Explanation: Open circuit test is performed to find the core loss in the transformer. In this test, the transformer's secondary winding is kept open-circuited, and a wattmeter is connected to the primary side. An ammeter (Used to measure the current) is connected in series with the transformer's primary winding, and rated voltage is applied at the primary side.
If normal voltage is applied at the primary side, flux is induced, and iron loss will occur. Hence the iron loss is maximum at rated voltage, and it is measured by using the wattmeter.
6) Lamination of the transformer core is made up of
Explanation: A high permeable material made up of thin silicon steel laminations are used for lamination of transformer core and other electrical devices for the given reasons:
Minimum hysteresis loss.
7) A power system has 3 synchronous generators. The turbine-control characteristics equivalent to the generators are X1 =60(60 - f), X2 = 120(70 - f), X3 = 140(80 - f) Where, f denotes the system frequency in Hz, and X1, X2, X3 are the power outputs of the turbines in MW. Considering the generators and transmission network to be lossless, the system frequency for a load of 800 MW is
X1 = 60(60-f)
X2 = 120(70 - f)
X3 = 140(80 - f)
X1+X2+X3 = 800
60(60-f) + 120(70 - f) + 140(80 - f) = 800
3600 - 60f + 8400-120f + 11200 - 140f = 800
22400 = 320f
f = 70 Hz
8) Transformer core are laminated in order to
Explanation: When a current-carrying conductor is placed in a magnetic field, a magnetic flux is induced and travels through the core of a transformer from primary to the secondary winding. During this process, some unwanted current is produced in the core of the transformer, usually known as eddy current, which is a primary cause of heat losses in the core. So, the transformer core is laminated to reduce the unwanted current (eddy current) and heat loss.
9) A transformer has a primary coil with 1600 loops and a secondary coil with 1200 loops. If the current in the primary coil is 6 Ampere, then what is the current in the secondary coil of a transformer.
Primary coil (Np) = 1600 loops
Secondary coil (Ns) = 1200 loops
The current in the primary coil (Ip) = 6 Ampere
As we know,
Is/Ip = Np/Ns
The current in the secondary coil is
Is/6 = 1600/1200
Is/6 = 1.33
Is = 1.33 * 6
Is = 7.98 Ampere
10) The short circuit test in the transformer is performed on
The short circuit test in the transformer is always carried on the high voltage side mainly due to the power supply limitations used to perform the transformer's short circuit test.
The primary purpose of performing a short circuit test in a transformer is to calculate the transformer's positive sequence impedance (Z). A short circuit test is also performed to calculate the copper loss.
11) The secondary voltage is 440 Volt, and primary voltage is 220 Volt, then a comparison of the secondary coil and primary coil is
Secondary voltage (Vs) = 440 Volt
Primary voltage (Vp) = 220 Volt
As we know,
Vs/Vp = Ns/Np
440/220 = Ns/Np
44/22 = Ns/Np
2/1 = Ns/Np
12) Which test determines the efficiency of two identical transformers under load conditions?
Explanation: In the back-to-back test or Sumpner's test on a transformer, two identical transformers are connected in such a way that one transformer is kept onto another transformer. The primary side of the two identical transformers is connected in parallel across the supply voltage. In contrast, the secondary side of the transformer is connected in series in such a way that emf's are in series opposition. It is a method for determining the efficiency, voltage regulation and heading under loaded conditions of the transformer.
13) The transformer ratings are expressed in terms of
Explanation: The rating of mostly electrical equipment shows its capability to sustain the mechanical load without overheating. There are two types of losses in the transformer due to the heating effect known as iron loss, and another is known as copper loss. The iron loss depends upon voltage, whereas copper loss depends upon current so that transformer is rated in KVA (KVA (Kilo-Volt-Ampere)
14) A transformer has 6 windings in its primary core and 3 in its secondary coil. If the primary voltage is 440 V, find the secondary voltage.
Explanation: As we know, electric power is the same in both the coils of the transformer, so that the given relationship must be true
N1/N2 = V1/V2 = I2/I1
In the given question, we only need the number of turns (n) and voltage (V)
N1/N2 = V1/V2
Rearranging the above equation, we get,
V2 = V1N2/N1
V1 = 440, N1 = 6 Turns, N2 = 3 turns
V2 = V1N2/N1 = (440 V) (3 turns)/ (8 turns)
= 165 V
15) Oil is provided in an oil-filled transformer for
Explanation: Transformers have broad applications for various industries; mainly, distribution transformers are filled with oil. The two-primary function of the transformer oil is given below
Insulator: The transformer oil has high dielectric strength; it means it can withstand very high voltage; this is the only reason it acts as an insulator in the transformer
Coolant: As we know, the transformer's coils are made up of copper and carrying a very high current, so it becomes hot in a very short time. The transformer oil is a good conductor of electricity and heat, so it reduces the coil's temperature.
16) A Buchholz relay can be installed on
Explanation: The primary function of the Buchholz relay is the detection of arcing in oil-cooled power transformers. The arcing emits gases that travel on the top of the tank head and increase the pressure over there that starts the relay.
17) The noise of the transformer primarily due to
Explanation: The noise of the transformer, also called humming, is mainly due to two reasons
Magnetostriction: Magnetostriction refers to the expansion and contraction of the laminations (iron core) due to the magnetic effect of the current flowing the coil of the transformer. Magnetostriction may be controlled, but it can't be totally removed depending on the transformer's design.
Stray magnetic fields: Generally, the steel used in the transformer core and wire used in the transformer coil vibrating at 50 Hz to 60 Hz frequency because of the interaction of the magnetic fields.
18) If a transformer is connected to a direct current supply, what happened?
Explanation: As we know, a transformer works on the principle of mutual induction, where you need a dynamic magnetic field to generate an induced emf in the secondary winding. In direct current supply, the change in frequency with respect to time is null, which means the dc supply can't produce a dynamic magnetic field; hence mutual induction can't possible on direct current supply since the primary winding of the transformer has a very low value of resistance so it can't oppose the excessive flow of current; therefore, the current having high value can damage the windings.
19) Cruciform shape is used in the transformer core to
Explanation: Cruciform shape is used in transformer core to reduce core loss, copper, in the winding, reduce core reluctance, and reduction of iron and copper losses. In a transformer, eddy current losses in the iron core are directly proportional to the core's thickness. To reduce eddy current loss, we usually use thin sheets of the iron core to reduce skin effect.
20) Which of the following does not change in an ordinary transformer?
Explanation: The transformer is a static (no moving parts) device, so the rate of magnetic flux change is constant for a certain operation. The moving flux in the transformer core is cut by the coils whose frequency is controlled by the primary supply. Since the coils or stationary conductors are static, the frequency does not change.
21) The dielectric strength of transformer oil is expected to be
Explanation: The dielectric strength refers to the withstand value; it means the electrical stress which oil can withstand safely. Usually, we monitored in transformed oil is BDV (breakdown voltage), i.e., the maximum electrical stress on which the transformer oil breaks down.
22) In the transformer, the function of a conservator is to
Explanation: In the transformer, the conservatory provides free space for the expansion of the oil. When the transformer is on full load, the oil heats up and expands. When the load is reduced from the transformer, oil temperature shrinks and oil contracts. Here, the air space in the conservatory enables breathing when oil expands or contracts. It is possible to manufacture a transformer without a conservator, but in that particular case, a tank with a gas cushion is required at the top of the transformer.
23) The efficiency of a transformer is maximum when
Explanation: The Efficiency of the transformer refers to the ratio of useful power output to the input power, the two being measured in the same unit. Its unit is either in Watts (W) or Kilowatt (KW). Transformer efficiency is denoted by Ƞ. Therefore, the efficiency of the transformer is maximum when iron loss = copper loss.
24) Which winding in a transformer has a greater number of turns
Explanation: In the transformer, the high voltage winding always has a greater number of turns, as a voltage is directly proportional to the number of turns.
25) In a transformer, tappings are usually provided.
Explanation: The tapings can be placed either on high voltage (HV) or low voltage (LV) windings.
Usually, the tapings are provided on high voltage (HV) winding due to the following reasons-
Good voltage regulation is possible with high voltage winding as it carries a large number of turns.
The low voltage winding of the transformer carries more current. So, if tapings are provided on the low voltage side, there are difficulties experienced in the interruption of high currents, making it unworkable. Hence, it is convenient to provide tapings on high voltage (HV) winding.
26) The highest voltage for transmitting electrical power in India is
Explanation: The highest voltage for transmitting electrical power in India is 400 kV
27) The purpose of a breather in a transformer to
Explanation: As the name suggests, "Breather" helps out the transformer to breathe, i.e., it assists in allowing the flow of air from the internal part of the transformer to the atmosphere and allowing the fresh air to come inside from the atmosphere. Generally, the breather comprises silica gel, which absorbs the moisture form air.
28) The necessary condition for the parallel operation of two single-phase transformers is that they must have the same.
Explanation: Parallel operation of single-phase transformer is done to increase the load capacity and reliability of the supply. It is done by adding another transformer in parallel simultaneously, given the continuous supply and frequent maintenance can be done evenly.
Some conditions must be fulfilled before the parallel operation of a transformer is given below.
The ratio of primary and secondary voltage ratings must be identical. It means the same transformation ratio for the transformer.
Both transformers must have the same polarity.
The X/R ratio must be the same.
29) The primary purpose of performing a short circuit test in a transformer is to measure its
Explanation: The short circuit test is performed to find the copper loss in the transformer. It is done by shorting the low voltage terminal and applying a low voltage across the high voltage terminal (because the current in the high voltage terminal will be less and simple to handle) and connect a wattmeter to measure the power dissipated in the low voltage terminal. The wattmeter reflects the full load copper loss.
30) The flashpoint of transformer oil must be greater than
Explanation: Usually, transformer oil has excellent insulating properties so that it is capable of withstanding high temperature. The Flashpoint of a transformer refers to the lowest temperature at which transformer oil gives vapor. Its primary function is to determine the self-ignition temperature of oil for safety reasons. The flashpoint of the transformer is directly proportional to the safe operation of the transformer. It means the higher the flash point value safer the operation. If the flash point value is lower than the specified value, the risk of fire in the transformer increases.
31) Which of the given test determine the iron loss of the transformer?
Explanation: An open circuit test is also known as a no-load test. It is used to find the iron loss at the rated applied voltage to the primary side. Iron loss of the transformer is not varying with a change in load. The iron loss also helps to determine the voltage regulation and efficiency of the transformer. Open circuit test of a transformer is carried out on the low voltage side by keeping the high voltage side open.
32) The frictional loss in a transformer is
Explanation: As we know, the transformer is a static device based on the principle of mutual induction with no rotating part, so there is no frictional loss.
33) Which type of winding is used in the three-phase shell type transformer
Explanation: Sandwich type winding used in three-phase shell-type transformers. It has a distinct circulation of leakage fluxes at two limbs. The total amount of leakage flux flowing through the two limbs depends on the leakage reactance. The sandwich-type winding is carried out, which depends on the flux distribution and applied voltage over two limbs.
34) Which of the given winding of the transformer has less cross-sectional area?
Explanation: High voltage winding of a transformer has the less cross-sectional area. The net current flowing through the winding calculates the cross-sectional area. It means if the current high current flows through the winding, a high cross-sectional area will be obtained since the high voltage winding of a transformer has a low current, so it has a less cross-sectional area.
35) The secondary winding of which of the given transformer is always kept closed?
Explanation: The secondary winding of the current transformer is always kept closed because if it open-circuited, the current transformer would develop an extremely high voltage across the secondary terminals. It may lead to damage to the transformer insulation as well as arcing across the terminal.
36) Which of the given losses changes with the load in the transformer?
Explanation: As we know, the transformer has primarily two types of losses Iron loss and copper loss. Iron loss is always constant, whereas copper losses are varying with a load.
Iron loss = Hysteresis loss + Eddy current loss
Both hysteresis loss and eddy current loss do not depend upon load current, which means iron losses are constant with varying load.
Copper loss = Conduction losses in conductor winding, which are directly proportional to the current square.
37) A transformer transforms
The transformer is a static electromechanical device that works on the principle of mutual induction. Transformer transforms:
Voltage: nU2 = Nu1, where U2 being the secondary side and U1 the primary side.
Current: I2 = I1/n, so when voltage is increased, the current is decreased.
38) A transformer has negative voltage regulation, only if its load power factor is
First, we need to understand the term "Voltage regulation" in brief:
When the secondary terminal of the transformer carried the full load, and the secondary voltage varies even though the primary supply voltage is held constant at rated value.
% Voltage Regulation = [ V2(NL) - V2 (FL)] *100 /[V2(NL)]
If the rated voltage is applied to the primary winding of the transformer, the secondary voltage changes with the load current and power factor, even the primary voltage is kept constant. These changes in voltage of windings are known as voltage regulation.
Voltage regulation depends on the voltage drop in the transformer's impedance, load current and load power factor.
39) In an autotransformer, the primary and secondary are ....... Coupled.
Explanation: Auto-transformers is a transformer with one common winding for primary and secondary, wound on a common core.
If you have a winding with a minimum of one tapping, you have an autotransformer. One extreme terminal in common for input and output. Two terminals are used for input, usually supply voltage, while the third terminal gives output with a common terminal. It is even possible to have more than one output.
In Auto Transformer, one single winding is used for primary winding and secondary winding, which means the primary and secondary windings have the common single winding. Therefore, the primary is electrically connected to the secondary and magnetically coupled to it.
Such an autotransformer saves on the cost of copper, and losses are less; flux linkage is better since there is just one winding. It is also smaller in size compared with a normal transformer.
40) Crushed rocks are provided in the substation to
Explanation: Stones or crushed rock are provided in the substation to provide an extra layer of high resistance between the soil and a person walking or standing over there. It provides a safety layer that reduces the step and touch voltages that occur during ground faults, which can happen both inside and outside the substation.
All substations are designed with a ground grid. Copper conductors are arranged in a grid pattern and laid in trenches dug into and buried in the soil when the substation is constructed. If the crushed rock were not used, more copper would be needed in the ground grid to achieve tolerable step and touch voltages during ground faults.
41) The maximum load that a power transformer can carry is limited by its
Explanation: The voltage ratio of a transformer refers to the ratio of the RMS terminal voltage of a high voltage winding to the RMS terminal voltage of a low voltage winding in a specified load condition. The total power a transformer can deliver depends primarily on the magnetic properties of the transformer and the volume of its core. The power of a transformer is directly proportional to the volume and frequency of the transformer.
42) When the supply frequency of a transformer increases, the secondary output voltage of the transformer
Explanation: As we know, the transformer is a static electrotechnical device that step-up or step-down voltage form one side of its coil to another side at a constant frequency. The transformer is based on the principle of mutual induction, which happens at a constant frequency, so the frequency doesn't change.
43) Open circuit test in a transformer is used to determine
Explanation: Open circuit test in a transformer is used to determine the core loss, i.e., eddy current loss and hysteresis loss. The core loss of a transformer is independent of load; it depends upon rated voltage.
44) In a transformer, the resistance between its primary and secondary is
Explanation: The primary and secondary windings of Transformers are magnetically coupled and electrically isolated; ideally, the resistance between the two windings should be infinity, but practically the insulation resistance is in the order of megaohms.
45) The direction of magnetic flux in a transformer must have
Explanation: Reluctance refers to analogous resistance. As we know, resistance opposes the flow of electric current in an electric circuit that same work does by a reluctance in a magnetic circuit.
Reluctance opposes the flow of magnetic flux in the magnetic circuit. It stores magnetic energy in the magnetic circuit. If the reluctance is low, less the opposite of the magnetic flux in the circuit; therefore, more flux can flow through the transformer core.
46) Which of the following part is not associated with the transformer?
Explanation: Exciter is usually used for starting the motor. It is a small generator mounted on the shaft of the main generator, which produces DC power for the main generator.
As we know, the transformer is a static device, so there is no moving part as in the motor.
47) If the transformer is working on, no-load is switched on to a specific source of voltage. It will draw a current
Explanation: When the transformer is in no-load condition, the secondary winding of the transformer has no load associated with it, and the transformer draws the zero current form the primary side of the winding. Practically whenever the transformer is in no-load condition, a small amount of current is drawn form the primary side of the winding to set up the required magnetic flux in the core of the transformer.
48) During short circuit test of the power Input to a transformer comprises predominately.
Explanation: Short circuit test is conducted on the HV side of the transformer, where the LV side of the transformer is short-circuited. Afterward, a wattmeter is connected to the primary side of the transformer as well as an ammeter is connected in series with the primary winding of the transformer. The total applied voltage for full load current in short circuit test on primary HV side of the transformer is quite low as compared to the rated voltage. With the help of this test, we get the total copper loss to happen in the transformer during full load condition.
49) Buck-boost transformers are used when
Explanation: Buck-boost transformers are used when the supply voltage does not match the voltage required by the load.
50) The transformer draws the current when its secondary winding is open.
Explanation: The transformer draws the no-load primary current when its secondary winding is open.