1. A transformer works on the principle of:
A) Self induction
B) Mutual induction
C) Eddy current
D) Hysteresis
Answer: B
Explanation: A transformer has a core on which the primary and secondary windings are wound. When the primary winding is energized, it draws a magnetizing current and produces magnetic flux in the core. This flux links with both the primary and the secondary windings. As a result, EMF is induced in the windings. We can take power from the secondary winding due to the induced EMF produced by the mutual flux linkage between the primary and secondary windings. Therefore, a transformer works on the principle of mutual induction.
2. The core of a transformer is laminated to:
A) Reduce copper loss
B) Increase flux
C) Reduce eddy current loss
D) Reduce hysteresis loss
Answer: C
Explanation: Lamination of the transformer core increases the resistance and therefore reduces the eddy current. The eddy current loss is expressed as I2R, where I represents the eddy current in this case. Obviously, the increase in resistance due to lamination has much less influence than the decrease in . Therefore, as a whole, the term reduces, and the overall eddy current loss of the core decreases due to lamination.
3. Transformer rating is expressed in:
A) kW
B) kVA
C) kVAR
D) HP
Answer: B
Explanation: The design of a transformer, especially its conducting portion along with its insulation system, mainly depends on two factors. The conducting portion depends on the current limit, while the insulation portion depends on the voltage limit. In fact, a transformer is designed to carry power without exceeding the specified temperature limit. This temperature limit depends on the heating of the conducting parts of the transformer, which is proportional to either or , that is, the ohmic loss. Obviously, this heating has no direct relation with the power factor. Therefore, a transformer is designed based on the current it can carry and the voltage it can withstand, or equivalently, based on its losses. That is why a transformer is rated in MVA, kVA, or simply VA (voltage × current), and not in kilowatts, because power factor does not come into consideration for its rating.
4. In a step-up transformer:
A) Ns < Np
B) Ns = Np
C) Ns > Np
D) Voltage decreases
Answer: C
Explanation: In a transformer, the voltage per turn is fixed and is the same for both the primary and secondary windings. The total voltage induced across a winding depends on the number of turns in that winding. Therefore, in a step-up transformer, the secondary winding has more turns than the primary winding. As a result, the voltage applied to the primary winding is transformed into a higher voltage across the secondary winding.
5. Copper loss depends on:
A) Voltage
B) Current
C) Frequency
D) Flux
Answer: B
Explanation: The copper loss of a transformer is also referred to as the ohmic loss of the transformer. It is nothing but the loss in the windings. Obviously, it depends on the magnitude of the current flowing through the transformer windings.
6. No-load current of transformer is about:
A) 50% rated
B) 25% rated
C) 2–5% rated
D) 80% rated
Answer: C
Explanation: When the secondary of a transformer is not loaded, it means the secondary is open-circuited. In this condition, no current flows through the secondary winding. However, the transformer still draws a small current from the primary source. This current is required to keep the core magnetized even under open-circuit conditions. It is known as the magnetizing current of the transformer. The primary draws this constant magnetizing current, which is typically about 2 to 5 percent of the rated primary current of the transformer.
7. Core loss depends mainly on:
A) Load current
B) Voltage & frequency
C) Resistance
D) Temperature
Answer: B
Explanation: If we examine the expressions for eddy current loss and hysteresis loss, we can see that both losses depend on frequency and voltage. Eddy current loss is directly proportional to the square of the frequency and the square of the maximum flux density, while hysteresis loss is directly proportional to the frequency and a power of the maximum flux density. Since the maximum flux density depends on the applied voltage, both eddy current loss and hysteresis loss ultimately depend on the supply frequency and voltage.
8. Ideal transformer has:
A) Zero copper loss
B) Zero core loss
C) No leakage flux
D) All of the above
Answer: D
Explanation: An ideal transformer implies that there are no losses and no leakage flux. It assumes zero copper loss, zero core loss, and perfect magnetic coupling between the primary and secondary windings, meaning all the flux produced by the primary links completely with the secondary.
9. Efficiency of transformer is maximum when:
A) Copper loss = Core loss
B) Copper loss > Core loss
C) Core loss > Copper loss
D) Voltage is maximum
Answer: A
Explanation: Maximum efficiency condition: Pcopper = Pcore.
10. OC test is conducted on:
A) HV side
B) LV side
C) Both sides
D) Any side
Answer: B
Explanation: The open-circuit test is always conducted from the low-voltage side of the transformer because it is more practical to apply a low voltage to the low-voltage winding to energize the transformer than to apply a high-voltage supply to the high-voltage winding. This arrangement is chosen purely for practical convenience.
11. SC test measures:
A) Core loss
B) Copper loss
C) Iron loss
D) Hysteresis loss
Answer: B
Explanation: In the short-circuit test, we apply a very small voltage to the high-voltage winding of the transformer to produce the rated current in both the primary and secondary windings. This rated current produces the required copper losses in the transformer. However, since the applied voltage is very low, the core loss is negligible. We know that the core loss of a transformer depends on the applied voltage, and in this case, the voltage is very small compared to the rated voltage of the transformer. Therefore, during the short-circuit test, the measured power mainly represents the copper loss.
12. Transformer frequency increase will:
A) Increase core loss
B) Decrease core loss
C) Not affect core loss
D) Burn transformer
Answer: A
Explanation: The core loss of a transformer consists of hysteresis loss and eddy current loss. Both of these losses depend on the supply frequency. Therefore, if the frequency increases, both hysteresis loss and eddy current loss increase. As a result, the total core loss of the transformer also increases.
13. Transformer works only with:
A) DC
B) AC
C) Pulsating DC
D) Both AC & DC
Answer: B
Explanation: A transformer works on the principle of Faraday’s law of electromagnetic induction. According to this law, whenever a changing magnetic flux links with a conductor, an EMF is induced across it. In a static device like a transformer, changing flux is possible only when an alternating supply is connected to it. If a DC source is applied, flux will be produced, but it will not change after the initial instant. Therefore, no continuous EMF will be induced in either the primary or the secondary winding. For this reason, a transformer operates only on an alternating supply.
14. In core type transformer:
A) Windings surround core
B) Core surrounds windings
C) Only LV winding present
D) Only HV winding present
Answer: A
Explanation: In a core-type transformer, the low-voltage winding is first wound around the limb of the core. Then, the high-voltage winding is wound over the low-voltage winding. Therefore, we can say that in a core-type transformer, the windings surround the core limb.
15. Shell type transformer has:
A) Single magnetic path
B) Double magnetic path
C) No leakage
D) No loss
Answer: B
Explanation: In a shell-type transformer construction, two parallel closed magnetic paths are formed on both sides of the central limb. The windings are wound on the central limb in a sandwich form. Therefore, we can say that a shell-type transformer always has a double parallel magnetic path in its core.
16. Voltage regulation is zero at:
A) Unity PF
B) Zero PF
C) Leading PF (slightly)
D) Lagging PF
Answer: C
Explanation: The voltage regulation of a transformer contains a term related to resistance. Even at unity power factor, the reactive term becomes zero, the resistive term remains with its maximum value. Therefore, at unity power factor, the voltage regulation is not zero because the resistive term still exists. Capacitive reactance produces a negative voltage drop. Therefore, when the load has a leading power factor, the reactive component of the voltage drop becomes negative. This negative reactive drop can offset the positive resistive voltage drop in the transformer. As a result, the overall voltage regulation can become zero or even negative at a slightly leading power factor.
17. Oil in transformer is used for:
A) Insulation
B) Cooling
C) Arc quenching
D) Both A & B
Answer: D
Explanation: Transformer oil performs two major functions: insulation and cooling. It also has arc-quenching properties. However, in transformers, the arc-quenching property is not required because no intentional arc is produced during normal operation. Therefore, transformer oil is used in transformers only for insulation and cooling purposes.
18. Buchholz relay is used in:
A) Dry transformer
B) Oil transformer
C) Distribution line
D) Generator
Answer: B
Explanation: Buchholz relay is a purely oil-actuated mechanical relay. Therefore, it can be used only in oil-filled transformers.
19. Tap changer is provided to:
A) Increase current
B) Change frequency
C) Control voltage
D) Reduce losses
Answer: C
Explanation: The output voltage of a transformer depends on its turns ratio. An on-load tap changer adjusts the tap position while the transformer is in operation. It changes the effective turns ratio to control and regulate the output voltage.
20. Turns ratio is defined as:
A) Vp/Vs
B) Ip/Is
C) Ns/Np
D) Np/Ns
Answer: D
Explanation: The turn ratio of a transformer is always defined as the ratio of the number of turns in the primary winding to the number of turns in the secondary winding. For example, in a 132 kV / 33 kV transformer, which is a step-down transformer, the 132 kV side is considered the primary side and the 33 kV side is the secondary side. Therefore, the turn ratio is defined as the ratio of the primary turns to the secondary turns.
21. Transformer core is made of:
A) Cast iron
B) Copper
C) Silicon steel
D) Aluminium
Answer: C
Explanation: The transformer core must be magnetized during operation. Therefore, it must be made of a magnetic material. Cast iron can be used for this purpose. However, it is not very efficient. It suffers from significant eddy current and hysteresis losses. In contrast, laminations made of cold-rolled grain-oriented (CRGO) silicon steel are much more efficient for this purpose. These laminations significantly reduce both eddy current loss and hysteresis loss. Therefore, CRGO silicon steel is commonly used for transformer cores.
22. Leakage reactance causes:
A) Voltage drop
B) Core loss
C) Copper loss
D) Hysteresis
Answer: A
Explanation: The leakage reactance of a transformer is produced due to leakage flux. This leakage flux links only with its own winding and produces a self-induced EMF. The self-induced EMF opposes the mutually induced EMF. As a result, an internal voltage drop occurs in the transformer.
23. Transformer with 100% efficiency is:
A) Practical
B) Ideal
C) Possible
D) Used in PSU
Answer: B
Explanation: A transformer is a machine. Although it is a static machine, it still has inherent losses. Therefore, a practical transformer cannot be 100% efficient. However, an ideal transformer is an imaginary, lossless transformer. Since it has no losses, its efficiency is 100%.
24. Transformer oil BDV test checks:
A) Viscosity
B) Insulation strength
C) Temperature
D) Density
Answer: B
Explanation: In the BDV test, a gradually increasing power-frequency voltage is applied across two electrodes immersed in the oil, with a specified gap between them. We observe the value of the applied voltage at which the oil breaks down. This value is called the breakdown voltage of the oil. It indicates the voltage-withstand capability of the transformer oil. Therefore, it is an insulation strength test of transformer oil.
25. Cooling type ONAN means:
A) Oil Natural Air Natural
B) Oil Natural Air Forced
C) Oil Forced Air Forced
D) Air Natural
Answer: A
Explanation: ONAN means Oil Natural Air Natural. In this cooling method, the heat generated in the transformer core and windings is carried to the radiator by the natural convection of transformer oil. From the radiator, the heat is transferred to the surrounding air and dissipated naturally.
Moderate Level Objective Questions (MCQ) on Transformers
26. Per unit impedance is important for:
A) Efficiency
B) Fault calculation
C) Core loss
D) Turns ratio
Answer: B
Explanation: The fault MVA of a transformer is the ratio of the rated MVA to the per-unit impedance of the transformer. Therefore, the per-unit impedance plays an important role in the fault calculation of a transformer. A lower per-unit impedance results in a higher fault MVA, while a higher per-unit impedance results in a lower fault MVA.
27. Guaranteed impedance is specified at:
A) 20°C
B) 75°C
C) 100°C
D) Ambient
Answer: B
Explanation: The actual impedance of a transformer has two components: a resistive part and a reactive part, because the impedance Z = R+jX. Here, R represents the resistance of the internal windings of the transformer. If the temperature changes, this resistance also changes. The transformer is designed to operate safely up to its maximum operating temperature. During a fault condition, a large fault current flows through the transformer windings. This current increases the temperature of the windings. Again, the magnitude of the fault current is governed by the transformer impedance. For this reason, the impedance must be guaranteed at the maximum operating temperature of the transformer winding, which is generally specified as 75°C according to standard practice.
28. Short circuit current is inversely proportional to:
A) Voltage
B) Impedance
C) Current
D) Power
Answer: B
Explanation: The short-circuit current of a transformer is given by the ratio of the rated voltage to the impedance of the transformer. Therefore, the short-circuit current is inversely proportional to the impedance of the transformer.
29. In parallel operation, transformers must have same:
A) kVA rating only
B) Impedance only
C) Voltage ratio & impedance
D) Color
Answer: C
Explanation: When two active elements are connected in parallel, circulating current between them can be avoided only if the terminal voltages and internal impedances of both elements are the same. Consider two batteries connected in parallel. If their voltages and internal resistances are not equal, a circulating current will flow between them. A transformer is also a source of power and therefore behaves as an active element. Hence, the parallel operation of transformers is possible without any local circulating current only when their voltage ratios and impedances are properly matched. It is better to say that the voltage ratio and the percentage impedance of both transformers connected in parallel must be the same.
30. Vector group Dyn11 indicates:
A) HV delta, LV star, 30° shift
B) HV star, LV delta
C) No phase shift
D) 60° shift
Answer: A
Explanation: Capital letters represent the high-voltage side. Small letters represent the low-voltage side. The numeric value represents the phase difference between the primary and secondary voltages. Here, D means delta connection in HV winding. small y means star connection in LV winding. Small n means neutral is brought out from LV winding. The number 11 represents the phase angle between the high-voltage and low-voltage sides.
In the clock notation method, the angle between the minute hand and the hour hand of a clock is 30 degrees at 11 o’clock. Here, the minute hand represents the high-voltage vector, and the hour hand represents the low-voltage vector.
31. Auto-transformer saves copper because:
A) Lower voltage
B) Shared winding
C) No leakage
D) No loss
Answer: B
Explanation: In an auto-transformer, different voltage levels are obtained by tapping a single common winding. This arrangement uses the same winding for both the primary and secondary windings. As a result, it saves a significant amount of winding material during the construction of the transformer.
32. Zero sequence current cannot flow in:
A) Star with neutral
B) Delta winding
C) Grounded star
D) Zig-zag
Answer: B
Explanation: Delta-connected winding forms a closed loop. However, it does not provide a common point like a star-connected winding. In a star connection, a neutral point is available. This neutral point can be connected to earth. In a delta connection, such a neutral point is not available. Therefore, the delta-connected winding cannot provide a path for zero-sequence current to flow.
33. Magnetizing current is:
A) In phase with voltage
B) 90° lagging
C) 90° leading
D) 180°
Answer: B
Explanation: Magnetizing current is a purely inductive current. Therefore, it lags the applied voltage across the winding by exactly 90 degrees.
34. Eddy current loss ∝:
A) f
B) f²
C) V
D) I
Answer: B
Explanation: The expression of eddy current loss is
Where is the eddy current constant, is the frequency, is the form factor, and is the maximum flux density of the core. If we go through the expression of eddy current loss, we find that it contains a term proportional to the square of frequency. Therefore, the eddy current loss is directly proportional to the square of the supply frequency.
35. Hysteresis loss ∝:
A) f
B) f²
C) I²
D) V²
Answer: A
Explanation: The expression of hysteresis loss is
Where is the hysteresis loss constant, is the frequency, and is the maximum flux density in the core. If we go through the expression of hysteresis loss, we find that it contains a term of frequency. Therefore, the hysteresis loss is directly proportional to the frequency.
36. Regulation is highest at:
A) Unity PF
B) Leading PF
C) Lagging PF
D) No load
Answer: C
Explanation: Voltage regulation of a transformer depends on the voltage drop in the transformer. The expression of voltage drop has two components: a resistive term and a reactive term. The voltage drop can be written as RIcosθ+XIsinθ, where R is the resistance and X is the reactance of the transformer. For a leading power factor, the value of sinθ is negative, and for unity power factor it becomes zero. Only for a lagging power factor does sinθ become positive. Therefore, the voltage drop becomes maximum for lagging power factor, and consequently the voltage regulation of the transformer is maximum at lagging power factor.
37. SC test voltage is about:
A) Rated voltage
B) 50%
C) 5–10%
D) 100%
Answer: C
Explanation: During the short-circuit test of a transformer, we normally short-circuit the LV winding. Then we apply voltage to the HV winding. The LV winding has almost zero external impedance. Therefore, the transformer can produce the rated current very easily. It requires only a small applied voltage on the HV side. This applied voltage is usually about 5 to 10 percent of the rated voltage of the transformer.
38. Core loss during SC test is:
A) Full
B) Half
C) Negligible
D) Maximum
Answer: C
Explanation: During the short-circuit test, the applied voltage is very small. It is usually about 5 to 10% of the rated voltage of the transformer. We know that core losses, such as hysteresis loss and eddy current loss, mainly depend on the applied voltage. In the short-circuit test, the applied voltage is very low. Therefore, the core loss becomes very small and almost negligible. For this reason, we perform the short-circuit test mainly to measure the copper loss (ohmic loss) of the transformer. We do not conduct this test to measure the core loss.
39. OC test gives:
A) Copper loss
B) Core loss
C) Leakage reactance
D) Efficiency directly
Answer: B
Explanation: During the open-circuit test, we apply the rated voltage to the LV winding of the transformer. We also measure the wattage taken by the transformer. As the other winding of the transformer remains open, there is no current in the open winding. Therefore, the entire wattage is mainly due to the core loss component of the transformer. Hence, by the open-circuit test, we mainly measure the core losses of the transformer.
40. Transformer tank is earthed to:
A) Increase efficiency
B) Reduce losses
C) Ensure safety
D) Improve cooling
Answer: C
Explanation: The transformer tank is made of steel or iron, which is a high conductive material. Due to the transformer action, a potential may be induced on the tank. To avoid this risk, we should always connect the transformer tank solidly to earth. This keeps the touch voltage within the safe limit even during fault conditions.