# Transformer MCQ

Qus1.  The secondary winding of which of the following transformers is always kept closed?

1. Current transformer
2. Voltage transformer
3. Power transformer
4. Step down transformer

Explanation.

The secondary winding of current transformers is always kept closed. If the current transformer secondary is not shorted when unused and kept open then it can develop a very high voltage across secondary which may damage transformer insulation.

Detail Explanation:-

Instrument transformers are used in conjunction with an ammeter and voltmeter to extend the range of meters. In dc circuit shunt and multipliers are used to extend the range of measuring instruments. The shunt is used to extend the range of the ammeter whereas the multiplier is used to extend the range of voltmeters

This type of ammeter is shown in Figure. The primary of the transformer is connected in series with the load, and the ammeter is connected to the secondary of the transformer. Notice that the range of the meter is changed by selecting different taps on the secondary of the current transformer. The different taps on the transformer provide different turn-ratios between the primary and secondary of the transformer. The working of the current transformer is explained in detail.

• The current transformer is used to step down the current to a lower value so that the current can be measured with a normal range ammeter.
• The current transformer has a primary coil of one or more turns of thick wire having high cross-sectional area and it is connected in series.
• The primary of the current transformer is connected to the load or feeder while the secondary of the current transformer is connected to an ammeter.
• The impedance of the primary is very low, and the currents very high. The primary current is dependent on the load on the line rather than the load on the secondary circuit.
• The current drawn by the secondary has little effect on the line current.
• The secondary of the transformer contains many turns of fine wire having a smaller cross-section area and has much higher impedance. If the secondary is not loaded, this transformer acts to step up the voltage to a dangerous level, due to the high turns ratio. Because of this, a current transformer should always have a short-circuit placed across its secondary winding when connecting or removing any device from its output. By heavily loading the secondary, the high voltage is reduced to the safe level.
• The nominal current rating of the secondary winding of the CT is 5A to 1 A.
• To illustrate the operation of a current transformer, assume that the current ratio of the primary winding is 100 A. The secondary winding has a standard rating of 5A.
• The primary winding consists of three turns of wire, and the secondary winding consists of 60 turns.
• The ratio between the primary and the secondary currents is 100 A/5 A, or 20:1
• In other words, the primary current is 20 times greater than the secondary current.
• Note that the number of turns and the current in the primary and secondary windings are related by an inverse proportion. i.e I1 / I2 = N2 / N1.
• By increasing the number of secondary windings, N2, the secondary current can be made much smaller than the current in the primary circuit being measured. In other words, as N2 increases, I2 goes down by a proportional amount.

Qus2.  If the supply frequency of a transformer increases, the secondary output voltage of the transformer

1. Increase
2. Decrease
3. Remain the same
4. Any of the above

Explanation:

The EMF Equation Of A Transformer is given by

$E = 4.44fBNA$

where

E = the voltage in the winding either primary or secondary,
f = frequency
B = flux density in the core
N = Number of turns on the winding
A = Cross area of the core

A transformer is designed for some constant parameters like frequency. So if the frequency increases, the secondary voltage or emf increases. But with high frequency, there is an increase in transformer losses like core loss and conductor skin effect. Also with high frequency, the magnetizing current becomes low and with low frequency the magnetizing current becomes high.

The higher the input frequency, the higher will be the rate of change of magnetic flux, which results in higher induced EMF

Frequency ~ Rate of Change of Magnetic Flux ~ Induced EMF

Note:-  There are some other conditions which should be kept in mind

Condition 1:- If you are maintaining the same voltage but higher frequency, flux ( V/f ) in the transformer falls, the induced emf would hence remain the same and would not increase. Although the rate of change of flux increases due to increased frequency but the value of flux is reduced and hence the overall effect is that voltage induced in the secondary remains the same but at a higher frequency of course.

Condition 2:- If you are increasing primary voltage along with increasing frequency so as to maintain constant flux, only in that case the secondary voltage will increase. Therefore you can safely conclude that secondary voltage depends only upon the primary voltage and turns ratio.

Qus3. Power transformers are designed to have maximum efficiency at

Explanation:

Transformers of the large size used in generating stations at the sending end of the transmission line to step up the voltage and at the receiving end of the transmission line to step down the voltage is known as power transformers. A number of such transformers are connected in parallel. They are operated up to full-load capacity by connecting or disconnecting transformers depending upon the load condition. Power transformers are mostly used near full load conditions and hence designed for maximum efficiency at or near full load.

Power transformers are used for transmission as a step-up device hence they are not directly connected to consumers therefore, load fluctuation is very less. So the power transformer can operate on full load.

Distribution transformers are comparatively smaller transformers of rating of the order of hundreds of KVA and are connected directly to supply the load at 400/230 volts. Thus, the secondary side of a distribution transformer is directly connected to the load. They are to be kept in operation all the time for all the days irrespective of whether the consumer is utilizing the power or not. Thus, the load on a distribution transformer varies throughout the day depending upon how the consumers utilize the load. The average load on a distribution transformer is much less than its rated capacity. That is why they are rated to have maximum efficiency at a load lower than their full-load capacity. They are designed to have good all-day efficiency rather than the highest efficiency at or near full-load.

Qus4. The open-circuit test in a transformer is used to measure

1. Copper loss
2. Winding loss
3. Total loss
4. Core loss

Explanation:

The purpose of the open-circuit test’ is to determine the excitation admittance of the transformer- equivalent circuit, the core loss, the no-load excitation current, and the no-load power factor.

The figure shows the connections for the open-circuit test. The low voltage winding is supplied with the rated voltage which should result in the rated voltage on the high voltage side. The current drawn should be low enough so that the copper losses are very low and the power measured 1s almost all from the core losses. The current drawn during the open Circuit test will be the excitation current.

• In the open-circuit test, the transformer load terminal is kept open. Open circuit test is also known as the no-load test.
• The current drawn by shunt parameters is a no-load current a very small current. Therefore the current that will flow in the circuit in the open circuit test is very low so the measurement of the quantities voltage, current, and power must be on the low voltage side so that the corresponding value will be readable in the instruments. And therefore, the open circuit test must be performed on the low voltage side. This means the high voltage side must be kept open and for the measurement of power, voltage and current on the low voltage side the wattmeter, voltmeter and ammeter must be connected
• We know that as the output terminal is open the parameters that are in the shunt can be found out by this test. Since the shunt circuit has the core parameter so we can say that the open circuit test gives the core parameter.
• The open-circuit test on the transformer is performed to determine magnetizing reactance and equivalent resistance due to iron loss.
• As the normal rated voltage is applied to the primary, therefore, normal iron losses will occur in the transformer core. Hence, a wattmeter will record the iron losses and small copper loss in the primary. Since the no-load current is very small (2 to 5% of rated current), copper losses in the primary under no-load conditions are negligible as compared with iron losses. Hence, wattmeter reading practically gives the iron losses in the transformer.

Qus5. The leakage flux in a transformer depends upon the value of

1. Frequency
2. Mutual Flux
4. Applied Voltage

Explanation:

Leakage flux:- The flux that escapes from the core and flux that passes through one winding only.

• The flux that links with the primary, but not with the secondary, is known as primary leakage flux, while that which links with the secondary, but not the primary- winding is called the secondary leakage flux. The value of leakage flux is proportional to the load on the transformer.
• Since each leakage flux is linked with one winding only, it induces back e.m.f. in that winding, which opposes the current flow in the winding. The greater the leakage flux, the greater the voltage drop. Good transformer design aims to reduce the leakage flux to a low level.
• The voltage drop caused by leakage flux is proportional to the load current. The greater the load current, the greater the magnitudes of both the primary and secondary ampere-turns, and hence the greater the respective leakage fluxes in both primary and secondary windings.
• Although leakage flux has an adverse effect on the transformer output voltage, it proves an asset under severe short-circuit conditions; the large voltage drop caused by the intense leakage flux limits the current to a lower value than would otherwise occur if no leakage were present and thus helps to avoid damage to the transformer.
• In a Transformer, Core flux is the difference between primary flux and Secondary flux which are opposite to each other in direction.
• When Current increases due to increased load (and voltage remains the same). Then both primary and secondary flux increase. Because both of them increase, so their difference remains the same. And all remaining flux is forced out. Hence leakage flux increases with current, but Core flux remains constant.

Qus6. Lamination of the transformer core is made of

1. Cast Iron
2. Silicon Steel
3. Aluminum
4. Cast Steel

Explanation:

Silicon steels are used for electrical transformer cores and cores of other electrical devices for the following reasons:-

1. Low hysteresis loss.
2. High permeability.
3. Virtually eliminated aging.
4. High resistance.

The function of a transformer’s core is to provide a low-reluctance/high permeability magnetic circuit by which magnetic flux, created by the magnetomotive force in the primary winding, can efficiently couple with the secondary winding.

Transformer cores are manufactured from a silicon-iron alloy (iron with around 3% silicon content), which is more generally termed ‘silicon steel, ‘electrical steel’, ‘transformer steel’, or by some trade name such as ‘Stalloy’. This ferromagnetic alloy is specifically designed to have all of the characteristics described.

⇒ Low-reluctance/high permeability magnetic circuit, in order to maximize the flux density within the core.

⇒ Low-remanence/low-coercivity/low-area hysteresis loop: To minimize the energy loss per magnetization/demagnetization cycle; this energy loss being termed its ‘hysteresis loss’

⇒ High resistivity conducting path: To minimize any ‘eddy currents’ (circulating currents) resulting from undesirable voltages induced into the core; eddy currents are further minimized by manufacturing the core from laminations.

The silicon content of the iron acts to both reduce the core’s hysteresis losses and to increase its resistivity by a factor of around 4.5, which acts to reduces the magnitude of any circulating currents (eddy currents) that result from voltages induced into the core.

There are two general categories of silicon-steel, termed ‘grain orientated steel‘ and ‘non-orientated steel‘:

⇒ Grain-orientated steel has a silicon content of around 3% and is manufactured in such a way that its magnetic properties are optimized along the direction of its grain, enabling its flux density to be increased by as much as 20% in that direction.

⇒ Non-orientated steel, with a silicon content of 2-3.5%, has a randomly orientated grain with similar magnetic properties in all directions, but the resulting flux density can be significantly lower than for grain-orientated steel.

From the point of view of the varying flux, the core of a transformer is simply another winding and, therefore, it will induce a voltage into it. If the core was manufactured from a solid piece of silicon steel, then it would behave like a heavy, short-circuited, single winding, and a large current would circulate around it.

In eddy current, the magnetic field that induces a voltage in the secondary of a transformer also induces a voltage in the core. This causes circulating (or eddy currents) in the core. Making the laminations as thin as possible reduces this loss. The laminations are coated with a thin insulating material to prevent current from flowing between them.

Qus7.  Breather is provided in a transformer to

1. Absorb moisture of air during breathing
2. provide cold air in the transformer
3. The filter of transformer oil
4. None of above

Answer 1. Absorb moisture of air during breathing.

Explanation:

Transformer oil should not be exposed directly to the atmosphere because it may absorb moisture and dust from the environment and may lose its electrical properties in a very short time. To avoid this problem a breather is provided on the top of the conservator.

Breather is the heart of the transformer, which is similar to the human heart. The breather transports fresh air in and out of the transformer. This component is required to maintain the cooling-medium level in the conservator.

Breather mainly consists of a silica gel. The silica gel absorbs the moisture content of air so that oil contamination can be prevented. The silica gel which is blue in color turns pink when it absorbs moisture fully. it is replaced periodically as routine maintenance.

Qus8. Which of the following losses varies with the load in the transformer?

1. Core loss
2. Copper loss
3. Both core & copper loss
4. None of the above

Explanation:

• Copper loss is due to the ohmic resistance of the transformer windings.
• The copper loss for the primary winding is I12 × R1 and for secondary winding is I22 × R2.
• Where I1 and I2 are current in primary and secondary winding respectively.
• R1 and R2 are the resistances of primary and secondary winding respectively.
• It is clear that Cu loss is proportional to the square of the current, and current depends on the load. Hence copper loss in the transformer varies with the load.

Qus9. A transformer transform

1. Current
2. Voltage & current
3. Frequency
4. Voltage

Explanation:

A transformer is a static device that is used to convert the voltage or current to a higher (step-up) or lower (step down) level.

The Distribution Transformer, Auto Transformer, and Tap changing Transformer basically belong to the family of Power Transformer with respective unique features.

In an Instrument Transformer, voltage or current on one side of the transformer, comparatively high (therefore not suitable for direct measurement), is scaled down to voltage or current to levels that is convenient for measurement with suitable instrumentation. Accordingly, the two types of Instrument Transformers are the Current Transformer and the Potential Transformer. Some applications may call for constant current (as in welding) or constant voltage. Such transformers am called Constant Current and Constant Voltage Transformers.

Qus10.  Transformer core are laminated in order to

1. Reduce hysteresis loss
2. Reduce hysteresis & eddy current loss
3. Minimize eddy current loss
4. Copper loss

Answer 3. Minimize Eddy current loss

Explanation:

• In a transformer, the eddy current loss is proportional to the square of the diameter of the core.
• Larger the diameter, the more the eddy current loss.
• In eddy current, the magnetic field that induces a voltage in the secondary of a transformer also induces a voltage in the core. This causes circulating (or eddy currents) in the core.
• Making the laminations as thin as possible reduces this loss.
• The laminations are coated with a thin insulating material to prevent current from flowing between them.
• Hence the transformer core is laminated so that the net effective diameter of the transformer core reduces and thus eddy current loss can be minimized.

High resistivity conducting path: To minimize any ‘eddy currents’ (circulating currents) resulting from undesirable voltages induced into the core; eddy currents are further minimized by manufacturing the core from laminations.

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