1. Potential transformers are used for measurement of _____voltages by means of _____ range voltmeters.

High, Low

Low, High

High, High

Low, Low

Answer.1. High, Low

Explanation:

Potential transformers are used for measurement of high voltages by means of low range voltmeters or for energizing the potential coils of wattmeters, energy meters. Measurement of voltage, power, etc., of high voltage lines, requires the high level of voltage to be stepped down before being applied to the measuring instrument.

2. The Primary MMF is least affected by the secondary terminal condition in a

Power Transformer

Potential Transformer

Current Transformer

Distribution Transformer

Answer.3. Current Transformer

Explanation:

In the current transformer, there is a few numbers of secondary turns therefore the effect on primary MMF is the least.

The secondary operating conditions for a current transformer are the opposite of those for a voltage transformer; the impedance must be as low as possible, to minimize the secondary voltage V2.

With a low impedance connected to the secondary, the magnetizing current I_{m} is small; the primary MMF is almost balanced by the secondary MMF, leaving the small MMF Im required to magnetize the core.

3. What is the effect of increasing secondary burden in potential transformer?

Increases in secondary current

Decreases in secondary current

Is constant

Decreases in primary voltage

Answer.1. Increases in secondary current

Explanation:

When the load is increased on the secondary side of the potential transformer, then the secondary current and the primary current also increase. This causes an increase in resistance and reactance drops in both primary and secondary windings.

4. Winding resistance of a potential transformer can be reduced by _________

Using thick conductors

Decreasing the length of the winding

Shorting the primary and secondary windings

Using thin conductors

Answer.1. Using thick conductors

Explanation:

The copper wire used for the windings in the potential transformer has some resistance and so, energy is lost due to heat produced in the wire (I^{2}R).

The energy loss is minimized by using thick conductors (i.e. low resistance coils) and taking care to reduce the length of mean turn of the windings.

5. Potential transformers are used to measure ________

Low voltages

High currents

High voltages

Low currents

Answer.3. High voltages

Explanation:

Potential transformers are used for measurement of high voltages by means of low range voltmeters or for energizing the potential coils of wattmeters, and energy meters. Measurement of voltage, power, etc., of high voltage lines, requires the high level of voltage to be stepped down before being applied to the measuring instrument.

6. In the case of a potential transformer (PT), the phase angle error is

Positive when the secondary winding voltage is reversed leads to the primary winding voltage

Negative when the secondary winding voltage is reversed leads to the primary winding voltage

Always positive

Always negative

Answer.1. Positive when the secondary winding voltage is reversed leads to the primary winding voltage

Explanation:

Phase angle error:

The error between the secondary terminal voltage is exactly in phase opposition with the primary terminal voltage.

The angle ′β′ between the primary system voltage Vp and the reversed secondary voltage vectors K_{T} × V_{s} is the phase error.

It will be positive when the secondary winding voltage is reversed leads to the primary winding voltage.

It will be negative when the secondary winding voltage reversed lags the primary winding voltage.

The ratio error is important when measurements of voltage are to be made and the phase angle error is important when the measurement of power.

This error depends upon the resistance, reactance of the winding, and no-load current of the transformer.

7. Resistance potential divider generally not used in voltage-controlled circuits due to

Non-linearity with variation

Lack of insulation

Distortion in waveform

High power losses

Answer.4. High power losses

Explanation:

A voltage divider (also known as a potential divider) is a passive linear circuit that produces an output voltage (Vout) that is a fraction of its input voltage (Vin). This method is not used in voltage-controlled circuits as it gives high power losses.

8. Induction Regulator is a

Current Transformer

Instrument Transformer

Autotransformer

Potential Transformer

Answer.4. Potential Transformer

Explanation:

The induction regulator is a special type of potential transformer with primary winding mounted on a cylindrical core that may be turned on its axis.

The secondary winding is stationary. Both windings are mounted in slots, much as are the windings of an induction motor. A small air gap separates the stator from the rotor.

Regulators of this type are used to regulate the voltage on single-phase feeder circuits.

Voltage drop in the feeder is compensated for by the voltage induced in the secondary of the regulator.

The maximum voltage induced in the secondary can be made to add directly or subtract directly from the line voltage by rotating the primary through an angle of 180°, the regulator is capable of boosting or bucking the line voltage; in the 90° position, and no voltage is induced in the secondary.

9. With increase in PT secondary burden the secondary voltage __________

Increases

Decreases

Remains constant

Depends on the supply

Answer.2. Decreases

Explanation:

With the increase in PT secondary burden, the secondary current is increased. This in turn will increase the primary current as well.

Both primary and secondary voltage drops are increased and hence, for a given value of the primary supply voltage V_{P}, secondary terminal voltage V_{S} is reduced with an increase of burden. The effect is therefore to increase the actual transformation ratio V_{P}/V_{S} with a resulting increase in the ratio error.

10. Leakage reactance in potential transformer is minimised by _________

Using thin conductors

Reducing leakage flux

Increasing flux density

Shorting the windings

Answer.3. Increasing flux density

Explanation:

The leakage reactance in the potential transformer can be minimized by keeping the primary and secondary windings as close as possible; however, there should be sufficient space to meet out the insulation requirements.

The objective while designing a PT should be to keep the operational flux density in the core as high.

Winding leakage flux and hence leakage reactance can be reduced by keeping the primary and secondary windings as close as permissible from the point of view of insulation requirements.