# Wattmeter MCQ || Wattmeter Questions and Answers

1. A wattmeter is used to measure _____

1. Active Power
2. Reactive Power
3. Apparent Power
4. Any of the above

Explanation:

1. The instrument most commonly used for active power measurement is the wattmeter.
2. It is essentially an inherent combination of an ammeter and a voltmeter and, therefore, consists of two coils known as the current coil and pressure coil.
3. The current coil is inserted in series with the line carrying the current to be measured and the pressure coil in series with a high non-inductive resistance connected across the load or supply terminals.

2. An induction wattmeter can be used for

1. Both DC and AC
2. DC only
3. AC only
4. None of these

Explanation:

The induction type wattmeter can be used to measure a.c. power only in contrast to dynamometer wattmeter which can be used to measure d.c. as well as a.c. power.

The principle of operation of an induction wattmeter is the same as that of an induction ammeter and voltmeter ie. the induction principle.

3. For balanced lagging power factor, according to two wattmeter method,

W1 = VL IL cos (30 – ϕ)

W2 = VL IL cos (30 + ϕ)

The total reactive volt-amperes Q is given by

1. Q = 0
2. Q = √3VLILsinφ
3. Q = 1
4. Q = VLILsin ϕ

Explanation:

In a two-wattmeter method,

The reading of first wattmeter (W1) = VL IL cos (30 – ϕ)

The reading of second wattmeter (W2) = VL IL cos (30 + ϕ)

Total power in the circuit (P) = W1 + W2 = √3 V­­LIL cos ϕ

Total reactive power in the circuit

Q = √3(W1 − W2) = √3VLILsinφ

4. Wattmeter cannot be designed on the principle of

1. Electrostatic instrument
2. Thermocouple instrument
3. Moving iron instrument
4. Electrodynamic instrument

Explanation:

We can use electrostatic instruments to measure voltage and power. These types of wattmeters are used for the measurement of a small amount of power, practically when the voltage is high and the power factor is low.

It is also used for measurement of dielectric loss of cables on alternating voltage and for calibration of wattmeter and energy meters.

Thermocouple instruments are used to measure current, voltage, and power.

We can use the moving iron instrument to measure current and voltage only. It can’t be used to measure power.

Electrodynamic instruments are the most preferred instruments for measurement.

5. A 3-phase balanced supply system is connected to a 3-phase unbalanced load. Power supplied to this load can be measured using:

1. Two wattmeters
2. One wattmeter
3. Three wattmeters
4. Both 1 and 3

Explanation:

For 3ϕ – 4 wire → neutral is present

2) As per Blondel’s theorem, if neutral is not present for ‘n’ wire system, then the ‘n-1’ number of wattmeter’s are required for ‘n’ wire system.

 Type of Load Star 3 – ϕ 3 – wire Star 3 – ϕ 4 – wire Delta 3 – ϕ 3 – wire Balanced 2 1, 2, 3 2 Unbalanced 2 2, 3 2

• The one-wattmeter method is used for the measurement of three-phase power in case balanced loads only
• It is used for both star and delta connected loads
• Two wattmeter method and three wattmeter methods are used for both balanced and unbalanced loads
• Two wattmeter method is best suitable for the three-phase three-wire system and the three wattmeter method is suitable for the three-phase four-wire system

Note: If neutral is available we shouldn’t use a 2- wattmeter method.

6. Which of the following are types of wattmeter?

1. Dynamometer wattmeter
2. Induction Wattmeter
3. Both 1 and 2
4. None of the above

Explanation:

There are two principal types of wattmeters

1. Dynamometer wattmeter
2. Induction wattmeter

Dynamometer Wattmeter:- A dynamometer wattmeter is almost universally used for the measurement of d.c. as well 3 as a.c. power. It works on the dynamometer principle i.e. mechanical force exists between two current-carrying conductors or coils.

Induction Wattmeters:-  The induction type wattmeter can be used to measure a.c. power only in contrast to dynamometer wattmeter which can be used to measure d.c. as well as a.c. power. The principle of operation of an induction wattmeter is the same as that of an induction ammeter and voltmeter i.e. induction principle.

7. Two wattmeters are used to measure the power in a 3-phase balanced system. What is the power factor of the load when one wattmeter reads twice the other?

1. 0
2. 0.5
3. 0.866
4. 1

Explanation:

Given that, W1 = 2 W2

Power factor = cos ϕ

$\phi = {\rm{ta}}{{\rm{n}}^{ – 1}}\left( {\frac{{\sqrt 3 \left( {{W_1} – {W_2}} \right)}}{{{W_1} + {W_2}}}} \right)$

$\tan \phi = \frac{{\sqrt 3 \left( {2{W_2} – {W_2}} \right)}}{{\left( {2{{\rm{W}}_2} + {W_2}} \right)}} = \frac{1}{{\sqrt 3 }}$

⇒ ϕ = 30°

Power factor = cos 30° = 0.866

8. In a compensated wattmeter, the readings are corrected for errors due to

1. Power consumed in current coil
2. Variations in voltage and frequency
3. Friction
4. Power consumed in potential coil

Answer.4. Power consumed in potential coil

Explanation:

In the compensated wattmeter, a compensating coil having the same number of turns as the current coil is wound on the current coil and is connected in series with the potential coil.

The current in the compensating coil flows in a direction opposite to that of the current in the current coil, and therefore its effect on the wattmeter field is nullified.

In a compensated wattmeter, the readings are corrected for errors due to  Power consumed in the potential coil.

When the potential-coil current flows through the current coil, the wattmeter so connected will read too high by the amount of the power lost in the potential coil.

When the power factor of the load is especially low, the wattmeter reading will be relatively low, and the error introduced by the potential-coil power may be too great to be ignored. In this case, a compensated wattmeter is convenient.

9. Z1 and Z2 are connected in series to form a load. A wattmeter’s current coil is connected in series with the load, whereas its pressure coil is connected across Z2. The wattmeter reads:

1. Zero always
2. Power consumed by Z1 and Z2
3. Power consumed by Z1
4. Power consumed by Z2

Explanation:

Wattmeter reads the power and it is given by

P = VPC ICC cos ϕ

VPC is the voltage across the pressure coil

ICC is current flows through the current coil

ϕ is the phase angle between VPC and ICC

Application:

The circuit representation of the given question is shown below.

• The potential coil is connected across Z2.
• It reads the voltage across Z2 only.
• So, Wattmeter reads only power consumed by Z2.

10. Poynting-vector wattmeter works on

1. Magnetic effect
2. Hall effect
3. Induction effect
4. Heating effect