# Induction Wattmeter MCQ || Induction Type Wattmeter Questions and Answers

1. Induction-type wattmeter instruments are used for _______

1. AC Measurement
2. DC Measurement
3. AC and DC Measurement
4. None of the above

Explanation:

The induction type wattmeter can be used to measure AC power only. The working of induction type wattmeter is based on the principle of electromagnetic induction. The induction wattmeter consists of two laminated electromagnets viz. Shunt Magnet and Series Magnet.

2. The two laminated iron-core electromagnets are _____

1. Shunt Magnets
2. Series Magnets
3. Shunt and Series Magnets
4. Bar Magnets

Explanation:

The induction type wattmeter can be used to measure AC power only. The working of induction type wattmeter is based on the principle of electromagnetic induction. The induction wattmeter consists of two laminated electromagnets viz. Shunt Magnet and Series Magnet.

The shunt magnet is excited by the current, proportional to the voltage across the load. flowing through the pressure coil and the series magnet is excited by the load current or a definite fraction of it flowing through the current coil.

3. The disc of Induction type wattmeter is made up of _____

1. Copper
2. Aluminum
3. Either copper and Aluminium
4. Steel

Explanation:

A thin disc made of copper or aluminum pivoted at the center is placed between the shunt and series magnets so that it cuts the flux from both of the magnets. The deflecting torque is produced by the interaction of eddy currents induced in the discs and the flux inducing it.

In order to cause the resultant flux in the shunt magnet to lag in phase by exactly 90° behind the applied voltage. one or more copper rings, known as copper-shading bands are provided on one limb of the shunt magnet.

4. The deflecting torque in shunt and series magnets is produced due to _____

1. Hysteresis Current
2. Eddy Current
3. Electromagnetic Current
4. Both Eddy and Hysteresis current

Explanation:

A thin disc made of copper or aluminum pivoted at the center is placed between the shunt and series magnets so that it cuts the flux from both of the magnets. The deflecting torque is produced by the interaction of eddy currents induced in the discs and the flux inducing it.

In order to cause the resultant flux in the shunt magnet to lag in phase by exactly 90° behind the applied voltage. one or more copper rings, known as copper-shading bands are provided on one limb of the shunt magnet.

5. The series magnet in induction type wattmeter is excited by the _______

1. Supply Current
4. Supply Voltage

Explanation:

• Series magnet in induction type wattmeter consists of a number of U-shaped laminations of silicon steel staggered together to form a core.
• A coil of thick wire having a few turns is wound on both the legs of a U-shaped magnet. This coil is connected in series with the load. Thus, it is excited by the circuit current or load current and is known as the current coil.
• This magnet is placed below the aluminum disc and produces the magnetic field φse proportional to and in phase with line current.

6. The shunt magnet in induction type wattmeter is excited by the _______

1. Supply Current
4. Current proportional to supply voltage

Answer.4. Current proportional to supply voltage

Explanation:

• Shunt magnet in induction type wattmeter consists of a number of M-shaped laminations of silicon steel assembled together to form a core.
• A coil of thin wire has a large number of turns in the wound on the central limb of the magnet.
• This coil is connected across the load. Thus, it is excited by the current proportional to the supply voltage and is known as a potential or pressure coil.
• This magnet is placed above the aluminum disc.

7. The series magnets in the Induction type wattmeter is wound with

1. Thick wire having Few turn
2. Thin wire having few turn
3. Thick wire having large turn
4. Thin wire having large turns

Answer.1. Thick wire having Few turn

Explanation:

• Series magnet in induction type wattmeter consists of a number of U-shaped laminations of silicon steel staggered together to form a core.
• A coil of thick wire having a few turns is wound on both the legs of a U-shaped magnet. This coil is connected in series with the load. Thus, it is excited by the circuit current or load current and is known as the current coil.
• This magnet is placed below the aluminum disc and produces the magnetic field φse proportional to and in phase with line current.

8. The shunt magnet in the Induction type wattmeter is wound with

1. Thick wire having Few turn
2. Thin wire having few turn
3. Thick wire having large turn
4. Thin wire having large turns

Answer.4. Thin wire having large turns

Explanation:

• Shunt magnet in induction type wattmeter consists of a number of M-shaped laminations of silicon steel assembled together to form a core.
• A coil of thin wire has a large number of turns in the wound on the central limb of the magnet.
• This coil is connected across the load. Thus, it is excited by the current proportional to the supply voltage and is known as a potential or pressure coil.
• This magnet is placed above the aluminum disc.

9. In induction type wattmeter the current in the pressure coil must _____ the supply voltage by ______.

2. Lag, 45°
3. Lag, 90°

Explanation:

• In order to obtain deflecting torque in induction type wattmeter, the current in the pressure coil must lag behind the supply voltage by 90°.
• For this, a copper shading band (short-circuiting copper ring) is provided on the central limb of the shunt magnet. The phase difference of 90° is obtained by adjusting the position of this shading band.
• The shading band acts as short-circuited transformer secondary.
• Since its resistance is negligibly small as compared to its inductance, and therefore, current circulating in the shading band lags behind the supply voltage nearly by 90°.

10. The type of damping used in induction type energy meter is

1. Fluid friction damping
2. Air friction Damping
3. Electromagnetic Damping
4. Eddy current Damping

Explanation:

• The eddy current damping is used in the induction-type energy meter.
• A permanent magnet positioned near the edge of the aluminum disc is used to provide necessary damping.
• When aluminium disc moves in the field of the permanent magnet, flux is cut and eddy currents are induced in the disc.
• The direction of induced currents is such that it opposes the rotation (Lenz’s Law), and thus, damping torque is produced.
• Since the induced currents are proportional to the speed of the disc (N), the damping torque is proportional to the disc speed.

11. The shading band in induction type wattmeter is used to obtain _____

1. Deflecting Torque
2. Phase Difference
3. Phase Difference and Deflecting Torque
4. Extra Power

Answer.3. Phase Difference and Deflecting Torque

Explanation:

• In order to obtain deflecting torque in induction type wattmeter, the current in the pressure coil must lag behind the supply voltage by 90°.
• For this, a copper shading band (short-circuiting copper ring) is provided on the central limb of the shunt magnet. The phase difference of 90° is obtained by adjusting the position of this shading band.
• The shading band acts as short-circuited transformer secondary.
• Since its resistance is negligibly small as compared to its inductance, and therefore, current circulating in the shading band lags behind the supply voltage nearly by 90°.

12. The current transformer is used in conjunction with induction type wattmeter for the current level above _____

1. 50 A
2. 100 A
3. 10 A
4. 30 A

Explanation:

In induction type wattmeter the currents upto about 100 A can be dealt with directly in such instruments. For currents above this, current transformers are used in conjunction with the wattmeter.

13. What are the advantages of Induction type wattmeter?

1. Good damping
2. Uniform scale
3. Both 1 and 2
4. Low power consumption

Explanation:

Advantages of Induction Type Wattmeter :

When compared with dynamometer wattmeters. these instruments have the advantages of greater working torque and length of the scale.

• The scale is uniform.
• They provide good damping.
• There is no effect of stray fields.

14. The voltage transformer is used in conjunction with induction type wattmeter for the voltage level above _____

1. 750 V
2. 200 V
3. 1000 V
4. 500 V

Explanation:

In induction type wattmeter the voltage level upto about 750 V can be dealt with directly in such instruments. For voltage above this, potential transformers are used in conjunction with the wattmeter.

15. What are the main disadvantages of Induction type wattmeter?

1. High power consumption
2. Low accuracy
3. Temperature effect
4. All of the above

Explanation:

When compared with dynamometer wattmeters. these instruments have certain inherent disadvantages. For example. they have less accuracy, the greater weight of the moving system. greater power consumption and they can be used only for the measurement of power on a.c. circuits. Since induction instruments are highly susceptible to variation in temperature and frequency. therefore induction wattmeters have industrial accuracy only at a stated frequency and temperature.

• Can be used only for ac power measurements.
• Low accuracy due to heavy moving system.
• Temperature changes can affect the readings by introducing errors.
• Power consumption is more.

16. Induction-type wattmeters can be used only with circuits having