1. _______ used as standard wattmeter for the purpose of calibration in various testing laboratories.
Suspended coil torsion head wattmeters
Pivoted coil direct indicating wattmeters
Ferrodynamic wattmeters
Low power factor wattmeters
Answer.1. Suspended coil torsion head wattmeters
Explanation:
Suspended coil torsion head wattmeters instruments are mainly used as standard wattmeters for the purpose of calibration in various testing laboratories.
In these instruments, the moving coil acts as a voltage coil that is suspended from a torsion head by a metallic strip which acts as a lead to the coil. This moving coil is completely surrounded by the fixed coils which serve as the current coil and the winding is so designed that the effect of the stray magnetic field is totally avoided.
2. To reduce the eddy current errors in suspended coil torsion head wattmeters _______ is used.
Stranded Conductors
Non-metallic parts
Both 1 and 2
Metallic Part
Answer.3. Both 1 and 2
Explanation:
To reduce the eddy current errors in suspended coil torsion head wattmeters, stranded conductors are used in the winding of fixed coils, and non-metallic parts are used within the region of the magnetic field of the instrument.
To make the instrument highly sensitive and accurate, pivot friction is avoided. The error due to mutual inductance can be completely eliminated by making the mutual inductance between fixed coils and moving coil zero.
3. In Pivoted coil direct indicating wattmeters instruments, the fixed coil is wound in two halves and connected in ______.
Series
Series Parallel
Parallel
Perpendicular
Answer.3. Parallel
Explanation:
In Pivoted coil direct indicating wattmeters, the fixed coil is wound in two halves and connected in parallel and so designed that the field produced is uniform.
The moving coil is pivoted centrally and so wound that no portion of the moving coil is projected outside the field coils at its maximum deflected position. The instrument is spring-controlled and two control springs are provided for this purpose.
4. ______ is acts as a lead in Pivoted coil direct indicating wattmeters.
Pivot
Spring
Damping Vane
Magnet
Answer.2. Spring
Explanation:
In Pivoted coil direct indicating wattmeters, the fixed coil is wound in two halves and connected in parallel and so designed that the field produced is uniform.
The instrument is spring-controlled and two control springs are provided for this purpose.
These springs are also used to carry the current to the moving coil and therefore, act as leads.
A damping vane is attached to the moving system and it moves in a sector-shaped box to provide the necessary damping.
5. When Iron core is used inside the wattmeter then such instrument is called as ________
Suspended coil torsion head wattmeters
Pivoted coil direct indicating wattmeters
Ferrodynamic wattmeters
Low power factor wattmeters
Answer.3. Ferrodynamic wattmeters
Explanation:
The operating torque in air-cored dynamometers is usually small. To increase the operating torque, iron cores are used inside the coils and such wattmeters are called ferrodynamic wattmeters.
6. The purpose of the iron core in ferrodynamic wattmeters is to ______
Increase current
Increase Torque
Decrease loading effect
Decrease Current
Answer.2. Increase Torque
Explanation:
The operating torque in air-cored dynamometers is usually small. To increase the operating torque, iron cores are used inside the coils and such wattmeters are called ferrodynamic wattmeters.
In ferrodynamic wattmeters, cores of low loss iron are used which causes the flux density to increase thereby increasing the operating torque considerably at the expense of slight loss inaccuracy.
7. In ferrodynamic wattmeters _______ coil is supplied with the voltage.
Fixed coil
Moving Coil
Either Fixed and Moving Coil
None of the above
Answer.2. Moving Coil
Explanation:
In ferrodynamic wattmeters, the fixed coil is wound on a laminated core and so shaped that a uniform radial field is obtained throughout the air gap.
The moving coil is pivoted asymmetrically and placed over a hook-shaped pole piece.
However, this causes the moving coil to creep over the hook when the meter is on no load and only the moving coil is supplied with the voltage since there is a tendency of the moving coil to take up such a position on the core where it can link with the maximum amount of flux.
8. The creep error in ferrodynamic wattmeters is reduced by using _____
Compensating Coil
Inductive coil
Resistive coil
Any of the above
Answer.1. Compensating Coil
Explanation:
In ferrodynamic wattmeters, the fixed coil is wound on a laminated core and so shaped that a uniform radial field is obtained throughout the air gap.
The moving coil is pivoted asymmetrically and placed over a hook-shaped pole piece.
However, this causes the moving coil to creep over the hook when the meter is on no load.
This creep causes an error which is compensated by using a compensating coil.
9. The scale range of the ferrodynamic wattmeter is upto
100 Degree
150 Degree
200 Degree
270 Degree
Answer.4. 270 Degree
Explanation:
Ferrodynamic wattmeters are more robust in construction and also less sensitive to stray magnetic fields. A long scale up to about 270° and a large deflecting torque almost proportional to average power can be obtained with the help of this type of construction.
10. Electrostatic wattmeter is used for the measurement of ______
High AC Power
High DC Power
Dielectric Losses
Frequency
Answer.3. Dielectric Losses
Explanation:
The electrostatic wattmeter is basically a laboratory instrument and not a commercial instrument like a dynamometer and induction wattmeter.
It is mainly used for the measurement of small amounts of power when the voltage is high and the power factor is low.
It is also used for the measurement of dielectric loss in cables.
