Electrical Measurements & Measuring Instruments Multiple Choice Questions on “Friction and Overload Compensation”.
1. Energy meter reads correctly when the ___________
a) torque is small
b) torque is large
c) torque is medium
d) torque is zero
Answer: a
Clarification: The reading in an energy meter is obtained correctly when the torque value is small at low loads. It is independent of the load on the meter an acts in the same direction as the driving torque.
2. Small torque for energy meter is provided __________
a) by a supply
b) by a shading loop
c) by unshaded loop
d) by a transformer
Answer: b
Clarification: Friction can be compensated in an energy meter by making use of a shading loop. It is placed between the central limb of the shunt magnet and a disc. Figure below illustrates the same.
3. Driving torque is small and is adjustable.
a) True
b) False
Answer: a
Clarification: Interaction between the parts of the shaded and unshaded fluxes, obtained through a shading loop leads to a small driving torque. The value of the torque can be adjusted through lateral movement of the loop.
4. Friction torque is eliminated by _________
a) using lubricating oil
b) by suspending the components in air
c) by adjusting the position of limb
d) by using steel alloy components
Answer: c
Clarification: We can eliminate the friction torque completely by adjusting the position of the shading loop. This enables in providing compensation for the frictional torque.
5. Frictional errors are dominant in an energy meter.
a) True
b) False
Answer: a
Clarification: Frictional errors exist in an energy meter at the top as well as bottom surfaces even at low value of loads. Even when the disc is rotating slowly errors due to friction exist in an energy meter.
6. At full load, disc __________
a) partially revolves and then stops
b) continuously revolves
c) does not revolve at all
d) revolves in an alternating fashion
Answer: b
Clarification: When an energy meter is operated in full load condition, disc revolves continuously due to the field of the series magnet. As a result, an e.m.f is induced in the disc.
7. Self braking torque is _________
a) proportional to cube of load current
b) proportional to load current
c) proportional to square of load current
d) proportional to reciprocal of load current
Answer: c
Clarification: In an energy meter, the self braking torque is dependent on the square of the load current. As a result the disc rotates at a slightly slower speed at high value of loads.
8. Self braking action is minimised by _________
a) maintaining high speed for disc
b) maintaining medium speed for disc
c) keeping the disc at rest
d) maintaining low speed for disc
Answer: d
Clarification: In an energy meter we can minimise or eliminate the self braking action by keeping the disc speed as low as possible at full load condition. This is achieved by maintaining the flux due to the current coil smaller than that due to the shunt coil. The figure below shows the compensating device.
