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PMMC instruments are unsuitable for a.c measurements because the torque reverses with the current, preventing the pointer from following rapid reversals, resulting in zero deflection. The basic principle of PMMC instruments is that a current-carrying coil in a magnetic field experiences a force proportional to the current. Sources of error in DC voltage measurement include friction in the moving system, heat affecting resistance, and aging of the magnet and control spring.

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5 views1 page

Devices

PMMC instruments are unsuitable for a.c measurements because the torque reverses with the current, preventing the pointer from following rapid reversals, resulting in zero deflection. The basic principle of PMMC instruments is that a current-carrying coil in a magnetic field experiences a force proportional to the current. Sources of error in DC voltage measurement include friction in the moving system, heat affecting resistance, and aging of the magnet and control spring.

Uploaded by

Sapitha
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as DOCX, PDF, TXT or read online on Scribd
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1.Why the PMMC instruments are not used for a.c measurement?

(Nov 2006)

When the PMMC instruments are connected to a.c, the torque reverse as the
current reveres and the pointer cannot follow the rapid reversals. Hence the
deflection corresponding to mean torque is zero thus making the PMMC
instrument not suitable for a.c measurements.

2. What is the basic principle PMMC instrument?

A current carrying coil is placed in the magnetic field experiences a force


proportional the current it carries.

3. What are the sources of error in DC voltage measurement? (Apr/May 2005)

i. Friction in the moving system

ii. Heat generated changes the resistance of the working coil.

iii. The ageing if the magnet and control spring.

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