In a PMMC instrument, if the controlling torque is too high, what effect will it have on the accuracy of the instrument?

The correct answer is option 3.

The controlling torque in a PMMC instrument is provided by the interaction of the magnetic field produced by a permanent magnet and the current flowing through the moving coil. The controlling torque is designed to be proportional to the quantity being measured (e.g., current or voltage).

If the controlling torque is too high, it can have both negative effects on the accuracy of the instrument:

1.) Overloading and Potential Damage: If the controlling torque is excessively high, it may lead to overloading of the instrument, especially if the current being measured exceeds the instrument's rated capacity. This can damage the instrument and decrease its accuracy.

2.) Reduced Damping: Excessive controlling torque can result in reduced damping. Damping is essential to stabilize the pointer and prevent it from oscillating excessively. If the controlling torque is too high, it may hinder the movement of the pointer, causing sluggish response and overshooting.

3.) Increased Friction and Hysteresis: High controlling torque can increase friction between the moving parts of the instrument. This friction, along with hysteresis effects, can introduce errors and affect the repeatability of measurements.

Additional Information Permanent Magnet Moving Coil (PMMC)

PMMC is used to measure an average DC value of electrical quantity.

The PMMC instrument has a linear scale, ​hence the deflecting torque produced is directly proportional to the current.

\(\tau_d=BINA\)

Td ∝ I 

The controlling torque in PMMC instruments can be produced by either spring control or gravity control.

For spring control: Tc ∝ θ  and For gravity control: Tc ∝ sin θ 

The damping torque in the PMMC instrument is provided by eddy current damping.