Correct Answer - Option 4 : Permanent magnet moving coil type
Advantages of PMMC Instruments :
- It has a uniformly divided scale because the response of the instrument is directly proportional to the deflection of the pointer.
- Very easy to measure the quantity from that instrument.
- It has the least power consumption (25 μW to 200 μW) among all kinds of analog instruments.
- Its torque to weight ratio is high.
- A single instrument can be used for measuring various quantities by using different values of shunts and series multipliers.
Permanent-Magnet Moving Coil (PMMC) Instruments:
- This instrument work on the principle that, when the current-carrying conductor placed in a magnetic field, a mechanical force acted.
- The movement of the moving coil pointer on the based of D’Arsonval movement.
Working:
- When the instrument is connected to the circuit to measure current or voltage, the measuring current flows through the coil.
- The current through the coil is placed in the magnetic field which produced a mechanical torque.
- This electrodynamic effect of mechanical torque is known as Deflecting torque.
- To control the position of the Pointer by means of Deflecting torque a hairspring is used.
- The main purpose of the hairspring is to control the deflecting torque and torque produced by it is known as Controlling torque.
Under the steady-state condition,
Controlling torque (Tc) = Deflecting torque (Td)
Tc ∝ θ
Tc = kθ …. (1)
Where θ is the angle of deflection made by the pointer
And, k is constant
Td = BINA
Where,
B is magnetic flux density in Wb / m2
I is current flow through the conductor coil in ampere
N is the number of turns of the coil
A is the area of the coil in m2
From equation (1),
kθ = BINA
\(\theta = \frac{{BINA}}{k}\)
