In a PMMC instrument, the torque weight ratio is

PMMC instruments:

PMMC instrument also called D'Arsonval meter.

These are used to measure the current and voltage in the DC circuit.

They are used eddy current type damping

The deflection angle can be calculated as

\(\theta = \frac {BANI}{K_c}\)

B = Flux density in Wb / sq.m 

A = Area of the frame

N = No. of turns

I = Current flow through the coil

Kc  = Spring control coefficient

Advantages:

• The scale is uniform (Linear scale)
• High torque to weight ratio compared to all the other type of instruments
• Frequency error or reactance error is absent
• High sensitivity
• Hysteresis loss is lesser
• No internal heating problem
• High accuracy
• Very less power consumption

Disadvantages:

• It works only for Dc not used for AC 
• Cost is very high