Torque Equation

The cquation for the developed torque can be obtained from the basic law of the
electromagnetic torque. The deflecting toque is given by,

1५ = NBAI
where Ty = deflecting torque in N-m

= flux density in air gap, Wb/ m?

= effective coil area ‏2م‎

B
N = number of turns of the coil
A
I = current in the moving coil, amperes
ஆ Ty = Gl
where G = NBA = constant

The controlling torque is provided by the springs and is proportional to the angular
deflection of the pointer.

Tr = 9
where T. = controlling torque

K = spring constant, Nm/rad or Nm/deg

0 = angular deflection
For the final steady state position,

Ty = T
2 GI = ۵

8 0 = (z)!

2
ಕ್‌
~
॥
ರಾಧಾ
೧೫7
‏مہہ‎
‎ஓ

Thus the deflection is directly proportional to the current passing through the coil. The
pointer deflection can therefore be used to measure current.

Key Point: As the direction of the current through to the coil changes, the direction of the
deflection of the pointer also changes. Hence such instruments are well suited for the
d.c. measurements.

10.
a)
1. Multiplier, m=I/Im = 100/10 mA= 10000
Rsh=m/(m-1) =l1mohm
Power dissipated, p= VI=100mAx100=10W

11. m=v/V=10000
Rs=Rm(m-1)= 99990 ohm
Power dissipated, p=10w