Page 826 - Basic Electrical Engineering
P. 826
For N number of turns
F = NBI ℓ
Torque = Force × Distance
Therefore, deflecting torque,
T = BI ℓ × 2r
d
= NBAI
= KI
Where K = NBA = meter constant.
A = ℓ × 2r = ℓ × d
d is the diameter or width of the coil.
Thus, we see that deflecting torque is proportional to the current flowing
through the coil. This deflecting torque is being opposed by the control
torque. If a spring is used to produce control torque (through winding and
unwinding of the spring), the control torque, T will be T = K θ where K is
c
c
s
s
the spring constant and θ is the angle of deflection of the moving coil.
It can be seen that more the deflection, more is the winding of the spring,
and hence, more is the control torque.
When T is higher than T , the coil will get deflected more and more, till
d
c
T , the opposing torque becomes equal to the deflecting torque. Thus, for
c
final deflection
Thus, deflection, θ is directly proportional to the current being measured.
