Spring control and Gravity control

Spring control

This is the most common method of providing controlling torque in electrical instruments. It may involve one or more springs. A spiral hairspring made of a non-magnetic material such as phosphor bronze is attached to the moving system. When the pointer deflects the spring twists in the opposite direction producing a restoring torque proportional to the angle of deflection of the moving system. The pointer comes to rest when T_d = T_c. In an instrument where the deflecting torque is uniform, spring control provides a linear or evenly spaced scale over a whole range. For example in a PMMC instrument, the deflecting torque is directly proportional to current flowing through the operating coil.

T_d α  I

With spring control T_c α θ.

In the final deflected position: T_d = T_c. Hence θ α  I.

 For the controlling torque to be proportional to the angle of deflection, the spring should have large number of turns so that the angular deformation per unit length, on full-scale deflection is small. The spring materials should have the following properties:

o   Should be non-magnetic.

o   Not subjected to much fatigue.

o   Have low specific resistance – especially in cases where they are used for leading current in or out of the instrument.

o   Have low temperature resistance coefficient.

The exact expression for controlling torque is T_c = Cθ where C is spring constant. Its value is given by

C = Ebt³/L N-m/rad. The θ angle is in radians.

Gravity control

 In gravity controlled instruments, a small weight is attached to the moving system in such a way that it produces a controlling torque, when the moving system is in deflected position. The controlling torque can be varied quite easily by adjusting the position of controlling weight upon the arm. Another adjustable weight is attached for zero adjustment and balancing purpose. This weight is called balancing weight. Gravity control is cheap, unaffected by change in temperature and is free from fatigue or deterioration with time but it gives a cramped scale (as the I α sin θ) and the instrument has to be kept in vertical position.

The controlling torque, T_c is given by: T_c = Wlsinθ = k_gsinθ

W – Control weight, l – distance of control weight from axis of rotation of moving system, k_g – gravity constant.

If deflecting torque is directly proportional to I, T_d α  I.

At equilibrium position: T_d = T_c (or) kI = k_gsinθ (or) I α sinθ. The relation shows that the current is proportional to sinθ and not θ. Hence in gravity controlled instruments, scale is not uniform. It is cramped for lower readings, instead of being uniformly divided.

Advantages

        1.  It is cheap and not affected by temperature variations.

        2.  It does not deteriorate with time.

3.      It is not subject to fatigue. 

Disadvantages

        1. Since the controlling torque is proportional to the sine of the angle of deflection, the scale is

            not uniformly divided but cramped at its lower end.

        2. It is not suitable for use in portable instruments (in which spring control is always preferred).

        3. Gravity control instruments must be used in vertical position so that the control weight may

            operate and also must be leveled otherwise they will give zero error.

In view of these reasons, gravity control is not used for indicating instruments in general and

portable instruments in particular.