Electrical Engineering ⇒ Topic : Spring Control method

Spring Control. This is the most common method of providing controlling torque in electrical instruments. A spiral *hairspring made of some non-magnetic material like phosphor bronze is attached to the moving system of the instrument as shown in Fig. (a). With the deflection of the pointer, the spring is twisted in the opposite direction. This twist in the spring provides the controlling torque. Since the torsion torque of a spiral spring is proportional to the angle of twist, the controlling torque is directly proportional to the deflection of the pointer i. e. T ∝ θ.

                                                                     (a)                                                                (b)

                                                                                         figure 

The pointer will come to rest at a position where controlling torque T_c is equal to the deflecting torque T_d i.e. T_d = T_c.In an instrument where the deflecting torque is uniform, spring control provides a linear or evenly-spaced scale over the whole range. For example, in a permanent-magnet moving coil instrument, the deflecting torque is directly proportional to the current flowing through the operating coil i.e.

Since the deflection is directly proportional to I, scale of such an instrument will be *linear (uniform) as shown in Fig. (b).

Spring Control

Figure (a) shows the arrangement of spring control.The following points should be noted:

                                                          figure (a)

• To reduce the deformity per unit length, number of turns should be increased.
• Controlling torque is directly proportional to the deflection angle of the pointer.
• To avoid the temperature variation, two springs are connected back to back.
• Silicon bronze, hard rolled silver or copper,platinum silver, and platinum iridium are generally chosen for spring materials. But german silver and phosphor bronze are the best options