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Damping Devices

Owing to the inertia of the moving system, when subjected to the deflecting and restoring torques, a number of vibration will be produced before coming finally to rest. To-avoid this, a damping torque is required which opposes the motion and ceases when the pointer comes to rest. The degree of damping should be adjusted to a value which is sufficient to enable the

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pointer to rise quickly to its deflected position without overshooting. In that case, the instrument is said to be dead-beat. If the instrument is over-damped the movement is very slow (and the instrument becomes lithargic) as shown in Fig. 3.

Damping can be provided by the following methods:

  1. Air damping.
  2. Eddy current damping.
  3. Fluid friction damping.

1. Air damping. Fig. 4 shows an arrangement for obtaining air damping. It consists of a thin metal vane MV attached to the spindle S ; the vane moves in a sector-shaped box B. Any tendency of the moving system to oscillate is damped by the action of the air on the vane.

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Fig. 4. Air damping.

2. Eddy current damping. This method of damping is based on the principle that when a conducting non-magnetic material is moved in a magnetic field an e.m.f is induced in it which causes

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Fig. 5. Eddy current damping.

currents called the eddy currents. Due to these eddy currents a force exists between them and the field. Due to Lenz’s law this force is always in opposition to the force causing rotation of the conducting, material, thus, it provides the necessary damping.

  • One form of eddy-current damping is shown in Fig. 5. Here a copper or aluminium disc, curried by a spindle, can move between the poles of a permanent magnet. If the disc moves clockwise, the e.m.f.’s induced in the disc circulate eddy currents as shown dotted. It follows from Lenz’s law that these currents exert a force opposing the motion producing them, namely the clockwise movement of the disc.
  • Another form of providing damping is used in the moving coil instruments using permanent magnet. The moving coil is mounted over a metallic former. When the coil is deflected eddy e.m.f..’s are induced in the two sides of the former, causing eddy forces as shown in Fig. 6.

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Fig. 6. Eddy current damping with permanent magnet.

3. Fluid friction damping. Fig. 7 shows the method of fluid friction damping. Here light vanes are attached to the spindle of the moving system. The vanes are dipped into a pot of damping oil and are completely submerged by the oil. The motion of the moving system is always opposed by the friction of the damping oil on the vanes. The damping force thus created always increases with the increase in velocity of vanes. There is no damping force when the vanes are stationary:

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Fig. 7. Fluid friction damping.

The damping oil used must have the following properties:

(i)                           Must be a good insulator.

(ii)                        Should be non-evaporating.

(iii)                      Should not have corrosive action upon the metal of the vane.

(iv)                      The viscosity of the oil should not change with the temperature.

Though in this method of damping, no case is required as in the air friction damping but it is not much used due to the following disadvantages:

(i)                           Objectionable creeping of oil.

(ii)                        Using the instrument always in the vertical position and its obvious unsuitability for use in portable instruments.

The principle types of electrical indicating instruments, together with the methods of control and damping, are summarized below:

S.No.

Type of instrument

Suitable for measuring

Method of control

Method of damping

1.

Moving-iron Current and voltage, D.C. and A.C. Hair springs Air

2.

Permanent magnet moving coil Current and voltage, D.C. only Hair springs Eddy current

3.

Thermocouple Current and voltage, D.C. and A.C. As for moving coil As for moving coil

4.

Electro-dynamic or dynamometer Current, voltage and power, D.C. and A.C. Hair springs Air

5.

Electrostatic Voltage only, D.C. and A.C. Hair springs Air or eddy current

6.

Rectifier Current and voltage, A.C. only As for moving coil As for moving coil

 

Note. Apart from the electrostatic type of voltmeter, all voltmeters are in effect milliammeters connected in series with non-reactive resistor having a high resistance.

Difference between an ammeter and a voltmeter

An ammeter and a voltmeter work on the same principle. The ammeter has a low resistance so that when it is connected in series with any circuit, it does not change the current. The voltmeter has a high resistance and it is so designed that when connected in parallel to the circuit for measuring voltages it does not take appreciable current.

An ammeter of low range can be used as a voltmeter by connecting an external resistance in series with it.