ELECTRODYNAMOMETER TYPE INSTRUMENTS

An electrodynamometer type instrument is a moving coil instrument in which the operating field is produced by another coil which is fixed. This type of instrument can be used either as an ammeter or as a voltmeter, but is generally used as a wattmeter.

ELECTRODYNAMIC INSTRUMENT AS AN AMMETER

ELECTRODYNAMIC INSTRUMENT AS A VOLTMETER

Electrodynamic instruments are also capable of functioning as transfer instruments. Besides, their use as an ammeter, voltmeter, and wattmeter; they are also used to transfer calibration of working instruments.

CONSTRUCTION –

The instrument consists of a fixed coil and a moving coil. The fixed coil is usually air-cored to avoid hysteresis effects when used on AC circuits. Fixed coils are wound with fine wire for use as a voltmeter.

But, if the instrument is to be used as an ammeter or wattmeter, then the fixed coils are wound with heavy wire carrying the main current.

 The moving coil is mounted on an aluminium spindle. It is wound either as self sustaining coil or else on a non-metallic former so as to prevent eddy currents. Moving coils are also air-cored.

CONTROLLING TORQUE –

Controlling torque is provided by two control springs. These springs act as leads to the moving coil.

DAMPING –

Air friction damping is provided by aluminium vanes attached to the spindle at the bottom.

DEFLECTING TORQUE –

Let the currents passing through fixed and moving coils be I₁ and I₂ respectively. Since, there is no iron, field strength and hence flux density is proportional to I₁.

                \[\therefore B=K{{I}_{1}}\] (K is constant)

Force on each side of the moving coil$=NB{{I}_{2}}l$

Torque produced on whole of the coil$=NB{{I}_{2}}l\times b=NB{{I}_{2}}A$

Which can be written as, ${{T}_{d}}=NK{{I}_{1}}{{I}_{2}}A$ N-m                                 (By putting B=KI₁ )

$\Rightarrow {{T}_{d}}\propto {{I}_{1}}{{I}_{2}}$

It shows that the deflecting torque is proportional to the product of currents flowing in the fixed and the moving coil.

Since, the instrument is spring controlled; the restoring or controlling torque is proportional to angular deflection$\theta $.

                                                                              

                                            \[{{T}_{c}}\propto {{K}_{2}}\theta \]

As, the deflecting torque is equal to the controlling torque, $\Rightarrow \theta \propto {{I}_{1}}{{I}_{2}}$ . The deflection produced in the instrument is proportional to the product of currents flowing in the fixed and the moving coil.

ADVANTAGES –

• These instruments are free from hysteresis and eddy current losses.
• They can be used on both AC and DC.
• They are used as transfer instruments.

DISADVANTAGES –

• Low torque/weight ratio and hence low sensitivity.
• Costlier than PMMC and moving iron type.
• Non-uniform scale.