The electrodynamometer instruments can be used as a voltmeter by connecting the
fixed and moving coils in series with a high non-inductive resistance. It is most
accurate type of voltmeter.

For using electrodynamometer instrument as a wattmeter to measure the power,
the fixed coils acts as a current coil and must be connected in series with the load.
The moving coil acts as a voltage coil or pressure coil and must be connected across
the supply terminals. The wattmeter indicates the supply power. When current passes
through the fixed and moving coils, both coils produce the magnetic fields. The field
produced by fixed coil is proportional to the load current while the field produced by
the moving coil is proportional to the voltage. As the deflecting torque is produced
due to the interaction of these two fields, the deflection is proportional to the power
supplied to the load.

Torque Equation

Let i) = Instantaneous value of current in fixed coil
iz = Instantaneous value of current in moving coil
L, = Self inductance of fixed coils
‏ہا‎ = Self inductance of moving coil
M =

Mutual inductance between fixed and moving coils
a

The electrodynamemeter instrument can be represented by an equivalent circuit as
shown in the Fig. 3.4. 0 1

The flux linkages of coil 1 are,

iy M ig .
t है t ಕ = 1.11 + M ३2
7 ‏و‎ ta 3 The flux linkages of coil 2 are,
| | 42 = 12 12 + ۸۸۷ ‏را‎ ∎

Fig. 3.4 Now e, = ٤
dt
~ ചേ
and ८2 31
Electrical input energy = ‏۱۰۲ر‎ + e215 dt

= i ‏رط ك‎ +i2do2
= td( Lit) + M iz) + டில்‌ (1.212 + M 11)
= it Li dip +i? ‏باك‎ +i; iz ‏الا ل‎ + 1 Mdi>+
१२.2५ i2+ ij ५1.2 + iy ४24 M+ 12 ۸۷ மய ... (1)
The energy stored in the magnetic field due to ‏ہا ,ا‎ and M is given by,

Energy stored = xh 12.1.) i} 1 iz M

ടെ

Change in stored energy = d டீ Li i? +

1 ∙ ⋅⋅
> ८2 13 ‏راع‎ 12 14]

a ಗ್ರ 1.141, + ऊ [41.1 + 2 Ladi. + $ மட, +

॥ Mdi;+i2Mdi, +i; izdM . (2)