The ferrodynamic type instruments are same as the electrodynamic type instruments. The main difference is that the ferrodynamic type instruments consists a magnetic circuit of soft magnetic steel.

Principle of operation:

The fixed coil i.e. also called current coil is made up of two similar parts which are perfectly fitted on the two limbs of the laminated magnet as shown in the Fig. 1.

Fig.1 Principle of ferrodynamic instrument |

The potential coil i.e. called as moving coil is fitted to a pivoted spindle along with the pointer. This coil is free to rotate about a cylindrical core place in the air gap as shown in the Fig.1 eddy currents are generated in the moving coil as it is without any metal frame.

When the current flows through the fixed or current coil, a radial uniform magnetic field proportional to the current flowing through coil is setup within air gap. The current flowing the potential coil or moving coil is proportional to the load voltage. This current interacts with magnetic field set up in the air gap and a deflecting torque is produced. It is proportional to the product of the r.m.s. values of the currents flowing through coil and cosine of phase angle between them. If I

_{c }is r.m.s. current flowing through potential coil and I_{p}is the r.m.s. current flowing through potential coil the torque developed is given by,
T = K I

_{c}I_{p}cosΦ = K' IV cosΦFig. 2 Phasor diagram |

Due to the ferromagnetic material core used in this type of instrument, there is significant rise in magnetic flux and then correspondingly the deflection torque.

The increase in magnetic flux, the effects of the external magnetic field are reduced. But on the other hand, this type of construction sets the complementary erros due to nonlinear magnetization curve, occurance of eddy currents and hysteresis in the iron core. Because of this iron losses, the magnetic flux Φ

_{c }linked with current coil lags behind current I by**α**.
Thus the actual wattmeter reading is given by,

W' = VI cosα

**.**cosθ**.**cos (Φ+α-θ)
The true reading is W = VI cosΦ

Hence error in reading is given by,

Error = VI [cos

**α**cosθ cos (Φ +**α**-θ) - cosΦ]
To reduce angle

**α**, low loss steel can be used. Also introducing air gaps in the magnetic path, can be reduced. The value of should be made equal to to θ reduce error. To increase value of θ, a series resistors made up of unifilar winding is used.
The accuracy of ferrodynamic instruments lowers due to the hysteresis and eddy currents. So to increase accuracy a permalloy magnetic circuit is preferred.

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