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.
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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 Ic is r.m.s. current flowing through potential coil and Ip is the r.m.s. current flowing through potential coil the torque developed is given by,
T = K Ic Ip cosΦ = K' IV cosΦ
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Fig. 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 Φc 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 [cosα 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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