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Dept. of Electrical Engineering
1st grade EE students
Electrical Measurements
Sheet 2
AC Moving Coil (Electrodynamic)
(Q.1) A 10 electrodynamic ammeter is controlled by spring having a constant of
0.1*10-6 Nm/deg. The full scale deflection is 110o. Determine the inductance of the
instrument when measuring a current of 10A. Initial inductance is 2µH and the
change is linear?
(Q.2) In a particular instrument the total resistance of the voltage coil circuit is 8200
ohm and the mutual inductance changes uniformly from -173 µH at zero deflection
to +175 µH at full scale, the angle of full scale is 95o. If a potential difference of 100
V is applied across the voltage circuit, and a current of 3 A at a power factor of 0.75
is passed through the current coil, What will be the deflection, If the spring constant
is 4.63*10-6 N.m/rad?
(Q.3) For a certain dynamometer ammeter the mutual inductance M varies with
deflection angle δ (expressed in degrees) as M= 6 cos (δ + 30o) mH. Find the
deflecting torque produced by a direct current of 50 mA corresponding to a
deflection of 60o?
(Q.4) A dynamometer ammeter is fitted with two fixed coils having a total resistance
of 3 ohm and a total inductance of 0.12 H. and a moving coil of resistance 30 ohm
and an inductance of 0.003 H. Calculate the error in reading when the instrument is
calibrated with d.c. and used on a.c. 50 Hz with moving coil shunted directly across
the field coils.
(Q.5) A 50 volt rang spring controlled electrodynamic voltmeter having a square
law scale take 0.05 A on d.c. for full scale deflection of 90o. The control constant is
0.5*10-6 N.m/deg and the initial inductance of the instrument is 0.25 H. Find the true
potential difference across the instrument when it reads 50V at 50 Hz?