Electrical Engineering ⇒ Topic : Measurement of Voltage and Current
MEASUREMENT OF VOLTAGE AND CURRENT
Detection of current flowing in a circuit is the basis of different types of meters. Ammeters are converted to voltmeters and ohmmeters. An ideal ammeter is a very low resistance instrument and is almost a short in the loop of the circuit in which it is connected. Hence, the ammeter should not be connected in parallel, as it would short the output and will get burnt
An ideal voltmeter is a high resistance instrument and hence presents an open circuit to the terminals across which it is connected. If it is connected in series, it will load the circuit with heavy voltage drop even for small currents.
The basic principle of operation of an ammeter is same as the motor principle, namely, if a current carrying conductor is placed at right angles to a magnetic field, it experiences a force,
F = BIL
As this force causes a rotation of the coil, it is called torque. The conductor is in the form of a coil wound on a rectangular former, therefore,
Torque T = Force x Arm = NBILD
or T = NBIA
where T is torque in Nm, N is number of turns, B is flux density in air gap in Wb/m2, I is current in amperes, and A is the effective coil area in m2.
For a given instrument, flux density B, number of turns N, and area A are fixed and denoted as torque constant Kt.
TD = KtI ......................(1)
Hence, deflecting torque TD is proportional to current I producing deflection.
The magnetic field B is produced by a permanent magnet which is made of Alnico. To the moving coil a pointer is attached to measure the current. Such meters are called Permanent magnet moving coil (PMMC) or D'Arsonval or Weston instruments.
The deflecting torque is opposed by the controlling torque of the spring constant and brings the needle to rest when they are equal.
At balance, TD = Tc .................... (2)
In addition, there is damping produced by the inertia of the movement mechanism decided by torque-weight ratio
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