2. Voltage and current are generally measured by voltmeter and ammeter , and
these are classified into two classes
Analog Meter Digital Meter
Classes of voltage and current
measuering instruments
3. Analog Ammeter
• Electrical measuring instrument used to
measure electric current in a circuit.
• Connected in series to measure the current
flowing through that circuit.
• Current flowing through all components in a
series circuit remains constant, allowing the
ammeter to measure the total current
passing through the entire circuit.
• Low resistance to minimize its impact on the
circuit, ensuring accurate current readings
by avoiding significant voltage drops across
the ammeter.
4. Analog voltmeter
• electrical measuring instrument used to
measure voltage in a circuit
• Connected in parallel across the points
where voltage is to be measured
• High resistance to minimize current draw
from the measured circuit, ensuring
accurate voltage readings.
5. Working principal of ammeter and voltmeter
When a current carrying conductor is placed in a magnetic field, it experiences a force and tends
to move in the direction as per Fleming’s left hand rule.
In case of analog or pointer indicating ammeter and voltmeter there are two classes of
instruments depending upon the operation.
• moving coil instrumentation (voltmeter/ammeter) / PMMC
• moving iron instrumentation (voltmeter/ammeter)
6. Moving coil instrumentation (voltmeter/ammeter)
Construction of PMMC Instrument
It has a permanent shoe magnet & a rectangular coil having large
number of turns wound on a light aluminum or copper former.
The coil is mounted on a spindle. The springs are used to supply current
to the moving coil. The pointer is provided on the spindle to indicate the
magnitude of quantity being measured.
7. • When the current-carrying coil is placed in the magnetic field produced by a
permanent magnet, then a mechanical torque acts on the coil and starts
moving.
• Therefore the pointer of the instrument attached to the moving coil moves
gradually in a clockwise direction, which indicates the measured value of current
in the circuit
• The pointer deflection is directly proportional to the current flowing through the
moving coil.
• Deflecting torque is provided by the force acting on the coil when current
flow through it.
• Controlling torque is provided by the springs provided on the spindle.
• Damping torque is provided by eddy current induced in the moving coil.
Working of Moving coil instrumentation (voltmeter/ammeter)
8. • The action of the spring produces the controlling torque, which opposes the
deflecting torque, and the pointer of the instrument starts deflecting in an
anticlockwise direction as the current in the coil is reduced.
• At steady-state conditions of the PMMC instrument, the controlling
torque and the deflecting torque are equal.
• At the steady-state position of the pointer, the damping torque is produced due to the
eddy current.
• The eddy current setup on the aluminum former opposes the movement of the coil,
which deflects the pointer and comes to rest position
• The pointer of the instrument indicates the magnitude of the measured current.
9. As per Fleming’s Left-hand rule, The direction of current flow and the magnetic field is perpendicular to the force F.
The formula for force F acting on the coil due to the current flow is given as,
F = NBIL -----------( 1 )
Hence, The deflecting torque Td is given as,
Td = Force x perpendicular distance
Td = NBIL x d
Td = NBIA----(2)
Mathematically, the deflecting torque equation is written as,
Td = GI G = Galvanometer constant = NBA
The controlling torque Tc of the coil produced due to the spring is given as,
Tc = Kθ Where ‘θ’ is the angle of pointer deflection
K’ is the spring constant.
At the steady-state condition of the instrument, the deflecting torque is equal to the controlling torque.
Td = Tc
GI = Kθ
θ = GI / K
From the above equation, we can conclude that the deflection of the pointer in the instrument is directly proportional to the current flowing through
the coil.
The scale is uniform for linear current/voltage measurements in DC circuits.
10. Advantages of moving coil Instrument
• Scale of these instrument is uniform.
• These are accurate & reliable.
• Eddy current damping is quit effective.
• There is no effect of stray magnetic field as it has a strong permanent magnet.
• These instrument require low driving power, hence their power consumption is low.
Disadvantages of moving coil Instrument
• These instrument can operate DC supply only.
• These are costlier than moving iron instrument.
