Chapter 2

Analog Electromechanical
instruments
 Introduction
• Definition: Instruments that measure physical
quantities and provide an analog output
(needle on scale).
• Working Principle: Converts physical
quantities into mechanical movement
displayed on a calibrated scale.
 Basic Components of Analog
Electromechanical Instruments
• Measuring Element:
o Converts physical property (e.g., voltage) to mechanical
movement.
• Moving System:
o Needle or pointer that indicates the reading.
• Damping Mechanism:
o Reduces oscillations for stable readings.
• Scale:
o Calibrated markings representing the measured quantity.
• Control/Zero Adjustments:
o Allows recalibration for accurate readings.
 PMMC Instrument

• PMMC (Permanent Magnet Moving Coil) Instrument: Measures DC current

or voltage.
• Called "Permanent Magnet Moving Coil" because of the use of permanent
magnets in its construction.
 Key Components
1. Permanent Magnet:
• Creates a strong magnetic field.
• Includes soft iron pieces to make the field uniform.
2. Rectangular Coil:
1. Made of fine wire with multiple turns.
2. Mounted on an aluminum former with an iron core to enhance sensitivity.
3. Spindle and Moving Element:
1. The coil is mounted on a spindle, allowing it to move freely in response to
current.
 Cont..
Working Mechanisms
1.Hair Springs:
– Provide current to the coil.
– Generate controlling torque, opposing the coil’s
movement.
2.Eddy Current Damping:
– Damping torque is produced to prevent oscillations,
ensuring stable readings.
3. Permanent Magnet: Produces a uniform field.
4. Iron Core: Enhances magnetic flux.
 Cont..
• Coil: Interacts with the field to produce deflection proportional to
the current/voltage.
• Advantages of PMMC:
• High accuracy and sensitivity.
• Linear scale for easier readings.
• Used for DC measurements.
 Working of PMMC instrument

• The Permanent Magnet Moving Coil (PMMC)

instrument is a type of analog instrument used for
precise measurement of DC quantities like current
or voltage.
• Its operation is based on the interaction of the
magnetic field produced by a permanent magnet
and the magnetic field generated by a moving coil
carrying current.
 Torque Equation for PMMC

• The equation for the developed torque of the PMMC can be obtained from
the basic law of electromagnetic torque.
• The deflecting torque is given by,
Td = NBAI
Where,
Td = deflecting torque in N-m
B = flux density in air gap, Wb/m2
N = Number of turns of the coils
A = effective area of coil m2
I = current in the moving coil, amperes
Therefore,
Td = GI
Where,
G = NBA = constant
 Cont..
• The controlling torque is provided by the springs and is proportional to
the angular deflection of the pointer.
Tc = KØ
Where,
Tc = Controlling Torque
K = Spring Constant Nm/rad or Nm/deg
Ø = angular deflection
For the final steady state position,
Td = Tc
Therefore
GI = KØ
So,
Ø = (G/K)I or I = (K/G) Ø
 Thus the deflection is directly proportional to the current passing
through the coil. Therefore pointer deflection can be used to measure
current.
Advantage of PMMC instrument
 Uniform scale.
 Power consumption is low.
 No hysteresis loss.
 They are not affected by stray field.
 Require small operating current.
 Accurate and reliable.
• Disadvantage of PMMC instrument
1. Only used for D.C measurement.
2. Costlier compared to moving iron instrument.
3. Some errors are caused due to the aging of the control springs
and the permanent magnets .
Errors in PMMC instrument
• Friction:
– Frictional errors caused by friction between moving parts.
– Ratio of torque to weight made very high to reduce
frictional errors.
• Temperature:
– Heat generated or changes in temperature lead to serious
errors.
– Changes in resistance of working coil due to temperature
variations cause large errors.
– For voltmeters, large series resistance with very low
temperature coefficient used to reduce temperature errors.
 Cont..
• Aging:
– Aging of temperature magnet and control springs causes
errors.
– Weakening of magnet results in less deflection.
– Weakening of control springs leads to larger deflection
for a specific current.
– Proper material selection and pre-stressing during
manufacturing can reduce errors due to weakening of
control springs.
 MOVING IRON INSTRUMENT

• There are classified in to two type

1. Attraction type moving iron instrument
2. Repulsion type moving iron instrument
• Attraction Type Moving Iron Instrument:
– Consists of a coil through which test current passes and a
pivoted soft-iron mass attached to pointer.
– Working principle:
• Magnetic polarity induced in coil end nearest iron mass.
• Opposite magnetic polarity induced in part of iron mass nearest coil.
• Iron mass attracted towards coil, deflecting pointer across scale.
– Structure:
• Flat coil with narrow slot-like opening.
• Moving iron: flat disc or sector eccentrically mounted.
 Cont..
– Operation:
• Current flow through coil produces magnetic field.
• Moving iron attracted from weaker field outside coil to
stronger field inside.
– Controlling Torque:
• Provided by springs; gravity control for vertically mounted
panel instruments.
– Damping:
• Air friction with light aluminum piston or vane attached to
moving system.
 Moving iron repulsion type instrument
• Features two vanes inside the coil: one fixed, one movable.
• Current flow in the coil magnetizes both vanes with like polarities.
• Repulsion between like polarities causes movement of the movable
vane.
• The two different designs of repulsion type instruments are:
i. Radial vane type and
ii. Co-axial vane type
 Radial van repulsion type instrument
• Characteristics:
– Most sensitive among moving iron mechanisms.
– Offers the most linear scale.
• Operation:
– uses radial vanes for repulsion.
• Advantages:
– High sensitivity and linear scale make it a preferred choice.
 Concentric Vane Repulsion Type Instrument
– Features concentric vane repulsion mechanism.
– Moderate sensitivity with a non-uniform scale due to square law response.
• Construction:
– Two concentric vanes: one fixed to the coil frame, the other rotating coaxially
inside.
• Operation:
– Both vanes magnetized to the same polarity by current in the coil.
– Repulsive force causes rotation of the movable vane attached to pivoted shaft.
– Pointer deflection proportional to current in the coil.
• Characteristics:
– Deflection proportional to the square of current through the coil.
– Non-uniform scale due to square law response.
• Utility:
– Moving iron instruments suitable for both AC and DC measurements.
– Direction of current in the coil does not affect deflection direction.
•
 Advantages of moving iron instruments
 Suitable for both A.C. and D.C. measurements.
 High torque-to-weight ratio reduces friction errors.
 Wide range coverage with a single moving
element, making them cost-effective.
 No current-carrying parts in the moving system,
ensuring ruggedness and reliability.
 Capable of providing good accuracy, with modern
models having ≤2% D.C. error.
 Withstand severe overloads without damage.
 Extendable range of measurement.
 Disadvantages Of Moving Iron Instruments

 Non-uniform scale, particularly cramped at lower

end, compromising accuracy.
 Significant errors due to hysteresis, frequency
variations, and stray magnetic fields.
 Temperature fluctuations affect readings due to
changes in coil resistance, spring stiffness, and
permeability.
 Calibration differences between A.C. and D.C. due
to meter inductance, requiring frequency-specific
calibration.
 Relatively high power consumption.
ELECTRODYNAMOMETER
 Construction of Electrodynamometer:
• The electrodynamometer is a device used to
measure electrical power or current based on the
interaction between fixed and moving coils.
• Working Principle:
– When current flows through both the fixed and moving
coils, a magnetic interaction generates a torque.
– The torque causes the moving coil to rotate.
– The amount of rotation corresponds to the electrical
parameter being measured, which the pointer indicates
on the scale.
 Cont..
 Components of Electrodynamometer
• Fixed Coil:
– A pair of coils mounted horizontally with cylindrical wire
windings.
– Generates a magnetic field when current flows through it.
• Moving Coil:
– Positioned within the fixed coil.
– Free to rotate and attached to a pointer and scale for
measurements.
• Pointer and Scale:
– The pointer indicates the current or power reading on a
calibrated scale.
 Cont..
• Spring:
– Provides controlling torque to bring the moving
coil back to its equilibrium position.
• Pivot:
– Ensures smooth rotation of the moving coil.
• Electrical Connections:
– Provide current to the fixed and moving coils.
 Advantages of Electrodynamometer Instruments:

1. Accurate Measurement of Power

– Electrodynamometers can measure true power (active
power) in AC circuits, including single-phase and three-
phase systems.
2. Used for Both AC and DC
– These instruments work for both alternating current
(AC) and direct current (DC) systems, making them
versatile for various electrical setups.
 Cont..
3. Wide Measurement Range
– With proper design, they can be used to measure a
wide range of currents and voltages.
4. Durability
– Electrodynamometer instruments have a sturdy
construction and can withstand overload conditions
to some extent, making them suitable for long-term
use.
 Disadvantages of Electrodynamometer
Instruments:
1. Low Sensitivity
– The torque generated by the electromagnetic
interaction is relatively weak, which reduces the
sensitivity of the instrument, especially for low-
power measurements.
2. High Power Consumption
The current flowing through the coils can lead to
significant power loss, making them less efficient
compared to other types of measuring instruments.
 Cont.…
3. Complex Construction
– The moving and fixed coil design, along with the
control springs and pointer, makes the
instrument mechanically complex.
 ELECTROSTATIC INSTRUMENTS

• Electrostatic instruments are devices used to

measure electrical quantities, primarily voltage,
by using the principles of electrostatics.
• They rely on the attraction or repulsion between
electrically charged plates to produce a
mechanical force, which is then translated into a
reading.
 Working Principle of Electrostatic
Instruments
• Electrostatic instruments operate based on
Coulomb's law,
– Which states that the force between two charges is
directly proportional to the product of their charges
and inversely proportional to the square of the
distance between them.
• The device typically uses two plates:
– One plate is fixed.
– The other plate is movable and connected to a pointer
or scale.
 Cont..
Advantages of Electrostatic Instruments:
• Negligible power consumption from mains.
• Compatible with both AC and DC.
• No frequency or waveform errors; deflection proportional to
voltage square, no hysteresis.
• Immune to errors from stray magnetic fields; operates on
electrostatic principle.
• Well-suited for high voltage measurements.
 Cont..
Disadvantages of Electrostatic Instruments:
• Limited applicability to specific high-voltage AC circuits due to
risk of erroneous readings from current draw of other
instruments.
• Protective resistor needed to limit current in case of short
circuits.
• Expensive, bulky, and lack robust construction.
• Non-uniform scale.
• Small operating force.
 RECTIFIER-TYPE INSTRUMENTS

• Overview
• Rectifier-type instruments are widely used for measuring the
average value of alternating current (AC) or voltage.
• They operate by converting the alternating current into a
direct current (DC) using rectification.
• Commonly used in applications where:
– Precise measurement of sinusoidal signals is required.
– The measurement of average values suffices, rather than the true
RMS (Root Mean Square) value
– Used to measure the average value of AC signals by
converting them into DC.
– Suitable for both voltage and current measurement.
 Advantage of Rectifier-Type
Instruments
• Advantages
– Simple design makes it affordable.
– Requires fewer components, allowing for
portability.
– Can measure both AC and DC signals.
– Fewer moving parts compared to complex
instruments.
 Cont..
Limitations of Rectifier-Type Instruments:
• Accuracy limited to calibrated waveforms:
– Calibration assumes pure sine waves.
– Presence of harmonics leads to erroneous readings.
• Temperature sensitivity:
– Rectifier is affected by temperature variations.
– Large temperature changes impact instrument
accuracy.
 Cont..
Applications of Rectifier-Type Instruments:
• Suitable for measuring alternating voltages in
the range of 50–250 V.
• Can function as a micrometer or low milli-
ammeter (up to 10–15 mA).
• Commonly used in communications circuits
due to high sensitivity and low power
consumption.
The end