Electromechanical instruments measure electrical quantities using a moving coil within a magnetic field and a pointer to indicate the measured value on a calibrated scale. Permanent magnet moving coil (PMMC) instruments are a common type of deflecting instrument. They contain a permanent magnet that produces a magnetic field, a coil that moves within the field, and a pointer attached to the coil. Three forces act on the moving coil: deflecting force from current in the coil, controlling force from a spiral spring, and damping force from eddy currents induced in the core. Together these forces allow the pointer to indicate the measured value while damping oscillations.

1. Electromechanical instruments

2. Electromechanical instruments
 The function of these instruments is based on electromechanical system.
These instruments are also known as deflecting type instruments
 A pointer is used to indicate the value of measured quantity.
 Some electromechanical instruments are
 Galvanometer
 Voltmeter
 Ammeter
 Ohmmeter
 Wattmeter
 Power factor meter

3. Permanent Magnet Moving Coil (PMMC) Instruments
 A deflection type instrument uses a pointer that moves over a
calibrated scale to indicate measured quantity.
 PMMC instrument is one of the types of deflecting type
instruments.
 Three forces are present in electromechanical mechanism.
 Deflecting force
 Controlling force
 Damping force
 Two methods are used for supporting the moving system of a
deflection instrument.
 Jeweled bearing suspension method
 Taut band method

4. PMMC Instruments construction
Main parts of the meter are
 Permanent magnet
 Coil
 Soft iron core
 Pole shoe
 Pointer
 Spiral spring
 Supporting system

5. Deflecting torque
 Deflecting in moving coil will be
T = NIAB
 This shows that if you are using a permanent magnet galvanometer
which is generally used, then torque will depend on the amount of
current through the coil.
 The practical coil area generally ranges from approximately 0.5 to 2.5
cm2.
 Flux densities for modern instruments usually range from 1,500 to
5,000 gauss (0.15 to 0.5 Wb/m2).

6. Controlling torque
 Controlling torque provides controlling force in PMMC meters.
 It is exerted by spiral springs.
 It is directly proportional to the deformation or windup of springs.
 It is directly proportional to the angle of deflection of the pointer.
T = Cθ

7. Damping torque
 Damping Torque provides damping force.
 It is provided by the induced currents in core on which the coil is wound
or in the circuit of the coil itself.
 As the coil moves in the field of the permanent magnet, eddy currents
are set up in the core.
 The magnetic field produced by the eddy currents opposes the motion
of the coil.
 The pointer will therefore swing more slowly to its proper position and
come to rest quickly with very little oscillation.

8. Dynamic behavior of coil
 The motion of a moving coil in a magnetic field is characterized by three
quantities :
 The moment of inertia (J) of the moving coil about its axis of rotation
 The opposing torque (S) developed by the coil suspension
 The damping constant (D).

9. Suspension galvanometer
 This instrument was the forerunner of the moving-coil/deflecting
type instruments.
 In modern Galvanometers, a pointer is attached with moving coil.
 Pointer deflects in the direction of current flowing through the coil.
 Galvanometers are often employed to detect zero current in the
circuit.
 It is also referred as null detector.

10. Galvanometer sensitivity
 Voltage sensitivity of galvanometer is often expressed for a given value
of critical damping resistance.
 It is expressed in µV/mm.
 Galvanometer sensitivity is specified as mega ohm sensitivity .
 Pointer galvanometers have current sensitivities ranging from 0.1 to 1
µA/mm.
 For light beam galvanometers current sensitivities ranging from 0.01 to
0.1 µA/mm.