PERMANENT MAGNET MOVING COIL
Principle of Operation
The principle on which a Permanent Magnet Moving Coil (PMMC) instrument operates is that a torque
is exerted on a current-carrying coil placed in the field of a permanent magnet. The coil C has a number
of turns of thin insulated wires wound on a rectangular aluminum former F. The frame is carried on
a spindle S mounted in jewel bearings J1, J2. A pointer PR is attached to the spindle so that it moves
over a calibrated scale. The whole of the moving system is made as light in weight as possible to keep
the friction at the bearing to a minimum.
The coil is free to rotate in air gaps formed between the shaped soft-iron pole piece (pp) of a permanent magnet PM and a fixed soft-iron cylindrical core IC. The core serves two purposes; (a) it intensifies the magnetic field by reducing the length of the air gap, and (b) it makes the field radial and uniform in the air gap.
Thus, the coil always moves at right angles to the
magnetic field. Modern permanent magnets are
made of steel alloys which are difficult to machine.
Soft-iron pole pieces (pp) are attached to the
permanent magnet PM for easy machining in order
to adjust the length of the air gap. A soft-iron yoke (Y ) is used to complete the flux path and to provide shielding from stray external
fields.
Deflecting Torque Equation of PMMC Instrument
Let, B = flux density in the air gap (wb/m2
)
i = current in the coil (A)
L = effective axial length of the coil (m)
b = breadth of the coil (m)
n = number of turns of the coil.
i = current in the coil (A)
L = effective axial length of the coil (m)
b = breadth of the coil (m)
n = number of turns of the coil.
Force on one side of the coil is=BiLsinθ
Damping Torque When the aluminum former (F) moves with the coil in the field of the permanent
magnet, a voltage is induced, causing eddy current to flow in it. These current exerts a force on the
former. By Lenz’s law, this force opposes the motion producing it. Thus, a damping torque is obtained.
Such a damping is called eddy-current damping.
The ratio of the total current to the current in the meter is called multiplying power of shunt.
Multiplying power,
Advantages of PMMC Instruments:
- Sensitive to small current .
- Very accurate and reliable .
- Uniform scale up to 270° or more .
- Very effective built in damping .
- Low power consumption, varies from 25 µW to 200 µW .
- Free from hysteresis and not effected by external fields because its permanent magnet shields the coil from external magnetic fields.
- Easily adopted as a multirange instrument .
Disadvantages of PMMC Instruments:
- This type of instrument can be operated in direct current only. In alternating current, the instrument does not operate because in the positive half, the pointer experiences a force in one direction and in the negative half the pointer experiences the force in the opposite direction. Due to the inertia of the pointer, it retains it’s zero position.
- The moving system is very delicate and can easily be damaged by rough handling.
- The coil being very fine, cannot withstand prolonged overloading.
- It is costlier.
- The ageing of the instrument (permanent magnet and control spring) may introduce some errors.
EXTENSION OF RANGE OF PMMC INSTRUMENTS :
Ammeter Shunts: The moving-coil instrument has a coil wound with very fine wire. It can carry only few mA safely to
give full-scale deflection. For measuring higher current, a low resistance is connected in parallel to the
instrument to bypass the major part of the current. The low resistance connected in parallel with the
coil is called a shunt.
The resistance of the shunt can be calculated using conventional circuit analysis
Rsh = shunt resistance (Ω)
Rm = coil resistance (Ω)
Im = Ifs = full-scale deflection current (A)
Ish = shunt current (A)
I = current to be measured (A)
The voltage drop across the shunt and the meter must be same as they are connected in parallel.
Voltmeter Multipliers
For measuring higher voltages, a high resistance is connected in series with the instrument to limit the current in the coil to a safe value. This value of current should never exceed the current required to produce the full scale deflection. The high resistance connected in series with the instrument is called a multiplier.
The value of multiplier required to extend the voltage range, is calculated as under:
Rsc = multiplier resistance (Ω)
Rm = meter resistance (Ω)
Im = Ifs = full scale deflection current (A) v = voltage across the meter for producing current Im (A)
V = voltage to be measured (A)
Sensitivity The moving-coil instrument is a very sensitive instrument. It is, therefore, widely used for
measuring current and voltage. The coil of the instrument may require a small amount of current (in
the range of µA) for full-scale deflection. The sensitivity is sometimes expressed in ohm/volt. The
sensitivity of a voltmeter is given by
Where, Ifs is the full-scale deflecting current. Thus, the sensitivity depends upon on the current to
give full-scale deflection.
