Permanent Magnet Moving Coil (PMMC)

PERMANENT MAGNET MOVING COIL (PMMC)

It is used only for the measurement of DC.

It consist of 'U' shaped permanent magnet, Aluminium former is placed in the gap between the poles. Former is cylindrical or rectangular in shape. current carrying coil is wound on the former coil is made from enamelled or silk covered 'cu'. Iron core is located inside the former core is used to provide a low reluctance flux path, so it helps to produce strong magnetic field for the coil to move.

This arrangement increases the deflecting Torque (T_d) and also the sensitivity of the instrument.

Nowadays permanent magnet is replaced by Alcomax or Alnico. These magnetic material has small length and high field intensities. These materials also serve as core.

The spindle is supported by Jewel bearing. In some case, to eliminate bearing friction, ribbon suspersion is used.

The construction of PMMC is similar to d'Arsonval galvanometer with a difference. The lamp & mirror arrangement is replaced by a pointer and a scale in PMMC.

Flux density near the pole tips is smaller than the flux density at the centre and the magnetic field is not radial near the pole tips. This is called fringing field. So the coil is alligmed at the centre and not on the fringing field.

Control torque is provided by two hair springs attached to the spindle one at the top & another at the bottom side. Spring is made up of phosphor bronze. Current enters and leaves the instrument through these springs.

Damping torque is produced by the eddy current in the Aluminium former in the case where PMMC is used for voltage measurement.

In case of ammeter measurement, PMMC is made from non metallic former, so damping torque is provided by low resistance, connected across the coil.

When the polarity is reversed, the torque is developed in the opposite direction the meter does not allow the deflection in the opposite direction and the pointer reads zero. Therefore polarity should be maintained with this PMMC instrument.

Principle:

When supply is given to the coil, current flows through it. The current carrying coil is placed in a magnetic fied, By faraday's law the coil experiences a force. This force produce a torque and deflects the coil.

Meter range:

If the measuring current is less than 20mA, the whole current is allowed to pasis through the coil, otherwise coil is shunted with a resistor.

The voltage drop across the DC ammeter is 0.50mV, or 0 to 100mV.

DC voltmeter: Normally allows mA to pass through for full scale deflection.

Torque equation:

The force produced in the coil is given by,

Force = N Bil sineƟ …... (1)

Where

N = no. of turns of the coil

B = Flux density in wb/m²

i = measuring current

l = length of the vertical side of the coil in 'm'.

Ɵ = angle between direction of magnetic field and the conductor.

Here Ө = 90° and sinᎾ = 1

Deflecting torque, T_d = Force x distance

T_d = N Bil × d …...(2)

where 'd' is the horizontal length of the coil in 'm'

A = ld = area of the coil in m².

T_d = NBi A ….... (3)

where G = NBA is the displacement constant

The spring control provides the controlling torque.

T_d = k Ө_F …….(4)

where k is the spring constant.

Ө_F is the meter deflection.

For final steady deflection.

T_e = T_d …….(5)

G.i = k Ө_F

current, i = ﴾k/G﴿ Ө_F ….....(6)

deflection, Ө_F = ﴾G/K ﴿.i ………(7)

PMMC instruments have linear scale.

Errors in PMMC are

1. Weakening of permanent magnet due to ageing and temperature effects.

It decreases the deflection.

2. Weakening of spring increases the deflection.

3. Change in the resistance of moving coil.

Temperature error is eliminated by providing a swamping resistance made up of mangamin in series with the moving coil. The value of swamping resistance is 20 or 30 times greater than the moving coil resistance.

Advantages:

1. Uniform scale

2. Low power consumption

3. High Torque - weight ratio

4. Instrument range can be extended.

5. Flux from stray magnetic field errors.

Disadvantage:

1. Used only for DC and not for AC because if current reverses, Torque also reverses, pointer cannot follow the rapid reversals and the deflection due to mean torque is zero.

2. Error occurs due to ageing.

3. High cost

4. Sometime frictional, error occurs.