Permanent Magnet Moving Coil Instrument (PMMC)

  • Aviinash 
  • 6 min read

Introduction:

In 1882, D’Arsonval developed a device (PMMC) that consists of a permanent magnet and moving coil suspended by fine wire which electric current to coil and also restores the torque to return to the initial position. An iron tube is placed between the magnetic poles as it has a gap through which the coil is rotated. This produces a radial magnetic field and results in a linear response of the instrument. As a mirror is attached to it, it will show the position of the coil. Later, Edward Weston made an instrument that places current-carrying coil between two magnets. According to faraday’s law, when the current-carrying conductor is placed in a magnetic field produced by the magnets, it will create a force. This magnitude of force will be proportional to amount of current flows through the coil and shows the value by deflecting the needle. This instrument is also called a Galvanometer. It helps in measuring only DC. When we try to measure AC current through this, as the AC current reverses from positive to negative simultaneously so that the torque reverses and shows zero deflection.

Principle:

It works on the principle that when torque is applied to moving coil placed between the magnets which produces magnetic field, the needle attached to it will deflects and it shows precise reading for DC current.

Construction of PMMC:

The moving coil and the permanent magnet are the important parts of this instrument. Some of the parts which are used in this instrument are explained below:

  • MOVING COIL:

This is used to carry the current in the instrument. It is placed between two magnets as it is free to move between them. The current passes though it helps to deflect the needle which will help to measure the value of current passes through it. The coil is made up of copper wire and it is placed in a rectangular former which is made up of aluminium. The former increases the radial and uniform magnetic field by the air gap between the poles.

  • MAGNET SYSTEM:

They use permanent magnets made up of Alcomax and Alnico to create magnetic field. They have high coercive force and high magnetic field intensities.

  • CONTROL:

In this instrument, the torque is being controlled by springs. These springs are made up of Phosphorous bronze and it is placed between jewel bearings. The spring are also used to provide the path for the current to move in and out of the coil.

  • DAMPING:

Damping torque is used to keep the movement of coil at rest. It is induced due to movement of aluminium core which is placed between two magnetic poles.

  • POINTER & SCALE:

The pointer is attached to moving coil and it is made up of light weight material for easy deflection. The pointer will deflects over a scale which shows the value of current flows through the coil.

pmmc

Working:

It is constructed as shown above. Two magnetic poles of north pole and south pole are placed and a moving coil is placed between them. The springs are attached to the moving coil at both the ends. The pointer is attached at one end to the moving coil. As the electromagnetic field will produce a deflecting torque which is been nullified by the damping torque produced by the springs. According to fleming’s thumb rule, when a current carrying conductor placed in a magnetic field a force will be created perpendicular to them. This magnitude of force will be perpendicular to deflection of pointer which shows the value of DC current.

Torque Equation:

The deflecting torque produced by movement of coil is given as,

Td=NBLDI

Where,

N-Number of turns of coil

B-Density of flux in air gap

L, D-Vertical and Horizontal length of sides

I-Current flowing through the coil

The damping torque produced by the springs is given by,

Tc=KѲ

Where,

K-Spring constant

Ѳ-Angle of deflection

By equating the above two equations, we get,

I= (K/G) Ѳ

This equations shows that deflecting torque is directly proportional to current flows through the coil.

Errors:

  • PERMANENT MAGNET:

The heat and vibrations of magnet will reduces the lifetime of it and also the field intensity. This will results in reduction of deflection of coil.

  • SPRINGS:

The spring will weaken due to rise in temperature. One degree raise in temperature will decrease the 0.004 percent life of it. As the life of spring reduces, the damping torque will also reduce and it show large deflection for small amount of current.

  • CHANGE IN RESISTANCE OF MOVING COIL:

The error occurs when the range is extended from the use of shunt limit. The error occurs because of change in coil resistance and this is because the coil made up of copper that has high shunt resistance. This error can be overcome by using swamping resistance (has low temperature coefficient) in series with it. It also reduces the effect of temperature.

Advantages:

  • The scales are correctly divided
  • The consumption of power is very low
  • They have high accuracy because of high torque weight ratio
  • A single device can be used for measuring various quantities.

Disadvantages:

  • It cannot measure AC Current.
  • The cost is high.

Applications On Ship:

As they are many electric gadgets used on ships. So it is important to prevent it from the flow of current beyond the limit of the gadgets. For this purpose they use this type of instruments to measure the flow of current through it to safeguard it. They are used as ammeter, voltmeter, ohm meter in field of marine. It is used as AMMETER by connecting the moving coil with low resistance shunt. It is used as VOLTMETER by connecting the moving coil with high resistance. A resistor of same alloy is placed in series for both ammeter and voltmeter to reduce the errors due to temperature. In case of OHM METER, it is used to measure resistance of the circuit by applying a constant voltage to the resistance.

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