Permanent Magnet Moving Coil Instruments-PMMC Instruments

As we know that there are different types of measuring instruments which we had discussed in previous articles. In this article we will discuss one of the type of moving coil instrument which is
permanent magnet moving coil type instrument.

What is Moving Coil Instrument

The coil rotates between the magnetic field of a permanent magnet in Moving coil instruments. Such instruments are used for dc measurements only. It works on the principle of that when a current carrying conductor (coil) placed in a magnetic field then it experiences a force. Because of this magnetic field the coil rotates.

There are two types of moving coil instruments namely the permanent magnet moving coil type and dynamometer type.

Permanent magnet moving coil instrument or PMMC instrument is the most accurate and useful for dc measurements. Dynamometer type moving coil instruments can be used both on dc as well on ac. Their calibration is same for both dc as well as ac. So these instruments are very useful as transfer measurement.

Permanent Magnet Moving Coil (PMMC) Instruments

Construction of PMMC instrument

It consists of permanent powerful magnet with soft iron pole pieces. Between the two poles of magnet a cylindrical iron core is mounted. They gives very narrow air gap in which the sides of a pivoted light rectangular coil lies. The rectangular coil is wounded over a light aluminum or copper former so acts as moving element. To make the field uniform we use core. It is to reduce the reluctance of the magnetic circuit. A low reluctance helps to retain permanence of the magnet for a longer period.

Pmmc Instrument construction
PMMC Instrument construction

The current is led into and out of the coil by means of phosphor bronze hair springs provided at both ends. The springs also provides the controlling torque. The springs are spiralled in opposite directions to neutralize the effect of change in temperature.

Working of PMMC Instrument

Torque Equation of PMMC instrument

When the current to be measured is passed through the coil in the direction as shown in figure above. Then a deflecting torque is produced. On account of reaction of the permanent magnetic field with the coil magnetic field. Deflection of torque direction is determined by Fleming’s left hand rule.

principle of PMMC instrument
principle of PMMC instrument
Permanent Magnet Moving Coil (PMMC) Instruments

You can also understand the deflection of the coil by watching the small video below.

Why PMMC instruments not used for AC measurement ?

Since theta is directly proportional to current (as derived in the above equation) because of this scale is linear. If the supply frequency is low to the moving coil. The pointer will deflect up-scale for one half-cycle of the input waveform and down-scale for the next half cycle. At power frequencies of 50 hertz and above. The pointer will not be able to follow the rapid variation in the direction. Hence will oscillate slightly around the zero mark and seeking the average value of AC, i.e. zero. That is why permanent magnet moving coil instrument is unsuitable for AC measurement. Unless the current is rectified before applying it to the moving coil.

Advantages of PMMC Instrument

  • Low power consumption because of small driving power.
  • No hysteresis loss is the former is of copper or aluminum.
  • Very effective and reliable Eddy current damping.
  • High torque-weight ratio resulting in high accuracy.
  • Use of single instrument for measurement of current and voltage by employing shunt and multiplier of different resistance.
  • No effect of stray magnetic fields because of use of intensive polarized or uni-directional field.
  • The instrument using magnet is very suitable in aircraft and Aerospace applications. Because there are multiplicity of instruments are mounted in close proximity to each other. This is because of self shielding property of magnets.

Disadvantages of PMMC Instruments

  • AC measurements is not suitable with PMMC instruments.
  • These are costlier in comparison to moving iron instrument because of delicate construction and the necessary accurate machining and assembly of various parts.
  •  Friction and temperature introduces errors as in case of other instruments which can be reduced.
  • Ageing  of control Spring and permanent magnet might cause error.
 Also read: introduction to measuring instrument
types of measuring instruments
moving iron instruments