Permanent Magnet Moving Coil Instrument or PMMC Instrument
Permanent Magnet Moving Coil InstrumentThe permanent magnet moving coil instrument or PMMC type instrument uses two permanent magnets in order to create stationary magnetic field. These types of instruments are only used for measuring the DC quantities as if we apply AC current to these type of instruments the direction of current will be reversed during negative half cycle and hence the direction of torque will also be reversed which gives average value of torque zero. The pointer will not deflect due to high frequency from its mean position showing zero reading. However it can measure the direct current very accurately.
Let us move towards the constructions of permanent magnet moving coil instruments. We will see the construction of these types of instruments in five parts and they are described below:
- Stationary Part or Magnet System: In the present time we use magnets of high field intensities, high coercive force instead of using U shaped permanent magnet having soft iron pole pieces. The magnets which we are using nowadays are made up of materials like alcomax and alnico which provide high field strength.
- Moving Coil: The moving coil can freely moves between the two permanent magnets as shown in the figure given below. The coil is wound with many turns of copper wire and is placed on rectangular aluminium which is pivoted on jeweled bearings.
- Control System: The spring generally acts as control system for PMMC instruments. The spring also serves another important function by providing the path to lead current in and out of the coil.
- Damping System: The damping force hence torque is provided by movement of aluminium former in the magnetic field created by the permanent magnets.
- Meter: Meter of these instruments consists of light weight pointer to have free movement and scale which is linear or uniform and varies with angle.
Errors in Permanent Magnet Moving Coil InstrumentsThere are three main types of errors:
- Errors due to permanent magnets: Due to temperature effects and aging of the magnets the magnet may lose their magnetism to some extent. The magnets are generally aged by the heat and vibration treatment.
- Error may appear in PMMC Instrument due to the aging of the spring. However the error caused by the aging of the spring and the errors caused due to permanent magnet are opposite to each other, hence both the errors are compensated with each other.
- Change in the resistance of the moving coil with the temperature: Generally the temperature coefficients of the value of coefficient of copper wire in moving coil is 0.04 per degree celsius rise in temperature. Due to lower value of temperature coefficient the temperature rises at faster rate and hence the resistance increases. Due to this significant amount of error is caused.
Advantages of Permanent Magnet Moving Coil Instruments
- The scale is uniformly divided as the current is directly proportional to deflection of the pointer. Hence it is very easy to measure quantities from these instruments.
- Power consumption is also very low in these types of instruments.
- Higher value of torque is to weight ratio.
- These are having multiple advantages, a single instrument can be used for measuring various quantities by using different values of shunts and multipliers.
Disadvantages of Permanent Magnet Moving Coil Instruments
- These instruments cannot measure ac quantities.
- Cost of these instruments is high as compared to moving iron instruments.