Embedded Files
WHAT IS A PERMANENT MAGNET MOVING COIL (PMMC)?
WHAT IS A PERMANENT MAGNET MOVING COIL (PMMC)?
A Permanent Magnet Moving Coil (PMMC) meter, also known as a D’Arsonval meter or galvanometer.
PMMC is an instrument that allows you to measure the current through a coil by observing the coil’s angular deflection in a uniform magnetic field.
Application; -Ammeter -Voltmeter -Galvanometer -Ohmeter
A PMMC meter places a coil of wire (i.e. a conductor) inbetween two permanent magnets in order to create stationary magnetic field. According to Faraday’s Laws of electromagnetic induction, a current carrying conductor placed in a magnetic field will experience a force in the direction determined by Fleming’s left hand rule.
PMMC INSTRUMENTS
PMMC INSTRUMENTS
Only used for measuring the Direct Current (DC) current.
If use Alternating Current (AC) current, the direction of current will be reversed during the negative half cycle, and hence the direction of torque will also be reversed.
This results in an average value of zero torque – hence no net movement against the scale.
Despite this, PMMC meters can accurately measure DC current.
-PMMC Instrument-
PMMC : CONSTRUCTION
A PMMC meter (or D’Arsonval meters) is constructed of 5 main components:
Stationary Part or Magnet System
Moving Coil
Control System
Damping System
Meter
1. Stationary Part or Magnet System
1. Stationary Part or Magnet System
Use magnets of high field intensities.
High coercive force instead of using U shaped permanent magnet having soft iron pole pieces.
The magnets nowadays are made up of materials like alcomax and alnico which provide high field strength.
2. Moving Coil
2. Moving Coil
The moving coil can freely moves between the two permanent magnets.
The coil is wound with many turns of copper wire and is placed on rectangular aluminium which is pivoted on jeweled bearings.
3. Control System
3. 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.
4. Damping System
4. Damping System
The damping force hence torque is provided by movement of aluminium former in the magnetic field created by the permanent magnets.
5. Meter
5. 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.
-PMMC Meter-
Errors in PMMC
Errors in PMMC
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
Advantages
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.
A high torque 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
Disadvantages
These instruments cannot measure AC quantities.
The cost of these instruments is high as compared to moving iron instruments.
Page updated
Google Sites
Report abuse