Variety of instruments are used onboard for measuring parameters of electrical machinery and systems. A permanent magnet moving coil (PMMC) is one such instrument which is popularly used onboard for several applications.
Principle of Operation
When a current carrying conductor is placed in a magnetic field, it experiences a force and tends to move in the direction as per Fleming’s left hand rule.
“If the first and the second finger and the thumb of the left hand are held so that they are at right angle to each other, then the thumb shows the direction of the force on the conductor, the first finger points towards the direction of the magnetic field and the second finger shows the direction of the current in the wire.”
Construction
A coil of thin wire is mounted on an aluminium frame (spindle) positioned between the poles of a U shaped permanent magnet made of magnetic alloys such as alnico.
The coil is pivoted on the jewelled bearing and is free to rotate. The current is fed to the coil through spiral springs, which are two in numbers. The coil current, which is to be measured, moves in a strong magnetic field produced by a permanent magnet and a pointer is attached to the spindle to show the measured value.
When a current flows through the coil, it generates a magnetic field that is proportional to the current when used as an ammeter. The deflecting torque is produced by the electromagnetic action of the current in the coil and the magnetic field.
The controlling torque is provided by two phosphorous bronze flat-coiled helical springs. These springs serve as a flexible connection to the coil conductors.
The eddy current set up in the aluminium coil dampens the oscillation.
Application
The PMMC has a variety of uses onboard ship such as:
1) Ammeter: When PMMC is used as an ammeter, except for a very small current range, the moving coil is connected across a suitable low resistance shunt, so that only small part of the main current flows through the coil.
The shunt consists of a number of thin plates made up of alloy metal, which is usually magnetic and has a low temperature coefficient of resistance, fixed between two massive blocks of copper. A resistor of same alloy is also placed in series with the coil to reduce errors due to temperature variation.
2) Voltmeter: When PMMC is used as a voltmeter, the coil is connected in series with high resistance. Rest of the function is same as above. The same moving coil can be used as an ammeter or voltmeter with an interchange of above arrangement
3) Galvanometer: Galvanometer is used to measure small value of current along with its direction and strength. It is mainly used onboard to detect and compare different circuits in a system.
4) Ohm Meter: The ohmmeter is used to measure resistance of the electric circuit by applying a voltage to a resistance with the help of battery. A galvanometer is used to determine the flow of current through the resistance. The galvanometer scale is marked in ohms and as the resistance varies, since the voltage is fixed, the current through the meter will also vary.
Advantages:
· The PMMC consumes less power and has great accuracy
· It has uniformly divided scale and can cover arc of 270 degree
· The PMMC has a high torque to weight ratio
· It can be modified as ammeter or voltmeter with suitable resistance
· It has efficient damping characteristics and is not affected by stray magnetic field
· It produces no losses due to hysteresis
Disadvantage:
· The moving coil instrument can only be used on D.C supply as the reversal of current produces reversal of torque on the coil
· It is very delicate and sometimes uses AC circuit with a rectifier
· It is costly as compared to moving coil iron instruments
· It may show error due to loss of magnetism of permanent magnet.
