Several electrical machines and panels are fitted onboard so that the ship can sail from one port to another, safely and efficiently. The electrical machinery and system require scheduled maintenance and checks to avoid any kind of breakdown during sailing.
Different instruments are used onboard for measuring several electrical parameters to analyse and keep these machinery in proper running condition. A permanent magnet moving coil (PMMC) is one such instrument which is popularly used onboard and has several applications. The other popular nomenclature of this instrument is D’alvanometer and galvanometer.
Permanent Magnet Moving Coil: Principle of Working
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.
Fleming 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.
The interaction between the induced field and the field produced by the permanent magnet causes a deflecting torque, which results in rotation.
The three important torque involved in this instrument are:
The force F which will be perpendicular to both the direction of the current flow and the direction of magnetic filed as per the fleming’s left hand rule can be written as
F = NBIL
where N: turns of wire on the coil
B: flux density in the air gap
I: current in the movable coil
L: vertical length of the coil
Theoretically the torque (here electro-magnetical torque) is equal to the multiplication of force with distance to the point of suspension.
Hence Torque on left side of the cylinder TL = NBIL x W/2 and torque on right side of the cylinder TR = NBIL x W/2
Therefore the total torque will be = TL + TR
T = NBILW or NBIA where A is effective area (A= LxW)
This torque is produced by the spring action and opposes the deflection torque so as the pointer can come to rest at the point where these two torques are equal (Electromagnetic torque = control spring torque). The value of control torque depends on the mechanical design of spiral springs and strip suspensions.
The controlling torque is directly proportional to the angle of deflection of the coil.
Control torque Ct =Cθ where, θ = deflection angle in radians and C = spring constant Nm /rad .
This torque ensures the pointer comes to an equilibrium position i.e. at rest in the scale without oscillating to give accurate reading. In PMMC as the coil moves in the magnetic filed, eddy current sets up in a metal former or core on which the coil is wound or in the circuit of the coil itself which opposes the motion of the coil resulting in slow swing of pointer and than come to rest quickly with very little oscillation.
A coil of thin wire is mounted on an aluminum frame (spindle) positioned between the poles of a U shaped permanent magnet which is made up of magnetic alloys like alnico.
The coil is pivoted on the jewelled bearing and thus the coil is free to rotate. The current is fed to the coil through spiral springs which are two in numbers. The coil which carries a 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 which shows the measured value.
When a current flow through the coil, it generates a magnetic field which is proportional to the current in case of an ammeter. The deflecting torque is produced by the electromagnetic action of the current in the coil and the magnetic field.
When the torques are balanced the moving coil will stopped and its angular deflection represent the amount of electrical current to be measured against a fixed reference, called a scale. If the permanent magnet field is uniform and the spring linear, then the pointer deflection is also linear.
The controlling torque is provided by two phosphorous bronze flat coiled helical springs. These springs serve as a flexible connection to the coil conductors.
Damping is caused by the eddy current set up in the aluminum coil which prevents the oscillation of the coil.
The PMMC has a variety of uses onboard ship. It can be used as:
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.
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
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.
5) Ohm Meter:
The ohm meter 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.
- 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.
- 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’s very delicate and sometimes uses ac circuit with a rectifier.
- It’s costly as compared to moving coil iron instruments.
- It may show error due to loss of magnetism of permanent magnet.
What are different reason that causes error in PMMC?
- Temperature effect: Error in the reading of the PMMC may cause due to change in the temperature which will effect the resistance of the moving coil. The temperature coefficients of the value of coefficient of copper wire in moving coil is 0.04 per degree celsius rise in temperature. Since the coil has a lower temperature coefficient, it will have a faster rate of temperature rises which will result in increase in the resistance causing an error
- Spring material and age: The other factor which may lead to error in the PMMC reading is the quality and contortion of the spring. Old ageing spring will not allow the pointer to show the correct reading making an error.
- Ageing of Magnet: Along with the age, the effect of heat and vibration will reduce the magnetic effect of the permanent magnet which will produce an error in the reading.
Can PMMC be used to measure AC?
If the frequency is low enough, the PMMC with addition to a rectifier can be used to measure AC which convert the measured quantity into a dc current typically less than 1mA. Add an appropriate scale and you have a meter.
What will happen if I use it on high frequency AC?
If frequency of ac is high, the meter just vibrates in and around the zero value (preferable to verify with a centre zero meter), and finally stops responding to ac
What are permanent magnets made of?
Permanent magnets are made of special alloys such as :
- Aluminum-Nickel-Cobalt (Alnicos)
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