Moving Coil Instrument Working Principle
A moving coil meter is a very commonly used form of an analog instrument because of its sensitivity, accuracy, and linear scale, although it only responds to d.c. signals.
The simple view of the construction of the moving coil instrument is shown in the figure. It consists of a powerful permanent shoe magnet. A light rectangular coil of many turns of fine wire is wound on a light aluminum former. An iron core is inserted inside the coil to reduce reluctance for the magnetic lines of force. The coil is mounted on the spindle and acts as the moving element.
Two phosphor bronze spiral hairsprings are attached to the spindle. The springs provide the controlling torque as well as act as incoming and outgoing leads for the current. Eddy current damping is provided by the aluminum former.
Moving Coil Instrument Working Principle
A moving coil instrument works on the principle of a DC motor. Which states that when a current-carrying conductor is placed in a magnetic field, a force F is exerted on the conductor, given by F = BIl.
When the flux density B is constant and the conductor length (i.e. coil length) is fixed then the force will be proportional to the current flowing in the conductor.
When the moving coil instrument is connected to the circuit, the operating current flows through the coil which is mounted on the spindle. Since the coil is placed in the strong field of permanent magnets, a force is exerted on the current-carrying conductors of the coil which produces deflecting torque. Thus the pointer attached to the spindle is deflected over the calibrated scale.
If the current in the coil is reversed, the direction of deflecting torque will be reversed because the field produced by the permanent magnets remains the same. This will give a wrong direction of rotation thus the instrument cannot be used on AC. Therefore, they can be used for the measurement of DC only.
The theoretical torque produced is given by T = B I h w N
Where B is the flux density of the radial field, I is the current flowing in the coil, h is the height of the coil, w is the width of the coil, and N is the number of turns in the coil.
If the iron core is cylindrical and the air gap between the coil and pole faces of the permanent magnet is uniform, then the flux density B is constant, and the above equation can be rewritten as
T = K I
That is, torque is proportional to the coil current and the instrument scale is linear.
As the basic instrument operates at low current levels of one milliamp or so, it is only suitable for measuring voltages up to around 2 volts. If there is a requirement to measure higher voltages, the measuring range of the instrument can be increased by placing a resistance in series with the coil, such that only a known proportion of the applied voltage is measured by the meter. In this situation, the added resistance is known as a shunting resistor.
While Figure shows the traditional moving coil instrument with a long U-shaped permanent magnet, many newer instruments employ much shorter magnets made from recently developed magnetic materials such as Alnico and Alcomax.
These materials produce a substantially greater flux density, which, in addition to allowing the magnet to be smaller, has additional advantages in allowing reductions to be made in the size of the coil and in increasing the usable range of deflection of the coil to about 120o.
Some versions of the instrument also have either a specially shaped core or specially shaped magnet pole faces to cater to special situations where a nonlinear scale, such as a logarithmic one, is required.
Advantages of Moving Coil Instruments
- They have a uniform scale.
- They are very effective and reliable.
- In these instruments, eddy current damping is used. As the former is made of aluminum, no hysteresis loss occurs in that.
- Since driving power is small they consume very low power.
- As the working field provided by the permanent magnets is very strong, stray magnetic fields do not affect them.
- They have a high torque/weight ratio, therefore, such instruments require a very small operating current.
- They are very accurate and reliable.
Disadvantages Moving Coil Instruments
- These instruments cannot be used for AC measurements.
- These are costlier in comparison to moving iron instruments.
- Friction and temperature might introduce some errors.
- Some errors are also caused due to the aging of control springs and the permanent magnets.
PMMC Ammeter Ranges
- Without shunt (i.e. instrument alone) 0 – 5 µA to 0 – 50 mA.
- With internal shunts, up to 0 – 200 A.
- With external shunts, up to 0 – 5000 A.
PMMC Voltmeter Ranges
- Without series resistance or multiplier (i.e. instrument alone) 0 – 50 mV.
- With series resistance, 0 – 30,000 V.
Errors in Moving Coil Instruments
The main sources of errors in moving coil instruments are due to:
- weakening of permanent magnets due to aging at temperature effects.
- weakening of springs due to aging and temperature effects.
- change of resistance of the moving coil with temperature.
Magnets: The magnets are aged by heat and vibration treatment. This process results in a loss of initial magnetism and may introduce some errors in the instrument.
Springs: The weakening of springs with time can be reduced by careful use of material and pre-aging during manufacture. However, the effect of the weakening of springs on the performance of the instrument is opposite to that of magnets.
The weakening of magnets tends to decrease the deflection for a particular value of current while the weakening of springs tends to increase the deflection.
In PMMC instruments, a 1oC increase in temperature reduces the strength of springs by about 0.04 percent and reduces flux density in the air gap of the magnet by about 0.02 percent per oC. Thus the net effect on the average is to increase the deflection by about 0.02 percent per oC.
Moving Coil: The moving coil of the measuring instrument is usually wound with a copper wire having a temperature coefficient of 0.004/oC. When the instrument is used as a micro-ammeter or a milli-ammeter and the moving coil is directly connected to the output terminals of the instruments, the indication of the instrument for a constant current would decrease by 0.04 percent per oC rise in temperature.
When the instrument is used as a voltmeter a large series resistance of negligible temperature coefficient (made of a material like manganin) is used. This eliminates the error due to temperature. This is because of copper coil forms a very small fraction of the total resistance of the instrument circuit and thus any change in its resistance has a negligible effect on the total resistance.
A situation when the instrument’s current range is extended by using a shunt is, however, different. The main source of error, in this case, is due to the relatively larger change in the resistance of the copper moving coil as compared to that of the manganin shunt. This happens because copper has a much higher resistance temperature coefficient as compared to manganin.
To reduce the error in this situation, it is usual to include in series with moving coil a ‘swamping resistance’ of manganin so that the copper coil forms only a small fraction of the total resistance comprising the coil and the additional swamping resistance. This swamping resistance is also used for the final calibration of the ammeter.
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