A permanent-magnet moving coil-type instrument works on the principle that “when a current-carrying conductor is placed in a magnetic field, it is acted upon by a force which tends to move it to one side and out of the field”. Construction
- The instrument consists of a permanent magnet M and a rectangular coil C which consists of insulated copper wire wound on light aluminium frame fitted with polished steel pivots resting in jewel bearings. The magnet is made of Alnico and has soft-iron pole-pieces PP which are bored out cylindrically.
- The rectangular coil C is free to move in air gaps, between the soft-iron pole pieces and a soft-iron cylinder A (central core), supported by a brass plate (not shown).
The functions of the central core A are: (i) To intensify the magnetic field by reducing the length of air gap across which the magnetic flux has to pass. (ii) To give a radial magnetic flux of uniform density, thereby enabling the scale to be uniformly divided. - The movement of the coil is controlled by two phosphor bronze hair springs BB (one above and one below), which additionally serve the purpose of leading the current in and out of the coil. The two springs are spiralled in opposite directions for neutralizing the effects of changes in temperature. - The aluminium frame not only provides support for the coil but also provides damping by eddy currents induced in it. Deflecting torque. Refer Fig. 12. When current is passed through the coil, forces are set up on its both sides which produce deflection torque. If I amperes is the current passing through the coil, the magnitude of the force (F) experienced by each of its sides is given by F = BIl newton where B = flux density in WB/m2, and l = length or depth of coil in metres.
Advantages and Disadvantages. The moving-coil permanent-magnet type instruments have the following advantages and disadvantages: Advantages: (i) Low power consumption. (ii) Their scales are uniform. (iii) No hysteresis loss. (iv) High torque/weight ratio. (v) They have very effective and efficient eddy-current damping. (vi) Range can be extended with shunts or multipliers. (vii) No effect of stray magnetic field as intense polarised or unidirectional field is employed. Disadvantages: (i) Somewhat costlier as compared to moving-iron instruments. (ii) Cannot be used for A.C. measurements. (iii) Friction and temperature might introduce errors as in case of other instruments. (iv) Some errors are set in due to the ageing of control springs and the permanent magnets. Ranges: D.C. Ammeters i. Without shunt …. 0/5 micro-amperes up to 0/30 micro-amperes. ii. With internal shunts …. upto 0/2000 amperes. iii. With external shunts …. upto 0/5000 amperes. D.C. Voltmeters i. Without series resistance …. 0/100 milli-volts. ii. With series resistance …. upto 20000 or 30000 volts. - Moving-coil permanent-magnet instruments can be used as : i. Ammeters …. by using a low resistance shunt. ii. Voltmeters …. by using a high series resistance. iii. Flux-meters …. by eliminating the control springs. iv. Basllistic galvanometers …. by making control springs of large moment of inertia. Extension of Range. The following devices may be used for extending the range of instruments :
- Current transformers and
- Potential transformers. Use of ammeter shunts and voltage multipliers is discussed below.
1. Ammeter shunts. An ammeter shunt is merely a low resistance that is placed in parallel with the coil circuit of the instrument in order to measure fairly large currents. The greater part of the current in the main circuit is then diverted around the coil through the shunt. The connection diagram for a shunt and milli-ammeter for measuring large currents is shown in Fig. 13. The shunt is provided with four terminals, the milli-ammeter being connected across the potential terminals. If the instrument were connected across the current terminals, there might be considerable error due to the contact resistance at these terminals being appreciable compared with the resistance of the shunt. The shunts are made of a material such as manganin (copper, manganese and nickel), which has a negligible temperature coefficient of resistance. The material, is employed in the form of thin strips, the ends of which are soldered to two large copper blocks. Each copper block carries two terminals-one current terminal and other potential terminal. The strips which form the shunt are spaced from each other to promote a good circulation of air and thus efficient cooling. Note. A ‘swamping’ resistor r, of material having negligible temperature co-efficient of resistance, is connected in series with the instrument (moving coil). The latter is wound with copper wire and the function of r is to reduce the error due to variation of resistance of the instrument with variation of temperature. 2. Voltmeter multipliers. The range of the instrument, when used as a voltmeter can be extended or multiplied by using a high non-inductive series resistance R connected in series with it as shown in Fig. 14.
Let I = full scale deflection current of volt meter, V = voltage of the circuit to be measured, Rv = resistance of the voltmeter, and R = external series resistance. Now, voltage across supply leads = voltage drop across the voltmeter + voltage drop across external resistance Example 2. A moving-coil milli-ammeter having a resistance of 10 ohms gives full scale deflection when a current of 5 mA is passed through it. Explain how this instrument can be used for measurement of: (i) Current upto 1 A. (ii) Voltage upto 5 V. Solution. Resistance of the milli-ammeter, Rm = 10 Example 3. If the moving coil of a voltmeter consists of 100 turns wound on a square former which has a length of.10 mm and the flux density in the air gap is 0. 09 Wb/m2, calculate the turning moment on the coil when it is carrying a current of 10 mA.
Solution. Number of turns, N = 100
Length of each side, 1= 30 mm : 0.03 m
Flux density, B : 0.09 Wb/m2
Current through the coil, I = 10 mA = 0.01 A
We know that the force on each side of the coil,
F = NBIl newton
Turning moment (i.e., deflecting torque),
T = F × breadth = F × I: NBIl2 N-m
= 100 × 0.09 × 0.01 × (0.03)2 N-m
= 8.1 × 10-5 N-m. (Ans.)
Example 4. A mooing-coil instrument has a resistance of 5 n between terminals and full-scale deflection is obtained with a current of 0.015A This instrument is to be used with a manganin shunt to measure 100 A full scale. Calculate the error caused by a 20°C rise in temperature.
(i) When the internal resistance of 5Ω is due to copper only.
(ii) When a 4 Ω manganin swamping resistor is used in series with a copper resistor of 1 Ω.
The temperature-resistance co-efficients are:
Copper: ac = 0.4% per °C, Manganin: αm = 0.015% per °C.
Solution. Resistance of the instrument, Rm= 5 Ω
Current through the instrument, Im= 0.015 A
Current to be measured, I= 100 A
Current through the shunt,
Is= I – Im= 100 – 0.015 = 99.985 A
Voltage across the shunt
ImRm= 5 × 0.015= 0.075 V
Example 6. A 15-volt moving iron voltmeter has a resistance of 300 Ω and an inductance of . 0.12 H. Assuming that this instrument reads correctly on D.C., what will be its readings on AC. at, 15 volts when frequency is (i) 25 Hz and (ii) 100 Hz ?
Solution. On D.C., only ohmic resistance is involved and the voltmeter reads correctly. But on AC., it is the impedance of the instrument which has to be taken into account.
(i) When frequency is 25 Hz :
Impedance at 25 Hz,