Permanent Magnet Moving Coil (PMMC) instruments are very sensitive and accurate. These instruments can be used only on d.c. as voltmeter and ammeter. These instruments have uniform scale.
The principle on which a Permanent Magnet Moving Coil (PMMC) instrument operates is that a torque is exerted on a current-carrying coil placed in the field of a permanent magnet.
It consists of a powerful U shaped permanent magnet. Alcomax and Alnico are used for making magnet. These materials have a high coercive force. The coil has a number of turns of thin insulated wires wound on a rectangular aluminium former. The frame is carried on a spindle mounted in jewel bearings. An iron core is inserted in between the coil sides to reduce reluctance for the magnetic lines of force.
Two phosphor bronze spiral hair springs are attached to the spindle. The springs provide the controlling torque as well as they act as incoming and outgoing leads for the current.
Eddy current damping is provided by the aluminium former.
The pointer is carried by the spindle and moves over a graduated scale. The pointer is of light-weight construction and apart from those used in some inexpensive instruments.
When the instrument is connected in 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 deflects over the calibrated scale.
If the current in the coil is reversed, the direction of the deflecting torque will be reversed. Since the direction of the field of the permanent magnet is the same. This will give a wrong direction and the pointer will try to deflect below the zero. Thus, such instruments can not be used on a.c. but can only be used on d.c.
When current is passed through the coil, forces acts on its both sides which produce the deflecting torque.
B = flux density in Tesla in the air gap;
L = effective length of the coil side in metres;
N = number of turns of the coil;
r = distance in metres between centre of the coil and force;
I = current flowing through the coil in ampere.
For N turns, force on each side of the coil is, F = B*I*l*N Newtons
Deflecting Torque = force * perpendicular distance
= 2 B*I*l*N
Since all other quantities are constant except I ;
∴ Td = I
The controlling torque is provided by the springs
∴ Tc = θ
In steady position of deflection Tc = Td ;
θ = I
Hence such instruments have uniform scale and the deflection of the instrument is directly proportional to the operating current.
Advantages of PMMC Instruments
- These instruments are sensitive to small current
- PMMC instruments are very accurate and reliable
- These instruments have uniform scale up to 270° or more
- Such instruments are very effective built in damping
- These instruments have low power consumption.
- Free from hysteresis and not affected by external fields because its permanent magnet shields the coil from external magnetic fields
Disadvantages of PMMC Instruments
- This type of instrument can be operated in direct current only.
- These instruments are costlier in comparison of moving iron instruments.
- Friction and temperature might introduce some errors.
Errors in PMMC instruments
The main source of errors in these instruments are due to
- Friction of the moving system while it moves.
- Change of resistance of the moving coil with temperature.
- Weakening the stiffness of springs due to ageing and rise in temperature.