Permanent Magnet Moving Coil or PMMC Instrument

Definition

Instruments that utilize a permanent magnet to generate a stationary magnetic field within which a coil moves are known as Permanent Magnet Moving Coil (PMMC) instruments. They operate on the principle that torque is exerted on a moving coil situated in the magnetic field of a permanent magnet. PMMC instruments provide accurate results for direct current (DC) measurements.

Construction of PMMC Instrument

The moving coil and the permanent magnet are the key components of a PMMC instrument. The following is a detailed explanation of the parts of a PMMC instrument.

Moving Coil

The coil is the current - carrying component of the instrument that moves freely within the stationary magnetic field of the permanent magnet. When current passes through the coil, it causes the coil to deflect, enabling the determination of the magnitude of current or voltage. The coil is mounted on a rectangular former made of aluminium. This former enhances the radial and uniform magnetic field in the air - gap between the magnetic poles. The coil is wound with silk - covered copper wire between the poles of a magnet.

Magnet System

In a PMMC instrument, a permanent magnet is used to create the stationary magnetic field. Alcomax and Alnico materials are employed for making the permanent magnet due to their high coercive force (the coercive force affects the magnetization property of the magnet). Additionally, these magnets possess high field intensities.

Control

In a PMMC instrument, the controlling torque is provided by springs. These springs are made of phosphorous bronze and are positioned between two jewel bearings. The springs also serve as the pathway for the lead current to flow into and out of the moving coil. The controlling torque is mainly a result of the ribbon suspension.

Damping

The damping torque is utilized to keep the coil's movement in a state of rest. This damping torque is induced by the movement of the aluminium core as it moves between the poles of the permanent magnet.

Pointer & Scale

The pointer is connected to the moving coil. It indicates the deflection of the coil, and the magnitude of this deflection is displayed on the scale. The pointer is made of lightweight material, allowing it to be easily deflected with the movement of the coil. Sometimes, parallax error occurs in the instrument, which can be easily minimized by properly aligning the pointer blade.

Torque Equation for PMMC Instrument

The deflecting torque is induced by the movement of the coil. It is expressed by the following equation.

N – Number of turns of coil
B – flux density in the air gap
L, d – the vertical and horizontal length of the side
I – current through the coil

The spring provides the restoring torque to the moving coil which is expressed as

Where K = Spring constant.

For final deflection,

By substituting the value of equation (1) and (3) we get,

The above equation indicates that the deflecting torque is directly proportional to the current flowing through the coil.

Errors in PMMC Instruments

In PMMC instruments, errors arise due to the effects of ageing and temperature. The main components of the instrument that contribute to these errors are the magnet, spring, and moving coil. The following details the different types of errors:

1. Magnet

Heat and vibration shorten the lifespan of the permanent magnet and also reduce its magnetism, which is the property of attraction or repulsion. A weakened magnet results in decreased deflection of the coil.

2. Springs

A weakened spring increases the deflection of the moving coil within the permanent magnet. As a result, even for a small current value, the coil shows a large deflection. The spring weakens due to temperature effects; a one - degree rise in temperature reduces the spring's lifespan by 0.004 percent.

3. Moving Coil

When the range of the coil is extended beyond the specified limit using a shunt, errors occur. This is due to the change in the coil resistance relative to the shunt resistance. Since the coil is made of copper wire with high shunt resistance and the shunt wire is made of Manganin with low resistance, this mismatch causes errors.

To mitigate this error, a swamping resistance is connected in series with the moving coil. A swamping resistance is a resistor with a low - temperature coefficient, which reduces the impact of temperature on the moving coil.

Advantages of PMMC Instruments

The following are the advantages of PMMC instruments:

The scale of PMMC instruments is accurately calibrated.
The power consumption of these devices is very low.
PMMC instruments have high accuracy owing to their high torque - to - weight ratio.
A single device can measure different ranges of voltage and current by using multipliers and shunts.
PMMC instruments use shelf - shielding magnets, which are beneficial for aerospace applications.

Disadvantages of PMMC Instruments

The following are the disadvantages of PMMC instruments:

PMMC instruments are only suitable for direct current. Alternating current varies with time, and the rapid variation of current changes the torque of the coil. However, the pointer cannot follow the fast reversals and deflections of the torque, so it cannot be used for AC.
The cost of PMMC instruments is significantly higher compared to other moving - coil instruments.
The moving coil itself provides electromagnetic damping. This electromagnetic damping opposes the motion of the coil as a result of the interaction between eddy currents and the magnetic field.