Permanent Magnet Moving Coil [PMMC] Instruments

PERMANENT MAGNET MOVING COIL [PMMC] INSTRUMENTS

The permanent magnet moving coil instruments are the most accurate type of DC measurements. The working principle of these instruments is same as that of a DC motor. When a current carrying conductor (coil) is placed in a magnetic field, the conductor (coil) experiences a force and tends to move. Hence, as the coil is moving and the magnetic field is produced by a permanent magnet the instrument is called as permanent magnet moving coil instrument. This basic principle is called as principle is called as D'Arsonal principle. The amount of force experienced by the conductor (coil) is directly proportional to the current passing through the coil. The PMMC instrument is a direct reading type, provided with a pointer and a scale.

Construction of permanent Magnet Moving Coil

Fig. 6.7 shows the general constructional features of a permanent magnet moving coil [PMMC] instrument.

1. Moving Coil

The moving coil is wound with many turns of enamelled (or) silk covered copper wire on a rectangular aluminium former which is pivoted on jewelled bearings. The coil can move freely in the field of a permanent magnet. Most voltmeter coils are wound on metal frames to provide the required electromagnetic damping. Most ammeter coils are wound on non magnetic formers, because coil turns are effectively shorted out by the ammeter shunt. Therefore the coil itself provides electromagnetic damping.

2. Magnet System

Normally a concentric structure is used, which is two sector shaped Alnico magnets, having round cylindrical faces. They are placed inside a soft iron steel ring and the assembly is diecast together. So the steel ring provides a return path for the magnetic circuit and shields the assembly from stray magnetic fields.

3. Control

The control torque is provided by two phosphor bronze hair springs. These springs also serve as leads to the coil.

4. Damping

Eddy current damping is produced by the movement of the aluminium former moving in the magnetic fields of the permanent magnet.

5. Pointer and Scale

The pointer is carried by the spindle and moves over a graduated scale.

Principle of Operation of PMMC Instrument

A current in the moving coil causes motor action as the coil is controlled by the control springs. Thus the necessary deflecting torque is produced, by which the moving coil deflects and a pointer attached to the spindle moves over a graduated scale. If the direction of the current in the coil is changed, then the direction of motion of the coil also changes as the magnetic field is a permanent magnet.

Thus in permanent magnet moving coil instruments, the operating torque depends on the direction of the operating current. This type of instruments are polarised. So, if PMMC meters are connected for AC measurements, then the deflecting torque is a pulsating one and the meter will not read. That is, the meter will read the average value of the measuring quantity.

Torque Equation

Let,

l = Length of vertical side of the coil in metre.

d = Length of horizontal side of the coil in metre.

A = area of the coil.

N = number of turns in the coil.

B = flux density in the coil.

I = current in the coil.

K_S = spring constant.

ϕ = Angle of deflection.

∝ = angle between the direction of magnetic field and conductor.

T_d = deflecting torque.

Tc = controlling torque.

G = displacement constant = NBA

The equation for deflecting torque can be obtained from the basic law of electromagnetic torque and given by,

Deflecting torque, T_d = Force × distance

= NBIA

T_d= GI

Where,

G=NBA

The controlling torque is provided by the spring and is proportional to the angular deflection of the pointer.

Controlling torque, Tc = K_Sθ

At the final steady state position of the pointer, Tc = T_d

K_Sθ = GI

 θ = (G/K_S)I

 I ⇒ θ ∝ I

Thus, the deflection is directly proportional to the current passing through the coil. The pointer deflection can therefore be used to measured current.

Shape of Scale

As the angle of deflection is directly proportional to the operating current, the scale of the PMMC instrument is linear (uniform).

Sources of error

1. Weakening of permanent magnets due to ageing and temperature effects.

2. Weakening of springs due to ageing and temperature effects.

3. Change of resistance of the moving coil with temperature.

4. When the instruments range is extended in an moving coil instrument with a manganic shunt, the error is relatively larger due to the larger change in resistance of copper moving coil to that of the manganic shunt.

Advantages of PMMC Instrument

1. Uniform scaling

2. The power consumption is very low (25 microwatts to 100 microwatts).

3. The torque - weight ratio is high which gives a high accuracy. The accuracy is of the order of generally 2 percent of full scale deflection.

4. A single instrument may be used for many different current and voltage range by of using different values of shunts and multipliers, respectively.

5. The operating forces vare larger due to large flux densities and so the error due to stray magnetic fields is small.

6. Self shielding magnets make the core magnet mechanism useful in air‒craft and aerospace applications where a multiplicity of instruments must be maintained in close proximity to each other.

7. Sensitivity is high.

8. It has high accuracy.

9. Instrument is free from hysteresis error.

Limitations of PMMC Instrument

1. These instruments are useful only for DC measurements. The torque reverses as the current reverses and so when it is used with AC the pointer cannot follow the rapid reversals. Since the deflection corresponds to the mean torque the pointer reads zero.

2. Ageing of permanent magnet and the control springs, introduces the errors.

3. The cost of these instruments are higher than the moving iron [MI] instruments.

4. The friction due to jewel‒pivot suspension, introduces errors. [This can be eliminated by using Tact Band suspension in the Instruments].