PMMC

A PMMC is also known as a D’arsonval meter which is used to measure current through a coil when the angular deflection takes place in the coil. It is accurate for DC measurement of current and voltage. Since they are polarity-sensitive, they are used to measure DC currents only. By adding some additional components to the PMMC, It can be used for other applications like ohm meter, ammeter, etc. At the end of this article, you will get a good understanding of PMMC instruments.

What is PMMC?

A Permanent Magnetic Moving Coil (PMMC) meter – also known as a Galvanometer or D’Arsonval meter is an instrument that measures the current through the coil by the coil’s angular deflection.

A PMMC is a conductor that is placed between two magnetic fields to create a stationary magnetic field. The strength of this force will be proportional to the amount of current through the wire which a pointer is attached to the end of the wire. The torque force which is balanced by the moving coil and its angular deflection can be measured by the scale. So basically it operates on the principle that the torque that is exerted on the coil- placed in the magnetic field.

PMMC METER

Working Principle of PMMC

A PMMC type instruments works on the principle which is the 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. It works on the electromagnetic effect ,as the coil carries the current and magnet is used to produce the flux ,a pointer is also connected to coil which gets deflected according to proportion of the current in that coil . The force in the field generates the deflection force, a damping force generates which maintains the pointer stable and thus equilibrium attained by controlling and deflection torque in order to provide the accuracy in the environment.

The magnetic field interaction with permanent magnet results in producing torque to the conductor (coil). The amount of torque produced is directly proportional to current passing the conductor (coil). The coil is attached to a pointer that points to the scale. The rotational force (torque) moves the pointer along a scale to get the reading of measurement it represents.

Construction of PMMC

• The Permanent Magnet (M) and a Rectangular Coil (C) which consists an insulated copper wire wound on the light aluminium frame which is fitted with a polished steel pivot resting in jewel brings.
• 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) supported by a brass plate.
  
  

  Construction of PMMC

Where, the instruments consists of

• M – Permanent magnet
• PP – Soft iron pole pieces
• A – Soft iron cylinder(central core)
• C – Rectangular Coil
• B – Spiral Springs
• D – Pointer

Permanent Magnet (M)

The Permanent magnet is that where the magnet is retain its Magnetic property without any inducing field or current. This Permanent magnet is used in PMMC in order to create a Stationary objects for which the coil move. It is the most accurate type measurement in Direct Current (D.C.) measurements of current and voltage.

Soft Iron Pole Pieces (PP)

This soft iron pole pieces is fixed between the air gaps in which the magnet are surrounding in it. It increase the magnetic flux which is passing through the coil, this instrument makes more sensitive to the small current. It is also the direct the magnetic field produced by the coil.

Soft Iron Cylinder (A)

It strengthen the magnetic field but also attracts the magnetic field of lines which increases the soft iron core. Soft iron cylinder strengthen the magnetic field of lines which increases the number of line in the core.

Rectangular Coil (C)

This rectangular coil is wounded by aluminium on wire which is copper acting as moving elements. Its help to retains its magnetic property. This is to make the uniform field and reduce reluctance of magnetic circuits.

Spiral Springs (B)

The spiral springs used in PMMC is used for controlling the torque. which these springs are made up phosphorous bronze and placed between these two jewel bearing. This lead to the current to flow in and out of the coil.

Pointer (D)

In which the pointer is linked with the moving coil. The pointer which shows the deflection of the coil and the magnitude of their deviation on the scale. hence, it is easily deflected with movement of the coil.

Function of Central Core

• To intensifying the magnetic field by reducing the length of the air gap between the soft iron pole pieces and a soft iron cylinder across which the Magnetic Flux has to pass.
• To give a radial Magnetic Flux of uniform density, there by enabling the scale to be uniformly divided.
• The Movement of the coil is controlled by two Phosphor Brome hair springs (BB), which additionally serve the purpose of leading the current in and out of the coil.
• The two springs which are spiraled in opposite direction for neutralizing the effect of the change in temperature.

Types of Errors in PMMC

In PMMC, error can be occurred that affects the accuracy of the measurement. The most common error occurs in moving coil instruments are due to the following types of errors.

• Friction Error: Friction error in PMMC occurs due to the friction in pivot and jewel bearing. It leads to affect the pointer movement that causes the pointer to point incorrect reading in the scale.
• Parallax Error: Parallax error is an observational error. This error occurs when the observer’s line of sight to pointer and scale is not accurate. The observer’s eye should align straight to the pointer and scale to avoid this error.
• Hysteresis Error: This error occurs due to change in external conditions. Hysteresis leads to provide different value (increased or decreased) at different external conditions like occurrence of external magnetic field. This error can be avoided by calibrating the pointer to zero when no input current and voltage at different external conditions.
• Damping Error: Damping in PMMC is the resistance to the motion of the coil occurred due to the friction or air. Variations in damping, whether increased or decreased damping can cause error in PMMC instrument that may provide incorrect measurement readings.
• Error due to Ageing of permanent magnets: Due to the use of the permanent magnets, it causes its weakening since the ageing there by resulting in a reduced magnetic field. Because of the decreases in magnetic field which reduce the deflecting torque that has to be produced. Therefore, there is a decrease in pointer deflection and the reading will obtain lesser than the actual value.
• Error due to Ageing of springs: This ageing of springs is occurred in the readings obtained. Due to ageing the spring losses its ability to tolerate the bending or twisting. Due to which the control torque is produced on the pointer decreases so the pointer deflects more than the actual. Whereas, the ageing of springs causes the reading obtained to be more than the actual value.
• Error due to Temperature variations: This variations in surrounding temperature can introduce errors in moving coil instruments. There are two variations that is, it affects the stiffness of the spring and the flux density in the air gap. The resistance of the conductor used for moving coil varies, though changing the amount of current flowing through the coil effects the deflection of pointer.

Deflecting Torque

When the current is passed through the coil, the forces are set up on its both sides which produce deflection torque. If the current measured in ampere (I) the current passing through the coil.

The Magnitude of the force (F) experienced by both sides of the coil and it is given by,

F = BIL Newton

where,

• B – flux density wb/m
• L – length of the coil in meters.

for N – turns the equation can be written as,

F = N BIL

Deflecting torque (Td) = Force x perpendicular distance.

Td = N BIL x b

Td = NBI (l x b)

Deflecting torque (Td) = NBI A

If B is constant

Deflecting torque (Td) = KI

K is constant for the given instrument

where,

• b- breadth of the coil in meter
• A- Face area of the coil.

Controlling Torque

Controlling torque of the spring (Tc) ∝ Angular deflection θ

Tc ∝ θ

Tc = C × θ

Here, C is constant of spiral spring.

For steady state deflection, Tc = Td.

We know that Td = K I.

C × θ = K I

Angular deflection, θ = KI/C

The steady state deflection is proportional to current and the scale is therefore uniformly divided. PMMC instrument is unsuitable for A.C measurements unless the current is rectified.

Applications of PMMC Instruments

The PMMC system can be converted into an instrument to measure the A.C as well as D.C quantities like current, voltage and resistance.

• It is used to detect small currents in DC galvanometers.
• It is used in ammeter.
• It is used in voltmeter.
• It is used in ohmmeter.
• It is used in galvanometer.
• It is used to measure the change in magnetic flux linkage in galvanometer.

PMMC can be used for various measurement application to measure various quantities by adding additional components in the circuit of PMMC.

The various modifications of PMMC meter movement are,

D.C. instruments

• D.C ammeter – By using a Shunt resistor.
• D.C Voltmeter – By using series multiplier resistor.
• Ohm meter – By using battery and series resistor.

A.C. instruments

• A.F ammeter (or) Voltmeter – By using rectifier
• R.F ammeter (or) Voltmeter – By using a thermocouple

Temperature Compensation

Temperature may be compensated by the use of series and shunt resistors. The Swamping resistor is made up of manganin and combined with copper in the ratio of 20:1 to 30:1.

The total resistance of the coil and the swamping resistor increases slightly with rise in temperature but only to contract the change of springs and magnet. Hence, that the overall temperature is zero.

Characteristics of PMMC

• Full scale deflection current (IM) : It is the current needed to deflect the pointer all the way to right to the last mark on the calibrated scale. The maximum value of the measured quantity for which a measuring instrument is calibrated.
• Internal resistor (RM) : It is D.C Ohmic resistance of the wire of the moving coil, RM ranges from 1-2 ohm for 30mA movement to 2 KΩ for 50k.Internal resistor refers to the opposition to the flow of current offered by the cells and batteries themselves resulting in the generation of heat.
• Sensitivity (S) : The sensitivity of PMMC instrument instrument is 10kΩ/v per volt. This instrument is used in rectifier type voltmeter with half wave rectification the sensitivity will became kΩ/v.

Advantages of PMMC

• No hysteresis loss
• High torque weight ratio
• Range can be extended with shunts or multipliers.
• Low power consumption
• The scale of this instrument is divided equally .
• High accuracy .

Disadvantages of PMMC

• PMMC instrument is costlier.
• This instrument cannot be used for A.C. measurements.
• Errors due to ageing of control springs and permanent magnets, friction and temperature.

Conclusion

In this article, we studied about PMMC instrument and its working principle. The PMMC instrument is a valuable and versatile device used for measuring direct current (DC) in various applications. The PMMC instrument can be used to measure various electrical parameter like current, resistance, voltage and etcetera by adding additional relevant component to the circuit. The measurements through the PMMC can be read by using the deflection angle of pointer along a scale.

FAQs on PMMC

Why PMMC does not use for A.C instruments ?

If Alternating current supplied, a reversing torque is produced. This torque cannot produced continuous deflection. So, it is not used in A.C. instruments.

Which type of damping is used in PMMC ?

Eddy current damping is used in PMMC in which the damping torque is provided. So, there is a presence of Permanent magnet.

What is PMMC also known as ?

PMMC is also known as D’Arsonval meter (or) Galvanometer.