PMMC Instrument : Construction, Working & Its Applications

In this lecture, we are going to learn about the PMMC instrument. How the PMMC instrument works, Its principle of working, the Construction of the PMMC instrument, and advantages, and disadvantages of the PMMC instrument. So let’s start the lecture with the full form of PMMC and its definition.

PMMC Instrument

• The term PMMC is the short form of “permanent magnetic moving coil”. These instruments use permanent magnets to create the stationary magnetic field in the coils. Then it is used with the moving coil that is connected to the electric source for generating deflection torque according to the Fleming left-hand rule theory.

Working principle of PMMC Instrument

• Whenever a current-carrying conductor is located within a magnetic field, it experiences a force that is perpendicular to the current & the field. Based on the rule of “Fleming left hand”, if the thumbnail of the left hand, middle, and forefinger are at 90 degrees with each other.

• After that the magnetic field will be in the forefinger, the flow of current will be across the middle finger and finally, the force will be through the thumb finger. Once the current flow within the coil on the aluminum former, the magnetic field can be generated in the Tesla coil in proportion to the current flow.

Construction and Working of PMMC Instrument

• The construction of the PMCC instrument can be done using several parts where the permanent magnet and moving coils are essential parts.

1. Moving Coil: The design of this coil can be done by wounding copper coils to a rectangular block among the magnetic poles. It is made with Aluminium and the rectangular block can be called Aluminium former rotated into the jewel bearing. The aluminum former is a non-metallic former used to measure current, while the metallic former including high electromagnetic damping is used to calculate the voltage.

2. Magnet system: Two permanent magnets of high-intensity magnetic field mostly soft iron cylinder used.

3. Control spring: torque can be controlled due to the springs which are fabricated with phosphorous bronze. These springs are arranged among the two jewel bearings. These springs are responsible for producing controlling torque.

4. Damping torque: Damping torque can be generated within the PMMC instrument using the aluminum core’s movement within the magnetic field. The aluminum frame is responsible for producing damping torque.

5. Pointer and Scale arrangement: The connection of the pointer can be done through the moving coil. It notices the moving coil’s deflection. The magnitude of their derivation can be displayed on the scale. The pointer is designed with lightweight material.

Torque Equation for PMMC Instrument

• Torque produced in PMMC is given by, The deflecting torque induced due to the coil’s movement and this can be expressed with the equation shown below;

T_d = NBLdI        …..(i)

Where,

‘N’ is the no. of turns in the coil

‘B’ is the density of flux within the air gap

‘L’ & ’d’ are vertical as well as horizontal lengths of the surface

‘I’ is the flow of current in the coil

• The restoring torque can be provided to the moving coil can be done with the spring and it can be expressed as

T_c = K\theta (‘K’ is the spring constant)     …..(ii)

• Final deflection can be done through the equation T_c = T_d

• Substitute the values of T_c \;and\; T_d in the above equation, then we can get

K\theta = NBLdI    ….(iii)

• Equation (iii) implies that deflection is directly proportional to the current flowing and hence scale in PMMC is linear. 

Errors in PMMC instruments

1. Error Due to Ageing of Permanent Magnets: The aging of permanent magnets is due to the loss of their magnetism with time. Also the effect of heat and vibration reduces the magnetic strength and coil deflection resulting in an error in the PMMC reading. To overcome these errors, the swamping resistance is connected to the moving coil in series. Where the swamping resistance is the resistor that has a low-temperature coefficient. This swamping resistance reduces the effect of temperature on the coil.
2. Error due to Spring: An error in the output reading of the PMMC depends on the contortion (twisting or bending) and quality of the spring. The pointer of the PMMC instrument doesn’t show the accurate reading due to the aging of the spring and hence causes an error. The errors caused by the permanent magnets and the errors caused by the aging of the spring are opposite and can be compensated.
3. Error due to Temperature: As the temperature changes, the resistance of the moving coil changes, which causes an error in the output reading of the PMMC instrument. The temperature coefficient of the copper wire in the moving coil is 0.04°C rise in temperature. The rate of temperature rises very fastly due to the lower temperature coefficient of the moving coil. Hence the resistance increases and causes an error.

Advantages of PMMC Instrument

• The scale in the instrument can be divided properly
• It generates no losses because of hysteresis.
• It uses less power
• It is not influenced by the stray magnetic field.
• High accuracy
• It is used as a voltmeter/ammeter with appropriate resistance.
• This instrument can measure the voltage & current with different ranges
• This instrument uses a shelf shielding magnet so it is applicable in aerospace.

Disadvantages of PMMC Instruments

• It works with only DC
• It is expensive compared with other alternative instruments
• It is delicate
• It shows an error because of the magnetism loss in the permanent magnet.

Application of PMMC Instruments

• The few applications of PMMC Instrument are given below;

• It is used in the measurement of direct voltages and currents
• Used to detect small currents in DC galvanometers.
• It is used to measure the change in magnetic flux linkage in Ballistic galvanometers
• It is used as an Ammeter
• It is used as a Voltmeter
• It is used as an Ohm meter
• It is used as a Galvanometer.
• At low frequencies, the PMMC instrument along with the rectifier is used to measure AC by converting it into DC (<1mA).

Frequently Asked Questions (FAQs) of PMMC Instrument

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