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PMMC Full Form: Permanent Magnet Moving Coil Explained in Detail

Author : Salim Yousaf | Published On : 31 Oct 2025

In the world of electrical and electronic measurements, accuracy and precision are everything. Whether you’re testing a small circuit or measuring current in an industrial setup, the instruments used must deliver reliable results. One such vital instrument is based on PMMC, which stands for Permanent Magnet Moving Coil. The PMMC instrument is widely recognized for its high accuracy, linear scale, and excellent sensitivity in measuring electrical quantities like current and voltage.

In this blog, we will explore the full form of PMMC, its construction, working principle, advantages, disadvantages, and its applications in the electrical field. Let’s dive deep into the fascinating working of the PMMC instrument.

What is the Full Form of PMMC?

The full form of PMMC is Permanent Magnet Moving Coil.

It refers to an analog type of measuring instrument that operates on the principle of the interaction between the magnetic field of a permanent magnet and the magnetic field produced by a current-carrying coil. This interaction generates a torque, causing the coil (and hence the pointer) to move in proportion to the magnitude of the current flowing through the coil.

PMMC instruments are primarily used as DC measuring devices. However, they can also be adapted for AC measurements when used with rectifiers.

Construction of PMMC Instrument

The PMMC instrument consists of the following main components:

  1. Permanent Magnet:
    The permanent magnet provides a constant magnetic field. It is usually made from materials like Alnico (an alloy of aluminum, nickel, and cobalt). The magnetic field ensures consistent and accurate movement of the coil when current flows through it.

  2. Moving Coil:
    The coil is made of thin copper wire wound on a lightweight aluminum frame. This coil is placed between the poles of the permanent magnet. When current flows through this coil, it interacts with the magnetic field and experiences a deflecting torque.

  3. Soft Iron Core:
    A soft iron cylindrical core is placed inside the coil to make the magnetic field uniform and to increase the sensitivity of the instrument.

  4. Pointer and Scale:
    The pointer is attached to the moving coil and moves over a graduated scale to indicate the reading. The scale is linear, making it easy to interpret readings directly.

  5. Control Springs:
    Two spiral hair springs are used, which provide the controlling torque to the moving system. These springs also serve as conductors to carry current to and from the moving coil.

  6. Damping Mechanism:
    The movement of the coil is damped using eddy current damping, which is achieved by the aluminum frame moving in the magnetic field. This ensures the pointer comes to rest quickly and does not oscillate.

Working Principle of PMMC Instrument

The PMMC instrument operates on the principle that:

When a current-carrying coil is placed in a magnetic field, it experiences a torque that causes it to move.

Here’s a step-by-step explanation of its working:

  1. When an electric current passes through the moving coil, it generates its own magnetic field.

  2. This magnetic field interacts with the fixed magnetic field of the permanent magnet.

  3. Due to this interaction, a deflecting torque is produced which causes the coil to rotate.

  4. The amount of deflection (rotation) is directly proportional to the current passing through the coil.

  5. As the coil moves, the control spring produces an opposing (restoring) torque.

  6. When the deflecting torque and restoring torque become equal, the coil stops rotating, and the pointer shows a steady reading on the scale.

Because the deflection is proportional to the current, the scale of a PMMC instrument is uniform.

Mathematical Expression

If

  • B = flux density of magnetic field,

  • I = current flowing through the coil,

  • N = number of turns of the coil,

  • A = area of the coil,

Then the deflecting torque (Td) is given by:

Td=B×I×N×ATd = B times I times N times ATd=B×I×N×A

Since the controlling torque (Tc) is proportional to the deflection angle (θ):

Tc=K×θTc = K times θTc=K×θ

At equilibrium,

Td=TcTd = TcTd=Tc BINA=KθBINA = KθBINA=Kθ θ=BINAKθ = frac{BINA}{K}θ=KBINA​

Hence, deflection (θ) is directly proportional to current (I). This gives the PMMC instrument its linear scale characteristic.

Advantages of PMMC Instruments

  1. High Accuracy:
    PMMC instruments are very precise and provide accurate readings due to the uniform magnetic field.

  2. Linear Scale:
    The deflection is directly proportional to the current, resulting in a linear scale that is easy to read.

  3. Low Power Consumption:
    The instrument consumes very little power because of its efficient design and lightweight coil.

  4. No Hysteresis Loss:
    Since it uses a permanent magnet, there is no hysteresis loss as in electromagnetic instruments.

  5. Effective Damping:
    Eddy current damping ensures the pointer comes to rest quickly and smoothly.

  6. Good Sensitivity:
    The lightweight coil and uniform field enhance the sensitivity, making it suitable for small current measurements.

Disadvantages of PMMC Instruments

  1. Cannot Measure AC Directly:
    PMMC instruments can only measure DC directly. For AC, they require a rectifier.

  2. Expensive:
    The use of permanent magnets and delicate construction makes it costlier than moving iron instruments.

  3. Temperature Sensitive:
    Coil resistance changes with temperature, affecting accuracy.

  4. Fragile Construction:
    The moving parts are delicate and can be damaged by mechanical shock or overload.

Applications of PMMC Instruments

PMMC instruments are widely used in both educational and industrial environments due to their reliability and accuracy. Some common applications include:

  1. DC Ammeters and Voltmeters:
    They are primarily used to measure DC current and voltage with high precision.

  2. Laboratory Instruments:
    Frequently used in laboratories for testing, calibration, and experimentation.

  3. Electronic Equipment:
    Used in multimeters and testing devices for DC circuit analysis.

  4. Rectifier Type Meters:
    When used with rectifiers, PMMC instruments can also measure AC quantities.

  5. Standard Instruments:
    Due to their linearity and accuracy, PMMC instruments are often used as standard reference meters.

Comparison Between PMMC and Moving Iron (MI) Instruments

Feature PMMC Instrument MI Instrument
Type of Current DC only AC and DC
Scale Linear Non-linear
Accuracy Very High Moderate
Damping Eddy current damping Air friction damping
Sensitivity High Low
Cost Expensive Cheaper

Conclusion

The PMMC (Permanent Magnet Moving Coil) instrument is a cornerstone of electrical measurement technology. Its precision, linear scale, and stability make it a preferred choice for measuring DC quantities in laboratories, industries, and educational setups. Although it cannot measure AC directly and is relatively expensive, its accuracy and sensitivity outweigh these limitations.

In today’s world of digital meters and electronic measurement devices, the PMMC instrument still holds a significant place as a fundamental analog measuring tool that laid the groundwork for modern electrical metering systems. Understanding its working principle helps engineers and students grasp the fundamentals of measurement techniques used across electrical engineering.