PMMC Full Form: Understanding Permanent Magnet Moving Coil Instruments in Detail

Author : Salim Yousaf | Published On : 10 Oct 2025

In the field of electrical and electronic measurements, accuracy and precision are crucial. Engineers, technicians, and students often come across various types of measuring instruments used to measure electrical quantities such as current, voltage, and resistance. One of the most widely used and accurate types of analog measuring instruments is the PMMC. The full form of PMMC is Permanent Magnet Moving Coil.

This device plays a vital role in laboratories, industries, and educational setups for measuring direct current (DC) and voltage with high accuracy. In this blog, we will explore what PMMC is, how it works, its construction, advantages, limitations, and applications.

What is PMMC?

The Permanent Magnet Moving Coil (PMMC) instrument is an analog type of measuring instrument used for measuring DC quantities such as current and voltage. It works on the principle that when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force.

In PMMC instruments, a lightweight coil is placed between the poles of a permanent magnet. When an electric current passes through the coil, it experiences a torque that causes it to move. This movement is proportional to the current flowing through the coil, and the attached pointer indicates the value on a calibrated scale.

PMMC Full Form Explained

• P – Permanent

• M – Magnet

• M – Moving

• C – Coil

The name itself describes the working components of the instrument. A permanent magnet creates a constant magnetic field, and a moving coil mounted on a pivot moves within this field when a current passes through it. This movement is then translated into readable measurements.

Construction of PMMC Instrument

A typical PMMC instrument consists of the following key parts:

1. Permanent Magnet

   • The permanent magnet is made of materials like Alnico or Neodymium, providing a strong and stable magnetic field.

   • It usually has a horseshoe shape, ensuring a uniform magnetic field between the poles.

2. Moving Coil

   • The coil is made of fine copper wire wound on a lightweight aluminum frame.

   • The coil is free to rotate in the magnetic field and is attached to the spindle or shaft.

3. Soft Iron Core

   • A soft iron cylinder is placed inside the coil to make the magnetic field more uniform and to increase the torque.

4. Control Springs

   • Two hair springs made of phosphor bronze are attached to the spindle.

   • These springs provide the controlling torque and also act as electrical leads to the coil.

5. Pointer and Scale

   • The pointer attached to the spindle moves over a calibrated scale to indicate the measured value.

   • The scale is usually linear because the deflection is directly proportional to the current.

6. Damping Mechanism

   • Aluminum frame and eddy currents provide damping torque to stop oscillations and bring the pointer to a steady position.

Working Principle of PMMC

The working of a PMMC instrument is based on Fleming’s left-hand rule, which states that when a current-carrying conductor is placed in a magnetic field, it experiences a force perpendicular to both the field and the direction of current.

Here’s how it works step by step:

1. When a DC current passes through the coil, it interacts with the magnetic field of the permanent magnet.

2. Due to this interaction, a torque acts on the coil, causing it to rotate.

3. The controlling springs provide a counter torque proportional to the deflection.

4. The coil comes to rest when the deflecting torque equals the controlling torque.

5. The angular deflection of the pointer is proportional to the current flowing through the coil.

Since the deflection is directly proportional to the current, the PMMC instrument provides a linear scale, which makes reading easier and more accurate.

Advantages of PMMC Instruments

1. High Accuracy

   • PMMC instruments provide very accurate readings, making them ideal for laboratory and research purposes.

2. Linear Scale

   • The deflection of the pointer is proportional to the current, resulting in a linear scale that is easy to read.

3. Low Power Consumption

   • The moving coil is lightweight and requires a very small current for operation.

4. No Hysteresis Loss

   • Since the magnetic field is produced by a permanent magnet, there are no hysteresis or eddy current losses.

5. Good Damping

   • Eddy current damping ensures quick settling of the pointer without oscillations.

6. Reliable and Robust

   • With fewer moving parts and a strong construction, PMMC instruments are durable and reliable for long-term use.

Disadvantages of PMMC Instruments

1. Used Only for DC

   • PMMC instruments cannot measure alternating current (AC) because the direction of current in AC changes continuously, causing the pointer to oscillate around zero without providing a steady reading.

2. Expensive

   • Permanent magnets made of materials like Alnico or Neodymium make the instrument relatively costly.

3. Temperature Sensitivity

   • The resistance of the copper coil changes with temperature, which can affect accuracy.

4. Delicate Construction

   • The moving coil mechanism is sensitive to mechanical shocks and vibrations.

Applications of PMMC Instruments

PMMC instruments are used in a wide variety of fields due to their precision and reliability. Some common applications include:

1. Ammeter – To measure direct current (DC) in circuits.

2. Voltmeter – To measure DC voltage by connecting a high resistance in series with the coil.

3. Galvanometer – For detecting small currents in bridge circuits and experiments.

4. Ohmmeter – When combined with a voltage source, PMMC can be used to measure resistance.

5. Testing Laboratories – For calibration and testing of other instruments.

6. Educational Institutions – Widely used in labs for practical experiments and demonstrations.

Precautions While Using PMMC Instruments

• Always ensure the instrument is used with DC, not AC.

• Avoid overloading the meter to prevent coil damage.

• Protect it from high temperatures and mechanical shocks.

• Regularly calibrate for accurate readings.

Difference Between PMMC and Moving Iron Instruments

Conclusion

The PMMC (Permanent Magnet Moving Coil) instrument is one of the most accurate and reliable devices for measuring DC quantities. Its working principle, based on the interaction between magnetic fields and current-carrying conductors, ensures high precision and linear readings. Despite being limited to DC measurements and having a slightly higher cost, PMMC instruments are still preferred in laboratories, testing facilities, and educational environments where accuracy is of utmost importance.

In the world of electrical measurement, understanding the full form of PMMC and its functioning helps students and professionals appreciate the engineering behind precise analog instruments — a foundation on which many modern digital measuring devices are built.