What is Pmmc (permanent magnet moving coil)?

Permanent Magnet Moving Coil (PMMC)

1. Basic Structure

The primary components of a Permanent Magnet Moving Coil (PMMC) include:

• • Permanent Magnet: Provides a stable magnetic field, typically using high-coercivity rare-earth magnets such as neodymium-iron-boron.

• • Moving Coil (Coil): Consists of fine wire wound into a coil, suspended within the air gap of the permanent magnet. When current flows through the coil, it experiences a force in the magnetic field, causing it to deflect.

• • Shaft and Bearings: Support the moving coil and allow it to rotate freely.

• • Spiral Spring (Hair Spring): Provides a restoring torque to return the coil to its zero position when no current is present. It also conducts the current to the coil.

• • Pointer and Scale: The pointer is connected to the moving coil and moves with its deflection, indicating the measured value. The scale allows for the reading of specific values.

2. Working Principle

The working principle of a PMMC is based on Ampère's Law and Faraday's Law of Electromagnetic Induction. The process is as follows:

• • When current flows through the moving coil, according to Ampère's Law, the current in the coil generates a force (Lorentz force) in the magnetic field, causing the coil to deflect.

• • The deflection angle of the coil is proportional to the current passing through it, allowing the current magnitude to be directly read from the pointer's movement.

• • The spiral spring provides a counteracting restoring torque, ensuring that the coil returns to its initial position (zero) when the current ceases.

3. Features and Advantages

The PMMC has several notable features and advantages:

• • High Precision: Due to its linear response characteristics, PMMC instruments offer high measurement accuracy, making them suitable for precision measurements.

• • Low Power Consumption: The coil has low resistance, consuming minimal power, which is ideal for low-power applications.

• • Excellent Stability: The stable magnetic field provided by the permanent magnet ensures reliable and consistent measurement results, unaffected by external magnetic fields.

• • High Sensitivity: The lightweight design of the moving coil makes it highly sensitive to small changes in current or voltage, enabling the detection of minor variations.

• • Unidirectional Deflection: PMMCs are designed to work only with direct current (DC) because alternating current (AC) would cause the coil to oscillate, preventing stable readings. Therefore, PMMC instruments are typically used for DC measurements.

4. Applications

PMMC is widely used in various precision measurement instruments, including:

• • Ammeter: Measures the direct current (DC) in a circuit.

• • Voltmeter: By connecting a high-resistance resistor in series, a current meter can be converted into a voltmeter to measure DC voltage.

• • Ohmmeter: Combining a current meter with a power source and a variable resistor allows for the measurement of resistance.

• • Multimeter: Modern multimeters often incorporate PMMC meters to measure current, voltage, and resistance.

5. Improvements and Variants

To expand the application range of PMMC, several improvements and variants have been developed:

• • Dual-Coil Structure: Adding a second moving coil allows for bidirectional deflection, making it suitable for AC measurements.

• • Electronic PMMC: Integrating electronic amplifiers and digital displays enhances measurement accuracy and ease of reading.

• • Thermocouple Compensation: In high-temperature environments, PMMC instruments may be affected by temperature changes. Some instruments include thermocouple compensation mechanisms to ensure accurate measurements.

Summary

A Permanent Magnet Moving Coil (PMMC) is a precision measurement device based on electromagnetic induction principles, widely used for measuring current, voltage, and power. It offers high precision, low power consumption, excellent stability, and high sensitivity, making it particularly suitable for DC measurements. Although PMMC instruments are primarily used for DC applications, improvements and variant designs have extended their use to AC measurements and other specialized scenarios.