What is a PMMC Instrument?

     

    A PMMC (Permanent Magnet Moving Coil) instrument is an electrical measuring device that uses a permanent magnet to create a fixed magnetic field and a moving coil connected to the electrical source to measure the current or voltage. The interaction between the magnetic field and the current flowing through the coil produces a deflection that is proportional to the magnitude of the current or voltage, providing accurate measurements. This type of instrument is primarily used for direct current (DC) measurements and is known for its precision and reliability.

    Working Principle of a PMMC

     

    The PMMC (Permanent Magnet Moving Coil) instrument operates based on the interaction between a magnetic field and an electric current.

    1. Current in a Magnetic Field:

      • When a current flows through a coil inside a magnetic field, the coil experiences a force.
      • This force causes the coil to move.

    2. Fleming’s Left-Hand Rule:

      • Thumb: Direction of force (movement of the coil).
      • Forefinger: Direction of the magnetic field.
      • Middle finger: Direction of the current.

    3. Coil Movement:

      • The coil is attached to a pointer.
      • As the coil moves, the pointer moves along a scale.

    4. Balancing Force:

      • A spring opposes the movement of the coil.
      • The coil and pointer stop moving when the force from the current equals the spring’s resistance.

    5. Damping:

      • To prevent the pointer from swinging back and forth, a damping force (like an aluminum piece moving in the magnetic field) stabilizes it quickly.

    6. Reading the Measurement:

      • The stable position of the pointer on the scale shows the precise measurement of the current or voltage.

    PMMC Instrument Construction

     

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

    A PMMC (Permanent Magnet Moving Coil) instrument is built using several key components. Here’s a breakdown of its construction:

    1. Moving Coil

    • Essential Component: The moving coil is crucial for the PMMC instrument.
    • Design: Made of copper wire wound around a rectangular block called the aluminum former.
    • Aluminum Former: Allows the coil to rotate freely in jeweled bearings.
    • Function: When current passes through the coil, it deflects in the magnetic field, indicating the current or voltage magnitude.
    • Types:
      • Non-metallic former: Used for measuring current.
      • Metallic former with electromagnetic damping: Used for measuring voltage.

    2. Magnet System

    • High-Intensity Magnets: Usually two, in a U-shaped design.
    • Materials: Made of Alnico or Alcomax for strong magnetic fields.
    • Soft Iron Cylinder: Sometimes placed between the magnetic poles to create a uniform field and reduce air reluctance, enhancing field strength.

    3. Control Mechanism

    • Springs: Made of phosphorous bronze, placed between the jewel bearings.
    • Function: These springs control the torque and guide the current in and out of the moving coil.
    • Torque Control: Achieved by the tension of the ribbon-like springs.

    4. Damping Torque

    • Generation: Damping torque is generated by the movement of the aluminum core within the magnetic field.
    • Function: Prevents pointer oscillations and ensures steady readings by creating eddy currents that resist the coil’s motion.
    • Result: The pointer gradually comes to rest at the correct position for accurate measurement.

    5. Pointer and Scale

    • Pointer: Attached to the moving coil, shows the coil’s deflection.
    • Material: Made of lightweight material for easy movement.
    • Scale: Displays the measurement.
    • Parallax Error: Reduced by proper alignment of the pointer’s blade to ensure accurate readings.

    Summary

    The PMMC instrument consists of a moving coil, a magnet system, control springs, damping mechanisms, and a pointer with a scale. The moving coil, placed in a strong magnetic field, deflects proportionally to the current flowing through it. Springs control the movement, while damping forces ensure steady readings. The pointer attached to the coil indicates the measurement on the scale.

    Advantages of PMMC 

    • The scale in the instrument can be divided properly
    • It generates no losses because of hysteresis.
    • It uses less power.
    • High accuracy

    Disadvantages of PMMC 

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

    Applications of PMMC

    • Ammeter
    • Galvanometer
    • Ohmmeter
    • Voltmeter