PMMC Instrument Damping Explained

Permanent Magnet Moving Coil (PMMC) instruments are widely used for measuring direct current (DC) quantities due to their high accuracy and uniform scale. The working principle relies on the interaction between the magnetic field produced by a permanent magnet and the magnetic field generated by the current flowing through a coil placed in that field. This interaction creates a deflecting torque that moves the pointer across the scale.

The Necessity of Damping in PMMC Instruments

When the instrument is energized, the pointer moves towards its final deflected position. However, due to inertia, the pointer tends to overshoot the final position. This causes the pointer to oscillate back and forth around the steady state position before finally settling down. These oscillations make it difficult to obtain an accurate reading quickly. To overcome this, a damping mechanism is employed to reduce the oscillations and bring the pointer to rest rapidly. The damping torque acts in opposition to the motion of the moving system.

Analysis of Damping Methods for PMMC

Several damping methods exist, but not all are suitable for PMMC instruments. Let's examine the options:

• Air Friction Damping: This method uses a light vane attached to the moving system, which moves inside a sector-shaped chamber. The air resistance provides damping. While simple, it's generally less effective for the required level of damping in precise instruments like PMMC.
• Fluid Friction Damping: In this method, the moving system is immersed in a viscous fluid (like oil) to provide damping. This provides smooth damping but increases the weight and friction, and is typically used in instruments like galvanometers where sensitivity is paramount and oscillations need significant reduction. It's not the standard choice for PMMC measuring instruments.
• No Damping: Without any damping, the pointer would oscillate uncontrollably for a long time, making the instrument practically useless for taking readings.
• Eddy Current Damping: This is the most effective and commonly used method for PMMC instruments. Here's how it works:
  • A lightweight rectangular conducting (but non-magnetic) former, usually made of aluminum or copper, is used to wind the moving coil.
  • This former moves within the strong radial magnetic field produced by the permanent magnets.
  • When the coil and former move, the conductors of the former cut the magnetic flux lines.
  • According to Faraday's law of electromagnetic induction, eddy currents are induced within the former.
  • These eddy currents flow in such a direction as to create their own magnetic field, which, according to Lenz's Law, opposes the motion that produced them.
  • This opposing torque is the damping torque. It effectively reduces the oscillations of the pointer and helps it settle quickly to the final reading. The damping torque is proportional to the speed of deflection, providing critical or slightly over-damped conditions.

Conclusion on PMMC Damping

Given the principles of operation and the need for effective damping without adding significant weight or friction, eddy current damping is the preferred and standard method employed in PMMC type instruments.