Understanding PMMC Instrument Spindle Support

The Permanent Magnet Moving Coil (PMMC) instrument is a fundamental device used for measuring DC current and voltage. Its operation relies on the interaction between a magnetic field from a permanent magnet and a current-carrying coil that is free to rotate. The accuracy and sensitivity of a PMMC instrument are significantly affected by friction in its moving parts.

The moving system in a PMMC instrument consists of a coil wound on a former, a spindle attached to the coil, control springs, damping mechanism, and a pointer. This entire assembly rotates on the spindle, which needs to be supported in a way that minimizes friction to ensure precise movement even with small currents.

The question asks about the supports for the spindle of this moving system. Let's look at the typical bearing types used in such instruments:

• Jewelled bearings: These are precision bearings often made from hard materials like synthetic sapphire or agate. They provide a low-friction pivot point for the spindle. The tip of the spindle (pivot) is usually hardened steel, resting in a cup-shaped jewel bearing. This combination is excellent for minimizing friction and wear in sensitive instruments.
• Steel bearings: While common in many mechanical applications, standard steel bearings might introduce more friction compared to jewelled bearings, which is undesirable in high-sensitivity measuring instruments.
• Gun metal bearings: Gunmetal is a type of bronze. Bearings made of gunmetal are used in various machines, but they are generally not preferred for the delicate, low-friction support required in precision electrical measuring instruments like PMMC meters.
• Bush bearings: Bush bearings are simple, cylindrical bearings that support rotating shafts. They can be made of various materials but typically involve more sliding friction than a pivot-and-jewel arrangement and are often used for heavier loads or in less sensitive applications compared to instrument pivots.

In PMMC instruments, minimizing friction is crucial for accurate readings, especially at low current values. Jewelled bearings are specifically designed to provide very low friction support for the pivots of the moving coil, making them the standard choice for supporting the spindle at both ends.

Therefore, the spindle of the moving system in a PMMC instrument is supported at both ends with the help of jewelled bearings.

Revision Table: PMMC Instrument Components

Additional Information: Friction in Measuring Instruments

Friction is a significant source of error in sensitive electrical measuring instruments like PMMC meters. If the friction in the bearings supporting the moving system is high, the pointer might not move accurately in response to small changes in the current or voltage being measured. This leads to inaccuracies, especially at the lower end of the scale.

Jewelled bearings help overcome this problem by providing a very smooth and hard surface for the pivot to rest on, drastically reducing the frictional torque that opposes the movement of the coil. This allows the instrument to respond to even very weak torques generated by small currents, improving its sensitivity and accuracy.