PMMC & Dynamometer Instruments Construction Difference

Difference between the construction of PMMC and dynamometer instruments

PMMC (Permanent moving magnet coil) Instrument

(1) Moving Coil: The moving coil is wound with many turns of enamelled or silk-covered copper wire. The coil is mounted on a rectangular aluminium former which is pivoted on jewelled bearings. The coils move freely in the field of a permanent magnet.

(2) Control: The coil is supported between two jewel bearings the control torque is provided by two phosphor bronze hair springs.

(3) Damping: Damping torque is produced by the movement of the aluminium former moving in the magnetic field of the permanent magnet.

(4) Pointer and Scale: The pointer is carried by the spindle and moves over a graduated scale. The pointer is of light-weight construction and, apart from those used in some inexpensive instruments, has the section over the scale twisted to form a fine blade.

Dynamometer Instrument

(1) Moving Coil: A single element instrument has one moving coil. The moving coil is wound either as a self-sustaining coil or else on a non-metallic former. A metallic former cannot be used as eddy currents would be induced in it by the alternating field. Both fixed and moving coils are air cored.

(2) Control: The controlling torque is provided by two control springs. These springs act as leads to the moving coil.

(3) Damping: Air friction damping is employed for these instruments and is provided by a pair of aluminium vanes attached to the spindle at the bottom. These vanes move in sector shaped chambers.

(4) Pointer and Scale: The scales are hand drawn, using machine sub-dividing equipment. Diagonal lines for fine sub-division are usually drawn for main markings on the scale.

Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Adblocker detected! Please consider reading this notice.

We've detected that you are using AdBlock Plus or some other adblocking software which is preventing the page from fully loading.

We don't have any banner, Flash, animation, obnoxious sound, or popup ad. We do not implement these annoying types of ads!

We need fund to operate the site, and almost all of it comes from our online advertising.

Please add electricalvoice.com to your ad blocking whitelist or disable your adblocking software.

×