moving iron instrument
- 2. Moving iron (M.I) Instrument • The most common ammeter and voltmeter for laboratory or switch board use at power frequency are the moving iron instruments. • The instrument can be constructed to measure current and voltage to an accuracy needed in most engineering works and still be cheaper as compared any other type of a.c. Instrument of same accuracy
- 3. Classification of moving iron instrument • Moving iron instrument are of two types Attraction type Repulsion type
- 4. Attraction type • The construction of an attraction type moving iron instrument. The coil is flat and has a narrow slot like opening • The moving iron disc or a sector essentially mounted. When the current flows through the coil a magnetic field is produced and the moving iron moves from the weaker field outside the coil to the stronger field inside it or in other words the moving iron is attracted
- 5. • The controlling torque is provided by spring but gravity control can be used for panel type of instrument which are vertically mounted • Damping is provided by air friction with help of light aluminum piston
- 6. Repulsion type • In the repulsion type there are two vanes inside the coil one fixed other is movable Radial vane type: In this type the vanes are radial strips of iron. The strips are placed within the coil The fixed vanes is attached to the coil and the movable one to the spindle of the instrument.
- 7. Co-axial vanes type in this type instrument the fixed and moving vanes are sections of co-axial cylinder as shown
- 8. • The controlling torque is provided by spring. • The damping torque is provide by air friction. • The operating magnetic field in moving iron instrument is very weak and therefore eddy current damping is not used in them as introduction of a permanent magnet required for eddy current damping would distort the operating magnetic field.