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Explain construction & working of attraction type M.I. instrument with neat diagram.

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Construction & Working of Attraction Type M.I. Instrument: 

Construction: It consists of a coil which is mounted vertically near the spindle carrying an oval shaped soft iron disc, called moving iron. The spindle also carries pointer, balance and control weights, airdamping vane etc. as shown in figure.

image

Working: When the instrument is connected in circuit, an operating current flows through the coil, this current sets up magnetic field and ovel shaped moving iron is so magnetized that force is exerted on it by which it moves from the weaker field outside the coil to the stronger field inside the coil or in other words the moving iron is attracted in, resulting the movement of pointer on the scale from zero position to give the reading.The controlling torque is provided by spring and damping torque is provided by air friction damping with the help of light aluminium piston. Whatever may be the direction of current, the moving iron is always attracted inside the coil, making the instrument suitable for measurement of both DC as well as AC quantities. The force of attraction is proportional to the square of current in the coil.

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image

1. Moving element: a small piece of soft iron in the form of a vane or rod. 

2. In Attracted type a cylindrical coil or solenoid which is kept fixed. An oval-shaped soft-iron piece are used 

3. Control torque is provided by spring or weight (gravity). 

4. Damping torque the damping device consisting of an air chamber and a moving vane attached to the instrument spindle. 

5. Deflecting torque produces a movement on an aluminum pointer over a graduated scale. 

6. The electrical quantity which is to be measure is provided to the input of meter coil so, that current will circulate through the meter coil 

7. The current to be measured is flowing in the coil, produces a magnetic field. Iron piece gets attracted towards center of the magnetic field and pointer deflects on the scale. 

8. The scale is non-linear. Mirror is provided to avoid parallax error. 

9. When = the pointer will be stable & we can get correct reading of the electrical quantity

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