Electrical Engineering ⇒ Topic : Attraction (or Single-iron) Type
The attraction type of moving iron instrument is shown diagrammatically in Fig. (a). It has a winding through which the current to be measured passes. The winding is housed in a metal former. It also contains a moving iron piece as shown. The current through the winding is proportional to the line current if the instrument is used as an ammeter, or it is proportional to the voltage to be measured if it is used as a voltmeter. When the current flows through the winding, the mmf of the coil produces a magnetic flux along the axis of the coil. The moving iron piece attached to the spindle will be attracted towards the coil due to magnetisation and for this reason the instrument is called the attraction type of instrument. The force of attraction that acts on the moving iron piece is independent of the direction of the current in the coil and therefore this type of instruments are called unpolarised instruments. The torque that is responsible for the deflection of the iron piece is called the deflecting torque (Td) and is proportional to the square of the current to be measured as shown below
Deflecting Torque (TD)
Let Φ be the initial angle between the vertical axis and the axis of the iron piece when the coil carries no current as shown in Fig. (b). Let I be the current in the coil and ∅ be the corresponding deflection of the axis of the iron piece from the vertical axis.Let H be the magnetic field intensity in A/m2, and m be the pole strength of the iron piece (which depends on the value of H). The components of H acting along the axis of the iron piece = H. sin ( ∅+Φ ). Let F be the magnetic force acting on each pole of the iron piece. F acts along the H field at the north pole. F, the force on the poles of the iron piece mH, is proportional to H2 sin ( ∅+ Φ). By resolving F along the perpendicular to the axis of the iron piece, it can be seen that the component F cos (∅ + Φ),which is normal to the axis of the iron piece will induce a deflecting torque (TD). This torque is proportional to FI, where lis the distance between the line of action of the forces F cos (∅ +Φ ), as shown in Fig(c).From this it follows that
FIGURE Moving-iron attraction type instrument
But we already know that H ∝ I,
Assuming that the gravity control is used in the instrument, we have the control torque,
When the instrument indicates a steady deflection, the deflecting torque and the control torque must be equal, i.e. from Equation (a) and (b), we have
It is clearly seen from the Equation (a) that the deflecting torque is proportional to the square of the current, and it is also clear from Equation (c) that the current is no proportional to the deflection, and therefore the scale is not uniform. In this type of instrument, air damping is generally used. When the instrument is used for the measurement of voltage, it will be connected across the mains supply. To limit the current in the winding, a suitable high resistance is connected in series with the winding
Attraction (or Single-iron) Type
Figure (a) shows an attraction type moving iron instrument. This instrument consists of a coil or solenoid and an oval-shaped iron. The oval-shaped iron is pivoted in such a way that it can move in or out of the solenoid. To this iron, a pointer is attached. The pointer deflects along with the moving iron over a graduated scale. The iron is generally made of sheet-metal, which is designed in such a way that the scale becomes as uniform as possible.
To measure the current (or a definite fraction of the current to be measured), it is passed through the solenoid. So a magnetic field inside the solenoid is set up such that it magnetizes the iron and is attracted towards the coil. These instruments generally have spring control and pneumatic damping. In Figure (a), gravity control instrument is shown which has been superceded by spring control instruments. Spring control instruments can be used in any position.
Let us suppose that Φ is initial angle between soft iron with the direction perpendicular to the field, I is current through the coil, H is magnetic field of strength H parallel to the axis of the coil due to flow of I through the coil, θ is deflection of soft iron due to flow of current I through it, m is pole strength of the disc due to magnetization, TD is deflecting torque, and Tc is controlling torque.
Now, from the Figure (b) it is clear that
The force pulling the disc inside is expressed as:
The deflecting torque is given by
where I is the distance between the points where force F acts and the pivot of the disc.
where K1 is the constant of proportionality.
In spring controlled instruments, the controlling torque is given by
In these instruments, the instantaneous deflecting torque is proportional to the square of the current when the instrument is connected to an ac circuit. It increases with the increase of r.m.s. value of current or voltage.
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