LECTURE (10) Moving –Iron meter

As well as measuring d.c. signals, the moving-iron meter can also measure a.c. signals at frequency up to 125 Hz. It is the cheapest form of meter available and, consequently, this type of meter is also commonly used for measuring voltage signals. The signals to be measured is applied to a stationary coil, and the associated field produce is often amplified by the presence of an iron structure associated with the fixed coil.

The moving element in the instrument consist of an iron vane that is suspended within the filed of the fixed coil. Types of moving –iron meter Repulsion type Attraction type

Indication Wattmeters. Principle of induction wattemeters is the same as that of induction ammeters and voltmeters. They can be used on a.c. supply only in constant with dynamometer wattemeters, which can be used both on d.c. and a.c. supply. Induction wattmeters are useful only when the frequency and supply voltage are constant.

the wattmeter has two laminated electromagnets, one of which is excited by the current in the main circuit-exciting winding being joined in series with the circuit, hence it is also called a series magnet. The other is excited by current which is proportional to the voltage of the circuit. Its exciting coil is joined in parallel with the circuit, hence this magnet is sometimes referred to as shunt magnet.

A thin aluminum disc is so mounted that it cuts the fluxes of both magnets. Hence , two eddy currents are produced in the disc . The deflection torque is produced due to the interaction of these eddy current and the inducing fluxes. Two or three copper rings are fitted on the central limb of the shunt magnet and can be so adjusted as to make the resultant flux in the shunt magnet lag behind the applied voltage by 90°.

The fig. below shown one of the two most common forms of the electromagnets

Ф1 proportional to I where Фis in the phase with I Theory let the load current I lag behind V by ø(load power factor angle). The phase angle between Ф1 and Ф2 is 𝛼 = ( 90-ø) Ф1 proportional to I where Фis in the phase with I Ф2 proportional to v/wl where Ф2 lag Load current (I) lag behind the voltage by ø Load power factor angle = ø The phase angle between Ф1 and Ф2 is proprtional

Proportional = 90-ø The torque acting on the disc is T=kw Ф1, Ф2 sin proportional T proportional with w.I.v/Lω. Sin(90 – ø) T proportional with V.I.cosø T proportional with power Td is proportional with Tc Tc proportional with θ θ proportional with power

Energy meter Induction type single- phase watt hour meter Construction:- the meter consist of two a.c ectromagnets as shown in fig. one of which is excited by the line current and is known is series magnet. The alternating flux 1produced by it is proportional to and in phase with the line current.

The winding of other magnet M2 called shunt magnet The winding of other magnet M2 called shunt magnet. Is connected across the supply line and carries current proportion to the supply voltage V. The flux 2 produced by it is proportional to supply voltage V and lag behind it by 90°. This phase displacement of shading band C on the shunt magnet M2 Major portion of 2 crosses the narrow gap between the center and side limbs of M2 but a small amount, which is the useful flux passes through the disc D.

The braking torque is produced by a pair of magnets .

n is proportion with P Theory :- current through the windings of M1= I current through the windings of M2= V/ωL =90 -  where  is the load power factor angle Td is proportion with ω. V/ωL. I sin( 90 - ) Td is proportion with V.I. cos Td is proportion with the power TB is proportion with n Where TB is the breaking torque and n is the speed of the disc Td = TB Td is proportion with power(P) TB is proportion with P n is proportion with P By take the integration with the time for n and p 𝛼

We get N= K . E Where N is the no. of revolution K is the meter constant E is the energy in KWh the errors in induction watt our meters Phase error. Speed error. Frictional error. Errors due to temperature variation.

Questions what is the main parts of induction wattmeter. Draw and explain the moving-iron meter ( attraction type). A 230V single phase watt-hour meter has a constant load of 4A passing through it for 6 hours at unity p.f. if the meter disc makes 2208 revolutions, during this period, what is the meter constant in revolutions / KWh