Single Phase Induction Type Energymeter

SINGLE PHASE INDUCTION TYPE ENERGYMETER

Energy is the total power consumed or delivered over a period of time.

Energy = power × time

= ₀∫^t VI cos ϕ dt; watt second (or) Joule.

kilowatt hour is the power delivered at an average rate of 1000 watts for one hour.

Energy consumed can be measured by induction type energymeter.

Induction type energymeter consist of four main systems. They are 1. Driving system, 2. Moving system, 3. Braking system and 4. counting system. These are explained as follows:

1. Driving system:

It is comprised of two electromagnets with silicon steel laminated core. coil of one of the electromagnet is split into two halves and excited by load current. It is also called as current coil. This coil forms the series magnet. The coil of another electromagnet is connected across the supply and carries the current proportional to the supply voltage. It is also called as pressure coil. This coil forms the shunt magnet. Adjustable copper shadding bands are introduced at the central limb to bring the flux produced by shunt magnet exactly in quadrature (90').with the applied voltage.

2. Moving system:

It is comprised of an aluminium disc mounted on the light alloy moving shaft. Aluminium disc is placed in a air gap between the two electromagnets (series and shunt). Sleeve pin type bearing forms the upper suspension. Pivoted Jewelled bearing forms the lower suspension. A pinion interconnects the shaft with the counting mechanism.

In floating shaft type, small magnets are used at each end to bear the shaft.

3. Braking system:

Permanent magnet is placed at the edge of the Aluminium disc. It provides the damping or breaking torques. This torque is adjusted by adjusting the permanent magnet to different radial positions.

4. Counting System / Registering mechanism:

This system is used to continuously record the revolutions made by the moving system. The pinion on the shaft along with a set of reduction gears drives a series of 5 or 6 pointers. Pointers rotates on round dials which are having 10 equal divisions. Cyclometer register can also be used.

Principle:

Two fluxes ϕ₁ and ϕ₂ are produced by the currents flowing in the coils (C.C and P.C) of the instrument.

These two alternating fluxes ϕ₁ & ϕ₂ which cut the disc.

flux ϕ₁ produces the eddy current i₁

flux ϕ₂ produces the eddy current i₂

Two fluxes produces two eddy current which inturn induces two torques.

First torque is produced by the interaction of ϕ₁ with i₂

Second torque is produced by the interaction of ϕ₂ with i₁

Total torque = Sum of two torques.

This net torque tends to rotate the aluminium disc.

Theory and Operation:

V = applied voltage

I = Load current

ϕ = Phase angle of load

Δ = phase angle between supply voltage and pressure coil flux

f = frequency

I_p = Pressure coil current

z = impedance of eddy current paths.

E_ep = eddy emf induced by flux ϕ_p

E_es =eddy emf induced by flux ϕ_s

∞ = phase angle of eddy current paths

I_ep = eddy current due to flux ϕ_p

I_es = eddy current due to flux ϕ_s

Due to large number of turns P.C winding is highly inductive. P.C current I_p is directly proportional to the supply voltage and lags by a few degrees less than 90. This is because the P.C winding has small resistance and iron losses in the magnetic circuit.

ϕ_pt is the flux produced by I_p. This flux is divided into two, ϕ_g and ϕ_p. ϕ_g flows across the side gaps and occupies the major portion of the flux. Reluctance of ϕ_g path is small. ϕ_p's magnitude is small because its reluctance path is large.

ϕ_p is alternating in nature. ϕ_p is in phase with I, and directly proportional to I_p.

ϕ_p produces eddy emf, E_ep which inturn produces eddy current I_ep. ϕ_p goes along with aluminium disc and responsible for driving torque.

Load current is inphase with ϕ_s. ϕ_s produces eddy emf E_es which inturn produces eddy current I_es

Two torques are developed first torque is developed by the interaction of ϕ_s with I_eq. Second torque is developed by the interaction of ϕ_p with I_es. These two torques are in opposite direction and net torque is the difference of these two torques.

T_d = k ϕ_p ϕ_s  ƒ/z sin (Δ - ϕ) cos ∞

T_d = k VI ƒ/z sin (Δ - ϕ) cos ∞

T_d = k₁ VI sin (Δ - ϕ)

At steady state condition, T_B = T_d

Breaking torque = k₂ N

k₂ N = k₁ VIsin(Δ - ϕ)

Speed,N = k₃ VI sin (90 – ϕ)

where angle A=90'

Total number of revolutions = ∫Ndt

= k₃ ∫VIcosϕ dt

= k₃. Energy.

Lag adjustment device:

Meter registers true energy if Δ =90° angle between ϕ_p and V shoud be 90.For this P.C should have high inductance and low resistance.

core should have small iron small iron loss. This is achieved by using lag coil.

Lag coil is the small additional coil introduced into the limb next to the shunt coil. It consist of few turns of thick wire.

I_L depends on reactance and resistance of lag coil.

Phase of ϕ_p can be adjusted by changing the mmf of shading coil either in magnitude or in phase or both.

Arrangements for changing the mmf of the lagcoil:

There are two methods or arrangements are used for changing the mmf of the lag coil. These are called lag adjustments or power factor adjustments or quadrature adjustments or inductive load adjustments.

1. Adjustable resistance:

Lag coil circuit is closed through a low adjustable resistor.

R↑ ⇒ I↓⇒ mmf of lag coil ↓ ⇒ lag angle↓

R↓I↑ mmf of lag coil ↑⇒ lag angle ↑ and Δ becomes 90

2. Shading bands:

'Cu' shading bands 'L₁' is placed around the central limb of shunt magnet.

Shading bands are movable along the axis of the central limb for adjusting the mmf of the lag coil.

If shading band moves up, it gives more flux and increases emf as well as current which inturn increases mmf and lag angle.

Compensations Provided in Energymeter

1. Light load (or) friction compensation:

Moving system cause frictional error in large manner at light loads.

To compensate this a small torque is produced by small shading loop (L₂) placed between central limb and disc. this torque is independent of load.

2. Creep compensation:

A slow but continuos rotation of the aluminium disc, when only P.C is energised and there is no current flowing through the C.C is called creep.

Creeping is caused because of over compensation provided for friction. Friction compensating torque is independent of load.

Creeping is also caused by excessive voltage across the P.C, vibrations and stray magnetic field.

Creeping is eliminated by the two diametrically opposite holes in the disc.

The disc will stop to rotate with one of the holes under the edge of a pole of shunt magnet.

When the hole is under the edge of a pole, circular eddy current path is distorted and produces a torque which opposes the rotation of the disc. Thus the rotation being limited to a maximum of half a revolution.

Creep is also eliminated by attacting a small iron piece to the disc.

Braking magnet exerts a force of attraction to this iron piece and stops the disc. These compensating torque is small in nature and does not affects the deflecting torque.

3. Over load compensation:

Under loaded condition, the disc rotates continuously in the field of the series magnet, this creates a dynamically induced emf which inturn self breaking torque in the disc.

At high load, the meter reads low because of self braking torque.

1.To minimize this torque, 1. Full load speed is kept small nearly equal to 40 rpm

2. The current coil flux o, is made small compared with op.

3. Otherwise an overload compensating device is used. Magnetic shunt provided in this device diverts series magnetic flux, so a large portion of the flux appears in the air gap and produces more driving torque.

4. Voltage compensation:

Variations in supply voltage causes error because

1. Relationship between the shunt magnetic flux and the supply voltage is non linear due to saturation in iron parts.

2. ϕ_p causes dynamically induced emf in the disc which inturn creates self breaking torque.

This is eliminated by using magnetic shunt or by drilling holes in the side limbs. Holes increases the reluctance of the side limbs of the shunt magnet.

5. Temperature compensation:

If temperature rises, resistance of copper and aluminium parts increases and causes the following results.

1. a small decrease in ϕ_p and Δ decreases.

2. the torque produced by shading bands are decreased.

3. an increase in the resistance of eddy current paths.

4. a decrease in the angle ‘∞' of eddy current.

This errors are high at low power factor loads. This causes the disc to run fast and to register high.

This is eliminated by using a temperature shunt on the breaking magnet or by using a special magnetic material like mutemp.

mutemp has the property of decreasing its permeability with increase in temperature.

Error:

Error caused by the driving system are

1. Incorrect magnitude of fluxes

2. Incorrect phase angles and

3. Lack of symmetry in magnetic circuits.

Errors caused by the braking system are

1. Variations in strength of the braking magnet.

2. Variations in the resistance of the disc.

3. Self braking effect of series magnetic flux and

4. Friction of moving parts.

Advantages:

1. Universally used for domestic and industrial ac circuits.

2. Low friction

3. High torque - weight ratio.

4. Maintain accuracy over wide range of load and temperature.

Three Phase Energymeter

As Per Blondel's Theorem, 'n' conductor system requires (n - 1) measuring elements for the measurement of total energy. Therefore 3 phase 3 wire system uses 2 element for the measurement of total energy.

Two element energymeter Consist of two discs. A current coil together with its pressure coil is called as an element. Each element consist of one disc. Magnetic shunt is provided to one or both the elements, to balance the torques of these two elements.

Pressure coils are connected in parallel and current coils are connected in series in such a way that the torque produced by each element are equal and opposite in nature with the single phase supply. Now the magnetic shunt is adjusted till no rotation of the disc occurs.

Lag adjustments, light load adjustments and unity power factor adjustments are done independently for each element.

after doing all these adjustments, the meter is connected to the three phase supply. Two discs are mounted on the same spindle. Therefore the total deflecting torque is the sum of the deflecting torques produced by the two elements separately.