Energy Meter (Measurement of Energy)

MEASUREMENT OF ENERGY

Energy is a measure of power over a period of time or energy is the time integral of power.

Energy = power × time.

If the time interval (t₂‒ t₁) is measured in seconds and the voltage 'e' in volts, current 'i' in ampheres the energy, E is given by,

If the unit of time is in hour and power is in watts, the energy unit will be in watt‒hour. Generally unit of energy is given by kilowatt‒hour (KWhr). An energy meter will record the energy consumed. Energy meters are indicating type of instruments.

Energy Meter

Construction

This is essentially a split phase induction motor whose output is largely absorbed by its braking system and dissipated as heat. It utilises two magnetic fields displaced in space and time. There are four main parts of operating mechanism in an energy meter. They are:

1. Driving system

2. Moving system

3. Braking system and

4. Registering (counting) system

The pole arrangements and connections of an induction type watt‒hour meter is shown in Fig. 6.16.

1. Driving system

The driving system of the meter consists of two electromagnets namely series and shunt. One is excited by the load current and the other is by a current proportional to the voltage of the circuit in which the energy is to measured. The coil wounded on the current poles (series magnet) is known as "current coil" and that on voltage pole (shunt magnet) is known as "pressure coil". The pressure coil has a larger number of turns of thin wires, unlike the current coil which has a few number of turns of thick wires.

Copper shading band is provided on the central limb of shunt magnet and the position of the band is adjustable. The function of the band is to bring the flux produced by the shunt magnet exactly in quadrature with the applied voltage. The two copper bands which are in the outer limbs of the shunt magnet is for friction compensation.

2. Moving system

It consists of a thin aluminium disc mounted on a light alloy shaft so that it is cut by the flux, both the magnets. A pinion cut on the shaft at the top meshes with a gear wheel on the revolution counter or register.

3. Braking system

As permanent magnet positioned near the edge of the aluminium disc as shown in Fig. 6.16, forms the braking system. In the field on this magnet, the disc moves and hence provides a braking torque. The position of the permanent magnet is radially adjustable and hence the braking torque can be adjusted.

4. Registering system

This system contains a train of reduction gears, a pinion on the shaft which drives a series of pointers. These pointers rotate on round dials which are equally marked with equal divisions. This system records continuously a number which is proportional to the revolutions made by the aluminium disc.

Working

It is working on the principle of induction i.e., on the production of eddy currents in the moving system by the alternating fluxes. These eddy current is induced in the moving system interact with each other to produce a driving torque due to which disc rotates to record the energy. In energy meters, there is no controlling torque and thus due to driving torque only, a continuous rotation of the disc is produced. To have constant speed of rotation, braking magnet is provided.

The working can be explained as shown in fig. 6.17 since the pressure coil is carried by shunt magnet M₂ which is connected across the supply, it carries current proportional to the voltage. Series magnet M₁ carries current coil which carries the load current. Both these coils produces alternating fluxes ϕ₁ and ϕ₂ respectively. These fluxes are proportional to currents in their coils. Parts of each of these fluxes links with the disc and an emf induces in it. Due to this emf, eddy current will flow in the disc. The eddy current induced by the electromagnet M₂ react with magnetic field produced by M₁. Also eddy currents induced by electromagnet M₁ react with magnetic field produced by M₂. Thus a mechanical force will be exerted in the disc (Motor action) and the disc rotates. The speed of the disc is controlled by braking magnet when disc rotates in the air gap, eddy currents are in the disc which opposes the cause producing it (Lenz's law). Hence braking torque T_b is generated. The spindle of the disc is connected to the recording mechanism through gears which record the energy supplied.

Theory of Energy Meter

Let,

V = Applied voltage.

I_S = I = Load current

I_p = Pressure coil current

E_ep = Eddy emf induced due to ϕ_p

E_es = Eddy emf induced due to ϕ_s

I_es = Eddy current due to ϕ_s

Z = Impedance of eddy current.

f = frequency.

ϕ = Phase angle between V and I

N = Speed or revolution of disc.

 T_d ∝ ϕ_pϕ_s (f/Z) sinβ cos∝                          ..... (1)

where,

 β = (∆‒ϕ)

T_d = K₁ϕ_pϕ_s (f/Z) sin(∆‒ϕ) cos∝

= K₂ϕ_pϕ_s sin(∆‒ϕ)

where,

 f, Z and ∝ are constants.

We know that,

ϕ_p ∝ V

ϕ_s ∝ I

T_d=K₄VI sin (∆‒ϕ)                        ..... (2)

The braking torque is proportional to speed of disc (N)

T_d ∝ N                        ..... (3)

T_d = K₃N                        ..... (4)

At steady speed, T_d=T_b

K₃N=K₄ VI sin (∆‒ϕ)

N=KVI sin(∆‒ϕ)

where,

K= K₄ / K₃

if, ∆=90

N = KVI sin (90 ‒ ϕ)

N = KVI cos ϕ

N ∝ VI cos ϕ                        ..... (5)

Adjustments in Energy Meter

Some adjustments are carried out in energy meters so that they read correctly and their errors are within allowable limits. The sequence of these adjustments are;

1. Preliminary light load adjustment

The disc is so positioned that the holes are not under neath the electromagnets. Rated voltage is applied to the potential coil with no current through the current coil. The light load device is adjusted until the disc just fails to start.

2. Full load unity power factor adjustment

The full load adjustment is made at rated load by changing the radial position or effective strength of the brake magnet.

3. Lag adjustment (Low power factor adjustments

The lag adjustment, usually made at resistance of a lag coil or by moving lag plate in radial direction.

4. With rated supply voltage, rated full load current and unity power factor, low power factor adjustments are repeated until the desired accuracy limits are reached for both conditions.

5. Light bad adjustment

The light load adjustment is made at 10% of rated load by moving a light load plate in a direction parallel to the direction of motion of the meter.

6. The performance is re‒checked at 0.5 power factor lagging.

7. Creep adjustment

As a final check on light load adjustment, the pressure coil is excited by 110% of rated voltage with zero load current. If the light load adjustment is correct the meter should not creep under these conditions.

Advantages

(i) Its construction is simple and strong.

(ii) It is cheap in cost.

(iii) Frictional errors are less and hence accurate.

(iv) Less maintenance is required.

Limitations

(i) Can be used only for ac circuits.

(ii) Creeping can cause errors.