POWER METER
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ELECTRIC POWER
Electric power is the rate at which electric energy is
transferred by an electric circuit.
The SI unit of power is the watt.
When electric current flows in a circuit, it can transfer
energy to do mechanical or thermodynamic work.
Device convert electrical energy into many useful forms
such as, heat (electric heaters), light (light bulbs),
motion (electric motors), sound (loudspeaker),
information technological processes (computer), or
even chemical changes.
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POWER METER
A METER FOR MEASURING THE AMOUNT OF
ELECTRIC POWER USED.
SOME EXAMPLES OF POWER METER ARE:
WATTMETER
KWH METER
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CLAMP METER
WATTMETER
SYMBOL OF ANALOGUE WATTMETER
BASIC PRINCIPLE OF ANALOGUE WATTMETER
A wattmeter has two coils that
multiply volts times amps.
P = IV
Watt = Ampere x Volt
One coil is wired as an ammeter
with a shunt, the other as a
voltmeter with a multiplier.
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This instrument is of the electrodynamic type.
It consist of a pair of fixed coils, known as
Current coils and movable coil known as the
potential coil.
The voltage is measured across (parallel)
the load and charges the pointer magnet.
The current is measured in series with the
load and charges the field magnets.
The movable coil carries a needle which
moves over a suitable marked scale.
Spiral coil springs hold the needle to a zero
position.
Fig1: A simplified electrodynamic wattmeter
circuit.
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Wattmeter connection for power measurement.
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A WATTMETER CONNECTED IN VARIOUS CIRCUITS.
TWO-PHASE SYSTEM .
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KWH METER
An electric meter or energy meter is a device that measures the
amount of electrical energy consumed by a residence, business, or
an electrically powered device.
Electric meters are typically calibrated in billing units, the most
common one being the kilowatt hour.
The most common type of electricity meter is the
electromechanical induction watt-hour meter.
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The electromechanical
induction meter operates by
counting the revolutions of an
aluminium disc which is made
to rotate at a speed
proportional to the power.
The metallic disc is acted
upon by two coils.
One coil is connected in
such a way that is produces
a magnetic flux in proportion
to the voltage and the other
produces a magnetic flux in
proportion to the current.
This produces eddy currents in the disc and the
effect is such that a force is exerted on the disc.
A permanent magnet exerts an opposing force proportional to the
speed of rotation of the disc. The equilibrium between these two
opposing forces results in the disc rotating at a speed proportional
to the power being used.
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EDDY CURRENT
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Mechanism of electromechanical
induction meter.
1 - Voltage coil - many turns of fine wire
encased in plastic, connected in parallel with
load.
2 - Current coil - three turns of thick wire,
connected in series with load.
3 - Stator - concentrates and confines
magnetic field.
4 - Aluminum rotor disc.
5 - rotor brake magnets.
6 - spindle with worm gear.
7 - display dials - note that the 1/10, 10 and
1000 dials rotate clockwise while the 1, 100
and 10000 dials rotate counter-clockwise.
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CLAMP METER
An electrical meter with integral AC current clamp.
This allows properties of the electric current in the conductor to be
measured, without having to make physical contact with it.
Clamp meters are often sold with a device that is plugged in between the
power outlet and the device to be tested. The device is essentially a short
extension cord with the two conductors separated.
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HOW TO USE CLAMP METER?
In order to use a clamp meter, only one conductor is normally passed
through the probe.
If more than one conductor is passed through then the measurement
would be the vector sum of the currents flowing in the conductors and
would depend on the phase of the current.
The reading produced by a conductor carrying a very low current can be
increased by winding the conductor around the clamp several times; the
meter reading divided by the number of turns is the current.
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Cost of the power consumption.
Calculate the cost of the power consumed by an
electrical range that used 5A current on a 230-Volt
line within 4 hours. The electricity tariff rate is
18cents per kilowatt hour.
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P IV
P(5A)(230V )
P 1150W
KWh  (1150W )(4h)
1000
KWh  4.6KWh
Cost  4.6KWh  RM 0.18
KWh
 RM 0.828
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Find the total power of the following appliances.
-A toaster that draws a current of 5A on 115-volt line.
-An electric range that uses a 5A current on a 230-volt line.
- A heater that draws a current of 5A on 230-volt line within 2 hours.
Then, calculate on how much the cost of the total power if the rate is 28cents per
kilowatt hour.
P = IV
= 5A x 115 V
= 575 w
Kwh = 0.575kwh
Cost = RM 0.28/kwh x 0.575kwh
= RM 0.161
Total cost = RM0.161 + RM0.322
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= RM 0.483
P= IV
= 5A x 230V
= 1150 w
kwh = 1.15kwh
cost = RM 0.28/kwh x 1.150kw
= RM 0.322
- A heater that draws a current of 5A on 230-volt line within 2 hours.
Then, calculate on how much the cost of the power if the rate is 28cents per kilow
hour.
P = IV
= 5A x 230V
= 1150w
Kwh = 1.15kwh x 2
= 2.3kwh
Cost = RM 0.28/kwh x 2.3kwh
= RM 0.644
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