A Wattmeter, as its name implies, measures the electrical power given to or developed by an electrical device or circuit. A wattmeter is rarely needed in D. C. circuits because the power (P = VI) can be easily determined from the voltmeter and ammeter readings.
However, in A. C. circuits, such a calculation is generally speaking impossible. This is because A. C. In the circuit, the power (P = VI cos Φ) depends not only on the voltage and current, but also on the phase shift between them. Therefore, a wattmeter is necessary for measuring A. C. power.
A wattmeter is a combination of an ammeter and a voltmeter. And therefore consists of two coils known as current coil and pressure coil. The operating torque is generated due to interaction of flux on account of currents in current and pressure coils.
There are two main types of wattmeter. So let’s know what they are.
- Dynamometer wattmeter
- Induction wattmeter
Dynamometer wattmeter use for both D. C. and A. C. power. And induction wattmeter use for A. C. power only. So let’s know a little more about dynamometer wattmeter. which we are telling below.
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(1) Dynamometer wattmeter
A dynamometer wattmeter is use almost universally for the measurement of D. C. as well as A. C. power. This dynamometer works on the principle that a mechanical force exists between two current carrying conductors or coils.
So here is a little information about dynamometer wattmeter. So now let’s know about some of its working and about some construction.
Dynamometer wattmeter construction and working
Construction :
Figure shows the major parts of an dynamometer wattmeter.
When a dynamometer instrument is used as a wattmeter, the stationary coils are connected in series with the load and carry the load current (I1), while the moving coil is connected to the load through a series multiplier R and carries a current (I2). proportional to the load voltage as shown in the figure.
A stationary coil (or coil) is know as a current coil and a moving coil is know as potential coil. Control torque is provided by two spiral springs which also serve the additional purpose of leading current in and out of the moving coil.
Air friction damping is provided in such devices. A pointer is attach to the movable coil.
Working :
When a wattmeter is connect to a circuit to measure power, the current carries the coil load current and the potential is proportional to the coil load voltage. Due to the currents in the coils, mechanical force exists between them.
The result is that the movable coil moves the pointer over the scale. The pointer comes to rest in a position where the deflecting torque is equal to the controlling torque.
Reversal of current reverse current in both fixed coil and the movable coil so that the direction of deflecting torque remains unchanged. Therefore, the use of such devices in D. C. as well as A. C. can be use to measure power.
So here is some important information about the working and construction of dynamometer wattmeter, which you have to remember and understand well.
But there are some errors in the wattmeter too, so let’s know where and why are those errors in it.
Errors in wattmeter
A wattmeter may not give correct reading due to several errors like (1) error due to connection of potential coil circuit (2) error due to inductance of potential coil (3) error due to capacitance in potential coil circuit (4) error due to stray fields and (5) error due to eddy current.
Here is the information about the working, construction and its errors of dynamometer wattmeter. So let’s have another wattmeter which is an induction wattmeter. So below we are going to tell you about the induction wattmeter, as well as its working, construction, etc.
(2) Induction wattmeter
Induction type wattmeter is use for A. C. power can only be use to measure dynamometer unlike wattmeter which can be use to measure D. C. as well as A. C. power can be use to measure. The principle of operation of induction wattmeter is similar to that of induction ammeter and voltmeter which is induction principle.
However, it differs so far from an induction ammeter or voltmeter in that two separate coils are use to produce a rotating magnetic field in place of one coil with a split phase arrangement. So this is energy meter which you can also call integrated meter.
so let’s know about the construction and working of induction wattmeter.
Construction and working of induction wattmeter
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Construction :
The figure shows the physical arrangement of the various parts of an induction wattmeter.
(1) It consists of two laminated electromagnets. One electromagnet, called a shunt magnet, is connect to the supply and carries a current proportional to the supply voltage. The coil of this magnet is make highly inductive so that the current in it lags 90° behind the supply voltage.
Another electromagnet, called a series magnet, is connect in series with the supply and carries the load current. The coil of this magnet is design to be highly non-inductive. So that the angle of the lag or lead is completely determine by the load.
(2) A thin aluminum disc mounted on the spindle is placed between the two magnets so that it cuts off the flow of both the magnets. The controlling torque is provided by spiral springs.
Damping is electro-magnetic and is usually provided by a permanent magnet embracing the aluminum disc. Two or more closed copper rings (called shading rings) are provided on the central limb of the shunt magnet. By adjusting the position of these rings, the shunt can be make exactly 90° behind the supply voltage to the magnet flux.
Working :
When a wattmeter is connect to a circuit to measure A. C. power, the shunt magnet carries a current proportional to the supply voltage. And the series magnet carries the load current.
The two fluxes produced by the magnets induce eddy currents in the aluminum disc. The interaction between the flux and the eddy currents produces a deflecting torque on the disc, causing the pointer to be connected to the moving system that moves the scale. The pointer comes to rest at a position where deflecting torque is equal to the controlling torque.
Let V = Supply voltage
IV = Current carried by shunt magnet
IC = Current carried by series magnet
cos Φ = Lagging power factor of the load
You can see the phasor diagram in the picture below. The current IV in the shunt magnet lags the supply voltage V by 90° and so does the flux ΦV produced by it. The current IC in the series magnet is the load current and hence lags behind the supply voltage V by Φ.
The flux ΦC produced by this current (that is IC ) is in phase with it. It is clear that phase angle θ between the two fluxes is 90° – Φ that is θ = 90° – Φ
Therefore Mean deflecting torque, Td ∝ ΦV ΦC sin θ
∝ VI sin (90° − Φ)
[ ∵ ΦV ∝ V and ΦC ∝ I ]
∝ V I cos Φ
∝ a. c. power
Since the instrument is spring controlled, TC ∝ θ.
For steady deflected position, Td = TC.
Therefore θ ∝ a. c. power
Hence such instruments have uniform scale. So let’s now know about the energy meter, also known as the integrating meters.
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What is energy meter? – Integrating meters
Induction type watt hour meter
This is the most commonly use meter on A. C. circuits for measurement of energy. Also, we are going to tell below about some of its advantages, which you have to remember in a random way. Because questions related to this are likely to come up frequently. so let’s know.
Advantage :
- Simple in operation
- High torque / weight ratio
- Cheap in cost
- Correct registration even at very low power factor
- Unaffected by temperature variations
- More accurate than commutator type energy meter on light load ( Owing to absence of a commutator with its accompanying friction).
Induction type single phase energy meter
Single phase induction watthour meters or energy meters are widely use for the measurement of electrical energy in A. C. circuits. Such meters can be install in homes.
An induction watthour meter is essentially an induction wattmeter in which the control spring and pointer are remove. But a brake magnet and counting mechanism are provided.
These are, by far, the most common forms of A. C. meters found in everyday domestic and industrial installations. These meters measure electrical energy in kWh.
Construction and working of single phase induction type energy meter
Construction :
(1) It consists of (a) two A. C. electromagnets that is series magnet and shunt magnet (b) an aluminum disc or rotor placed between the two electromagnets (c) Brake magnet and (d) counting mechanism.
(2) The shunt magnet is wound with a fine wire of many turns and connected to the supply so that it flows current proportional to the supply voltage. Since the coil of a shunt magnet is highly inductive, the current in it lags 90° behind the supply voltage.
Wrap the series magnet with a heavy wire of few turns and connect it in series with the load so that it can carry the load current. The coil of this magnet is highly non-inductive so that the angle of the lag or lead is completely determined by the load.
(3) A thin aluminum disc mounted on the spindle is placed between the shunt and the series magnets so that it cuts off the flux of both the magnets.
(4) Breaking torque is achieved by placing a permanent magnet near the rotating disc so that the disc continuous to rotate in the field established by the permanent magnet. The induced eddy currents in the disc produce a braking or retarding torque which is proportional to the speed of the disc.
(5) A short-circuit copper loop ( also known as a power factor compensator) is provided on the central limb of the shunt magnet. By adjusting the position of this loop, the shunt magnet flux can be make to move exactly 90° back from the supply voltage.
Frictional compensation is achieve by means of two adjustable short-circuit loops placed in the leakage gaps of the shunt magnet. There is a calculation mechanism designed for the rotating element that directly indicates the energy consumed in kilowatt hour (kWh).
Working :
This is also very important. Read this thoroughly and understand it. So let’s start.
The shunt electromagnet generates a magnetic field that is of pulsating character, it cuts through the rotation disc and induces eddy currents there, but generally produces no driving force by itself.
Similarly the series electromagnet induce eddy currents in the rotating disc, but does not in itself produce any driving force. In order to obtain driving force in this type of meter, phase displacement of 90° between the magnetic field set up by shunt electromagnet and applied voltage V is achieve by adjustment of copper shading band C ( also known as power factor compensator or compensating loop).
The reaction between these magnetic fields and eddy currents sets up a driving torque in the disc.
NOTE : It is possible to measure power in a single phase A. C. circuit without using a wattmeter by using the following methods
- Three ammeter method : It uses three ammeters and a known non-inductive resistance.
- Three voltmeter method : It uses three voltmeters and known non-inductive resistance.
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What are the errors in induction type energy meter?
(1) Phasor error
The meter will read correctly only when the shunt magnet flux is exactly 90° behind the supply voltage. Since the shunt magnet coil has some resistance and is not completely reactive, the shunt magnet flux does not lag exactly 90° behind the supply voltage. As a result, the meter does not read correctly at all power factors.
Adjustment :
The flux in the shunt magnet can be made to lag behind the supply voltage by exactly 90° by adjusting the position of the shading coil placed around the lower part of the central limb of the shunt magnet.
The current is induce in the shading coil by the shunt magnet flux and causes further displacement of the flux. By moving the shading coil up or down, the displacement between the shunt magnet flux and the supply voltage can be adjusted up to 90°. This adjustment is known as lag adjustment or power factor adjustment.
(2) Speed error
Sometimes the disc speed of the meter is either faster or slower, resulting in incorrect recording of energy consumption.
Adjustment :
The speed of the disc of the energy meter can be adjust to the desired value by changing the position of the break magnet.
If the break magnet is move toward the center of the spindle. So the barking torque decreases and the disc speed increases. If the break magnet is moved away from the center of the spindle so the reverse will happen.
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(3) Frictional error
Frictional forces in rotor bearings and counting mechanism cause a noticeable error, especially at light loads. At light loads, the torque produced by friction greatly increases the braking torque.
Since friction torque is not proportional to the speed but is roughly constant. And it can cause considerable error in the meter reading.
Adjustment :
To compensate for this error, it is necessary to add a constant to the driving torque that is equal and opposite to the friction torque.
This shunt is produce by means of two adjustable short-circuit loops placed in the leakage gap of the magnet. These loops upset the symmetry of the leakage flux and generate a small torque to oppose the friction torque.
This adjustment is know as the light-load adjustment. The loops are adjusted in such a way that when no current is passing through the current coil (which is the exciting coil of the series magnet), the torque produced is sufficient to overcome the friction in the system, actually rotate the disc without.
(4) Creeping
Sometimes the disc of the meter rotates slowly but continuously without any load. When the potential coil is excited but no currents flow through the load. This is know as creeping.
This error can be cause by overcompensation for friction, excessive supply voltage, vibrations, stray magnetic fields etc.
Adjustment :
In order to prevent this creeping, two diametrically opposite holes are drilled in the disc. It causes substantial distortion of the field. The result is that when there is a hole under one of the poles of the shunt magnet, the disc will remain stationary.
(5) Temperature error
Since watthour meters are often require to operate in outdoor installations and are subject to extreme temperatures. Hence the effects of temperature and their compensation are very important.
The resistance of the disc, the potential of the coil and the characteristics of the magnetic circuit and the strength of the break magnet are affect by changes in temperature.
Therefore, great care is take in the design of the meter to remove errors due to temperature variations.
(6) Frequency variations
The meter is design to give minimum error at a particular frequency (usually 50 Hz). If the supply frequency changes, the response of the coil also changes, resulting in a small error, Fortunately, this is not very significant as commercial frequencies are kept within close range.
(7) Voltage variations
The shunt magnet flux will increase with increase in voltage. The driving torque is proportional to the first power of flux. While the braking torque is proportional to the square of the flux. Hence, if the supply voltage is higher than the normal value, the braking torque will increase much more than the driving torque and vice-versa.
As a result, the meter has a tendency to operate at a slower at than normal voltage and faster at a lower voltage. However, for most meters the effect is small. And does not exceed 0.2 % to 0.3 % for a voltage change of 10 % from the rated value.
Small error due to voltage variation can be eliminate by proper design of magnetic circuit of shunt magnet.
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So friends, here is some important information about wattmeter and wattmeter types. So what can you learn from here, comment on us. We are sure that you will get to know and learn a lot from this article. Thank you