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An analog single-phase wattmeter is a device which measures the power transmitted from source to load in the circuit.
The simplest type is an ammeter whose scale is calibrated watts. This device can provide satisfactory indications of power generated or consumed if:
A) The line voltage is known and varies little under any conditions where an indication is needed, and
B) The circuit uses direct current (DC), or alternating current (AC) where the load is purely resistive.
An application where such a meter would provide useful indications of power consumed is a circuit connected to a well-regulated 120 volt source, AC or DC, has a load consisting of incandescent lamps and/or heating elements and the meter is calibrated to 120 volts. If the line current is 9 amperes, the meter indicates 1080 watts. It works on the formula E x I = P, where E is the electromotive force in volts, I is the line current in amperes and P is the power, in watts.
This type of meter has a number of shortcomings. If the mains voltage varies, such as when few loads are connected the voltage may be higher than 120 and the instrument will underestimate the power. When the mains are heavily laden, the reverse is true. If the voltage is well-regulated, this error may be small enough to ignore as it can be less than that of the meter movement, especially where the effort and expense of obtaining precise readings is not justified.
If the circuit operates from a AC source and the load is an electric motor, this type of instrument will indicate the apparent power, in volt-amperes but this will differ from the true power, measured in watts.
The cause of this difference is the inductance, or opposition to change in current flow inherent in magnetic devices such as motors or iron-core fluorescent lamp ballasts. In motor circuits, the current will lag in time behind the voltage and the amount of this lag depends on a number of factors, chiefly the type of motor and the torque demanded from it. In the case of lamp ballasts, the lag is affected by ballast design, ambient temperature, age of the bulb, etc.
We can calculate the difference, or power factor, between apparent power and true power if we know the lag angle (360 degrees = 1 cycle):
Watts = VA x Cos(q)
or,
True Power = Apparent Power x Power Factor.
One way determine the lag angle is with an oscilloscope, which can indicate the amplitude of the voltage, current and their phase relationship. This method is rarely used for AC power circuits as an oscilloscope is costly, requires a skilled operator and these three values must then be plugged into a formula to determine the true power.
The method long used for practical measurements is the dynamometer-type wattmeter. This is a meter movement which has one coil which is in series with one conductor to the load, usually it induces a magnetic field in the stator and has relatively few turns of large-diameter wire. The second coil is attached to the pointer and suspended in the magnetic gap of the stator. It has many turns of fine wire and is connected parallel to the load. A magnetic field in either coil by itself will produce no movement of the pointer, as its flux is unopposed. The torque produced, and the degree of deflection of the pointer is the result of the interaction both of amplitude and of phase of the two magnetic fields, one related to current and the other to voltage. Thus it remains accurate for variable voltages, currents and phase relationships.
An example of this instrument's value: A certain home has a central air conditioner. On a muggy evening, the homeowner measures the current at 21 amperes using a clamp-on type ammeter and the voltage at 234 VAC, for an apparent power of 4194 VA. On a sweltering summer afternoon the compressor demands more torque from the motor driving it, so 23 amperes and 228 VAC is seen for 5244 VA.
Now let's check that power factor. In the first case the lag angle is 45 degrees so the true power is 2966 watts. As more torque is demanded from the motor the lag angle changes much more than the current, so in the second case if the lag angle is only 15 degrees, the true power is 5065 watts.
As we can see from this example, the ammeter can provide us with useful information on choosing the size of the conductors, Transformers, alternator windings or circuit breaker ratings for motor circuits but only a very rough guess as to the actual energy consumption.
Incidentally the electric meter which the utility company uses to determine the monthly household electrical use is a variation of the dynamometer wattmeter movement and the rotation of the wheel is in proportion to true, not apparent power.
Recent advances in microelectronics allow construction of dynamometers using semiconductor devices to measure voltage, current and phase relationship and such a device may be less expensive to construct than the traditional meter. Also they can provide a digital display, eliminating parallax and scale interpretation error.
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Why you use upf wattmeter for short circuit test on load test on single phase transformer?
If, by 'upf', you mean 'unity power factor', then allwattmeters measure the in-phase component of the load current, so the term is quiet unnecessary.A short-circuit test is used to determine the (true) power loss in the transformer, which is exactly what a wattmeter measures
Does a wattmeter indicate real or apparent power?
As its name implies, a 'wattmeter' measures 'real' or 'true power'. It does this by measuring the supply voltage and the in-phase component of the load current.
Is it possible to measure power factor of the balanced three-phase load by two-wattmeter method?
yes.
Single phase and three phase circuit?
Yes, there is a difference between single phase and three phase circuits.
What is effect of power factor on the readings of wattmeter?
If you are asking whether power-factor improvement has any effect on a wattmeter reading, then the answer is no, it doesn't. Improving the power factor of a load has absolutely no effect on the power of the load, but it can act to reduce the value of the load current.
Why use two wattmeter to calculate the power of a three phase load?
Two wattmeter method is used in a three phase sysstem to measure the true power without regard to the balance ie. It can be used to measure both balanced loads and unbalanced loads.The three phase power could be measured using single phase wattmeters in each loads of the system. But using three wattmeters is quite unnecessary as we can measure it by using only two wattmeters.The three phase power could be measued by using single phase wattmeters , haing a current coil in one line and a potential coil connected between the line and some arbitrary common junction point. If that arbitrary common point is chosen on one of the three lines, then the wattmeter connected to that line will indicate zero power because it's potential coil has no voltage across it. hence, that wattmeter may be eliminated, and the three phase power can be determined by means of only two single phase wattmeters.
Why you use upf wattmeter for short circuit test on load test on single phase transformer?
If, by 'upf', you mean 'unity power factor', then allwattmeters measure the in-phase component of the load current, so the term is quiet unnecessary.A short-circuit test is used to determine the (true) power loss in the transformer, which is exactly what a wattmeter measures
What may happen if you accidentally select the 10 volt d c range on an analog meter when testing a single phase supply?
If by single phase, you mean 120/240 AC; it could blowup in your hand!
Does a wattmeter indicate real or apparent power?
As its name implies, a 'wattmeter' measures 'real' or 'true power'. It does this by measuring the supply voltage and the in-phase component of the load current.
Why does not wattmeter measure the reactive power?
A wattmeter is designed so that it measures the supply voltage and the in-phase component of the load current. The product of these two quantities is the true power of the load.
How would you measure power factor of the balanced three phase load by two wattmeter method?
Yes. The wattmeter's current coil will have to be connected into one of the line conductors, and its voltage coil between that same line conductor and the neutral point of the load. Connected this way, the wattmeter's voltage coil is measuring one of the three phase voltages (line-to-neutral voltage) while its current coil is measuring the corresponding phase current (for a 4-wire system, the phase current = line current). The power factor (cosine of the phase angle) is accounted for automatically within the wattmeter. So the wattmeter will measure the true power (in watts) of one phase. The total power, therefore, will be 3x the wattmeter reading -providing, of course, that the load is balanced (i.e. each phase is identical). WebRep currentVote noRating noWeight
In which Analog to analog modulation methods does the peak amplitude?
Frequency modulation Phase modulation
Is it possible to measure power factor of the balanced three-phase load by two-wattmeter method?
yes.
How would you measure power using three wattmeter in three phase?
Each of the wattmeter's current coils are inserted into line conductors, and the voltage coils are connected in wye between each line conductor.
Explain in detail about construction woking of single phase wattmeter?
WattmeterDefinitionThe wattmeter is an instrument for measuring the electric power (or the supply rate of electrical energy) in watts of any given circuit.Electric power is measured by means of a wattmeter. This instrument is of the electrodynamic type. It consists of a pair of fixed coils, known as current coils, and a movable coil known as the potential coil. The fixed coils are made up of a few turns of a comparatively large conductor. The potential coil consists of many turns of fine wire. It is mounted on a shaft, carried in jeweled bearings, so that it may turn inside the stationary coils. The movable coil carries a needle which moves over a suitably marked scale. Spiral coil springs hold the needle to a zero position.The figure below shows the circuit of a simple wattmeter.
Why do you use two phase watt meter?
You may be thinking of the 'two-wattmeter method' for measuring three-phase power?As a rule, it's always possible to use one less wattmeter than the number of conductors supplying a three-phase load to measure the power of that load. So, for a three-phase, three-wire, system, two wattmeters may be used to measure the total active power of the load -regardless of whether the load is balanced or unbalanced. The algebraic sum of the two wattmeter readings will give the total power.
Will the two-wattmeter method work for all three-phase loads eg unbalanced loads inductive loads capacitive loads and delta-connected loads?
Blondel's Theorem tells us that, to measure the total power of a three-phase load (balanced or unbalanced), we can use one less wattmeter than there are conductors supplying that load.So the two-wattmeter method will work for anythree-phase load, provided there are only threeconductors supplying that load, e.g. three-wire delta or three-wire star (wye).Bear in mind that wattmeter's read true power (expressed in watts) and ignores the reactive power of inductors and capacitors.
