Ammeter

Measurement instrument

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Ammeter,  instrument for measuring either direct or alternating electric current, in amperes. An ammeter can measure a wide range of current values because at high values only a small portion of the current is directed through the meter mechanism; a shunt in parallel with the meter carries the major portion.

Ammeters vary in their operating principles and accuracies. The D’Arsonval-movement ammeter measures direct current with accuracies of from 0.1 to 2.0 percent. The electrodynamic ammeter uses a moving coil rotating in the field produced by a fixed coil. It measures direct and alternating current with accuracies of from 0.1 to 0.25 percent. In the thermal ammeter, used primarily to measure alternating current with accuracies of from 0.5 to 3 percent, the measured current heats a thermoconverter (thermocouple); the small voltage thus generated is used to power a millivoltmeter. Digital ammeters, with no moving parts, use a circuit such as the dual slope integrator to convert a measured analogue (continuous) current to its digital equivalent. Many digital ammeters have accuracies better than 0.1 percent.

Active, Reactive, Apparent and Complex Power

Active, Reactive, Apparent and Complex Power. Simple explanation with formulas.

(1) Real Power: (P)

 Alternative words used for Real Power (Actual Power, True Power, Watt-full Power, Useful Power, Real Power, and Active Power)

In a DC Circuit, power supply to the DC load is simply the product of Voltage across the load and Current flowing through it i.e., P = V I. because in DC Circuits, there is no concept of phase angle between current and voltage. In other words, there is no Power factor in DC Circuits.

But the situation is Sinusoidal or AC Circuits is more complex because of phase difference between Current and Voltage. Therefore average value of power (Real Power) is P = VI Cosθ is in fact supplied to the load.

In AC circuits, When circuit is pure resistive, then the same formula used for power as used in DC as P = V I. 

You may also read about Power Formulas in DC, AC Single Phase and and AC Three Phase Circuits.

Real Power formulas:

P = V I                                  (In DC circuits)

P = VI Cosθ                          (in Single phase AC Circuits)

P = √3 V_L I_L Cosθ         or     (in Three Phase AC Circuits)

P = 3 V_Ph I_Ph Cosθ 

P = √ (S² – Q²)^or

P =√ (VA² – VAR²) or  

Real or True power = √ (Apparent Power²– Reactive Power²) or

kW = √ (kVA² – kVAR²)

 

(2) Reactive Power: (Q)  

Also known as (Use-less Power, Watt less Power)

The powers that continuously bounce back and forth between source and load is known as reactive Power (Q)

Power merely absorbed and returned in load due to its reactive properties is referred to as reactive power

The unit of Active or Real power is Watt where 1W = 1V x 1 A.

Reactive power represent that the energy is first stored and then released in the form of magnetic field or electrostatic field in case of inductor and capacitor respectively.

Reactive power is given by Q = V I Sinθ which can be positive (+ve) for inductive, negative (-Ve) for capacitive load.

The unit of reactive power is Volt-Ampere reactive. I.e. VAR where 1 VAR = 1V x 1A.

In more simple words, in Inductor or Capacitor, how much magnetic or electric field made by 1A x 1V is called the unit of reactive power.

Reactive power formulas:

Q = V I Sinθ

Reactive Power=√ (Apparent Power²– True power²)

VAR =√ (VA² – P²)

kVAR = √ (kVA² – kW²)

 

(3) Apparent Power: (S)

The product of voltage and current if and only if the phase angle differences between current and voltage are ignored.

Total power in an AC circuit, both dissipated and absorbed/returned is referred to asapparent power

The combination of reactive power and true power is called apparent power

In an AC circuit, the product of the r.m.s voltage and the r.m.s current is called apparent power.

It is the product of Voltage and Current without phase angle

The unit of Apparent power (S) VA i.e. 1VA = 1V x 1A.

When the circuit is pure resistive, then apparent power is equal to real or true power, but in inductive or capacitive circuit, (when Reactances exist) then apparent power is greater than real or true power. 

Apparent power formulas:

S = V I

Apparent Power = √ (True power² + Reactive Power²)

kVA = √kW² + kVAR²

AlsoNote that;

Resistor absorbs the real power and dissipates in the form of heat and light.

Inductor absorbs the reactive power and dissipates in the form of magnetic field

Capacitor absorbs the reactive power and dissipates in the form of electric or electrostatic filed

∴ These all quantities trigonometrically related to each other as shown in below figure.

Click image to enlarge

Power Factor.

 1).  The Cosine of angle between Current and Voltage is called Power Factor.
P = VI Cosθ OR
Cosθ = P / V I OR
Cosθ = kW / kVA
Cosθ = True Power/ Apparent Power
 
2). The ratio between resistance and Impedance is Called Power Factor.
Cosθ = R/Z
 
3). The ratio between Actual Power and Apparent Power is called power factor.
Cosθ = kW / kVA

   Beer Analogy of Active or True power, Reactive power, Apparent Power and Power factor.

Power Factor Improvement

 Methods for Power Factor Improvement

The following devices and equipments are used for Power Factor Improvement.

1. Static Capacitor
2. Synchronous Condenser
3. Phase Advancer

1. Static Capacitor

We know that most of the industries and power system loads are inductive that take lagging current which decrease the system power factor. For Power factor improvement purpose, Static capacitors are connected in parallel with those devices which work on low power factor.  These static capacitors provides leading current which neutralize (totally or approximately) the lagging inductive component of load current (i.e. leading component neutralize or eliminate the lagging component of load current) thus power factor of the load circuit is improved. These capacitors are installed in Vicinity of large inductive load e.g Induction motors and transformers etc, and improve the load circuit power factor to improve the system or devises efficiency.

 Suppose,here is a single phase inductive load which is taking lagging current (I) and the load power factor is Cosθ as shown in fig-1.

In fig-2, a Capacitor (C) has been connected in parallel with load. Now a current (Ic) is flowing through Capacitor which lead 90° from the supply voltage ( Note that Capacitor provides leading Current i.e., In a pure capacitive circuit, Current leading 90°  from the supply Voltage, in other words, Voltage are 90°  lagging from Current). The load current is (I). The Vectors combination of (I) and (Ic) is (I’) which is lagging from voltage at θ₂ as shown in fig 3.

It can be seen from fig 3 that angle of θ₂ < θ₁ i.e. angle of θ₂ is less than from angle of θ₂. Therefore Cosθ₂ is less than from Cosθ₁ (Cosθ₂> Cosθ₁). Hence the load power factor is improved by capacitor.

Also note that after the power factor improvement, the circuit current would be less than from the low power factor circuit current. Also, before and after the power factor improvement, the active component of current would be same in that circuit because capacitor eliminates only there-active component of current. Also, the Active power (in Watts) would be same after and before power factor improvement.

Advantages:

• Capacitor bank offers several advantages over other methods of power factor improvement.
• Losses are low in static capacitors
• There is no moving part, therefore need low maintenance
• Itcan work innormalairconditions (i.e. ordinary atmospheric conditions)
• Do not require a foundation for installation
• They are lightweight so it is can be easy to installed

Disadvantages:

• The age of static capacitor bank is less (8 – 10 years)
• With changing load, we have to ON or OFF the capacitorbank, which causes switching surges on the system
• If the rated voltage increases, then it causes damage it
• Once the capacitors spoiled, then repairing is costly

2. Synchronous Condenser

When a Synchronous motor operates at No-Load and over-exited then it’s called a synchronous Condenser. Whenever a Synchronous motor is over-exited then it provides leading current and works like a capacitor. When a synchronous condenser is connected across supply voltage (in parallel) then it draws leading current and partially eliminates the re-active component and this way, power factor is improved. Generally, synchronous condenser is used to improve the power factor in large industries.

Advantages:

• Long life (almost 25 years)
• High Reliability
• Step-less adjustment of power factor.
• No generation of harmonics of maintenance 
• The faults can be removed easily
• It’s not affected by harmonics.

Require Low maintenance (only periodic bearing greasing is necessary)

Disadvantages:

• It is expensive (maintenance cost is also high) and therefore mostly used by large power users.
• An auxiliary device has to be used for this operation because synchronous motor has no self starting torque
• It produces noise

3. Phase Advancer

Phase advancer is a simple AC exciter which is connected on the main shaft of the motor and operates with the motor’s rotor circuit for power factor improvement. Phase advancer is used to improve the power factor of induction motor in industries. As the stator windings of induction motor takes lagging current 90° out of phase with Voltage, therefore the power factor of induction motor is low. If the exciting ampere-turns are excited by external AC source, then there would be no effect of exciting current on stator windings. Therefore the power factor of induction motor will be improved. This process is done by Phase advancer.

 Advantages:

• Lagging kVAR (Reactive component of Power or reactive power) drawn by the motor is sufficiently reduced because the exciting ampere turns are supplied at slip frequency (fs).
• The phase advancer can be easily used where the use of synchronous motors is Unacceptable

Disadvantage:

• Using Phase advancer is not economical for motors below 200 H.P. (about 150kW)
• Power factor improvement in three phase system by connecting a capacitor bank in

(1). Delta connection

(2). Star Connection)

AC CURRENT

AC CURRENT

Most students of electricity begin their study with what is known as direct current (DC), which is electricity flowing in a constant direction, and/or possessing a voltage with constant polarity. DC is the kind of electricity made by a battery (with definite positive and negative terminals), or the kind of charge generated by rubbing certain types of materials against each other.

As useful and as easy to understand as DC is, it is not the only “kind” of electricity in use. Certain sources of electricity (most notably, rotary electro-mechanical generators) naturally produce voltages alternating in polarity, reversing positive and negative over time. Either as a voltage switching polarity or as a current switching direction back and forth, this “kind” of electricity is known as Alternating Current (AC): Figure below

Direct vs alternating current

Whereas the familiar battery symbol is used as a generic symbol for any DC voltage source, the circle with the wavy line inside is the generic symbol for any AC voltage source.

One might wonder why anyone would bother with such a thing as AC. It is true that in some cases AC holds no practical advantage over DC. In applications where electricity is used to dissipate energy in the form of heat, the polarity or direction of current is irrelevant, so long as there is enough voltage and current to the load to produce the desired heat (power dissipation). However, with AC it is possible to build electric generators, motors and power distribution systems that are far more efficient than DC, and so we find AC used predominately across the world in high power applications. To explain the details of why this is so, a bit of background knowledge about AC is necessary.

If a machine is constructed to rotate a magnetic field around a set of stationary wire coils with the turning of a shaft, AC voltage will be produced across the wire coils as that shaft is rotated, in accordance with Faraday's Law of electromagnetic induction. This is the basic operating principle of an AC generator, also known as an alternator: Figure below

Alternator operation

Notice how the polarity of the voltage across the wire coils reverses as the opposite poles of the rotating magnet pass by. Connected to a load, this reversing voltage polarity will create a reversing current direction in the circuit. The faster the alternator's shaft is turned, the faster the magnet will spin, resulting in an alternating voltage and current that switches directions more often in a given amount of time.

While DC generators work on the same general principle of electromagnetic induction, their construction is not as simple as their AC counterparts. With a DC generator, the coil of wire is mounted in the shaft where the magnet is on the AC alternator, and electrical connections are made to this spinning coil via stationary carbon “brushes” contacting copper strips on the rotating shaft. All this is necessary to switch the coil's changing output polarity to the external circuit so the external circuit sees a constant polarity: Figure below

DC generator operation

The generator shown above will produce two pulses of voltage per revolution of the shaft, both pulses in the same direction (polarity). In order for a DC generator to produce constant voltage, rather than brief pulses of voltage once every 1/2 revolution, there are multiple sets of coils making intermittent contact with the brushes. The diagram shown above is a bit more simplified than what you would see in real life.

The problems involved with making and breaking electrical contact with a moving coil should be obvious (sparking and heat), especially if the shaft of the generator is revolving at high speed. If the atmosphere surrounding the machine contains flammable or explosive vapors, the practical problems of spark-producing brush contacts are even greater. An AC generator (alternator) does not require brushes and commutators to work, and so is immune to these problems experienced by DC generators.

The benefits of AC over DC with regard to generator design is also reflected in electric motors. While DC motors require the use of brushes to make electrical contact with moving coils of wire, AC motors do not. In fact, AC and DC motor designs are very similar to their generator counterparts (identical for the sake of this tutorial), the AC motor being dependent upon the reversing magnetic field produced by alternating current through its stationary coils of wire to rotate the rotating magnet around on its shaft, and the DC motor being dependent on the brush contacts making and breaking connections to reverse current through the rotating coil every 1/2 rotation (180 degrees).

So we know that AC generators and AC motors tend to be simpler than DC generators and DC motors. This relative simplicity translates into greater reliability and lower cost of manufacture. But what else is AC good for? Surely there must be more to it than design details of generators and motors! Indeed there is. There is an effect of electromagnetism known as mutual induction, whereby two or more coils of wire placed so that the changing magnetic field created by one induces a voltage in the other. If we have two mutually inductive coils and we energize one coil with AC, we will create an AC voltage in the other coil. When used as such, this device is known as a transformer: Figurebelow

Transformer “transforms” AC voltage and current.

The fundamental significance of a transformer is its ability to step voltage up or down from the powered coil to the unpowered coil. The AC voltage induced in the unpowered (“secondary”) coil is equal to the AC voltage across the powered (“primary”) coil multiplied by the ratio of secondary coil turns to primary coil turns. If the secondary coil is powering a load, the current through the secondary coil is just the opposite: primary coil current multiplied by the ratio of primary to secondary turns. This relationship has a very close mechanical analogy, using torque and speed to represent voltage and current, respectively: Figure below

Speed multiplication gear train steps torque down and speed up. Step-down transformer steps voltage down and current up.

If the winding ratio is reversed so that the primary coil has less turns than the secondary coil, the transformer “steps up” the voltage from the source level to a higher level at the load: Figure below

Speed reduction gear train steps torque up and speed down. Step-up transformer steps voltage up and current down.

The transformer's ability to step AC voltage up or down with ease gives AC an advantage unmatched by DC in the realm of power distribution in figure below. When transmitting electrical power over long distances, it is far more efficient to do so with stepped-up voltages and stepped-down currents (smaller-diameter wire with less resistive power losses), then step the voltage back down and the current back up for industry, business, or consumer use.

Transformers enable efficient long distance high voltage transmission of electric energy.

Transformer technology has made long-range electric power distribution practical. Without the ability to efficiently step voltage up and down, it would be cost-prohibitive to construct power systems for anything but close-range (within a few miles at most) use.

As useful as transformers are, they only work with AC, not DC. Because the phenomenon of mutual inductance relies on changing magnetic fields, and direct current (DC) can only produce steady magnetic fields, transformers simply will not work with direct current. Of course, direct current may be interrupted (pulsed) through the primary winding of a transformer to create a changing magnetic field (as is done in automotive ignition systems to produce high-voltage spark plug power from a low-voltage DC battery), but pulsed DC is not that different from AC. Perhaps more than any other reason, this is why AC finds such widespread application in power systems.

• REVIEW:
• DC stands for “Direct Current,” meaning voltage or current that maintains constant polarity or direction, respectively, over time.
• AC stands for “Alternating Current,” meaning voltage or current that changes polarity or direction, respectively, over time.
• AC electromechanical generators, known as alternators, are of simpler construction than DC electromechanical generators.
• AC and DC motor design follows respective generator design principles very closely.
• A transformer is a pair of mutually-inductive coils used to convey AC power from one coil to the other. Often, the number of turns in each coil is set to create a voltage increase or decrease from the powered (primary) coil to the unpowered (secondary) coil.
• Secondary voltage = Primary voltage (secondary turns / primary turns)
• Secondary current = Primary current (primary turns / secondary turns)