OHMMETERS

The instrument, which is used to measure the value of resistance between any two points in an electric circuit is called ohmmeter. It can also be used to find the value of an unknown resistor. The units of resistance are ohm and the measuring instrument is meter. So, the word “ohmmeter” is obtained by combining the words “ohm” and “meter”.

Types of Ohmmeters

Following are the two types of ohmmeters.

• Series Ohmmeter
• Shunt Ohmmeter

Now, let us discuss about these two types of ohmmeters one by one.

 Active, Reactive and Apparent Power

Active Power

Definition: The power which is actually consumed or utilised in an AC Circuit is called True power or Active power or Real power. It is measured in kilowatt (kW) or MW. It is the actual outcomes of the electrical system which runs the electric circuits or load.

Reactive Power

Definition: The power which flows back and forth that means it moves in both the directions in the circuit or reacts upon itself, is called Reactive Power. The reactive power is measured in kilo volt-ampere reactive (kVAR) or MVAR.

Apparent Power

Definition: The product of root mean square (RMS) value of voltage and current is known as Apparent Power. This power is measured in kVA or MVA.

It has been seen that power is consumed only in resistance. A pure inductor and a pure capacitor do not consume any power since in a half cycle whatever power is received from the source by these components, the same power is returned to the source. This power which returns and flows in both the direction in the circuit, is called Reactive power. This reactive power does not perform any useful work in the circuit.

In a purely resistive circuit, the current is in phase with the applied voltage, whereas in a purely inductive and capacitive circuit the current is 90 degrees out of phase, i.e., if the inductive load is connected in the circuit the current lags voltage by 90 degrees and if the capacitive load is connected the current leads the voltage by 90 degrees.

Hence, from all the above discussion, it is concluded that the current in phase with the voltage produces true or active power, whereas, the current 90 degrees out of phase with the voltage contributes to reactive power in the circuit.

Therefore,

True power = voltage x current in phase with the voltage

Reactive power = voltage x current out of phase with the voltage

Active component of the current

The current component, which is in phase with the circuit voltage and contributes to the active or true power of the circuit, is called an active component or watt-full component or in-phase component of the current.

Reactive component of the current

The current component, which is in quadrature or 90 degrees out of phase to the circuit voltage and contributes to the reactive power of the circuit, is called a reactive component of the current.

VOLTMETER

Definition: The instrument which measures the voltage or potential difference in volts is known as the voltmeter. It works on the principle that the torque is generated by the current which induces because of measurand voltage and this torque deflects the pointer of the instrument. The deflection of the pointer is directly proportional to the potential difference between the points. The voltmeter is always connected in parallel with the circuit.

Symbolic Representation of voltmeter

The voltmeter is represented by the alphabet V inside the circle along with the two terminals.

Why is Voltmeter connected in Parallel?

The voltmeter constructs in such a manner that their internal resistance always remains high. If it connects in series with the circuit, it minimises the current which flows because of the measurand voltage. Thus, disturb the reading of the voltmeter.

AMMETERS

Definition: The Ammeter is a measuring instrument used to find the strength of current flowing around an electrical circuit when connected in series with the part of the circuit being measured.

The ideal ammeter has zero internal resistance. But practically the ammeter has small internal resistance. The measuring range of the ammeter depends on the value of resistance.

Symbolic Representation

The capital alphabet A represents the ammeter in the circuit.

 

 

 Three Phase Wyes and Deltas

Three phase voltages can be generated and transformed as either a Wye or a Delta. Wye and Delta refer to the connections between the single phase windings which connect to create three phase voltages. Notice the polarity of each winding at the connection points. In a Wye all the ends or 2's are connected together while the beginnings or l's are connected to the three phase power source, Ll, L2, and L3. In a Delta the end (2) of one winding is connected to the beginning (1) of .another winding. Three phase power is connected to each of these 1, 2 junction points.

 Star or Wye (Y) Connection

In this method of interconnection, the similar ends say, ‘star’ ends of three coils (it could be ‘finishing’ ends also) are joined together at point N. 

Delta (Δ) or Mesh Connection

In this form, of interconnection the dissimilar ends of the three phase winding are joined together i.e. the ‘starting’ end of one phase is joined to the ‘finishing’ end of the other phase and so on. In other words, the three windings are joined in series to form a closed mesh.

3-PHASE POWER

   

Most power is distributed in the form of 3-phase AC. Basically, instead of just one coil turning in agenerator, there are three coils, spaced 120 degrees apart.

As the coils turn through the magnetic field, power is sent out on three lines. Three current and voltage sine waves are generated, 120 degrees out of phase with each other. Each sine wave represents the current or voltage on one of the phases.

Three-phase electricity powers large industrial loads more efficiently than single phase electricity. When single-phase electricity is needed, it is available between any two phases, or, in some systems, between one of the phases and ground.

 POWER IN AC CIRCUITS

Power is dissipated in AC resistive circuits the same way as in DC resistive circuits. Power is measured in watts and is equal to the current times the voltage in the circuit.

The power consumed by the resistor in an AC circuit varies with the amount of current flowing through it and the voltage applied across it. 

The relationship of power, current, and voltage in resistive AC circuit.

Picture shows the relationship of power, current, and voltage. The power curve does not fall below the reference line because the power is dissipated in the form of heat. It does not matter in which direction the current is flowing, as power is assumed to have a positive value.

Power varies between the peak value and 0. Midway between peak value and 0 is the average power consumed by the circuit. In an AC circuit, the average power is the power consumed. This can be determined by multiplying the effective voltage value by the effective current value. This is expressed as

P=IE

PARALLEL AC CIRCUITS

A simple parallel AC circuit.

The voltage in a parallel circuit remains constant across each individual branch. However, the total current divides among the individual branches. In a parallel AC circuit, the total current is in phase with the applied voltage. The individual currents are also in phase with the applied voltage.

The in-phase relationship of the applied voltage, total current, and individual branch currents in a parallel AC circuit.

All current and voltage values are the effective values. These values are used the same way DC values are used.

 

 

 

 

 

 

 

 

 CAPACITIVE AC CIRCUITS

Capacitors are key components of AC circuits. Capacitors combined with resistors and inductors form useful electronic networks.

CAPACITORS IN AC CIRCUITS

When an AC voltage is applied to a capacitor, it gives the appearance that electrons are flowing in the circuit. However, electrons do not pass through the dielectric of the capacitor. As the applied AC voltage increases and decreases in amplitude, the capacitor charges and discharges. The resulting movement of electrons from one plate of the capacitor to the other represents current flow.

The current and applied voltage in a capacitive AC circuit differs from those in a pure resistive circuit. In a pure resistive circuit, the current flows in phase with the applied voltage. Current and voltage in a capacitive AC circuit do not flow in phase with each other.

                                               Note the  out-of-phase relationship between the current and the voltage in a capacitive AC circuit.                         The current leads the applied voltage. 

 INDUCTIVE AC CIRCUITS

Inductors, like capacitors, oppose current flow in AC circuits. They may also introduce a phase shift between the voltage and the current in AC circuits. A large number of electronic circuits are composed of inductors and resistors.

INDUCTORS IN AC CIRCUITS

Inductors in AC circuits offer opposition to current flow. When an AC voltage is placed across an inductor, it creates a magnetic field. As the AC voltage changes polarity, it causes the magnetic field to expand and collapse. It also induces a voltage in the inductor coil. This induced voltage is called a counter electromotive force (cemf); the greater the inductance, the greater the cemf. The cemf is out of phase with the applied voltage by 180° and opposes the applied voltage. This opposition is as effective in reducing current flow as a resistor.

The applied voltage and the induced voltage are 180° out of phase with each other in an inductive circuit.

 

The amount of voltage induced in the inductor depends on the rate of change of the magnetic field. The faster the magnetic field expands and collapses, the greater the induced voltage. The total effective voltage across the inductor is the difference between the applied voltage and the induced voltage. The induced voltage is always less than the applied voltage.

Figure below shows the relationship of the current to the applied voltage. In a purely inductive circuit, the current lags behind the applied voltage by 90°. 

The current lags the applied voltage in an AC inductive circuit.

BASIC AC RESISTIVE CIRCUITS

The relationship of current, voltage, and  resistance  is similar in DC and AC circuits. The simple AC circuit must be understood before moving on to more complex circuits containing capacitance andinductance.

 BASIC AC RESISTIVE CIRCUITS

A basic AC circuit consists of an AC source, conductors, and a resistive load. The AC source can be an AC generator or a circuit that generates an AC voltage. The resistiveload can be a resistor, a heater, a lamp, or any similar device.

When an AC voltage is applied to the resistive load, the AC current’s amplitude and direction vary in the same manner as those of the applied voltage. When  the applied voltage changes polarity, the current also  changes. They are said to be in phase. 

AC Circuits

The usual waveform of alternating current in most electric power circuits is a sine wave, whose positive half-period corresponds with positive direction of the current and vice versa. In certain applications, like guitar amplifiers, different waveforms are used, such as triangular waves or square waves. Audio and radio signals carried on electrical wires are also examples of alternating current. These types of alternating current carry information such as sound (audio) or images (video) sometimes carried by modulation of an AC carrier signal. These currents typically alternate at higher frequencies than those used in power transmission.