Magnetic and Electric Circuit

A magnetic circuit is made up of one or more closed loop paths containing a magnetic flux. The flux is usually generated by permanent magnets or electromagnets and confined to the path by magnetic cores consisting of ferromagnetic materials like iron, although there may be air gaps or other materials in the path.

The closed path followed by magnetic lines of forces is called the magnetic circuit. In the magnetic circuit, magnetic flux or magnetic lines of force starts from a point and ends at the same point after completing its path.

Some examples of magnetic circuits are:

• horseshoe magnet with iron keeper (low-reluctance circuit)

• horseshoe magnet with no keeper (high-reluctance circuit)

• electric motor (variable-reluctance circuit)

• some types of pickup cartridge (variable-reluctance circuits)

Flux is generated by magnets, it can be a permanent magnet or electromagnets.

A magnetic circuit is made up of magnetic materials having high permeability such as iron, soft steel, etc.

Magnetic circuits are employed to efficiently channel magnetic fields in many devices such as electric motors, generators, transformers, relays, lifting electromagnets, galvanometers, and magnetic recording heads.

The concept of a "magnetic circuit" exploits a one-to-one correspondence between the equations of the magnetic field in an unsaturated ferromagnetic material to that of an electrical circuit. Using this concept the magnetic fields of complex devices such as transformers can be quickly solved using the methods and techniques developed for electrical circuits.

There are two types of magnetic circuits 1.Series and 2.Parallel

The interconnection of various active and passive components in a prescribed manner to form a closed path is called an electric circuit. The system in which electric current can flow from the source to the load and then back to the other terminal of the source is referred to as an electric circuit.

The main parts of an ideal electric circuit are:

1. Electrical sources for delivering electricity to the circuit and these are mainly electric generators and batteries

2. Controlling devices for controlling electricity and these are mainly switches, circuit breakers, MCBs, and potentiometer like devices etc.

3. Protection devices for protecting the circuit from abnormal conditions and these are mainly electric fuses, MCBs, switchgear systems.

4. Conducting path to carry electric current from one point to other in the circuit and these are mainly wires or conductors.

5. Load.

   
   

   Basic Properties of Electric Circuits

   1.A circuit is always a closed path.

   2.A circuit always contains at least an energy source which acts as a source of electrons.

   3.The electric elements include uncontrolled and controlled source of energy, resistors, capacitors, inductors, etc.

   4.In an electric circuit flow of electrons takes place from negative terminal to positive terminal.

   5.Direction of flow of conventional current is from positive to negative terminal.

   6.Flow of current leads to potential drop across the various elements.

   Types of Electric Circuits

   The electric circuit can be categorized in three different ways

   1. Open circuit

   2. Closed circuit

   3. Short circuit

      Open Circuit: If due to disconnection of any part of an electric circuit if there is no flow of current through the circuit, is said to be an open circuited.
      Closed Circuit

      If there is no discontinuity in the circuit and current can flow from one part to another part of the circuit, the circuit is said to be closed circuit.

      Short Circuit

      If two or more phases, one or more phases and earth or neutral of AC system or positive and negative wires or positive or negative wires and earth of DC system touch together directly or connected together by a zero impedance path then the circuit is said to be short circuited.

      Electric circuits can be further categorized according to their structural features into either:

      1. Series Circuits

      2. Parallel Circuits

      3. Series Parallel Circuits

      Series Circuit:

      When all elements of a circuit are connected one after another in tail to head fashion and due to which there will be only one path of flowing current then the circuit is called series circuit. The circuit elements then are said to be series connected. In the series electrical circuit, same current flows through all element connected in series.

      If components are connected in such a way that the voltage drop across each component is same then it is known as parallel circuit. In parallel circuit the voltage drop across each component is same but the currents flowing through each component may differ. The total current is the sum of currents flowing through each element.

      An example of a parallel circuit is the wiring system of a house. If one of the electric lamp burns out, current can still flow through the rest of the lights and appliances.

      Series Parallel Circuit

      An electrical circuit in which some of the elements are connected in series and some of the elements are connected in parallel is called a series parallel circuit. Most of the practical circuits are series parallel circuits. A very common example is the connection of conductors in the rotor of DC motor.

      All AC circuits are made up of the combination of resistance R, inductance L and capacitance C. The circuit elements R, L, and C are known as circuit parameters. To study a general AC circuit it is necessary to consider the effect of each parameter separately.

      Purely Resistive AC Circuits

      A purely resistive circuit is a circuit which has inductance so small that at normal frequency its reactance is negligible as compared to its resistance. In a purely resistive circuit whole of the applied voltage is utilized in overcoming the ohmic resistance of the circuit. A purely resistive circuit is also known as the non-inductive circuit.

      The power factor of the purely resistive circuit (cos φ) is 1.

      Purely Inductive AC Circuits

      Inductive Reactance

      The opposition offered to the flow of an alternating current by the inductance of the circuit is known as inductive reactance. In fact, it is a property of all inductors. It is denoted by XL.
      XL = ωL = 2πfL ohms
      where, f = frequency in Hz; L = inductance in henrys.
      The unit of inductive reactance is ohm. The inductive reactance of a given inductor is directly proportional to the frequency of applied voltage. It means inductive reactance increases linearly with frequency.

      Purely Capacitive AC Circuits

      When an alternating voltage is applied to a purely capacitive circuit, the capacitor is charged first in one direction and then in the opposite direction.

      Capacitive Reactance

      Capacitive reactance is the opposition offered to the flow of alternating current by the capacitance of a capacitor. In fact, it is a property of all the capacitors. It is denoted by XC.
      
      XC =1/ωC = 1/2πfC ohms
      where, f = frequency in Hz; C = capacitance in farad.
      The unit of capacitive reactance is ohm.

      
      

      
      

      A pure inductive coil is that which has no ohmic resistance and hence no I2R loss. A pure inductance is practically not attainable, though it is nearly approached by a coil wound with such thick wire that its resistance is negligible.

      Whenever an alternating voltage is applied to a purely inductive circuit a back EMF is produced due to self-inductance of the coil. The back EMF, at every step, opposes the rise or fall of current through the coil. As there is no ohmic voltage drop, the applied voltage has to overcome this self-induced EMF only.

      In actual practice, AC circuits contain two or more than two components connected in series. In a series circuit, each component carries the same current. An AC series circuit may be classified as under:

      
      

      • RL series circuit

      • RC series circuit

      • RLC series circuit

        RL Series Circuit :In an RL series circuit, a pure resistance (R) is connected in series with a coil having the pure inductance (L). To draw the phasor diagram of RL series circuit, the current I (RMS value) is taken as reference vector because it is common to both elements.

        Voltage drop VR is in phase with current vector, whereas, the voltage drop in inductive reactance VL leads the current vector by 90o since current lags behind the voltage by 90o in the purely inductive circuit. The vector sum of these two voltage drops is equal to the applied voltage V (RMS value).

        The power waveform for RL series circuit is shown in the figure. In this figure, voltage wave is considered as a reference. The points for the power waveform are obtained from the product of the corresponding instantaneous values of voltage and current.

        Since the area under the positive loops is greater than that under the negative loops, the net power over a complete cycle is positive. Hence a definite quantity of power is consumed by the RL series circuit. But power is consumed in resistance only; inductance does not consume any power.

        RC Series Circuit:

        In an RC series circuit, a pure resistance (R) is connected in series with a pure capacitor (C). To draw the phasor diagram of RC series circuit, the current I (RMS value) is taken as reference vector. Voltage drop VR is in phase with current vector, whereas, the voltage drop in capacitive reactance VC lags behind the current vector by 90o, since current leads the voltage by 90o in the pure capacitive circuit. The vector sum of these two voltage drops is equal to the applied voltage V (RMS value).

        Since the area under the positive loops is greater than that under the negative loops, the net power over a complete cycle is positive. Hence a definite quantity of power is consumed by the RC series circuit. But power is consumed in resistance only; capacitor does not consume any power.

        RLC Series Circuits:

        In an RLC series circuit a pure resistance (R), pure inductance (L) and a pure capacitor (C) are connected in series. To draw the phasor diagram of RLC series circuit, the current I (RMS value) is taken as the reference vector. The voltages across three components are represented in the phasor diagram by three phasors VR, VL and VC respectively.
        
        The voltage drop VL is in phase opposition to VC. It shows that the circuit can either be effectively inductive or capacitive. In the figure, phasor diagram is drawn for the inductive circuit. There can be three cases of RLC series circuit.

        • When XL > XC, the phase angle φ is positive. In this case, RLC series circuit behaves as an RL series circuit. The circuit current lags behind the applied voltage and power factor is lagging. In this case,
          if the applied voltage is represented by the equation;
          v = Vm sin ωt
          then, the circuit current will be represented by the equation;
          i = Im sin (ωt – φ).

      
      

   Alternating currents (ac) are currents that alternate in direction (usually many times per second). Such currents are produced by voltage sources whose polarities alternate between positive and negative. By convention, alternating currents are called ac currents and alternating voltages are called ac voltages. Under AC fundamentals some important terms comes like Alternating quantity, Instantaneous value, Amplitude, Frequency,Time Period,Peak Value,Root Mean Square Value, Average Value,Form Factor,Crest Factor.

   Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude.

   
   

   Peak Value: The maximum value attained by an alternating quantity during one cycle is called its Peak value.

   It is also known as the maximum value or amplitude or crest value.

   Average Value:The average of all the instantaneous values of an alternating voltage and currents over one complete cycle is called Average Value.

   RMS Value:That steady current which, when flows through a resistor of known resistance for a given period of time than as a result the same quantity of heat is produced by the alternating current when flows through the same resistor for the same period of time is called R.M.S or effective value of the alternating current.

   The average of all the instantaneous values of an alternating voltage and currents over one complete cycle is called Average Value.

   Cycle :When one set of positive and negative values completes by an alternating quantity or it goes through 360 degrees electrical, it is said to have one complete Cycle.

   Frequency :The number of cycles made per second by an alternating quantity is called frequency. It is measured in cycle per second (c/s) or hertz (Hz) and is denoted by (f).

   Time period: Time taken by an AC quantity to complete one cycle. It is measured in seconds.

   Form Factor (Kf): It is defined as the ratio of rms value to average value

   Kf=RMS value/Average value

   Peak Factor(Kp): It is defined as the ratio of Peak value to average value

   (Kp)=Peak value/RMS value

Applications of Magnetic circuits: Magnetic circuits are used in various devices like electric motor, transformers, relays, generators galvanometer, etc.

Applications of Electric circuits: Electric motors, or computers; and the connecting wires or transmission lines.