The three-point starter is mainly used for starting shunt and compound motors.Schematic Diagram of Three Point StarterThe circuit diagram of the three-point starter is shown in the figure. It is called three-point starter because it has three terminals viz. L, Z and A. It consists of a graded starting resistance to limit the starting current and is connected in series with the armature of the motor. The tapping points of the starting resistance are taken out to a number of studs.The three terminals L, Z and A of the starter are connected to the positive terminal, the shunt field terminal ... Read More

The open circuit characteristics (O.C.C) or magnetization characteristics is the curve that shows the relationship between the generated EMF at no-load (E0) and the field current (If) at constant speed. It is also known as no-load saturation curve. Its shape practically the same for all types of DC generator whether separately excited or self-excited.ExplanationIn order to determine the open circuit characteristics of a DC generator, the field winding is disconnected from the machine and is excited by an external DC source. The generator is run at its normal speed. The field current (If) is increased gradually from zero and the ... Read More

The losses in DC machine (generator or motor) may be divided into three categories as −Iron or Core LossesCopper LossesMechanical LossesThe losses in DC machines appear in the form of heat and hence increases the temperature of the machine. Also, the losses reduces the efficiency of the machine.Iron or Core LossesThe iron losses occur in the armature core of the DC machine since the armature core is subjected to the magnetic field reversal i.e. changing magnetic field. The core losses are of two types viz.Hysteresis LossEddy Current LossHysteresis LossThe armature core of DC machine is subjected to magnetic field reversal ... Read More

The four-point starter is used to overcome the drawbacks of the 3-point starter.Schematic Diagram of Four Point StarterThe circuit diagram of the four-point starter is shown in the figure. It consists of a graded starting resistance to limit the starting current and is connected in series with the armature of the motor. The tapping points of the starting resistance are taken out to a number of studs. It is called 4-point starter because it has 4 terminals viz. L, N, F and A.The one end of the armature coil is connected to the terminal A and of the shunt field ... Read More

A DC motor operates in two modes – motoring and breaking.In motoring operation, it converts electrical energy into the mechanical energy, which assists its motion.In breaking operation, it converts mechanical energy into electrical energy, thus works as a generator and opposes its motion.The motor can provide motoring and breaking operations in both forward and reverse directions. Therefore, a DC motor can operate in both directions of rotation and of producing both motoring and breaking, hence it is known as four quadrant operation of DC motor.The power developed by the motor is given by the product of angular speed and the ... Read More

The DC motors can be stopped using one of the following methods −Mechanical (Friction) BreakingElectric BreakingIn mechanical breaking, the motor is stopped due to friction between the moving parts of the motor and the break shoe. The mechanical breaking has several disadvantages as non-smooth stop, wear and tear of moving parts, breaking power wasted as heat and greater stopping time etc.In electric breaking, the kinetic energy of moving parts of the motor is converted into electrical energy which is either dissipated in a resistance or returned to the supply source. Types of Electric BreakingThere are three types of electric breaking methods ... Read More

Efficiency of DC GeneratorThe efficiency of a DC generator is defined as the ratio of mechanical input power to the output electrical power.$$\mathrm{Efficiency, \:\eta\:=\frac{Electrical\:Power\:Output(P_{o})}{ Mechanical\:Power\:Input(P_{i})}}$$ExplanationConsider the power flow diagram of a DC generator (see the figure), here the power is represented in three stages asBy referring the power flow diagram, $$\mathrm{Iron\:and\:Friction\:Losses\:=\:𝐴\:−\:𝐵}$$$$\mathrm{Copper\:Losses\:=\:𝐵\:−\:𝐶}$$Therefore, the efficiency of a DC generator can also be defined for the three stages as followsMechanical Efficiency  −$$\mathrm{\eta_{mech}\:=\frac{B}{A}=\:\frac{Power\:Developed\:in\:Armature\:(E_{g}I_{a})}{Mechanical\:Power\:Input\:(P_{i})}}$$Electrical Efficiency −$$\mathrm{\eta_{elect}\:=\frac{C}{B}=\:\frac{Electric\:Power\:Output\: (VI_{L})}{Power\:Developed\:in\:Armature\:(E_{g}I_{a})}}$$Commercial Efficiency − (always consider this unless stated otherwise)$$\mathrm{\eta\:=\frac{C}{A}=\:\frac{Power\:Output\:(P_{o})}{Power\:input\:(P_{i})}}$$Condition for Maximum EfficiencyThe efficiency of a DC generator is not constant but changes with the change in load.Let, for a ... Read More

Efficiency of a DC MotorThe efficiency of a DC motor is defined as the ratio of output power to the input power.Mathematically, $$\mathrm{Efficiency, \eta \:=\:\frac{Power\:Output}{Power\:Intput}\times 100 \%\:=\:\frac{P_{out}}{P_{in}}\times100\%\:...(1)}$$Since, $$\mathrm{Power\:input, \:P_{in} = Power\:Output(P_{out}) + Losses}$$Therefore, $$\mathrm{Efficiency, \eta \:=\:\frac{P_{out}}{P_{out}\:+\:losses}\times100\%\:....(2)}$$Condition for Maximum Efficiency of DC MotorThe efficiency of a DC motor is not constant but varies with the load. Consider a shunt motor (as shown in the figure) drawing an armature current of Ia amperes and the back emf is Eb.The mechanical power developed by the motor is (neglecting the mechanical losses), $$\mathrm{P_{m}=P_{out}=E_{b}I_{a}}$$The input power to the motor is, $$\mathrm{P_{in}=P_{out}\:+\:variable\:losses\:+\:constant\:losses}$$$$\mathrm{Efficiency, \eta\:=\:\frac{P_{out}}{P_{in}}\:=\:\frac{E_{b}I_{a}}{P_{out}\: +\:variable\:losses\:+\:constant\:losses}}$$$$\mathrm{⇒\eta\:=\:\frac{E_{b}I_{a}}{E_{b}I_{a}\:+\:I_a^2R_{a}\:+\:W_{c}}}$$$$\mathrm{⇒\eta\:=\:\frac{1}{1+(\frac{I_{a}R_{a}}{E_{b}})+\frac{W_{c}}{E_{b}I_{a}}}\:....(3)}$$The efficiency of ... Read More

An autotransformer is a one winding transformer in which a part of the winding is common to both primary and secondary windings.An autotransformer has a single continuous winding with a tap point between the primary and secondary windings. The tap point can be adjusted to obtain the desired output voltage, hence this is an obvious advantage of the autotransformer. The main disadvantage of an autotransformer is that the secondary winding is not electrically isolated from the primary.Theory of AutotransformerCase 1 – Autotransformer on No-LoadThe connection diagram of an unloaded autotransformer (step-down and step-up) is shown in the figure. In this, ... Read More

A current transformer (C.T.) is an instrument transformer which is used for the protection and measurement purposes in a power system. The C.T. is primarily used to measure high alternating currents in a power system.Construction of Current TransformerThe magnetic core of the current transformers is made up of thin laminations of silicon steel. The primary winding of a CT has a single turn (also called bar primary) and carries full-load current whereas the secondary winding has large number of turns. Therefore, the current transformer is a voltage step-up and current step-down transformer (see the figure).The primary winding of the current ... Read More