The transformers are designed such that it approaches the characteristics of an ideal transformer. For this, following design considerations are incorporatedThe core is made of magnetic materials (like silicon steel) which have high permeability and low hysteresis loss. Also, the core is made laminated to reduce the eddy current loss. Therefore, by doing these the iron losses or core losses can be reduced considerably and hence the no-load current.Instead of placing primary winding on one limb and the secondary winding on the other limb, in practice, the one half of each winding is wound on one limb and the other ... Read More

In DC machines, there are mainly three methods to obtain good commutation. They areResistance CommutationVoltage CommutationCompensating WindingsResistance CommutationIn the resistance commutation method of improving commutation, the high resistance carbon brushes are used. This increases the resistance between the contact of commutator segments and brushes. This high contact resistance has the tendency to force the current in the short-circuited coils to reverse according to commutation requirements and then increase in the reverse direction.Voltage CommutationIn voltage commutation method, the arrangements are made to induce a voltage in the coil undergoing commutation, which will neutralise the reactance voltage. This induced voltage is in ... Read More

The performance of a DC motor is given by the relation among the armature current, torque and speed. These relations are given graphically in the form of curves, which are called as characteristics of DC motors. These characteristics show the behaviour of the DC motor under different load conditions.Following are the three important characteristics of a DC motorTorque and Armature Current CharacteristicsIt is the graph plotted between the armature torque (τa) and the armature current (Ia) of a DC motor. It is also known as electrical characteristics of the DC motor.Speed and Armature Current CharacteristicsIt is the graph plotted between ... Read More

Characteristics of DC GeneratorsThe speed of a DC generator is made constant by the prime mover. Under such conditions, the performance of the generator is given by the relation among the excitation, terminal voltage and load. These relations are given graphically in the form of curves, which are called as characteristics of DC generators. These characteristics show the behaviour of the DC generator under different load conditions.The following are the main characteristics of a DC generatorOpen Circuit Characteristics or Magnetisation CurveThis is the graph plotted between the generated EMF at no-load (E0) and the field current (If) at a given ... Read More

Since the armature of a DC motor rotates in a magnetic field, an emf is induced in the conductors of the armature due to electromagnetic induction (as in a generator). This induced emf acts in the opposite direction to the applied voltage (according to Lenz’s law) and hence is known as back emf or counter emf. It is denoted by Eb and is given by, $$\mathrm{E_{b}=\frac{NP\varphi Z}{60A}}$$The magnitude of the back emf is always less than the magnitude of the applied voltage. But under normal operating conditions the difference of these two is small.ExplanationConsider a DC shunt motor as shown ... Read More

Armature ReactionThe current flowing through the armature conductors creates a magnetic field, which is called as armature flux. This armature flux distorts and weakens the magnetic flux produced by the main poles. This effect of armature flux on the main flux is known as armature reaction.Case 1Consider a two pole generator on no-load. Thus, the current in the armature conductors is zero. Under this condition, there is only the main flux (φm) in the machine which is produced by the main poles. This main flux is distributed symmetrically with respect to the polar axis (i.e. centre line of field poles).The ... Read More

The open-circuit test and the short-circuit test being performed on a transformer to determine the circuit parameters, efficiency and the voltage regulation without actual loaded of the transformer. The open-circuit and short-circuit tests give more accurate results than those are obtained by performing the measurements on a fully loaded transformer. The principle of advantage of these tests is the power consumption is very small as compared to the full-load output of the transformer.Open-Circuit Test of TransformerThe circuit arrangement for the open-circuit test of a transformer is shown in the figure. In this test the high voltage side of the transformer ... Read More

Working of AutotransformerThe connection diagram of an ideal step-down autotransformer is shown in the figure. In which the winding ab is the primary winding having N1 turns and the winding bc is the secondary winding having N2 turns. Here, the current I1 is the input primary current and the current I2 is the output secondary current or load current.Now, the number of turns in the 'ac' portion of the winding is N1 – N2 turns and the voltage across this portion is V1 – V2. The current in the common portion (winding 'bc') of the winding is I2 – I1.Consider ... Read More

A three-phase star-connected autotransformer is shown in the figure. The three-phase autotransformers are used for small ratios of transformation.In practice, the star connected three-phase autotransformers are used and the delta connected autotransformers are avoided. The primary application of three phase autotransformers is for interconnecting two power systems of different voltage levels such as 66 kV to 132 kV power systems, 110 kV to 220 kV power systems, 220 kV to 400 kV power systems etc.Numerical ExampleA three-phase star-connected autotransformer supplies a balanced three-phase load of 100 kW at 380 V and at 0.85 power factor lagging. If the supply voltage ... Read More

The non-sinusoidal nature of the magnetising current in a three phase transformer produces some undesirable phenomena. The phase magnetising current should contain third and higher order harmonics which is necessary to produce a sinusoidal flux.If the phase voltage across each phase is sinusoidal, then the phase magnetising currents are given as follows −$$\mathrm{𝐼_{𝑅𝑁} = 𝐼_{𝑚}\:sin\:𝜔𝑡\:+\:𝐼_{3𝑚 }sin(3𝜔𝑡\:+\:φ_{3})\:+\:𝐼_{5𝑚}\:sin(5𝜔𝑡\:+\: φ_{5})\:+\:… (1)}$$$$\mathrm{𝐼_{𝑌𝑁} = 𝐼_{1𝑚}\:sin(𝜔𝑡 − 120°)\:+ \:𝐼_{3𝑚}\:sin[3(𝜔𝑡 − 120°)\:+\:φ_{3}]\:+\:𝐼_{5𝑚} \:sin[5(𝜔𝑡 − 120°)\:+\:φ_{5}] +....}$$$$\mathrm{\Rightarrow\:𝐼_{𝑌𝑁} = 𝐼_{1𝑚}\:sin(𝜔𝑡 − 120°)\:+\:𝐼_{3𝑚}\:sin(3𝜔𝑡\:+\:φ_{3})\:+\:𝐼_{5𝑚}\:sin(5𝜔𝑡\:+\:120°\:+\:φ_{5}) +..…\:(2)}$$And$$\mathrm{𝐼_{𝐵𝑁} = 𝐼_{1𝑚}\:sin(𝜔𝑡 − 240°)\:+ \:𝐼_{3𝑚}\:sin[3(𝜔𝑡 − 240°)\:+\:φ_{3}]\:+\:𝐼_{5𝑚}\:sin[5(𝜔𝑡 − 240°)\:+\:φ_{5}] +...}$$$$\mathrm{\Rightarrow\:𝐼_{𝐵𝑁}= 𝐼_{1𝑚}\:sin(𝜔𝑡 − 240°)\:+\:𝐼_{3𝑚}\:sin(3𝜔𝑡\:+\:φ_{3})\:+\:𝐼_{5𝑚}\:sin(5𝜔𝑡\:+\:240°\:+\:φ_{5}) +\:… (3)}$$Hence, from the eqns. (1), (2) and (3), it is clear that ... Read More