The Ward Leonard Method of speed control is based on the fact that the speed of DC motors can be changed by varying the applied voltage to the armature. The schematic diagram of the ward-leonard method is shown in the figure.In Ward-Leonard system, M is the DC motor whose speed is to be controlled and G is a separately excited DC generator which is driven by either a 3-three phase induction motor or synchronous motor. The combination of AC driving motor and the DC generator is known as the motor-generator set or M-G set.By varying the field current of the ... Read More

A self-excited DC generator has its own field excitation. Consider a shunt generator in which the field winding is connected in parallel with the armature, thus armature voltage supplies the field current.Assume that the generator has no-load connected to it and the armature is driven at a certain speed by a prime mover. The voltage build up in a DC generator depends upon the presence of the residual flux in the field poles of the generator. Because of this residual flux, a small voltage E0r of the order of 1 V or 2 V will be generated and is given ... Read More

According to the type of connection of the field winding with the armature, the DC motors are classified as follows −Permanent Magnet DC MotorsSeparately-Excited DC MotorsSelf-Excited DC MotorsSeries Wound DC MotorShunt Wound DC MotorCompound Wound DC MotorPermanent Magnet DC MotorA DC motor in which the main field flux is produced by the permanent magnets is known as permanent magnet DC motor. In this type DC motor, there is only one external source of DC supply is required, for supplying electrical power to the armature. The permanent magnet DC motors are mainly used in small scale application like in toys.Separately-Excited DC ... Read More

In DC machines, the drum type armature windings are used. In the case of drum type winding, the armature conductors (in the form of coils) are placed in the slots around the surface of cylindrical or drum shaped armature core. The coils are connected in series through the commutator segments such that their EMFs are added to each other.In DC machines, two types of armature windings are used −Wave WindingLap WindingWave WindingIn the case of wave winding, the armature coils are connected in series through the commutator segments in such a way that the armature winding is divided into two ... Read More

The power losses which occur in a transformer are of two types −Core or Iron Losses (Pi)Copper Losses or I2R Losses (Pcu)The losses in a transformer appears in the form of heat which increases the temperature and reduces the efficiency.Core or Iron Losses (Pi)The iron losses occur in the core of the transformer due to the alternating flux. These losses consist of hysteresis loss (Ph) and eddy current loss (Pe), i.e., $$\mathrm{P_{i}=P_{h}+P_{e}}$$The hysteresis and eddy-current losses are given by, $$\mathrm{Hysteresis\:loss, \: P_{h} \:= \:k_{h}f_{max}^{1.6} \:1.6 V \:Watt}$$$$\mathrm{Eddy\:current \:loss, \:P_{e} \:= \:K_{e}f^2B_{max}^{2}t^2 V \:Watt}$$Where, kh = hysteresis coefficient, ke = eddy ... Read More

Transformers are always rated in kVA instead of kW. Let's see why it is so.Reason 1The first reason is the power loss in the transformer. As, the copper loss or I2R loss depends on the current and the iron or core loss depends upon the voltage of the transformer. Thus, the total losses in a transformer depend upon volt-ampere (VA) only and not on the power factor of the load. That is why the transformer rating is given in kVA and not in kW.Reason 2At the time of designing a transformer, the manufacturer does not know which kind of load ... Read More

The transformers may also be constructed with a third winding in addition to the primary and secondary windings. This third winding is known as tertiary winding. The primary winding has the highest voltage rating, the tertiary winding has the lowest voltage rating and the secondary winding has the intermediate voltage rating. Also, the kVA ratings of the three windings of the 3-winding transformer are usually unequal. The figure shows the schematic diagram of a 3-winding transformer.The tertiary winding is always connected in delta. The primary advantage of the delta connected tertiary winding is that it suppresses any harmonic components that ... Read More

The Scott-T connection is a method of connecting two 1-phase transformers to perform the 3- phase to 2-phase conversion and vice-versa. In the Scott connection, the two 1-phase transformers are connected electrically (not magnetically), where one transformer is known as main transformer and the other is called as auxiliary or teaser transformer.The connection diagram of the Scott connection is shown in the figure. The main transformer is the centre tapped transformer at point D and is connected across the lines Y and B of the 3- phase side. Thus, the primary of the main transformer is YB and the secondary ... Read More

The delta-delta connection of the primary and secondary windings of a three-phase transformer is shown in the figure. Here, the secondary winding r1r2 corresponds to the primary winding R1R2, and the terminals R1 and r1 have same polarity. Also, the polarity of terminal 𝐫 connecting the r1 and b2 is the same as that of R connecting R1 and B2.The phasor diagram is drawn for the usual case of lagging power factor load. Here, the magnetising current and voltage drops in the impedances have been neglected. Under balanced condition, the line currents are √3 times of the phase currents and ... Read More

Transformers do not work on DC supply. Let us see why.The transformer works on the principle of mutual inductance i.e. current in one winding must change uniformly to induce the EMF in the other. When a DC voltage V is applied to the primary of a transformer, the primary will draw a constant current or direct current and hence, the magnetic flux produced in the core is also constant. As a result of it, there is not mutual inductance between the primary and secondary windings so no EMF will be induced in the primary and secondary windings because, $$\mathrm{EMF, \:𝑒 ... Read More