A potential transformer (P.T.) is an instrument transformer which is used for the protection and measurement purposes in the power systems. A potential transformer is mainly used to measure high alternating voltage in a power system.Potential transformers are step-down transformers, i.e., they have many turns in the primary winding while the secondary has few turns. The figure shows a typical potential transformer for the measurement of high alternating voltage. From the figure, it is clear that a P.T. is a well designed step down transformer.The stepped down voltage by the Potential transformer can be measure using a low range AC ... Read More

When the primary windings of two or more transformers are connected to a common voltage supply and their secondary windings are connected to a common load. Then, the transformers are said to be connected in parallel, i.e., parallel operation of the transformers. The figure shows two transformers T1 and T2 connected in parallel.Reasons for Parallel Operation of TransformersThe primary reasons for operating transformers in parallel are as follows −For large loads, it may be impractical or uneconomical to have a single large transformer so that many small transformers are paralleled to meet the load demand.By operating the transformers of standard ... Read More

If one transformer of delta-delta connected transformer bank is damaged or opened, then rest of the system will continue to supply the 3-phase power. If this damaged transformer is isolated, the remaining two transformers will function as a 3-phase bank with rating reduced to about 58 % that of the original delta-delta bank. This type of arrangement is known as open-delta or V-V connection.Therefore, in the case of open-delta or V-V connection, two instead of three 1-phase transformers are used for three-phase operation.Now, let VRY, VYB and VBR be the applied voltages to the primary winding. The voltage induced in ... Read More

There are four main types of transformers viz.Distribution TransformersPower TransformersAutotransformersInstrument TransformersDistribution TransformersThe transformers which are used to step down the transmission voltage to the distribution voltage or the distribution voltage to a standard service voltage are known as distribution transformers. These transformers have variable load which is considerably less than the full load rating of the transformer. Hence, the distribution transformers are designed to have their maximum efficiency at between ½ and ¾ of the full load.These transformers are kept in operation 24 hours a day whether they are carrying any load or not. The distribution transformers have a good ... Read More

When the load is connected to a DC generator, the load current flows through the armature conductors and set up a magnetic flux called armature flux. This armature flux is always 180° phase apart from the main field flux and distorts or opposes it. This opposition of armature flux on the main field flux is known as armature reaction.The armature reaction has two undesirable effects. The first is it reduces the main field flux called as demagnetising effect and the second is it distorts the main field flux called as cross magnetising effect.ExplanationIn order to calculate the demagnetising and cross-magnetising ... Read More

DC Motor Voltage EquationConsider a DC shunt motor as shown in the figure.Let, $$\mathrm{V\:=\:Applied\:DC\:voltage}$$$$\mathrm{E_{b}\:=\:Back\:EMF}$$$$\mathrm{R-{a} \:=\:Resistance\:of\:the\:armature\:circuit}$$$$\mathrm{I_{a}\:=\:Armature\:current}$$By applying the KVL in the armature circuit, we get, $$\mathrm{V\:=\:E_{b}\:+\:I_{a}R_{a} … (1)}$$The expression in eqn. (1) is known as the voltage equation of the DC motor.Power Equation of DC MotorIf the eqn. (1) is multiplied by Ia on both sides, we obtain, $$\mathrm{VI_{a}\:=\:E_{b}I_{a}\:+\:I_a^2R_{a}\:.... (2)}$$The eqn. (2) is known as the power equation of DC motor. The mechanical losses are neglected.Where, $$\mathrm{VI_{a}\:=\:Input\:electrical\:power\:to\:the\:armature}$$$$\mathrm{E_{b}I_{a}\:=\:Electromechanical\:power\:developed\:by\:armature}$$$$\mathrm{I_a^2R_{a}\:=\:Electric\:power\:wasted\:in\:the\:armature\:Cu\:losses}$$Condition for Maximum Mechanical PowerThe mechanical power developed by the motor (neglect mechanical losses) is given by, $$\mathrm{P_{m}\:=\:E_{b}I_{a}}$$$$\mathrm{⇒\:P_{m}\:= VI_{a}I_a^2R_{a}}$$Since, the V and Ra are constant ... Read More

A DC motor is an electromechanical energy conversion device, which converts electrical energy input into the mechanical energy output.The operation of the DC motor is based on the principle that when a current carrying conductor is placed in a magnetic field, a mechanical force acts on the conductor. The magnitude of the force is given by, $$\mathrm{F\:=\:BIl\:Newtons}$$The direction of this is given by the Fleming's left hand rule.Construction of a DC MotorHere is the schematic diagram of a DC MotorA DC motor consists of six main parts, which are as followsYokeThe outer frame of a DC motor is a hollow ... Read More

DC GeneratorA DC generator is an electromechanical energy conversion device that converts mechanical power into DC electrical power through the process of electromagnetic induction.A DC generator operates on the principle of electromagnetic induction i.e. when the magnetic flux linking a conductor changes, an EMF is induced in the conductor. A DC generator has a field winding and an armature winding.The EMF induced in the armature winding of a DC generator is alternating one and is converted into direct voltage using a commutator mounted on the shaft of the generator. The armature winding of DC Generator is placed on the rotor ... Read More

Critical Field Resistance of DC Shunt GeneratorThe critical field resistance (RC) of a DC shunt generator is defined as the maximum field circuit resistance with which the shunt generator would just excite.The voltage build-up in a shunt generator depends upon field circuit resistance. If the field circuit resistance is R1, then the generator will build-up a voltage V0 (see the E0 versus If curve). If the field circuit resistance is increased to R2, then the generator will build-up a voltage V0 slightly less than V0. As the resistance of the field circuit is increased, the slope of the field resistance ... Read More

When the transformer is in operation, heat is generated due to copper losses in the windings and iron losses in the core. The removal of heat from the transformer is known as cooling of the transformer.Transformer Cooling MethodsFor the dry type transformersAir Natural (AN) CoolingAir Forced (AF) or Air Blast (AB) CoolingFor oil immersed type transformersOil Natural Air Natural (ONAN) CoolingOil Natural Air Forced (ONAF) CoolingOil Forced Air Forced (OFAF) CoolingOil Forced Water Forced (OFWF) CoolingAir Natural (AN) CoolingThe air natural (AN) cooling is used in dry type, self-cooled transformers. In this method, the natural circulation of surrounding air is ... Read More