Found 1006 Articles for Electronics & Electrical

The frequency of the generated voltage by the alternator depends upon the number of field poles and the speed at which the field poles are rotated. One complete cycle of the voltage being generated in an armature coil when a pair of field poles i.e. one north pole and one south pole passes over the coil.Let, 𝑃 = Number of rotor field poles𝑁 = Speed of rotor or field poles in RPM𝑓 = Frequency of the generated voltage in HzIn one revolution of the rotor, an armature coil is cut by (P/2) north poles and (P/2) south poles. Since one ... Read More

Coil span or coil pitch is defined as the distance between the two sides of a coil.Pole pitch is defined as the angular distance between the central line of one pole to the central line of the next pole. A pole pitch is always 180° electrical regardless of the number of poles on the machine.When a coil is having a span of 180° electrical, it is called as a full-pitch coil.When the coil is having a span less than 180° electrical, it is known as a short-pitch coil or fractional-pitch coil.The fractional-pitch coil is also known as chorded coil. A ... Read More

For a salient-pole synchronous generator or alternator, the per phase reactive power is given by, $$\mathrm{𝑄_{1𝜑} =\frac{𝑉𝐸_{𝑓}}{𝑋_{𝑑}}cos\:𝛿 −\frac{𝑉^{2}}{2𝑋_{𝑑}𝑋_{𝑞}}\lbrace{(𝑋_{𝑑} + 𝑋_{𝑞} ) − (𝑋_{𝑑} − 𝑋_{𝑞})\:cos\:2\delta}\rbrace … (1)}$$Where, V is the terminal voltage per phase.Ef is the excitation voltage per phase.$\delta$ is the per phase angle between Ef and V.Xd is the direct-axis synchronous reactance.Xq is the quadrature-axis synchronous reactance.For reactive power to be maximum, $$\mathrm{\frac{𝑑𝑄_{1𝜑}}{𝑑\delta}= 0}$$$$\mathrm{\Rightarrow\:\frac{𝑑}{𝑑\delta} \left(\frac{𝑉𝐸_{𝑓}}{𝑋_{𝑑}}cos\:\delta −\frac{𝑉^{2}}{2𝑋_{𝑑}𝑋_{𝑞}}\lbrace(𝑋_{𝑑} + 𝑋_{𝑞}) − (𝑋_{𝑑} − 𝑋_{𝑞}) cos\:2\delta\rbrace \right)= 0}$$$$\mathrm{−\frac{𝑉𝐸_{𝑓}}{𝑋_{𝑑}}sin\:\delta −\frac{2𝑉^{2}}{2𝑋_{𝑑}𝑋_{𝑞}}(𝑋_{𝑑} − 𝑋_{𝑞})sin\:2\delta = 0}$$$$\mathrm{\Rightarrow\:𝐸_{𝑓}\:sin\:\delta +\frac{𝑉}{𝑋_{𝑑}}(𝑋_{𝑑} − 𝑋_{𝑞})(2\:sin\:\delta\:cos\:\delta) = 0}$$$$\mathrm{\Rightarrow\:cos\:\delta = −\frac{𝐸_{𝑓}𝑋_{𝑞}}{2\:𝑉(𝑋_{𝑑} − 𝑋_{𝑞} )}… (2)}$$By putting the value of from eq. (2) ... Read More

Losses in an AlternatorThe losses that occur in an alternator can be divided into the following categories −Copper Losses or I2R LossesThe copper losses or I2R losses occur in the armature winding and rotor winding of the alternator. These losses occur due to the resistance of the windings.$$\mathrm{Armature\:winding\:cu\:loss = {𝐼^{2}_{𝑎}} 𝑅_{𝑎}}$$$$\mathrm{Rotor\:winding\:cu\:loss={𝐼^{2}_{𝑟}} 𝑅_{𝑟}}$$Core LossesThe core losses or iron losses occur in the pole faces, teeth and stator core of the alternator. The core losses in the alternator occur because the various iron parts of the machine are subjected to the varying magnetic field. The core losses consist of eddy current loss ... Read More

Hydrogen gas is used as cooling medium for cooling the alternators because of its superior cooling properties. The hydrogen cooling of the alternators requires completely seated circulated system. Mainly, oil-seated glands are used between shaft and casing of the alternator. The oil absorbs both hydrogen leaking-out and air leaking-in, so it should be purified periodically.In the hydrogen cooling of the alternators, the hydrogen gas is circulated by blowers and fans through the stator and rotor. Then, the heated hydrogen gas is passes over cooling coils inside the seated casing. The cooling coils carry oil or water to extract the heat ... Read More

Excitation in an electrical machine means production of magnetic flux by passing current in the field winding. In order to excite the field winding on the rotor of the synchronous machine the direct current is required.Excitation System in Small-size Synchronous MachinesFor small size synchronous machines (motor or generator), the direct current is supplied to the rotor field winding by a DC generator, known as exciter. The exciter is mounted on the shaft of the synchronous machine. The DC output of the exciter is fed to the field winding of the synchronous machine through brushes and slip rings.Excitation System in Medium-size ... Read More

In a concentrated winding, the coil sides of a given phase are concentrated in a single slot under a given pole. The induced EMFs in individual coils are in phase with each other. Therefore, these EMFs may be added arithmetically. To determine the induced EMF per phase in case of concentrated winding, a given coil EMF is multiplied by the number of series connected coils per phase.In actual practice, the coils of the windings of the alternator in each phase are not concentrated in a single slot, but are distributed in a number of slots in space to form a ... Read More

The equivalent circuit of an alternator or synchronous generator is shown in Figure-1. The equivalent circuit of the alternator is redrawn in Figure-2 by taking synchronous reactance $𝑋_{𝑆} = 𝑋_{𝐴𝑅} + 𝑋_{𝑎}$. By using the equivalent circuit, the phasor diagram of the alternator can be drawn as given below.By referring the equivalent circuit, we can write, $$\mathrm{𝑽 = 𝑬_{𝒂} − 𝑰_{𝒂}(𝑅_{𝒂} + 𝑗𝑋_{𝑆}) … (1)}$$Phasor Diagram of Alternator for Lagging Power Factor LoadThe phasor diagram of an alternator supplying a load of lagging power factor is shown in Figure-3. Here, the power factor is 𝜑 lagging. To draw this phasor ... Read More

Voltage Generation in AlternatorThe rotor of the alternator is run at its proper speed by the prime mover. The prime mover is a device which supplies mechanical energy input to the alternator. For slow and medium speed alternators, the water turbines are used as prime mover while the steam and gas turbines are used for the large and high speed alternators.When the poles of the rotor move under the armature conductors on the stator, the magnetic field cuts the armature conductors. Therefore, EMF is induced in these conductors by the electromagnetic induction. This induced EMF is of alternating nature because field ... Read More

Cooling is the very important aspect in the construction and operation of the alternators. The cooling in the alternators or synchronous generators are classified into two types −Open-circuit coolingClosed-circuit coolingOpen Circuit CoolingIn open-circuit cooling, the air is drawn into the alternator by the means of fans and blowers and circulated inside. This air is later released back into the atmosphere. The open-circuit cooling method is suitable for small sized alternators.Closed Circuit CoolingThe closed circuit cooling method is a totally enclosed system and clean hot air from the alternator is cooled by oil or water-cooled heat exchanger and then, this cooled ... Read More