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

Following are the assumptions being made in the synchronous impedance method for finding the voltage regulation of the alternator or synchronous generator −The synchronous impedance is constant. For the synchronous generator, the synchronous impedance is determined from the open-circuit characteristics (O.C.C.) and short-circuit characteristics (S.C.C.). The synchronous impedance is the ratio of open-circuit voltage to short-circuit current i.e.$$\mathrm{Synchronous\:impedance, \:𝑍_{𝑠}=\frac{open\: circuit\:voltage\:per\:phase}{short\:circuit \:armature\:current}}$$When the O.C.C. and S.C.C. are linear, then the synchronous impedance is constant. Above the knee point of the O.C.C. when the saturation starts, the synchronous impedance decreases because in this condition, the O.C.C. and S.C.C. approach each other.Therefore, the synchronous ... Read More

In this article, we will take a look at some of the basic terms used in Electrical Machines and their definition.Field WindingThe winding of the electrical machine through which a current is passed to produce the main flux is known as field winding.Armature WindingThe winding of the electrical machine in which voltage is induced by the action of electromagnetic induction is known as armature winding.Some of the basic terms associated with the armature winding are as follows −Turn – When two conductors are connected to one end by an end connector, it is known as a turn.Coil – When several ... Read More

Consider the direction of rotation of the rotor is clockwise. The direction of the generated voltage in various conductors can be determined by applying the right-hand rule keeping in view that the direction of rotation of the conductors with respect to the rotor poles is anticlockwise.Consider the alternator is supplying current at unity power factor. The phase currents 𝐼𝑅 , 𝐼𝑌 and 𝐼𝐵 will be in phase with their respective generated voltage 𝐸𝑅 , 𝐸𝑌 and 𝐸𝐵 as shown in Figure-1. The positive direction of fluxes φ𝑅, φ𝑌 and φB are shown in Figure-2. The projection of a phasor on ... Read More

Armature Leakage ReactanceIn an AC electrical machine, the magnetic flux set up by the load current which does not contribute to the useful magnetic flux of the machine is known as leakage flux. This leakage flux sets up a self-induced EMF in the armature winding of the machine. The leakage flux may be classified into three categories as follows −Slot leakageTooth head leakageOverhang or coil end leakageThe leakage fluxes in the electrical machines induce EMFs in the armature windings. These leakage EMFs are taken into account by introduction of leakage reactance drops and lead the current producing them by 90°.Armature ... Read More

Consider a 2-pole alternator as shown in Figure-1. Suppose that the alternator is loaded with an inductive load of zero power factor leading.. Thus, the phase currents 𝐼𝑅 , 𝐼𝑌 and 𝐼𝐵 will be leading their respective phase voltages 𝐸𝑅 , 𝐸𝑌 and 𝐸𝐵 by 90°. Figure-2 shows the phase diagram of the alternator.Now, at time t = 0, the instantaneous values of currents and fluxes are given by, $$\mathrm{𝑖_{𝑅} = 0;\:\:φ_{𝑅} = 0}$$$$\mathrm{𝑖_{𝑌} = 𝐼_{𝑚}\:cos 30° = \frac{\sqrt{3}}{2}𝐼_{𝑚};\:\:\:φ_{𝑌} = \frac{\sqrt{3}}{2}φ_{𝑚}}$$$$\mathrm{𝑖_{𝐵} = −𝐼_{𝑚}\:cos 30° = −\frac{\sqrt{3}}{2}𝐼_{𝑚};\:\:\:φ_{B} = −\frac{\sqrt{3}}{2}φ_{𝑚}}$$The space diagram of the magnetic fluxes is shown in Figure-3. By the ... Read More