Consider a 2-pole alternator as shown in Figure-1. Suppose that the alternator is loaded with an inductive load of zero power factor lagging. In this case, the phase current 𝐼𝑅 , 𝐼𝑌 and 𝐼𝐵 will be lagging with their respective phase voltages 𝐸𝑅 , 𝐸𝑌 and 𝐸𝐵 by 90°. The phasor diagram of the alternator is shown in Figure2.Now, at time t = 0, the instantaneous values of currents and fluxes are given by, $$\mathrm{𝑖_{𝑅} = 0;\:\:φ_{𝑅} = 0}$$$$\mathrm{𝑖_{𝑌} = 𝐼_{𝑚}\:sin(−120°) = −\frac{\sqrt{3}}{2}𝐼_{𝑚};\:\:\:φ_{𝑌} = −\frac{\sqrt{3}}{2}φ_{𝑚}}$$$$\mathrm{𝑖_{𝐵} = 𝐼_{𝑚}\:sin(120°) = \frac{\sqrt{3}}{2}𝐼_{𝑚};\:\:\:φ_{𝐵} = \frac{\sqrt{3}}{2}φ_{𝑚}}$$The space diagram of the magnetic fluxes is shown in ... Read More

Practically, most of the synchronous generators or alternators have stationary armature and rotating field.A stationary armature-rotating field alternator has several advantages over a rotating armature type alternator, as given below −When a stationary armature is used in the alternator, the output current can be taken directly from the fixed terminals on the stationary armature without using slip rings, brushes, etc.The armature windings of the rotating field alternator, being stationary, are not subjected to vibration and centrifugal forces.A stationary armature can be easily insulated for the high voltage for which the alternator is designed. This generated voltage may be as high ... Read More

The synchronous impedance method or EMF method is used to determine the voltage regulation of the larger alternators. The synchronous impedance method is based on the concept of replacing the effect of armature reaction by an imaginary reactance.For an alternator, $$\mathrm{𝑽 = 𝑬_{𝒂} − 𝑰_{𝒂}𝒁_{𝒔} = 𝑬_{𝒂} − 𝑰_{𝒂}(𝑅_{𝒂} + 𝑗𝑋_{𝑠}) … (𝟏)}$$At first, the synchronous impedance (𝑍𝑠) is measured and then, the value of actual generated EMF (𝐸a) is calculated. Thus, from the values of (𝐸a) and V, the voltage regulation of the alternator can be calculated.Measurement of Synchronous ImpedanceIn order to determine the value of synchronous impedance, following ... Read More

The voltage regulation of an alternator or synchronous generator is defined as the rise in the terminal voltage when the load is decreased from full-load rated value to zero. The speed and field current of the alternator remain constant.In other words, the voltage regulation of the alternator can be defined as the change in terminal voltage from no-load to full load rated value divided by the full-load rated voltage, i.e., $$\mathrm{Per\:unit\:voltage\:regulation =\frac{|𝐸_{𝑎}| − |𝑉|}{|𝑉|}}$$Also, the percentage voltage regulation of the alternator is given by, $$\mathrm{Percentage\:voltage\:regulation =\frac{|𝐸_{𝑎}| − |𝑉|}{|𝑉|}× 100\%}$$Where, |𝐸𝑎| is the magnitude of generated voltage (or no-load voltage) perphase|𝑉| ... Read More

Unsaturated Synchronous ReactanceAt low excitations, the open-circuit characteristic (OCC) of an alternator coincides with the air-gap line, thus, the synchronous impedance of the alternator is constant and this value of the synchronous impedance is known as linear or unsaturated synchronous impedance.The unsaturated synchronous reactance (denoted by Xsu) can be obtained from the air-gap line voltage and the short-circuit current of the alternator for a particular value of the field current.Note – SCC is the short-circuit characteristic.From the characteristics, the unsaturated synchronous impedance is given by, $$\mathrm{𝑍_{𝑠𝑢} =\frac{𝐸_{𝑎𝑑}}{𝐼_{𝑎𝑏}}= 𝑅_{𝑎} + 𝑗𝑋_{𝑠𝑢}}$$If the armature resistance of the alternator is neglected, then the ... Read More

In an alternator or synchronous generator, the actual generated voltage consists of the summation of two component voltages. One of these component voltages is the excitation voltage  (𝐸exc); it is the voltage that would be generated because of the field excitation only. The excitation voltage is the voltage that would be generated when there is no armature reaction.The other component of the generated voltage is the voltage that must be added to the excitation voltage to take care of the effect of the armature reaction on the generated voltage. This component voltage is known as the armature reaction voltage and ... Read More

In a synchronous machine, the short-circuit ratio (SCR) is an important factor. The SCR affects the physical size, operating characteristics and cost of the synchronous machine.Low Value of SCRWith a low value of SCR, the synchronous machine is very sensitive to the load variations, i.e., the alternator has a large variation in terminal voltage with a change in load. In order to keep the terminal voltage constant, the field current of the machine is to be varied over a wide range. Also, for a small value of SCR, the synchronising power of the machine is small. The synchronising power keeps ... Read More

What is Hunting in Alternator?When an alternator or synchronous generator is running under normal condition, the armature field and the rotor magnetic field will be rotating in the same direction at synchronous speed.When either a sudden change in the load of the alternator or a change in the torque of turbine occurs, then the speed of the rotor decreases with respect to that of armature (or stator) field by some angle called as load angle. Once this happens, the alternator rotor starts to hunt, i.e., search for a new equilibrium position. Although the rotor has high inertia, it cannot achieve ... Read More

Why doesn't a Single-Phase Induction Motor Self-Start?A single-phase induction motor consists of a squirrel cage rotor and a stator carrying a single-phase winding. But, a single-phase induction motor is not self-starting like a 3-phase induction motor since it requires some starting means.When a single-phase supply is fed to the stator winding of the single-phase induction motor, it produces a magnetic field that pulsates in strength in a sinusoidal manner. The polarity of the magnetic field reverses after each half cycle but the magnetic field does not rotate in the space.As a result, the alternating flux cannot produce rotation in a ... Read More

The working principle of a single-stack variable reluctance (VR) stepper motor is based on the property of the flux lines to occupy the low reluctance path. Therefore, in a variable reluctance stepper motor, the stator and rotor get aligned such that the magnetic reluctance is minimum.Construction of Single-Stack Variable Reluctance Stepper MotorA single-stack variable reluctance stepper motor consists of a salient pole stator. The stator has concentrated windings placed over the stator poles. The number of stator phases depends upon the connection of stator coils. The rotor of the single-stack variable reluctance stepper motor is a slotted structure made up ... Read More