Synchronous Generators Questions and Answers

Answer: B

At zero power factor leading, the armature reaction in an alternator is purely magnetising. When the armature current leads the voltage by 90°, its flux acts in the same direction as the main field, and so it assists the field and raises the terminal voltage.

At zero power factor lagging the reaction is purely demagnetising, at unity power factor it is cross-magnetising, and for intermediate power factors it is part cross and part demagnetising or magnetising. So the second option is correct.

Answer: B

Both statements are false. Salient pole rotors have projecting poles and are used in low speed machines, such as those driven by hydraulic turbines and diesel engines. Turbo alternators run at high speed and use cylindrical (non-salient) rotors, so the first statement is false.

Cylindrical rotor alternators are driven by steam or gas turbines, and not by a solar cell, which is a source of electrical power and not a prime mover. So the second statement is also false. The key gives 'False, False', the second option.

Answer: D

The statement that is not true is that a salient type rotor is used in high speed turbine driven generators.

High-speed turbo-alternators driven by steam or gas turbines run at 3000 or 1500 rpm and use smooth cylindrical (non-salient) rotors of small diameter and great length, which can withstand the centrifugal forces. Salient pole rotors, with large diameter and many poles, are used in slow-speed machines such as hydro generators.

The other statements are true: synchronous generators normally have a stationary armature and rotating field, the cores are built of laminated steel to reduce eddy current loss, and the armature winding is distributed to improve the waveform.

Answer: B

Short pitch coils and distributed windings are used to make the generator emf more sinusoidal.

Distributing the winding in several slots and short-chording the coils both reduce the harmonics in the generated emf: the coil emfs are slightly out of phase, which suppresses harmonics strongly, and a suitable short pitch can eliminate a chosen harmonic altogether. The result is a waveform much closer to a sine wave.

The price is a slight reduction in the fundamental emf, since the distribution and pitch factors are each a little less than one. So these measures do not increase the magnitude of the emf, which rules out the first and third options, and they reduce rather than increase the distribution factor.

Answer: B

The normal arrangement is a stationary armature and rotating field poles.

In practical alternators the three-phase armature winding is placed in the stator and the field poles on the rotor. The high-voltage armature can then be insulated easily and connected to the load through fixed terminals, while only the low-voltage, low-power field current has to pass through slip rings. The rotor is also lighter and can run at high speed.

A rotating armature with stationary poles is used only in very small machines. Both parts stationary, or both rotating, would produce no relative motion and so no emf.

Answer: C

The per phase resistance is 0.25 Ω.

In a star (Y) connection, a measurement between two line terminals passes through two phase windings in series, so the measured value is twice the phase resistance.

\[ R_{ph} = \frac{0.5}{2} = 0.25\ \Omega \]

0.5 Ω forgets that two windings are in the path, and 1 Ω doubles instead of halving. In practice this dc value is then raised by a factor of about 1.2 to 1.6 to allow for skin effect when the ac resistance is wanted.

Answer: C

An alternator is rated in kVA.

The heating of an alternator, which limits its output, depends on the current in its windings and the voltage (which fixes the core loss), not on the power factor of the load. Since the load power factor is decided by the consumer and is not known when the machine is built, the rating is given as the product of voltage and current, in kVA.

A rating in kW or h.p. would depend on the power factor, and kVAR measures only reactive power.

Answer: C

The synchronous reactance is generally 10 to 100 times greater than the armature resistance.

Synchronous reactance combines the leakage reactance of the armature with the effect of armature reaction, and in practical alternators it is large, often of the order of one per unit, while the armature resistance is only about 0.01 per unit. That is why the resistance is often neglected in calculations of regulation and power.

The reactance is far larger than the resistance, so the 'smaller' options are wrong, and five times greater understates the usual ratio.

Answer: B

The direction of active power flow reverses.

If the prime mover fails while the generator stays connected to the grid, it keeps running at synchronous speed, but now it draws active power from the bus to cover its losses and drive the turbine. It runs as a synchronous motor, so active power flows into the machine instead of out. Reverse power relays detect exactly this.

The reactive power exchange is set by the field excitation, which the prime mover failure does not touch, so it is not the effect the question describes. 'Relative power' is not a power-system quantity, and active power certainly does change direction.

Answer: D

The induced emf phasor lags behind the flux phasor by 90°.

The emf in an armature coil is \(e = -N\,d\phi/dt\). If the flux linking the coil is \(\phi = \Phi_{m}\sin\omega t\), then:

\[ \begin{aligned} e &= -N\omega\Phi_{m}\cos\omega t \\ &= N\omega\Phi_{m}\sin(\omega t - 90^{\circ}) \end{aligned} \]

The emf is greatest when the flux is passing through zero and changing fastest, and zero when the flux is at its peak. It therefore follows the flux a quarter cycle later. It is neither in phase nor in opposition, and 'leads by 90°' reverses the sign that Lenz's law fixes.