What is a synchronous generator?

In comparison to an asynchronous generator, it is a machine where the rotor speed is equal to the rotating magnetic field produced by the stator, i.e., mechanical speed is equal to the electrical speed, thus called synchronous, and not asynchronous.

It is a doubly excited machine that produces alternating voltage and current. Since this machine is the bulkiest of all the electrical machines, armature winding is placed on the stator, and field winding is placed on the rotor.

Why most of the power is generated using a synchronous generator?

The synchronous generator is capable of delivering real and reactive power. If it delivers reactive power, it can be absorbed by the load. Asynchronous generators fail to do so.

We can also control the synchronous generator's power factor by controlling the magnetic field's excitation, which is not possible for an asynchronous generator.

The synchronous generator generates power at a fixed frequency because sinusoids of different frequencies are orthogonal to each other and transfer zero power losses.

Structure of a synchronous generator

The stator structure is made of CRGO (cold rolled grain oriented) steel and laminated to reduce eddy current loss. It is comprised of short pitch and distributed winding. The advantage of distributed winding is that weight is evenly distributed. The advantage of short pitching is that dominant harmonics can be eliminated by choosing a suitable value of the chording angle. Since the length of the coil reduces, its resistance and reactance also reduce, which leads to a reduction in voltage drop, thus improving voltage regulation.

The structure of the rotor is mainly of two types: salient pole rotor and non-salient pole rotor

The salient pole rotor of the synchronous generator is also known as the projecting pole rotor. Here poles are projected along the rotor periphery. The air gap across the rotor periphery is non-uniform. It can have a maximum of 120 poles. Since the weight is non-uniformly distributed, centrifugal forces are generated, which are non-uniform and can damage the rotor. Thus, this type of rotor rotates at low-speed to prevent mechanical unbalancing. These rotors are employed in hydroelectric power plants.

Non-salient pole rotor is also known as a cylindrical rotor. A uniform air gap exists across the rotor periphery. It is only wound for two or four poles, and it is mechanically well balanced so it can be rotated at higher speeds.

Principle of a synchronous generator

The armature winding of the synchronous generator delivers three-phase AC, and the field winding is energized from the DC source.

The DC energized field winding produces field emf (electromotive force), which interacts with the armature emf to produce three-phase power, i.e., three-phase current and voltage.

The three-phase high voltage generated in the armature winding need not be brought to an external circuit through slip rings and sliding contacts, but a direct connection to the terminals can be made. Compared to an asynchronous generator, low voltage is supplied to the field through slip rings.

Synchronous speed is the conversion of electrical frequency into mechanical speed in terms of rpm.

$N_s=\frac{120f}{P}$

Here, $N_s$ is the synchronous speed, $f$ is the frequency and $P$ is the number of poles. 

The emf equation of synchronous generator is

$E_{rms}=4.44f\varphi N_s$

Here, f is the frequency, $\varphi$ is the flux per pole and $N_s$ is the synchronous speed. 

As the rotor rotates, it cuts the stator conductor and induces an emf balanced in the three-phase stator winding. If the stator supplies load, this emf causes the balanced current in the stator winding. This current produces rmf (rotating magnetic field) that rotates at synchronous speed.

Methods to eliminate harmonics in a synchronous generator

On the field side,

• using chamfered salient poles.
• skewing the field pole to vary the conductor length sinusoidally.
• by increasing air gap and reluctance.

On the armature side,

• by short pitching the winding.
• by using distributed winding.

Voltage regulation in a synchronous generator

Voltage regulation is a significant parameter for a synchronous generator because higher voltage regulation means higher synchronous impedance. Knowing the value of synchronous impedance is crucial to determine power-sharing in parallel operation. If a load is disconnected or connected from the synchronous generator, we must know the magnitude of voltage rise or fall, respectively.

Also, it is used to design AVR (Automatic Voltage Regulator), an automatic control system used to regulate field current.

Parallel operation of a synchronous generator

The parallel operation of alternators is when multiple alternators are connected in parallel to meet huge power demand because a single alternator cannot supply such an amount of power.

Suppose we replace a single large unit with a large number of small units. In that case, as the load reduces, we can switch off certain units and only operate some units so that the units operating operate at full load and other units are off, which reduces the operating cost.

By connecting a thermal power plant and hydropower plant in parallel, we can supply power at minimum cost by letting the hydropower plant handle the maximum load during the rainy season and the thermal power plant handle the maximum load during the summer season.

The different methods used to synchronize a new alternator to the grid or existing parallel-connected alternators are the following:

• Dark lamp method
• Two bright and one dark lamp method
• Synchroscope  method

What happens if the prime mover or turbine is coupled to the alternator and excitation is switched on?

If the excitation is removed, but the turbine is still coupled to the alternator, the electrical output of the alternator reduces.

If prime mover is decoupled, but excitation is present, the alternator will work as a synchronous motor with the infinite bus.

If generator terminals are connected to load instead of the infinite bus, then after some time, prime mover decoupled, the load will not supply electric power, and the motor will gradually stop.

Uses of a synchronous generator

Some important applications of the synchronous generator are provided below:

• It is used in power plants because of zero power losses.
• It is used wherever stability is required.
• It is used to maintain the power factor of the grid.
• It helps in providing constant voltage for a long period.

Common Mistakes

Remember that only 50 Hz and 60 Hz are used because in the power system, the turbine generally gives more efficiency at 3000 rpm or 3600 rpm (calculated according to the above-mentioned formula), and according to that, 50 Hz and 60 Hz frequencies are suitable.

Note that there is a limitation of size for an alternator.

Context and Applications

In each of the expert exams for undergraduate and graduate publications, this topic is mainly used in the following context:

• Bachelor of Technology in the Electrical and Electronics Department
• Bachelor of Science in Physics
• Master of Science in Physics

Related Concepts

• Transformers
• Inductor and inductance
• Capacitor

Practice Problems

Q1. The synchronous generator requires _____________ for its operation.

1. AC
2. DC
3. Both AC and DC
4. None

Correct option: (c)

Explanation: Alternating current and direct current are required for its proper working.

Q2.  The excitation provided to the synchronous generator is in the form of _____________.

1. Thermal energy
2. Hydro energy
3. Nuclear energy
4. Thermal, hydro, and nuclear energy

Correct option: (d)

Explanation: The excitation is in the form of steam which can be generated using thermal, hydro, and nuclear energy.

Q3. Which of the following is not considered for synchronism of synchronous generators?

1. Current
2. Voltage
3. Frequency
4. Phase

Correct option: (a)

Explanation: Voltage, frequency, and phase are necessary conditions required to be matched for synchronism.

Q4. Which of the following is not a method to eliminate harmonics in alternator?

1. Champhered pole shoe
2. Skewing field poles
3. Reducing air gap and reluctance
4. Using short-pitched coils

Correct option: (c)

Explanation: The harmonics can be eliminated by increasing the air gap and reluctance rather than reducing it. On the other hand, reducing the air gap provides better DC in DC machines.

Q5. The 4-pole alternator has excitation so that it rotates at 1500 rpm. Determine the frequency of AC voltage in H.

1. 45
2. 50
3. 55
4. 60

Correct option: (b)

Explanation: Use the equation

$$\begin{gathered} N_s=\frac{120f}{P} \\ f=\frac{N_s\times P}{120} \end{gathered}$$

Substitute the values in above expression.

$$\begin{gathered} f=\frac{1500\times 4}{120} \\ f=50 \mathrm{Hz} \end{gathered}$$