Difference Between Induction Generator And Synchronous Generator


What Is a Synchronous Generator?

A synchronous generator is an alternator with the same rotor speed as the rotating magnetic field of the stator. According to the structure, it can be divided into two types: a rotating armature and a rotating magnetic field. Synchronous generators are one of the most commonly used alternators. In the modern power industry, it is widely used in hydropower, thermal power, nuclear power, and diesel power generation.

What Is an Induction Generator?

An induction generator is an alternator that utilizes an air gap rotating magnetic field between a stator and a rotor to interact with an induced current in a rotor winding. They are usually referred to as asynchronous generators. The speed is slightly higher than the synchronous speed. The output power increases or decreases with the slip rate. It can be excited by the power grid or self-excited with a power capacitor.

How Does an Induction Generator Work?

In the previous section, we gave you two simple definitions of what induction and synchronous generators are. In the following, we will show you how these two generators work differently.

An induction generator produces electrical power when its rotor is turned faster than the synchronous speed. For a typical four-pole motor where there are two pairs of poles on a stator operating on a 60 Hz electrical grid, the synchronous speed is 1800 rotations per minute. The same four-pole motor operating on a 50 Hz grid will have a synchronous speed of 1500 rotations per minute. The motor normally turns slightly slower than the synchronous speed; the difference between synchronous and operating speed, as you may know, is called slip and is usually expressed as a percent of the synchronous speed. For example, a motor operating at 1450 rotations per minute that has a synchronous speed of 1500 RPM is running at a slip of +3.3%.

In normal motor operation, the stator flux rotation is faster than the rotor rotation. This causes the stator flux to induce rotor currents, which make a rotor flux with magnetic polarity opposite to the stator. In this way, the rotor is dragged along behind stator flux, with the currents in the rotor induced at the slip frequency.

In generator operation, a prime mover such as a turbine or any kind of engine drives the rotor above the synchronous speed (negative slip). The stator flux still induces currents in the rotor, but since the opposing rotor flux is now cutting the stator coils, an active current is produced in stator coils and the motor now operates as a generator, sending power back to the electrical grid.

How Does a Synchronous Generator Work?

The synchronous generator working principle is the same as a DC generator. It uses Faraday’s law of electromagnetic induction. This law states that when the flow of current is induced within the conductor in a magnetic field then there will be a relative motion among the conductor as well as the magnetic field. In a synchronous generator, the magnetic field is immobile & conductors will rotate. However, in practical construction, armature conductors are motionless & field magnets will rotate between them.

The rotor in the synchronous generator can be fixed mechanically toward the shaft to turn at synchronous speed under some mechanical force which consequences in magnetic flux cutting in the stationary armature conductors of the stator. Due to this direct flux cutting result, an induced emf and flow of current will be there in armature conductors. For each winding, there will be a current flow in the first half cycle after that in the second half cycle with a specific time lag of 120°.

Difference Between Induction Generator And Synchronous Generator

Induction Generator

Synchronous Generator

In case of inductions generators, the output voltage frequency is regulated by the power system to which the induction generator is connected. If induction generator is supplying a standalone load, the output frequency will be slightly lower (by 2 or 3%) that calculated from the formula f = N * P / 120.

In a synchronous generator, the waveform of generated voltage is synchronized with (directly corresponds to) the rotor speed. The frequency of output can be given as f = N * P / 120 Hz. where N is speed of the rotor in rpm and P is number of poles.

Induction generator takes reactive power from the power system for field excitation. If an induction generator is meant to supply a standalone load, a capacitor bank needs to be connected to supply reactive power.

Separate DC excitation system is required in an alternator (synchronous generator).

Construction of induction generator is less complicated as it does not require brushes and slip ring arrangement.

Brushes are required in the synchronous generator to supply DC voltage to the rotor for excitation.

If induction generators are meant to supply standalone loads, a capacitors bank must be connected to supply reactive power.

An alternating or synchronous generator requires a separate DC excitation system, whereas an induction generator takes reactive powers from the powers system for field excitations.

READ HERE  What is a Synchronous Machine? Synchronous Speed


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