In a synchronous generator, a DC current is applied to the rotor winding, which produces a rotor magnetic field. The rotor of the generator is then turned by a prime mover, producing a rotating magnetic field within the machine. This rotating magnetic field induces a three-phase set of voltages within the stator windings of the generator.
Two terms commonly used to describe the windings on a machine are field windings and armature windings. In general, the tenn “field windings” applies to the windings that produce the main magnetic field in a machine, and the term “armature windings” applies to the windings where the main voltage is induced.
For synchronous machines, the field windings are on the rotor, so the terms “rotor windings” and ” field windings” are used interchangeably. Similarly, the terms “stator windings” and “armature windings” are used interchangeably.
In case of induction machines such as motors and generators whose field current is supplied by magnetic induction (transformer action) into their field windings. The field circuits of most induction machines are located on their rotors. An induction generator produces electrical power when its rotor is turned faster than the synchronous speed.
Induction generator is not a self excited machine therefore in order to develop the rotating magnetic field, it requires magnetizing current and reactive power. The induction generator obtains its magnetizing current and reactive power from the various sources like the supply mains or it may be another synchronous generator. The induction generator can’t work in isolation because it continuously requires reactive power from the supply system. However we can have a self excited or isolated induction generation in one case if we will use capacitor bank for reactive power supply instead of AC supply system.
That’s why induction generator usually draws its excitation power from an electrical grid; sometimes, however, they are self-excited by using phase-correcting capacitors. Because of this, induction generators cannot usually “black start” a de-energized distribution system.
- Synchronous generator generates the power only at synchronous speed whereas induction generator generates the power only at asynchronous speed.
- Induction generator needs AC supply for it’s excitation, but synchronous generator requires DC supply for
- No synchronization to the supply line is required in induction generator like a synchronous generator.
- Synchronous Generators/Alternators use a separate excitation field whereas in IG(Induction Generator) the field is induced in the rotor.
- An Synchronous Generator/alternator works on a standalone independent of a network supply, its output frequency would be dependent on rpm/poles. but when synchronized with network in stable operation it works at network frequency.
An induction or asynchronous generator is, in very simple terms, an electric induction motor driven at speeds above its synchronous speed. It has a solid armature, or squirrel cage, that is an electrical short circuit. When the current is connected, the machine will start turning like a motor at a speed that is just slightly below the synchronous speed of the rotating magnetic field from the stator. If we manually drive this rotor at exactly the synchronous speed of the generator (1800, 3600 RPM etc.), the magnetic field rotates at exactly the same speed as the rotor, we see no induction phenomena in the rotor, and it will not interact with the stator. If we increase speed above the synchronous speed of the generator, the rotor moves faster than the rotating magnetic field of the stator, and the stator induces a current in the rotor. The more mechanical power that is delivered to the rotor, the more power will be transferred as an electromagnetic force to the stator, and in turn converted to electricity, which is fed into the electric grid.
Because most single stage steam turbines develop their peak efficiencies between 3600 and 5500 RPM, the 2-pole (3600 RPM) induction generator allows for a direct drive application, eliminating the need for a speed reduction gear and a costly lubrication system, as the turbine is usually ring lubricated and the generator has grease packed ball bearings. Although induction motors / generators are available in very large sizes, they are typically utilized in smaller (<1000 kw) applications.
There are several reasons for this. One reason is power factor correction. We will explain this in depth later, but suffice to say at this juncture, induction generation works out to be most favorable when the generated load is small compared to your total plant load. Another reason is that the single stage turbine with an inexpensive, self-contained lubrication system is limited in its power producing capabilities. At higher powers more expensive multi-stage turbines and pressurized lubrication systems are required. The major advantage of the single stage steam turbine induction generator package is its simplicity: no gear, no pressure lube, simple controls, and simple connection to electric grid.
Note: connection to grid is required for excitation. All these factors result in a relatively low cost package, which when applied in a simple topping application, results in extremely fast payback on investment.
A synchronous generator runs at a constant speed and draws its excitation from a power source, external to, or independent of, the load or transmission network it is supplying. A synchronous generator has an exciter that enables the synchronous generator to produce its own “reactive” power and to also regulate its voltage. Synchronous generators can run in parallel with the utility, or in “stand-alone” or “island” mode. Synchronous generators in the <15 mw power range are only available as 4-pole (1800 RPM) units, are almost always geared, and require large pressure lube systems. Most synchronous packages over 1.0 mw are driven by multi stage steam turbines and typically involve more advanced controls and monitoring equipment. Although more complex and costly, larger synchronous generators can have a lower cost per kw due to their economy of scale.