Renewable energy plants convert mechanical motion from wind, water, or other sources into electrical energy. Two important machine types are synchronous generators and induction generators. They both use electromagnetic induction, but they behave differently when connected to the grid.
Understanding the difference helps explain how renewable plants control voltage, frequency, and power flow.
A synchronous generator produces electricity with a rotor magnetic field that locks to the grid frequency, so its speed is directly tied to the number of poles and the AC frequency. An induction generator usually runs slightly above synchronous speed and relies on the grid or power electronics to provide magnetizing current. Synchronous machines are common in hydro plants and some large renewable systems where voltage control is important.
Induction machines are common in many wind turbine designs because they are rugged, simple, and can tolerate some speed variation.
Key Facts
- Synchronous speed is n_s = 120f / P, where n_s is in rpm, f is frequency in Hz, and P is the number of poles.
- A synchronous generator rotor rotates at the same electrical speed as the grid magnetic field.
- An induction generator produces power when rotor speed is greater than synchronous speed.
- Slip is s = (n_s - n_r) / n_s, where n_r is rotor speed; generator operation has negative slip.
- Synchronous generators can supply or absorb reactive power by adjusting field excitation.
- Induction generators usually need reactive power from the grid, capacitors, or power converters to build magnetic flux.
Vocabulary
- Synchronous generator
- A generator whose rotor magnetic field turns in step with the rotating magnetic field set by the AC frequency.
- Induction generator
- A generator that produces electric power when its rotor is driven faster than the synchronous speed of its stator field.
- Synchronous speed
- The rotational speed of the magnetic field in an AC machine, determined by frequency and number of poles.
- Slip
- The fractional difference between synchronous speed and rotor speed in an induction machine.
- Reactive power
- Electrical power associated with magnetic and electric fields that helps control voltage but does not deliver net mechanical work.
Common Mistakes to Avoid
- Treating synchronous and induction generators as the same device is wrong because their rotor speed behavior and grid support needs are different.
- Using n_s = 120P / f is wrong because frequency belongs in the numerator and pole number belongs in the denominator: n_s = 120f / P.
- Assuming an induction generator can generate at any low speed is wrong because it must be driven above synchronous speed to send real power to the grid.
- Ignoring reactive power is wrong because induction generators need magnetizing reactive power, while synchronous generators can often control reactive power with excitation.
Practice Questions
- 1 A 60 Hz synchronous generator has 4 poles. Calculate its synchronous speed in rpm using n_s = 120f / P.
- 2 A 50 Hz induction generator has 6 poles and is driven at 1020 rpm. Calculate the synchronous speed and the slip s = (n_s - n_r) / n_s.
- 3 A wind plant must help regulate grid voltage during changing load conditions. Explain why a synchronous generator or a full power converter system may be preferred over a simple grid-connected induction generator.