The main difference between a **synchronous generator** and an **asynchronous generator** (also known as an **induction generator**) lies in their operating principles, rotor speed, and method of generating electricity. Here’s a detailed comparison:
### 1. **Rotor Speed and Synchronous Speed**
- **Synchronous Generator**: The rotor rotates at a speed exactly equal to the synchronous speed (the speed of the magnetic field generated by the stator). The rotor speed is directly proportional to the supply frequency and is constant.
- Formula for synchronous speed:
\[
N_s = \frac{120f}{P}
\]
where \(N_s\) = synchronous speed, \(f\) = frequency (Hz), and \(P\) = number of poles.
- **Asynchronous (Induction) Generator**: The rotor speed is not equal to the synchronous speed. It runs slightly faster or slower than the synchronous speed, depending on load and operating conditions. When used as a generator, it operates above synchronous speed.
### 2. **Excitation Mechanism**
- **Synchronous Generator**: Requires an external DC source to excite the rotor winding and create a magnetic field. This DC excitation is provided by either a separate excitation system (like a brushless exciter) or permanent magnets in some cases.
- **Asynchronous Generator**: Does not require an external excitation source. Instead, it relies on residual magnetism and the interaction between the rotor and the stator to induce current in the rotor, generating electricity.
### 3. **Self-Starting Capability**
- **Synchronous Generator**: Requires external assistance for starting. It cannot start generating electricity without external excitation and a prime mover that matches the rotor speed to synchronous speed.
- **Asynchronous Generator**: Can self-start when driven by a prime mover (like a wind turbine) at a speed higher than the synchronous speed.
### 4. **Power Factor**
- **Synchronous Generator**: Can operate at a controllable power factor (leading, lagging, or unity), making it useful for power factor correction in power systems.
- **Asynchronous Generator**: Always operates at a lagging power factor and cannot control the power factor independently.
### 5. **Efficiency and Applications**
- **Synchronous Generator**: More efficient, especially in large power plants (hydropower, thermal power stations), where steady, high power output is needed. It is widely used in centralized power generation.
- **Asynchronous Generator**: Simpler in design, lower maintenance, and cheaper, but less efficient. It is commonly used in wind turbines and small-scale renewable energy applications.
### 6. **Complexity of Control**
- **Synchronous Generator**: Requires more complex control systems for maintaining synchronous speed and excitation control.
- **Asynchronous Generator**: Easier to operate, but requires reactive power compensation (capacitors) to maintain voltage stability.
### 7. **Stability and Load Handling**
- **Synchronous Generator**: Can maintain constant voltage and frequency under varying loads, making it stable for grid-connected applications.
- **Asynchronous Generator**: Output voltage and frequency can vary with load changes, which is a disadvantage for grid stability in large-scale applications.
### Summary:
- **Synchronous Generator**: Runs at synchronous speed, requires external excitation, and is highly efficient and stable.
- **Asynchronous Generator**: Runs at speeds not equal to synchronous speed, self-excites from residual magnetism, and is more suitable for decentralized, small-scale applications like wind power.
Each type of generator has its advantages depending on the application and the nature of the power source.