Answer:

The machine designed to run without an active load but with its excitation to produce reactive power is called a synchronous compensator. Let's understand this concept in detail.

Definition of Synchronous Compensator:
A synchronous compensator is a type of synchronous machine that operates without a mechanical load but is connected to the electrical system to generate or absorb reactive power. It is also known as a synchronous condenser or synchronous capacitor.

Working Principle:
A synchronous compensator consists of a synchronous motor that is not mechanically coupled to any load. Instead, it is connected to the power system through a shaft or directly to the electrical grid. The machine runs at synchronous speed and its excitation is adjusted to control the reactive power flow.

Key Points:
1. Reactive Power Compensation: The main purpose of a synchronous compensator is to provide reactive power compensation in electrical power systems. It absorbs or generates reactive power to maintain the power factor or voltage stability.

2. Voltage Regulation: By adjusting the excitation of the synchronous compensator, it can control the voltage magnitude in the power system. It can absorb excess reactive power when the voltage is high and generate reactive power when the voltage is low.

3. Power Factor Improvement: Synchronous compensators are used to improve the power factor of the system. They absorb reactive power from the grid, reducing the burden on generators and transmission lines.

4. Stability Enhancement: Synchronous compensators also help in enhancing the stability of the power system. They provide dynamic voltage support and help in damping out oscillations caused by sudden changes in load or faults.

Comparison with Other Machines:
- Induction Motor: An induction motor requires a mechanical load to operate and does not provide reactive power compensation. It is not suitable for producing or absorbing reactive power without an active load.

- Synchronous Motor: A synchronous motor is mechanically coupled to a load and operates at synchronous speed. It does not provide reactive power compensation unless it is specifically designed as a synchronous compensator.

- Synchronous Generator: A synchronous generator is mechanically coupled to a prime mover and is designed to produce active power. While it can also provide reactive power, it requires an active load to operate.

Conclusion:
A synchronous compensator is a specialized synchronous machine that operates without an active load but provides reactive power compensation and voltage regulation. It is an essential component in power systems to improve power factor, voltage stability, and system stability.