There are two main types of alternating current machine used for the generation of electricity; synchronous and asynchronous. The difference between them begins with the way the magnetic field of the rotor interacts with the stator. Both types of machine can be used as either a generator or motor.
Let’s start by describing a synchronous generator. The rotor is basically just a magnet on a shaft. In practice the magnet is generally an electromagnet. The stator consists of three coils of wire placed to intersect with the rotor’s magnetic field, equally spaced around the circumference, 120o apart. Each coil supplies current for one phase of the grid. As the rotor rotates past each coil the induced current in each coil rises and falls in positive and negative directions as the north and south poles of the rotor passes by. Each revolution generates one cycle of current. The frequency generated is directly related to the speed of rotor revolution. For a single magnet (two pole) machine, 50 Hz is generated when rotating at 3000 RPM. A rotor with four poles generates 50 Hz when rotating at 1500 RPM.
Therefore the operating rotational speed of a synchronous machine is essentially a constant (within a small window). Its speed is tied to system frequency. Synchronous machines are governor controlled. The governor monitors the system frequency and adjusts the machine’s prime mover power to bring correction to the frequency. This is of course subject to the power capability of the machine and whether it is operating at power setting where increases (and reductions) can readily be made.
As mechanical power is applied to the shaft the rotor advances in relation to the rotating field generated by the system voltages on the stator coils. The machine still remains in rotational synchronism with the system voltages, but the rotor is now in advance of the system by an angle d. The angle d varies with the power being applied and generated, where the power is proportional to Sin(d). If d is positive the machine is in advance of the system and is acting as a generator. If d is negative the machine is being pulled along by the system, and it is acting as a motor. If d is zero, the machine is spinning but no energy transfer is occurring. Observe that Sin(d) maximises at 90o. This is the rotor advance angle limit relating to the theoretical maximum torque that the machine is capable of delivering.
Here’s a mechanical analogy of a synchronous machine that might help. Imagine the magnetic torque between the rotor and stator as being a spring connecting two rotating wheels. The first wheel is connected to the driving source, ie: the rotor. The second wheel represents the power system load. As some extra loading is applied to the second wheel the angle between the wheels begins to increase as the spring stretches. More torque is transferred via the stretched spring and kinetic energy moves from the directly connected spinning mass of the first wheel to the second.
As would be expected by the naming, the main difference between asynchronous and synchronous machines is about rotor synchronism. The rotor of an asynchronous generator does not run synchronism with system voltages. An asynchronous machine operates with ‘slip’. ‘Slip’ is a percentage measure of how much slower or faster the rotor runs compared to its synchronous speed. When the rotor is rotating slower than synchronous speed the machine acts as a motor. When the rotor is rotated faster than synchronous speed the machine acts as a generator.
Here’s a mechanical analogy of an asynchronous machine that might help. Imagine the magnetic torque between the rotor and stator as being a hydraulic fluid coupling between two wheels. The first wheel is connected to driving source, ie: the rotor. The second wheel represents the power system. As some extra loading is applied to the second wheel the hydraulic coupling slips more, but the flow of kinetic energy from the first wheel is largely decoupled by the hydraulic coupling.
Asynchronous generators are typically used where control of the prime mover is not possible, typically wind turbines or run of river hydro. While control systems are implemented to make best use of these resources, they cannot adjust output in response to a frequency change. (Some increase might be possible if the generator has been intentionally set sub-optimally, e.g.: to draw less energy from the wind than is potentially available. This being done so that on command the machine can hopefully adjust settings and thereby draw and increased amount of energy from the source).
There are two key differences affecting their contribution to stability.
- The kinetic energy of the synchronous machine’s rotor is closely coupled to the power system and therefore available for immediate conversion to power. The rotor kinetic energy of the asynchronous machine is decoupled from the system by virtue of its slip and is therefore not easily available to the system.
- Synchronous generators are controllable by governors which monitor system frequency and adjust prime mover input to bring correction to frequency movements. Asynchronous generators are typically used in applications where the energy source is not controllable, eg: wind turbines. These generators cannot respond to frequency movements representing a system energy imbalance. They are instead a cause of energy imbalance.
Short -term stability
The spinning kinetic energy in the rotors of the synchronous machines is measured in megawatt seconds. Synchronous machines provide stability under power system imbalances because the kinetic energy of their rotors (and prime movers) is locked in synchronism with the grid through the magnetic field between the rotor and the stator. The provision of this energy is essential to short duration stability of the power system.
Longer term stability is managed by governor controls. These devices monitor system frequency (recall that the rate of system frequency change is proportional to energy imbalance) and automatically adjust machine power output to compensate for the imbalance and restore stability.