IntermediateTopic 61 of 106Electrical Engineering
Synchronous machines
Rotor runs at synchronous speed Ns=120f/P; used as generators and for power factor control.
In depth
Rotor is driven at synchronous speed by DC field or permanent magnets. As generators (alternators) they supply almost all grid power; as motors they run at constant speed and can supply reactive power. Loss of synchronism occurs if load angle exceeds 90°.
Key points
- Power-angle P=(EV/X)sinδ
- V-curves
- Damper windings
ExampleA turbo-generator spins at 3000 rpm for 50 Hz with 2 poles.
What is Synchronous machines?
Rotor runs at synchronous speed Ns=120f/P; used as generators and for power factor control.
Why is it important to understand synchronous machines?
Synchronous machines run at a speed locked to the supply frequency, and they can supply reactive power to correct the power factor of a grid.
What sets the synchronous speed?
The supply frequency times 120, divided by the number of poles, so a four-pole machine on 50 Hz runs at 1500 rpm.
What happens if the load is too large for a synchronous motor?
It falls out of step and stops, so synchronous motors need correct excitation and protection.
How is Synchronous machines used in practice?
Typical applications include using a synchronous motor to correct the power factor of a large plant, driving a constant-speed compressor that must stay in step with the grid and generating power in a large power station turbine generator.
What should I learn before Synchronous machines?
Study "Induction motors" first (topic 60 of 106 in Electrical Engineering). It covers the groundwork this topic builds on.
What are the key points of Synchronous machines?
Power-angle P=(EV/X)sinδ. V-curves. Damper windings.
Can you give an example of Synchronous machines?
A turbo-generator spins at 3000 rpm for 50 Hz with 2 poles.
What comes after Synchronous machines?
The next topic is "Single-phase motors", which builds directly on this one.
How difficult is Synchronous machines?
It is classed as Intermediate in Electrical Engineering (topic 61 of 106). It is an intermediate topic. Work through the beginner topics in Electrical Engineering first.