What is a synchronous generator (alternator)?
Working principle
A synchronous generator turns shaft power into three-phase AC. A DC field current magnetises the rotor, and the turning poles induce an EMF E_f in the stator windings. The rotor must turn at exactly the synchronous speed n = 120·f/p, so a 4-pole machine at 1500 rpm gives 50 Hz. Per phase, E_f = V + jXs·I: the terminal voltage is the excitation EMF minus the drop across the synchronous reactance, and E_f leads V by the load angle δ. Almost all of the world’s electricity is produced this way, by turbines driving alternators.
Stand-alone operation and voltage regulation
When the alternator feeds its own load, the speed fixes the frequency and the field current fixes the voltage, while the load decides the current. A lagging load such as a motor makes the armature reaction demagnetising, so the voltage falls and the field must be raised. A unity-power-factor load lowers it less, and a leading, capacitive load magnetises the machine and can push the voltage above its no-load value. Voltage regulation is (E_f − V)/V: positive for lagging loads, negative for leading ones.
Parallel operation and synchronising
Before an alternator is connected to the grid, four conditions must hold: equal voltage, equal frequency, the same phase sequence and zero phase angle, which a synchroscope shows. Closing the breaker out of step causes a large current surge and a violent torque shock. Once on the grid, voltage and frequency belong to the bus: the turbine torque sets the active power and the field current sets the reactive power. Over-excited, the machine delivers reactive power; under-excited, it absorbs it.
Load angle, stability and capability
The active power is P = 3·V·E_f·sin δ / Xs, so δ grows with the turbine power. At δ = 90° the power peaks at about 3·V·E_f/Xs; beyond that the rotor slips poles and synchronism is lost. A stronger field raises the limit. The capability (P–Q) chart collects all the limits: stator heating (armature current circle), rotor heating (field current circle), the prime-mover power and the stability limit.
Frequently asked questions
- Why does the terminal voltage fall when a lagging load is added?
- A lagging current produces an armature reaction flux that opposes the field flux, and it also drops voltage across the synchronous reactance. Both act almost in line with E_f, so V falls more than with a unity-power-factor load. To hold the voltage you raise the field current. A leading load does the opposite and can make the voltage rise.
- How do you control active and reactive power of an alternator on the grid?
- The grid holds the voltage and frequency, so you cannot choose them. Opening the turbine valve raises the shaft torque and with it the active power and the load angle. The field current changes the reactive power: increase it to deliver more, decrease it to absorb. Try both in the simulator and follow P and Q on the capability chart.
- What happens if the turbine power is too high?
- The load angle grows until it reaches 90°, the steady-state stability limit. Beyond it the electrical power can no longer balance the turbine power, so the rotor accelerates and slips poles against the grid field, and the machine has to be tripped. A larger field current raises the limit, so a weakly excited machine is the first to fall out of step.