Why does more electrical current drawn from generator leads to more fuel consumption? I understand that it's due to conservation of energy, I just want to know what the phenomenon or reaction is called that leads to more power being transferred to the engine via the gears.
The increased output current in the generator windings increases the magnetic field strength which opposes rotation of the generator shaft. To maintain the same RPM (and thus maintaining frequency and voltage) the engine throttle must be opened further, using more fuel.
The generator has a speed governor.
As electrical load increases the mechanical load increases. If the throttle position were fixed the speed would drop. The governor opens the throttle to allow more fuel in to maintain speed at setpoint.
Figure 1. Mechanical speed governor. Source.
Figure 1 shows an old-fashioned mechanical governor.
- As the shaft rotation speed increases the flyweights are thrown outwards, lift the control sleeve against the speeder spring and drive the crank to shut off the fuel valve.
- If the shaft slows down the speeder spring pushes the control sleeve back down and the crank opens the fuel rack again.
- Spring pressure is controlled by the speed control lever to set the running speed.
Modern speed regulators are electronic. They monitor the output frequency and adjust the fuel valve electrically.
Here is a more detailed account of the energy conservation involved.
The generator output power is given by Power (watts) = Voltage (volts) X Current (amps). For AC, there are two additional multipliers, power factor to account for the phase relationship between voltage and current and the square root of three for a three-phase generator. Voltage is regulated to a constant value. Current and power factor vary with load.
The generator converts mechanical power to electrical power. Mechanical power is given by Power = Torque X Rotational Speed. An additional multiplier may be required depending on the units of measurement used. Speed is normally regulated to be a constant value in order to keep the voltage an frequency constant. Torque varies as required to supply the mechanical power that is converted to electrical power.
The engine speed is regulated to maintain a constant generator speed. If the generator requires more torque, the fuel flow must increase to supply the additional power. Power available from burning fuel is proportional to fuel mass per unit of time. The throttle regulates the mass rate of fuel flow.
For any electromechanical device, the electromagnetic torque or force produced is directly proportional to the current flowing through them, if the excitation field is unchanged. A generator has its torque opposing that produced by the engine (or whatever is turning the generator). The engine requires energy to turn the shaft against the torque of the generator, much like you need energy to lift a weight vertically against earth's gravity.
When the load draws more current, more generator torque is induced due to the higher currents, the engine faces more resistance to its motion, just like how you would feel when you are lifting a 10 kg mass and someone adds another 5 kg mass to what you already have. The speed (of the generator and you) would drop as a result (which is not good), so the control system sends additional fuel to bring the speed back.
Loading: If the load on a power system is increased, the load current on the system increases, which results in increased armature current in the generator. This increased armature current results in an increased magnet field in the armature. The rotor (field magnet) located in this armature magnetic field will then experience more attractive force which will in effect reduces the speed of the rotor, which in turn reduces the voltage. As the speed reduces the frequency in the system also reduces. N=120F/P. This reduction in rotor speed is sensed by the turbine governor. To keep the speed and frequency constant at 50hz, the governor valve will be opened and the turbine output power is increased to restore the generator voltage and frequency.
Unloading: If the load on a power system is reduced, the load current on the system reduces, which results in decreased armature current in the generator. This reduced armature current results in a reduced magnet field in the armature. The rotor (field magnet) located in this armature magnetic field will then experience a less attractive force which will in effect increases the speed of the rotor which in turn increases the voltage. As the speed increases the frequency in the system also increases. N=120F/P. This increase in rotor speed is sensed by the turbine governor. To keep the speed and frequency constant at 50hz, the governor valve will be closed and the turbine output power is reduced to restore the generator voltage and frequency.