Understanding how an Electric Generator Works

Whether powering a house or a city, every electrical generator works on a principle called the Faraday effect. Michael Faraday, a nineteenth century physicist, found that when a magnet moves through a wire loop it causes, or “induces,” electrical current in the wire. The amount of induced current depends on the strength of the magnet, the speed and direction at which it moves, and the properties of the wire loop or coil. When the magnet stops moving, the current ceases. The principal works in reverse, too: current is generated in a wire when it moves through a magnetic field.

When first discovered, induced current was mostly a scientific curiosity, but two centuries later it is essential to electrical generation (solar power is the main exception.) The basic principles of electrical generation haven’t changed since the nineteenth century, and are a simple extension of Faraday’s discovery. Every modern electrical generator consists of a combination of a stationary magnet with a wire coil rotating within its field to induce current. That coil is mounted on a shaft that some kind of mechanical energy causes to spin.

The energy needed to spin the generator coil can come from many sources. Most commercial power plants use steam heated by burning fossil fuel (oil, natural gas, coal,) or the heat of a controlled nuclear reaction. The kinetic energy in falling water spins hydroelectric generators, and the energy of moving air makes wind turbines spin wire coils. Some residences have standby generators powered by engines that use natural gas, gasoline, or diesel for fuel. Whatever their power source, these generators convert mechanical energy to electrical energy using the phenomenon Faraday discovered.

A generator’s output voltage is determined by the magnet’s strength, the configuration of the spinning coil, and the speed at which the coil rotates. Since the required voltage is fixed (120 volts in the US) and neither the magnetic field nor the coil can be altered, generators therefore spin at constant speed. The amperage, or rate of current output by the generator, can and does change.

Generators are designed with a feedback circuit to vary the torque (rotational force) applied to the spinning rotor. Increased torque on the coil at constant  rotation speed increases the amperage, or rate of current flow. When the power demand of devices plugged into a generator increases, the torque is increased to raise the output amperage.  The amperage in the circuit  is power demand on the generator divided by its output voltage. Since voltage is a constant value increasing output amperage provides electricity to power more or larger devices. Adding torque to the spinning coil generates greater amperage, so more watts are available.

Consumer generator ratings reflect the maximum wattage they can provide. When the load (amount of power required) on a generator is low, its engine seems to be idling. When the load approaches its maximum rating, however, the engine will run at the same speed but work harder, which makes it sound louder. You hear a similar change in a lawnmower engine when the mower moves from short grass to taller or damp grass.

Portable generators usually have an output of four to nine kilowatts (4,000 to 9,000 watts.) When the power demand of all the devices the generator is running exceeds the generator’s rating, a “brownout” (insufficient wattage) can occur. Brownouts can damage many electrical devices. It is important to buy a generator with enough capacity to power all your essential devices during a power failure, and to reduce use of non-essential devices while using your generator.


University of New South Wales (Electric Motors and Generators)

Georgia State University   (Faraday’s Law)