Generators, Alternating Current and Direct Current

Spinning makes power

Turn the crank on an emergency radio or flashlight and it lights up. Inside, you are spinning a small generator. Ontario's power stations do the same thing on a giant scale: falling water at Niagara, steam from nuclear reactors or burning natural gas, or wind turns a turbine, and the turbine spins a generator. The physics is the electromagnetic induction you learned in the last lesson.

The AC generator

A coil (the armature) is turned in a magnetic field. As it turns, the field through the coil keeps changing, so a potential difference is induced (Faraday's law).

• Each end of the coil is connected to its own slip ring. Brushes press on the rings and carry the current to the external circuit.
• During one half-turn, one side of the coil moves up through the field while the other moves down. During the next half-turn, the sides swap. So the induced current reverses every half-turn: alternating current.
• The output is greatest when the coil's sides are moving straight across the field lines (the plane of the coil is parallel to the field). It is zero at the instant the sides move along the field lines (the plane of the coil is perpendicular to the field).
• Plotted against time, the output is a sine wave. For a simple two-pole generator, one turn of the coil gives one cycle.

Large power-station generators usually do it the other way round: a powerful rotating electromagnet (the rotor) spins inside stationary coils (the stator). It is still a changing field through a coil, and it avoids sending huge currents through sliding contacts.

The DC generator

Replace the slip rings with a split-ring commutator, just like the one in a DC motor. Every half-turn, just as the induced current in the coil reverses, the commutator swaps the connections. The current in the external circuit always flows the same way, but it rises and falls: pulsating DC.

Notice the symmetry: a DC motor and a DC generator have the same parts. Supply current and it turns (motor); turn it and it supplies current (generator).

Lenz's law in a generator

When a generator supplies current, that current flows through the coil, which sits in a magnetic field. By the motor principle there is a force on it, and by Lenz's law that force opposes the rotation. So:

• With no load (open circuit), no current flows and the generator turns easily.
• With a bigger load (more current drawn), the opposing force is bigger, and the turbine must push harder.

That is why a power station must burn more fuel, or let more water through its turbines, when Ontario's demand rises on a hot summer afternoon. The electrical energy comes from the work done turning the generator.

AC versus DC

• DC flows one way. Sources: batteries, solar cells, DC generators, phone chargers' outputs.
• AC reverses direction regularly. In Canada and the rest of North America the grid alternates at 60 Hz, so each cycle lasts T = 1 ÷ 60 ≈ 0.0167 s, and the current reverses 120 times a second.
• A household outlet is rated 120 V. That is an effective value. For a sine wave, the peak is √2 × 120 ≈ 170 V. (Stoves and dryers use 240 V.)

Why AC for the grid? AC can be stepped up and down easily with transformers, which need a changing current. Sending electricity at very high voltage means a small current, which wastes far less energy in the lines. In the 1880s and 1890s, Thomas Edison promoted DC systems while George Westinghouse and Nikola Tesla promoted AC; AC won mainly because of the transformer. You will see the details in the lessons Transformers and Transmitting Electrical Energy.

DC is still used inside every electronic device, and some very long-distance lines use high-voltage DC, which needs special converter stations at each end.