A generator does not make electricity out of nothing. It converts the chemical energy in fuel into mechanical motion with an engine, and then converts that motion into electrical energy with an alternator. Understanding those two steps, and the electronics that sit between them in an inverter model, explains most of what you read on a specification sheet.
Quick answer
An engine spins a magnet inside coils of wire (or coils inside a magnetic field). Moving a magnetic field past a conductor induces a voltage, a principle discovered by Michael Faraday in 1831. In a conventional generator the engine runs at a fixed 3,600 rpm to produce 60 Hz power directly. An inverter generator produces raw electricity first, converts it to DC, then builds a clean 60 Hz output electronically, which lets the engine speed vary with the load.
In this guide
The parts of a generator
- Engine. Usually a single-cylinder, air-cooled gasoline engine on portables, from about 80 cc in a small inverter to 450 cc and more in a 10,000 W unit. Larger standby and diesel sets use multi-cylinder, liquid-cooled engines.
- Alternator. The part that actually produces electricity, sometimes called the generator head or the gen end.
- Fuel system. Tank, fuel valve, filter and carburetor, or a regulator for propane and natural gas.
- Voltage regulator. Keeps the output voltage steady as the load changes.
- Control panel. Outlets, circuit breakers, hour meter, and on newer models a carbon monoxide sensor that shuts the engine down.
- Cooling and exhaust. A fan and cooling fins or a radiator, and a muffler, often with a spark arrestor.
- Starting system. A recoil pull cord, an electric starter with a battery, or both.
How the alternator makes electricity
Inside the alternator is a rotor that turns with the engine and a stationary set of windings called the stator. One of them provides a magnetic field, either from permanent magnets or from an electromagnet, and the other has the coils where electricity is induced. As the rotor turns, the magnetic field sweeping past the coils pushes electrons back and forth. The result is alternating current: the voltage rises, falls, reverses and rises again many times a second.
How many times a second depends on how fast the rotor turns and how many magnetic poles it has. A two-pole alternator spinning at 3,600 rpm produces 60 cycles per second, which is the 60 Hz standard in North America. That is why conventional generators hold a fixed engine speed. If the engine slows, the frequency drops, and clocks, motors and electronics notice.
A governor on the engine opens the throttle as load increases to hold that speed, and a voltage regulator adjusts the field strength to keep the output near 120 or 240 volts. Simpler generators use a capacitor to regulate voltage; better ones use an automatic voltage regulator (AVR).
Why inverter generators are different
An inverter generator starts the same way, with an engine turning an alternator. The difference is what happens next.
- The alternator produces high-frequency, variable AC power, often at several hundred hertz.
- A rectifier converts that into direct current.
- An inverter switches the DC back into a clean 60 Hz sine wave at 120 volts.
Because the final waveform is built electronically, it no longer depends on engine speed. The engine can idle when you are only charging a phone and speed up when a refrigerator compressor kicks in. That is why inverter generators are quieter, use less fuel at light loads, and produce power with very little distortion. The cost is more electronics, a lower maximum output for a given engine, and a higher price per watt. Our inverter vs conventional comparison looks at when that trade-off is worth it.
Clean power and total harmonic distortion
Utility power is a smooth sine wave. The output of a basic generator is a rougher approximation, and the measure of that roughness is total harmonic distortion (THD). Inverter generators are typically under 3 percent. Conventional open-frame generators vary widely, and some makers advertise models under 5 percent. Motors and heaters do not care much. Sensitive electronics, battery chargers and some furnace control boards prefer low distortion, which is why the THD figure appears on spec sheets.
Alternator or generator?
People ask whether a machine is an alternator or a generator, and the honest answer is that the words overlap. Historically, a “generator” in a car was a DC dynamo that used a commutator to produce direct current, and the “alternator” that replaced it in the 1960s produces alternating current that is then rectified. In the equipment world, “generator” or “genset” means the complete machine, engine included, and “alternator” means the electrical end of it. Nearly every modern generator uses an alternator.
What this means when you shop
- Rated versus peak watts: the alternator can briefly supply more than its continuous rating, which is where the starting-watts figure comes from. Size on the rated figure. See what size generator you need.
- Engine size and fuel use: a bigger engine means more output and more fuel. Inverters save fuel mainly at part load. See how much gas a generator uses.
- Copper or aluminum windings: some makers advertise copper-wound alternators, which tolerate heat better than aluminum. It is one factor among many, not a deciding one.
- Brushless alternators: have fewer wearing parts and are common on better portables and most standby units.
Frequently asked questions
Why does a generator run at 3,600 rpm?
Because a two-pole alternator must turn 3,600 times a minute to produce 60 Hz. Four-pole alternators, used on many larger diesel and standby sets, produce 60 Hz at 1,800 rpm, which is quieter and easier on the engine but needs a larger, heavier alternator.
Does a generator need a battery to work?
Only for electric starting and, on some models, for the control panel. Most portable generators with a pull cord generate their own field current once running, although some need a weak residual magnetism in the rotor to start producing power.
What happens if I overload a generator?
The output circuit breaker should trip, or the engine bogs down and the voltage and frequency fall. Repeated overloading overheats the alternator windings and shortens its life. Size the generator with headroom and add loads one at a time.
