Alternating current (AC) is created when a conductor cuts through a changing magnetic field, inducing an electromotive force (EMF) that periodically reverses polarity. In a real circuit, this periodic reversal dictates that you must calculate power using Root Mean Square (RMS) values rather than simple peak voltage, and it enables the use of transformers to step voltages up for efficient transmission or down for safe branch-circuit use. Makers and apprentices frequently confuse the physical generation of AC (rotating magnetic fields in electromechanical alternators) with the solid-state synthesis of AC (switching DC via H-bridges in inverters), or they mistakenly conflate AC frequency (Hz) with voltage amplitude.
The Physics: How is AC Current Created in an Alternator?
At the utility scale and in portable gas generators, AC is created through electromagnetic induction, governed by Faraday’s Law. When a magnetic field rotates inside a stationary coil of wire (the stator), the magnetic flux linking the coil changes continuously. As the north pole approaches the coil, voltage induces in one direction; as it passes and the south pole approaches, the voltage induces in the opposite direction.
The frequency of the created AC depends entirely on the physical speed of the rotor and the number of magnetic poles. The formula is:
Where f is frequency in Hz, P is the number of poles, and N is rotational speed in RPM.
If you are running a standard 4-pole portable generator to create US-standard 60 Hz mains power, the engine must be governed to exactly 1800 RPM:
f = (4 × 1800) / 120 = 60 Hz.
Furthermore, the physical peak voltage ($V_{peak}$) induced in the stator will be higher than the nominal wall voltage. To achieve a standard 120V RMS output, the alternator must actually generate a peak sine wave of roughly 170V, because $V_{RMS} = V_{peak} / \sqrt{2}$ (170 / 1.414 ≈ 120V). For a deep dive on why multimeters read this differently, refer to Fluke's guide on True-RMS vs average responding measurements.
Solid-State Synthesis: How Inverters Create AC from DC
When you don't have a spinning prime mover—like in a solar battery bank or an uninterruptible power supply (UPS)—you must create AC synthetically from a DC source. This is done using an H-bridge circuit combined with Pulse Width Modulation (PWM).
An H-bridge consists of four switches (usually MOSFETs or IGBTs) arranged in an 'H' pattern across the DC supply. By closing the top-left and bottom-right switches, current flows through the load in one direction (creating the positive half-cycle). By opening those and closing the top-right and bottom-left switches, current flows in the exact opposite direction (creating the negative half-cycle).
When switching the H-bridge from the positive to the negative half-cycle, you must introduce a microsecond delay called dead-time. If the high-side and low-side MOSFETs on the same leg are on simultaneously for even a fraction of a microsecond, you create a dead short across your DC bus (known as shoot-through), which will instantly vaporize the silicon and blow your DC fuses. Always use a dedicated gate driver IC like the IR2110 or the EG8010, which have hardware dead-time generation built in.
Cheap modified-sine-wave inverters just slam these switches on and off, creating a choppy square wave that causes transformers to hum and motors to overheat. High-quality Pure Sine Wave inverters use Sinusoidal PWM (SPWM). The controller (often a dedicated chip like the EG8010) rapidly switches the MOSFETs at high frequencies (e.g., 23 kHz), varying the width of the pulses to perfectly trace the shape of a 60 Hz sine wave before passing it through an LC low-pass filter to smooth it into clean AC.
Where You Meet This in Practice
Understanding how AC is created dictates how you troubleshoot and design power systems in the field:
- The Utility Grid: Massive synchronous alternators at power plants create 3-phase AC. Because it is created electromechanically, the grid frequency is a direct proxy for the physical balance of mechanical power input vs. electrical load output. If frequency drops below 59.8 Hz, it means turbines are physically slowing down under heavy load.
- Portable Inverter Generators: Units like the Honda EU2200i do not create AC directly at the output. They use a multi-pole alternator spinning at variable RPMs to create wild, high-frequency AC, rectify it to DC, and then use a solid-state inverter to synthesize a perfect 60 Hz sine wave. This is why they can idle down under low loads without the AC frequency dropping.
- Solar Microinverters: Devices like the Enphase IQ8 mount directly to solar panels. They take the 30-40V DC from the panel and use high-frequency H-bridges to synthesize 240V AC, pushing it directly into the grid. For more on how these integrate with the grid, see the All About Circuits AC textbook chapter on grid integration.
Decision Path: Choosing Your AC Creation Method
When designing an off-grid system, repairing a power supply, or building a bench tool, you must choose between electromechanical generation and solid-state synthesis. Use this decision matrix to terminate your design phase with a concrete hardware selection.
| Your Scenario | Requirement | Concrete Pick / Action |
|---|---|---|
| Need continuous, high-power (5kW+) backup for a whole house. | High surge capacity, long runtime, true sine wave. | Electromechanical: Generac Guardian 24kW (Standby generator with synchronous alternator). |
| Need silent, portable AC for a 120V jobsite saw from a truck battery. | Low noise, clean power for sensitive electronics, battery-fed. | Solid-State: Victron Phoenix 12/3000 Inverter (Toroidal transformer, pure sine). |
| Building a DIY 1000W 12V-to-120V pure sine wave inverter from scratch. | Need SPWM logic, dead-time control, and MOSFET driving on a budget. | Solid-State Core: EGS002 V2.2 SPWM Driver Board (Features the EG8010 chip and IR2110 drivers). |
Default Recommendation: If you are a maker attempting to build your own DC-to-AC inverter for a 12V battery system, do not attempt to write your own SPWM code on an Arduino and wire discrete gate resistors. The timing tolerances are too tight, and a software glitch will result in catastrophic shoot-through. Buy the EGS002 V2.2 SPWM Driver Board (typically under $15). It handles the 60 Hz sine lookup table, enforces hardware dead-time, and drives the high-side MOSFETs via bootstrap capacitors natively. Pair it with four IRFP4468 MOSFETs and a 12V-0-12V center-tapped transformer, and you will have a robust, repairable AC creation circuit.
Frequently Asked Questions
Why is AC created at 60 Hz in the US and 50 Hz in Europe?
This is a historical compromise between physics and metallurgy. Lower frequencies (like 25 Hz) were easier to generate with early slow-speed steam engines and worked well for heavy motors, but they caused noticeable, headache-inducing flicker in early incandescent lighting. Higher frequencies (like 400 Hz used in aircraft) allow for much smaller, lighter transformers and motors, but they suffer from severe voltage drop over long transmission lines due to inductive reactance ($X_L = 2\pi fL$). 50 Hz and 60 Hz emerged as the optimal middle ground where lights don't visibly flicker, transformers remain reasonably sized, and transmission losses are manageable.
Can I create AC using a 555 timer?
Yes, but you are creating a square wave, not a sine wave. A 555 timer in astable mode can easily generate a 60 Hz square wave to drive a relay or an H-bridge. While this will successfully run resistive loads like incandescent bulbs or simple heaters, the high-frequency harmonics inherent in a square wave will cause inductive loads (like AC motors or transformer-based power supplies) to overheat, buzz loudly, and potentially fail. For anything beyond a basic heater, you must filter the square wave or use SPWM to create a true sine wave.
What is the difference between an alternator and a generator?
In strict electrical terminology, a generator is a broad term for any machine that converts mechanical energy into electrical energy. An alternator is a specific type of generator that produces alternating current (AC). In automotive and casual jobsite slang, 'generator' usually implies a standalone portable AC unit, while 'alternator' refers to the DC-producing device under the hood of your car (which is actually a 3-phase AC alternator with an internal rectifier bridge converting the output to DC).
