A Generac Portable Inverter Generator provides power to a Class Mobile Home. A Low THD Generator provides clean, safe power for appliances and electronics.
What is THD and Why Does it Matter When Buying a Generator?
Have you ever looked through a window that distorted your view of the world? Some windows made of many differently shaped pieces of glass can make objects appear out of place and have the wrong shape. That’s what harmonics do to AC current—they distort it out of its perfect waveform.
The term THD gets thrown around quite a bit. If you’re shopping for a generator, you’ve probably concluded that a low THD generator is better than one with high THD. But what does THD stand for? Total Harmonic Distortion, or THD, is a measure of how badly harmonics have distorted the AC signal from the generator.
In the above graph, a fundamental sine wave travels through 360 degrees. At far left, voltage starts at 0 degrees and zero volts, rises to a peak at 90 degrees, falls back to zero volts at 180 degrees and continues falling through a negative peak at 270 degrees, and rises again to zero volts at 360 degrees where it starts all over again. On the North American power grid, this cycle repeats 60 times per second, or at a frequency of 60 Hertz.
A harmonic is a sinusoidal wave (sine wave) that is a positive integer multiple of a periodic signal.
More simply put, a harmonic is a sine wave at a higher frequency than the fundamental sine wave. If 60 Hertz is the fundamental frequency, its third harmonic is three times the frequency at 180 Hertz, and has 1/3 the peak value.
This graph illustrates a fundamental 60 Hertz sine wave and its third, fifth, and seventh harmonic frequencies. The fundamental wave (red) reaches its first peak at 90 degrees. With respect to the 60 Hertz cycle, the third harmonic (blue) peaks at 30 degrees, the fifth harmonic (yellow) at 18 degrees, and the seventh (green) at 12.86 degrees.
Because voltage adds, we can expect the third, fifth, and seventh harmonics to increase the fundamental signal’s voltage through 12.86 degrees and then have the opposite effect. By the time the fundamental wave peaks at 90 degrees, both blue (third) and green (seventh) are nearly at their negative peaks, which drags the fundamental voltage down, as seen in the next image.
In this last graph, the black line represents the distorted output caused by adding the harmonics to the fundamental wave. Instead of a sine wave, it looks more like a square wave, but its frequency is still 60 Hertz. The distorted signal is the total of all the frequencies. We express the difference between the distorted signal and the fundamental signal as a percentage called Total Harmonic Distortion, or THD.
Some electronics and appliances block distorted electrical signals and shut down to avoid damage.
What Causes Generator THD
Generator THD is important because distorted current might not power all the appliances or electronics that you want it to power. A heavily distorted signal could damage some sensitive electronics or appliances that rely on electronics.
In an ideal world, we’d never worry about harmonics unless we wanted them. Music relies heavily on harmonics to turn simple sounds into a complex experience that we find pleasing.
However, the world is not ideal, and the harmonics associated with electrical power are neither pleasing nor beneficial. As we’ve already stated, harmonic distortion can damage electronics and appliances that use electronics.
In the real world, every alternating current has some degree of distortion. Non-linear loads like power supplies, LED lights, motor controls, and even the components of a generator all contribute to harmonic distortion. Ideally, THD measuring less than 6% is acceptable, while 1-3% is close to ideal.
Remember that harmonics add and subtract from the fundamental signal. Notice how the 3rd harmonic is not just 3 times faster, but also has 1/3 the peak value of the fundamental. The 5th harmonic is 5 times faster and 1/5 the peak value of the fundamental. The same is true for the 7th, 9th, and 11th—each is faster and has a lower peak value. As the peak value diminishes and the frequency increases, the higher harmonics have less effect on the output.
This distorted “dirty” output runs a space heater just fine because the heater doesn’t require a clean signal. It’s just a resistive load that heats up. However, a distorted signal doesn’t play well with electronics. Electric motors might run okay, but depending on the level of distortion, they may have lower power output, encounter heating issues, or have trouble starting under load.
Damage from harmonic distortion is often incremental. A refrigerator with a ten-year life might stop working after five or six, for example.
If you have a home office and use an uninterruptible power supply, a heavily distorted signal over five or six percent may put it in power-loss mode, because the design protects your equipment from dirty or distorted power.
Generator THD comes from non-linear loads, and the sources are many, including the generator components and how the generator regulates voltage.
Related: 4 Ways to Keep the Power On
Low THD Generator
A good Automatic Voltage Regulator (AVR) reduces generator THD. An AVR stabilizes generator voltage by adjusting the magnetic field within the generator. One method of reducing harmonics isolates the power supply to the coils from the generator output. Better AVRs may use a Permanent Magnet Generator (PMG) to regulate the magnetic field instead of using the AC output that supplies the loads which can feed harmonics back to the AVR.
Home Standby Generators from reputable brands produce low THD by using a quality alternator (generator) with a good automatic voltage regulator operated by a microprocessor, typically the generator controller. Standby generators usually have a Total Harmonic Distortion of less than 5% while transitioning from a no-load to full-load condition, such as when the transfer switch moves the house from utility power to generator power after an outage hits.
Better portable generators also employ good alternators with low THD, usually less than 5 or 6 percent. Some RV generators are available with THD below six percent.
Another way to produce clean, distortion free power uses electronics to invert DC power into AC Power. Newer portable inverter generators produce power with as little as 3% Total Harmonic Distortion. The generator motor turns an alternator, rectifies it to DC power, and then inverts the output to AC at 60 Hertz.
Portable vs Inverter Generator Whats the Difference?
The benefit of this arrangement is threefold:
- The output frequency relies on the electronics instead of the engine speed, which means the engine can match the speed to the power required rather than running at a continuous speed of 3600 RPM.
- Varying the engine speed can provide exceptional fuel efficiency, especially when power demand is low. Lower engine speed also reduces wear, and the generator runs quieter. Some inverter generators produce as little as 50 dBA when measured at 23 feet—about the same as quiet operation.
- Small inverter generators are easy to carry, with some models weighing in at less than 50 pounds. They are compact and easy to pack in a car trunk or van. They won’t take up much space in your RV or the back of your pickup.
Depending on your need and application, low THD generators are available in all forms. Check the generator specifications before making a purchase. A generator with a total harmonic distortion of less than 6% will power virtually every type of appliance or electronics.
