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Noisy power from typical backup generators
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IDEngineer:
Without disassembling one, I don't understand why typical gasoline powered generators generate such low-quality AC power. Seems to me that an alternator rotating through a magnetic field should be the literal definition of a sine wave, yet their output is often bad enough to trigger UPS's and there's a whole subcategory of generators that rectify the AC just so they can synthesize a sine wave with less distortion!

I understand that the stability of the frequency could be poor in a gas-powered generator, especially under varying loads, but why would the shape of the output sine wave be poor?
Stray Electron:
 IIRC they pulse modulate the field coil to regulate the output voltage and that causes a pulsating DC output instead of a true sine wave.  I've wondered if running the output through an isolation transformer would help.
amyk:
I believe some of the newer designs are actually a multiphase DC generator (like a car alternator) that doesn't run at a fixed frequency, and the output of that is fed to an inverter which then generates a very poor approximation of a sine wave. It's easier to generate a fixed frequency under varying load this way.
james_s:

--- Quote from: amyk on February 22, 2021, 04:51:10 am ---I believe some of the newer designs are actually a multiphase DC generator (like a car alternator) that doesn't run at a fixed frequency, and the output of that is fed to an inverter which then generates a very poor approximation of a sine wave. It's easier to generate a fixed frequency under varying load this way.

--- End quote ---

They are, they're called "inverter generators" and it will be prominently displayed on the unit. The one I have is a Honda EU2000i and it's fantastic, I can leave mine running at night during an extended outage and it will idle down and keep the refrigerator, clocks, internet router and phone chargers going for around 12 hours on a gallon of fuel. It uses a 3 phase brushless permanent magnet alternator which is built into the flywheel. The other side of the engine where the output shaft would normally be is blanked off with a plate.

Conventional generators are far less efficient at light load which is what most domestic generators spend most of their time doing. As far as the noisy power, I started a thread about the same thing probably a couple years ago. I don't remember whether there was ever a solid answer but I know there are multiple types of conventional generator. Some have an automatic voltage regulator feature which is an electronic module that regulates the field current. Others are simpler and have a self exciting rotor that just has a rectifier mounted on it. 
floobydust:
I'm used to standby generators with rotating diodes in the (brushless) rotor and a "voltage regulating capacitor" or "capacitor excitation":

"A separate excitation winding in the stator has a capacitor connected directly across its output as shown in Fig. 5.19. The rotor is usually of salient-pole construction as described previously, but in this case the rotor winding is shorted through a diode. On starting, the residual flux in the rotor body induces a small voltage in the stator excitation winding and a current flows through the capacitor. This current produces two waves of magnetic flux around the air gap of the generator. One wave travels in the same direction as the rotor, to create the armature reaction described in section 5.3.2. The second wave travels in a direction opposite to the rotor, and induces a voltage in the rotor windings at twice the output frequency. The current circulated in the rotor windings by this induced voltage is rectified by the diode to produce a dc current. This dc current increases the magnetic flux in the machine, which in turn drives more current through the stator excitation winding, which in turn produces more rotor current. This self-excitation process continues until the flux reaches a point at which the magnetic circuit is saturated, and a stable voltage results. The process also produces an inherent AVR action, since any load current in the output stator winding induces more rotor current to offset the armature reaction effect."

I think it's just old tech. I've never really figured out how they work as far as regulation and harmonics.
http://machineryequipmentonline.com/electrical-power-generation/generatorsbrushless-excitation/
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