This is more of a general technical topic and not really ham radio related. I couldn't find much information when searching so posting publicly in case it helps out someone else and so that the other technically minded folks here can fill any gaps in my analysis. Question: Can I run a generator head at under it's rated speed? Executive Summary: Only for a narrow subset of loads. For practical purposes: no. Context: I recently acquired a PTO generator at auction. This unit runs off a tractor PTO and is basically just a gearbox to up the PTO speed from 540 rpm to 1800 rpm, and the electrical generator itself. Due to running off a tractor I can easily vary the generator speed from about 800-1800rpms. As the tractor is much calmer at about 300 PTO rpm, and rather noisy at 540 PTO rpm, I began to wonder if I could underspeed the unit and use the 240v output as a 120v output. Example numbers assume a 240v/120v split-single phase generator with a load power factor of 1. Problem 1: Current at the lower voltage for the rated generator wattage: This concern is fairly straightforward, and not really an issue. If the generator is rated for, say, 24kW under normal circumstances then at full load at 240v the current in the windings will be 100a. If we underspeed the generator so that it's only putting out 120v then to get the full 24kW the current will be 200a, which is an over current overload. As such the generator must be derated by a minimum factor of 2 to avoid a current overload. This isn't a blocking issue, though, as it's easy to plan, monitor, and correct for. Problem 2: Field power: In the generator I'm working with one of the phases is converted to DC through a regulator transformer and a rectifier to provide DC for the field. At the lower speed the field will be running at 1/2 voltage. As the output voltage is controlled by the field strength (1), and because we intend to use the 240v output as 120v (discussed more in the next problem), this is desired. Problem 3: Output frequency. This is the main issue, and more than likely a blocker. Assuming we're running at a generator speed of 900rpm instead of 1800rpm, then the output frequency is going to be 30Hz, not 60Hz. This is a problem for any transformers or inductors involved as they are designed for a specific frequency. As we derate the frequency, we also need to derate the voltage to avoid core saturation (2). This isn't a problem in the generator itself, as we've already done that by using the 240v windings for 120v. However, it will be a problem for loads. Unfortunately this can potentially cause any load with a motor or an input transformer to not work correctly or burn out. So ultimately the question is can our load handle the lower frequency, which is hard to predict: - Old-fashioned incandescent bulbs should be fine, as they're resistive loads. - Resistive heaters should be fine. - Old-fashioned fluorescent lamps won't work, due to the ballast being a transformer - Cheap LED bulbs which just rectify the input and dump it into a capacitor should work, but might flicker - Anything with a transformer won't work, including cheap battery chargers. - Non-universal AC motors won't work. They'll either turn at 1/2 speed or burn out. - More modern electronics with a switch-mode power supply may or may not work. Most switch-mode supplies rectify the input to high-voltage DC and use that internally to create the lower output voltage. However, they also have input filters which may or may not be impacted by the lower voltage, and the input capacitors may not be large enough to handle the lower frequency. So it might work, but more likely won't work, and could even let the magic smoke out. Larger LED lamps fall into this category. Now, there is a final class of "might work" which is tools with universal motors that can run off 120VDC. These are apparently common for use by welders using motor-driven welder generators that output DC instead of AC (3). So it might be possible to rectify the AC output to DC and drive a motorized tool off that. The DC will be choppy, though, and this is really only viable with a 3 phase generator. This is also tool dependent as the tool may or may not be able to handle DC depending on the motor and speed control. There are also potentially safety issues working with 120VDC, but that's outside the scope of this write-up. This option definitely falls into the "you have to know exactly what you're doing" category. Finally, and not really in the scope of the main problem, I want to briefly mention diesel engine glazing. When researching these PTO generators this topic kept coming up. However, this appears to be more of an issue with standard generator sets where the engine is permanently attached to the generator, and can only drive the generator. Under consistent light loading the engine can glaze the cylinder walls causing engine problems (4). I don't believe this is a major concern with a PTO generator, however, because while the engine may be run at a light load for a prolonged period of time when generating, it can be easily moved to another load to run the engine hot and hard. Also this is an argument for running the generator slower, as the engine will need to work harder for a given power level at the lower speed, which some say is better for a diesel. I have not researched this last point in depth, however. Hope that's helpful or at the minimum interesting. And hopefully accurate, too (5). References: (1) https://www.pjm.com/-/media/trainin...20160104-basics-of-elec-gen-theory.ashx?la=en slide 5 (2) https://www.edn.com/using-a-power-transformer-at-a-frequency-it-wasnt-designed-for/ (3) https://www.garagejournal.com/forum/threads/who-uses-dc-to-power-a-corded-tool.285431/ (4) https://www.allpowersolutions.com.au/blog/generator-sets-and-diesel-engine-glazing (5) https://en.wikipedia.org/wiki/Ward_Cunningham#"Cunningham's_Law"