Running a generator underspeed

Discussion in 'General Technical Questions and Answers' started by TJNII, Jul 30, 2021.

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  1. TJNII

    TJNII QRZ Member

    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).


    (1) slide 5
  2. TJNII

    TJNII QRZ Member

    > 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.

    Made a mistake here and the edit window has closed: "may or may not be impacted by the lower voltage" should be "may or may not be impacted by the lower frequency"

    Also, since "Most switch-mode supplies rectify the input to high-voltage DC" is probably going to annoy someone: yes, all the supplies I'm aware of work this way by design. However I wanted to avoid speaking in absolutes especially in a "might work maybe" context. If you know of switchers that don't do this I'm curious to know about them.
  3. K6BRN

    K6BRN XML Subscriber QRZ Page

    Rather than underspeed the alternator, which is clearly not a good idea, would it be possible to build a pully-based 1.8:1 "gearbox" to attach between the alternator and PTO? The pullys and belts are relatively inexpensive, though some machine work/welding might be needed to construct a frame.

    Brian - K6BRB
  4. TJNII

    TJNII QRZ Member

    Honestly that's probably the better way to go. I'm also kicking around the idea of using a old truck manual transmission so that I can select the ratio. Difficulty, as you said, is setting up the rotating components properly which is a bit more work than just switching some wires around. I haven't really looked into this too much yet as I only ruled out the easier underspeed option this morning.
  5. KB9DT

    KB9DT Platinum Subscriber Platinum Subscriber QRZ Page

    540 and 1000 rpm are two common PTO speeds. Step up drives to change 540 to 1000 are commonly used on older tractors without a 1000 rpm PTO. Find one of these and convert the 1000 rpm output shaft to a 540 and you're done. Does you tractor have enough power at 1/2 PTO rpm to drive the generator?
    K0UO likes this.
  6. KD4UPL

    KD4UPL Ham Member QRZ Page

    Running under speed is fraught with problems as you're aware and completely unnecessary. As suggested, a gear box is all that's needed if your tractor has the HP at lower RPM. Does your tractor have a 1,000 RPM PTO option? If it does you may not even need a gear box. Some of the tractors I remember from the 80's had both, I think you just changed something at the rear where the PTO shaft was.
    N2EY, PU2OZT and K0UO like this.
  7. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    I have several tractors driven generators.
    You just run off your pto even a small tractor 25-35hp will run 10 to 25K.

    An AC generator needs to be run at the designed power take off speed. For AC/ You end up getting something besides 60 cycle If you change PTO speeds.
    The AC Generator frequency is the number of electrical cycles per second, measured in Hertz (Hz) and this is directly proportional to the engine speed.
    Last edited: Jul 31, 2021
    N2EY likes this.
  8. TJNII

    TJNII QRZ Member

    That's a great idea. I'll keep my eyes out for one.

    I'm not worried about power, I found my tractor's engine power curves and even at slow speeds I should still be up around 10kW out the generator. That's a lot of power, and if I ever need more than that I'll probably want to run the engine at full PTO speed anyway. Researching more I'm actually wondering about running too slow, loading these engines at too low a speed isn't good for them. If I go with a mechanical solution I probably want a 1:1.5 ratio instead of a 1:1.85 or 1:2.

    In any case, not a pressing problem. It works fine at 540RPM, but 360RPM would be nicer. Simply a nice to have if I can solve it cheaply and easily.
  9. TJNII

    TJNII QRZ Member

    No, it does not unfortunately. If it did the whole "will running it at a lower RPM work? Why not?" thought experiment wouldn't have come up.
  10. AA5MT

    AA5MT Ham Member QRZ Page

    Reminds me of cutting the grass with the engine idling. Works fine until it hits a clump of weeds. A car doesn't go under load at idle either.

    K0UO likes this.

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