# Running a generator underspeed

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

1. ### N1IPUHam MemberQRZ Page

I would find peak HP speed for you tractor at rpm first. If its lower than RPM required for the Genny then use pulleys and belt drive to get the gennys required rpm. You could also look for a used Reeves drive in the required HP range. Be cautioned in higher HP and torque values reeves drives used segmented belts. They are noisy as all get out and could easily kill someone if they came apart unguarded.

2. ### KF4ZGZHam MemberQRZ Page

Put a better muffler on the tractor and run it at the correct speed.
Cheaper and solves all problems.

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3. ### HUSSELQRZ Member

Why generators shut down or slow down on low frequency or under speed and how to fix it

What is generator low frequency?
Generator frequency comes down to the number of electrical cycles per second. Frequency is always measured in Hertz (Hz) and this is directly proportional to the engine speed of the generator producing power. So now when the frequency goes down, it only happens due to the speed of the generator engine going down.

Why generator engine speed go down?
When someone applies load to a generator it will always slow down the engine, that means the engine speed comes down and proportionally frequency also goes down. But the engine speed recovers after sometime .

When I apply load my generator slows down or just stops?
This is a special case when the load is very large the speed will not return to its normal level, and hence the generator's engine will stall and shutdown entirely. The power of a generator is limited to the amount of fuel and air that is injected into the cylinders for combustion. A 10kW (ESP rating) generator can only use enough fuel to produce 10kW of power (plus a little bit for speed governing usually.) A generator slows down and slowly stalls totally if the load applied is more than the rated load for ex, if the load carrying capacity of the generator is 10kw and instead you apply 12kW, it can't combust enough fuel, but the speed will start to drop and the engine gets stall. This is called overloading the generator. This is same as electrical transformers when applied load more than its rated voltage and current , short-circuit happens.

Some generators, have protection for overload. Overload protection works when the generator is not big enough for the items you are trying to connect to it. For example, if you have a 10kW generator with overload protection and try and connect 11kW, the generators control panel, or engines ECM will shut down the engine and display a warning on the control panel.

A fuel restriction or air restriction would also cause this problem, try changing your fuel filter and your air filter to see if this helps. You could also have poor quality fuel - if your fuel is old (especially petrol) or has been contaminated it won't be providing the same amount of power per litre, so you will end up with an under powered generator.

There are of course many reasons that a diesel generator can shut down, so this is just one answer to why it might be happening. If the generator only shuts down when you apply the load and connect it to the devices you want to power, then overload is a strong possibility.

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4. ### KM3FHam MemberQRZ Page

First you need the Generator to run at a 60 hertz output voltage.
Running at a lower speed will not allow this to happen. The generator design dictates this.
Second, the driving power needs a Governor to keep the Generator up to speed under loads changes.
Third, the ability to power the Generator to keep RPM up under load changes depends a lot on Engine Flywheel weight keeping Centrifugal force to keep Generator speed changes to a min. in an engineered Genset.
Over powered Engines minimize speed changes under load changes.
You cannot get around these parameters without causing issues from under speed, under powered drives.
Rough power of drive engines is usually about 1.5 HP per 1000 watts min..
My 13 KW Emg Genset uses a 22 Hp motor to produce a full load of near 50 amps at 240ac/60 hertz.
Experience: 35 years in part in Emg power generation at the 500 kw level for a communications company.
Most small engines in general run at about 3500 rpm for rated power out.
Those for Gen use are designed special for the application, with governors and other controls.

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5. ### N3HGBHam MemberQRZ Page

A conventional AC alternator can only run at design speed. The "inverter generators" you see like the Honda units save gas and noise by varying the engine speed to match the load and using an inverter to convert the random output frequency of the alternator to 60 Hz. There are drives used on boats and ships that use a mechanical/hydraulic drive of some kind to convert varying engine RPMs to a constant RPM.
On my boat I just use the 12 volt DC alternator to feed the batteries to feed an inverter.

I think for your case a PTO step-up gearbox might be just the thing.

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6. ### KM1HHam MemberQRZ Page

And here I have to suffer 3600 rpm from my generator to get 60 Hz! Is there any such thing as a converter down to 1800 Hz or so?

7. ### KM1HHam MemberQRZ Page

Wishful thinking if you stick with small engine size mufflers, more "muffle" = less HP. One from a very quiet vehicle may work.

8. ### K7JEMHam MemberQRZ Page

Most commercial diesel generators run at 1800 RPM. Only the smaller units that are designed for intermittent use run at 3600. The higher speed means more horsepower, so the engine can be smaller and lighter, which is a big selling point for portable generators.

9. ### KM1HHam MemberQRZ Page

Not if the governor is working properly to hold engine speed constant. Many governors for a typical home size are not set properly or have mechanical problems.

Then toss in dirty and/or worn carbs to add to the confusion.

10. ### KM1HHam MemberQRZ Page

Im aware of that as Ive looked at several mil surplus diesels but Id have to construct an outbuilding for it to run in, 15-20+ KW sizes get plenty noisy and that is far more than I need; 10KW is all I run and that is from a 6 and 4 KW genny to their own loads which includes a 660' deep 1.5 hp well pump.

Carl