Why does a generator shut down on low frequency?

The output frequency of the alternator is always directly proportional to the speed of the engine. If you find that the frequency is low, while the engine speed remains constant, the effect is caused by harmonic distortion instead, which is a different problem.

If the engine speed is decreasing, because the alternator frequency and engine speed are always directly proportional, this is causing the frequency reduction. To solve the problem, solve why your engine speed is decreasing. The main reasons why you will see the engine speed change are:

Engine speed change due to an increase in load

On a fixed speed engine, when you apply a load to a diesel generator engine speed will decrease. Read about what happens to an engine when you apply load. The engine speed should recover within a few seconds. If the generator frequency drop is too large for your requirements, you need to reduce the amount of load you are applying, break the amount of load you are applying down into smaller steps (that means turn some things off until you have transferred the load, then turn them back on again one after the other) or purchase a bigger generator.

When you apply the load to the generator, you will also see a voltage drop at the alternator terminals.

Engine speed change due to too little fuel

The engines speed will drop if there is a fuel restriction. A fuel restriction can be caused by a clogged fuel filter, clogged fuel lines or the engine stop solenoid closing. Check the filters and fuel lines for blockages and air leaks (if air is being sucked in, that will have the same effect) and check that the fuel solenoid is fully open.

Engine speed change to poor quality fuel

If the engine is being fed by poor quality fuel that is not to the engine manufacturers specification, or the fuel has been contaminated, the engine speed may fall, or it may hunt up and down. Always ensure the fuel is of good quality.

To do a simple sample, put some fuel into a glass jar and let it settle – often contamination such as water will settle out. If you see layers forming in the jar, you will need to polish your fuel, or change your fuel supply.

Engine speed change due to too little air

Like the fuel, the engine needs sufficient air to run. If the air flow is restricted, the fuel cannot completely combust in the engines cylinders and the engines power will be reduced. You may also see black smoke from the engine, as unburnt fuel will be ejected from the exhaust.