Basic Engine Speed Fault Finding

[merryengineer]

The scope of this article is to determine a course for fault finding generator engines that are running at low speed, or displaying a low speed condition. It is concerned with rectifying general conditions that can be present on initial starting and electical faults. It is not within the scope of this article to correct sever mechanical faults.

Firstly low speed can be split into two catagories. Genuine low speed conditions and sensed low speed conditions. The first step is to isolate which of these catagories apply.

All of these things should be done off load, therefore the circuit breaker should be OPEN and kept open for the whole duration on these tests.

Assuming the unit is running at 50hz, 1500RPM. Monitor the speed and frequency readings on control panel together until the low speed condition is reached. How does the engine perform? What is the speed and frequency during this time? Does the speed decrease slowly? Does the engine hunt (ie the speed is going up and down)? Does the speed stay at 1500RPM? Does the Frequency stay at 1500RPM?
It is useful to listen to the sound of the engine as well, as the pitch of the sound will change if the speed changes.
If you are a competant electrician, you can monitor the frequency of the generated AC current, using a multimeter.
It is important to note that if the control module is shutting the unit down on underspeed, then you should monitor the readings on the control module, not on any analogue meters.

Many generators have seperate frequency and speed sensors, so it is important to monitor both as either could potentially cause a shutdown condition.

If the engine speed remains at 1500RPM and the frequency remains at 50Hz up to the point of shutdown,then there are two main options.

1) MOST LIKELY
When a generator engine shuts down itself (either because an ECU is telling the engine to shutdown, or because of an external reason (no fuel for example) then as the engine shuts down, the controller may well see an UNDERSPEED condition and shutdown the unit.

Suggested remedies:
(these items are known for various engine types and may seem unrelated, we suggest you try all of them to ensure you have tried as far as possible to rectify the fault).
 Ensure there is plenty of diesel and that the tank is vented sufficiently.
 Ensure the radiator is fully topped up with water, right past its max line to the top.
 Ensure that the oil filter, fuel filter and air filter are not blocked and clean.
 Check the ECU (if installed) for any fault codes and rectify if required.


2) VERY UNLIKELY a controller fault.

If the speed is zero, the frequency is 50Hz:

There is a fault with the magnetic pickup. Check the wires if one is fitted, check it is fitted correctly, check the end is clean. Most magnetic pickups (certainly the standard Perkins ones) product around 3V AC when then are working correctly. Check this with a multimeter on the terminals.

The frequency is Zero, the speed is 1500RPM.

1) Check the frequency sensing circuit.
2) Check the fuse in the frequency sensing circuit.

If either the frequency changes, while the speed stays the same:

1) There may be a loose connection in the frequency sensing circuit.
2) Check its not on load - an incorrectly wired inductive load can cause this fault, normally leading to an underspeed error.

Speed high, frequency 50Hz:

1) Clean the Magnetic Pickup.
2) Check the number of teeth programmed int he magnetic pickup controller, matches the number of teeth on the engine flywheel.

These are the most common faults we know of. We encourage people to add more.

[merryengineer]

On electronic engines, you shoudl also ensure that the earthing is correct.

Check with the manufacturer before altering, but on most engines it is essential that the engine block is at the same potential as battery negative. A small error can lead to strange behaviour.

[wellandpower]

The most important thing when investigating a speed fault is to watch and measure and build a picture of what is happening, specifically determining if the fault is a genuine reduction/increase in speed or is of some other cause.
It should always be off load initially as the faults can be caused by the load (badly wired or faulty loads, excessive load, etc.)

With steady, methodical methods, you can find the cause of most problems and this is no different.