A Little Electrical One

[JK]

Here's another one.
It is a diesel electric plant. Three diesels alternators with two propulsion motors. The diesel alternators are spring mounted and run at 900 RPM.

Two engines are on load, the third is running but off load, either warming up or cooling down.
I am working beside the off load engine, on the JW cooler, when I became aware that the engine is vibrating a bit more then normal.
I look up at it to see it start to rock back and forth dramatically, scared the hell out me actually since it looked like it was moving about a foot each way. Probably wasn't, but when you are on your knees looking up. you get a powerful impression of something is wrong!
It was following the other engines as they loaded up as the ship maneuvered, but since it was not on line, there was no load. As I watched, the engine load dropped on the other engines and as they dropped in power output the third engine reacted the same way but with a slight delay.

I beat it into the MCR and shut it down and called the electrician.

When we tied up, a electrical contractor was brought in.

Any idea what he found?

[D Winsor]

I believe the auto synchronization control for the alternator remained active and the engine was reacting, via the governor speed control, to variations in buss frequency as the load changed on the other 2 engines.

[The Dieselduck]

Jesus, these problems are way too complicated for me. ehehehe.

[Big Pete]

With a traditional mechanical/hydraulic Governor, (Woodward) each governor would be adjusted to maintain the engines at constant RPM with a slight "Droop". (i.e. the RPM will drop off slightly as load increases, preferably in an identical way on all engines that can be run in parrellel, to automatically maintain load sharing.

I don't see that you could have had this problem with this system.

The next stage is that each of the governors has a speeeder motor to change the "set point" or "desired value of the RPM/ frequency. This is used to synchronise the alternators, load and unload them gradually and to give more precise load sharing than can be obtained purely from the governor "Droop" characteristic.
In Diesel Electric plants the % changes in load can be very large, and rapid which would require the governors to have perfectly matched Integral and derivative action as well as droop in order to maintain load sharing in transient conditions.

I think D. Winsor is probably correct and that the speeder motor was still being energised although the engine was off load.
With no load, and hence no droop, the engine would run too fast even if all the engines were receiving the same "set point" speed signal.
If the alternator control system was trying to maintain equal load (kw) on each alternator then the engine would presumably have been running at maximum RPM permitted by the system all the time. ( with the Alternator off load the power feedback signal to the control system would be zero).
I suspect that the problem was a stuck relay or similar, in the switchboard/ power management system. This could either be telling the system that the alternator was "on line" or failing to disconnect the speeder motor from the control circuit, depending exactly on how the system was designed. I have had problems with either the increase or decrease relays sticking so that the engine can only increase or decrease RPM /load but never anythiung exacrly as you describe.
B.P.

[D Winsor]

I was assuming that the governor was electronic, Woodward 2301, EGCP-3 or similar with an electrical actuator over hydraulic governor
The Electronic governor would monitor both engine rpm and buss frequency and would send a signal to the actuator to maintain the off line engine at synchronous speed.

[JK]

Jesus, these problems are way too complicated for me. ehehehe.

Martin, that's the best part about of all of these. The answers are usually pretty simple, it just takes a lot of head scratching to get to them. And it is usually the last thing you look at that is the problem.


Good answers. It took a couple of days to figure this out, everything had to be checked out and the circumstances had to be recreated.
The problem was the wiring to the electronic governors. It was the wrong type and unshielded and bundled together from the MCR for a distance before separating to the engines. The engine off load followed the other engines because of literally a ghost signal through the wiring
Because it wasn't under load, the vibration was magnified.
It had been like this for years, just never noticed because the circumstances were never correct.
The wiring was corrected and rerouted and the problem disappeared.

A similar problem existed on another ship I was on. It started having mysterious motor trips-very violent. The shafting and prop weighed 55 tons and there was a real fear that the coupling would slip. Being in the MCR was like being in a shooting gallery as the breakers tripped out, pretty scary.
After a huge effort, it was traced to one wire. As it heated up, the impedance changed, which changed the signal back to the controls. Wire was changed and the problem disappeared.

[JK]

I have another one, that no one can figure out. And I mean no one, everything has been tried to correct this. Some very experienced people have been stumped.

New electronic governors are installed on the Cat 3608 engines..I will have to check the models in the morning as I am not directly involved in this one, but aware of it.
Everything works fine, the generators synch and share the load, no problem BUT

When the ship start to roll, the engines start to hunt. If the ship is not rolling they are fine.
If you go to one of the engines and manually start rocking it on the spring mounts, they will start hunting.

Any ideas?

[Big Pete]

Wild guess, and it is only that, worn feedback potentiometers on the fuel rack. When the engine rolls the feedback values jump erratically.

[JK]

No that was all checked.
When I called my friend to get the models, he was telling me that they found that the 24V charger was screwed up and had to be replaced. He isn't sure, but he is thinking that might be part of the problem as it was the only thing not checked when they were troubleshooting.
They are replacing the engines so they are not going to spend any more time chasing it.

[D Winsor]

Was the cable to the magnetic pickup checked to insure it had enough slack to allow for engine travel as the engine rocked on the residual mounts without momentarily disconnecting from the connector plug or a loose conductor in the cable?
Was the clearance on the main bearings and thrust bearing checked? If the crank was moving away from the magnetic pickup as result of worn bearings or a misaligned sensor ring, there could be a momentarily loss of signal to the Electronic Governor as the engine moved on it's mount