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generator problem

I have an unusual but not unknown problem with a 230 volt generator.
The system consists of a Vetus 6.5kW generator coupled into a shore power / off / generator switch and then into a Victron inverter / charger / controller. There is also DC battery charging from the main engine via its own charger controller.
The system is 24 volts and the batteries (which were replaced a few months ago) charge fully from both the main engine and the generator.
The problem is that when the generator that when the generator is started there is the usual delay whilst the controller allows for the generator coming up to speed then as soon as the generator is connected the 30mA RCD trips but the batteries still charge. When the generator is stopped the trip can be reset.
I spoke to a local boat yard and they said they had seen the same problem but put the fault down to the control board in the controller. This board was sent to Holland for repair but was returned three weeks later as not being faulty. By this time the customer was not happy and took the repair to another boatyard. The first boatyard therefore did not know the resolution to the fault.
I contacted Vetus in the UK but never received a reply to my email.
I then contacted Merlin who was the supplier and designer of the electrical system. They were extremely pleasant and helpful. I spoke a few times to a young lady called Vicky who was one of the system engineers. She gave me a number of ideas to try one of them being to connect to a shore supply or to a pure sine wave external generator fed in as shore supply. Neither of these were available so we connected to another boat which had a pure sine wave inverter. The trip did not 'blow'. The conclusion was that the control board was working correctly and the fault was definitely in the generator.
When we bought the boat the generator only had 37 hours on the clock because the boat had been mainly used on shore power. The problems started around the 100 hours mark.
I brought in a local boat electrician who was very good but in some thirty odd years of working on boats had not come across. The generator was checked for earth connections, shorts and speed. The speed was out by 16%. This was corrected with difficulty because his test equipment was giving erratic readings. This did not cure the fault.
On the electricians next visit I had set up an oscilloscope with an inductive pickup. The results were unexpected. The speed was exactly 50 Hz and the output was a sine wave but the sine wave consisted of spikes and troughs up to about 40 volts all along the sine wave.

Asked by: pat addy  | 11.37am, Thursday 17 July

WW says:

When you looked with the oscilloscope, was the generator connected to the Victron?

Rupert Smedley  | 10.35PM, Thursday 17 July

Is the generator a conventional one or the newer DC generator/inverter types?

Rupert Smedley  | 10.33PM, Monday 21 July

It sounds like a problem with the generator; specifically the exciter circuit or the brushes that supply the exciter amature. This would cause the spikes seen on the mains waveform, and it is likely to be these spikes that are causing the trips; they would also have interfered with the test equipment as you describe.

Rupert Smedley  | 2.13PM, Wednesday 23 July

Readers say:

The generator was connected to Victron when the oscilloscope was used. But the same sine wave 'interference'was there whether the generator was plugged in or not

pat addy  | 9.37AM, Sunday 20 July

The generator is a conventional one based on a small Yanmar diesel engine.
There are two outputs - the 6.5kW 230 volt supply and small 12 volt DC output for a dedicated starter battery which also runs the safety features such as oil pressure, temperature and cooling water temperature.

pat addy  | 2.03PM, Tuesday 22 July

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