Electrical Interference

[doug748]

My new Beta has a super 40amp alternator, but it does cause a rosette of interference on the Seafarer when it is charging the main battery. I know there is a simple fix but could someone spell it out because my electrical knowledge is....er basic.

 

[mortehoe]

RF noise filters

1) Ask Beta for a solution.

2) Go to Maplins/RS or any electronics shop and get a .47uF or greater 60V or greater class X2 or Y2 non-polarised capacitor and insert it across the alternator output/return lugs. (cost <£1.60)

IMO, as this is such a common problem with marine engines, I ask the question, why if a new engine costs mega£££s then why can't the manufacturer add a simple capacitor to the alternator .... or failing that, then why can't the alternator manufacturer incorporate that self same capacitor into the control gear??????

 

[mortehoe]

Re: RF noise filters (Cont)

or (more expensive) goto Halfords or any automotive spares place and get a DC RF filter alternator capacitor. If that doesn't work then thread a 6mm ferrite ring (£0.06) onto the alternator supply as close to the alternator as possible.

 

[doug748]

Re: RF noise filters

Thanks for that. Yep, it is a mystery to me as well. Now I have some info I shall speak to Beta and ask them...

 

[William_H]

Re: RF noise filters

Firstly if an alternator has a dead diode then it will often appear ok if you don't demand much current except it will give a lot more interference.
To explain there are 3 phases of AC from the alternator stator coils. 2 diodes rectify each phase. So one phase rises to +ve and that feeds the output followed by the next phase 120 degrees of turn later to feed the output followed by the 3 rd phase followed by the first again. if a diode is dead you get an output which is relatively smooth until it comes to the missing phase when there is hole in the output. This happens in the order of 400 times per second. Depending on engine speed. The interference sound will change with engine revs.

You should try to determine with an ampmeter that the alternator is capable of full output. Obviously with one phase dead the max output will be far less than half. However with little current demand the output voltage may appear OK. ie at least 30 amps at somewhere near 12 volts output. You might however find this difficult in practice. You need a really heavy drain on the electrics.
failing that you might like to remove the alkternator for a load check at a auto electrics shop.

Alternators can still give interference. Sometimes caused by tiny in balance in the diodes.

Suppressor Capacitors

We used to be able to get a suppressor capacitor made for the job. A tin can with mounting bracket It has a screw connection for the power from the alternator and another on the other end for the output. The connection input to output is just a piece of steel or copper but because the capacitor is wound around the conducting tube it gives less inductance from the power line to the capacitor.

This is more a nicety than essential. A normal capacitor mounted on the output lug with a short connection direct to earth should do the same job.
As suggested already if you have spare lenght of wire from the alternator you could wind it around a ferrite rod as many turns as possible or loop it through a ferrite toroid (doughnut shaped ring)
The capacitor goes near the alternator output terminal and the coil from there to the rest of the electrics.

A ferrite rod can be salvaged from an old AM radio. It is black carbon like material either 6mm in diameter round or even in a flattened rod. It has a coil of fine wire which you remove. Toroids of various sizes can be salvaged from old computer keyboards cables monitor cables or computer power supplies.

A further capacitor can be connected at the equipment end of the coil to ground. Almost any capcitor in the range .01 to ,1 microfarrad should be usefull. It must be rated at least at 60 volts. (not usually a problem) and have robust wires. good luck olewill