Alternator and Inverter weirdness

[Paul Coleman]

Last weekend, my alternator stopped charging. Having checked the wiring and belt, I took it off the boat to have it tested, and it came back fine.

 

After re-installing it, I started the engine and it was indeed working, charging at a (maybe too) healthy 14.8v

 

When I switched on the inverter however, the alternator stopped all charging and the voltage dropped down to about 12.5, fluctuating with inverter load. As soon as I switched off the inverter, the alternator started charging again! The inverter was switched on whilst I was checking the alternator the first time.

 

The inverter is new and a cheap Chinese pure sine wave unit. As far as I know, it has always done this to the alternator, I just haven't noticed until now as I run on my solar panel when the sun shines!

 

Has anybody experienced this, or can anybody suggest how I might fix it?

 

Also, after charging for a while, the alternator voltage drops down to just over 14.2v. I put this down to some form of temperature compensation by the alternator as it heats up. Any other views?

Edited December 4, 2012 by Paul Coleman

[blodger]

How are you measuring that the alternator cuts out when the inverter is on?

 

If you are running biggish loads on the inverter then the alternator output will be diverted to supplying the inverter with power possibly augmented by power from the batteries; Hence a voltage at the batts of 12.5 would be expectable.

 

The batteries clamp down the voltage output by the alternator which should steadily rise but you need to be measuring the voltage at the batteries rather than at the alternator. The bits of wire, isolators, diodes or relays in between drop the voltage and consume power as heat due to their resistance.

 

How big is the alternator and on what engine? Sometimes an alt spinning too slow struggles hard to give a good voltage on start up but relaxes and gives what it can as it settles down, sort of.

[RobinJ]

When I switched on the inverter however, the alternator stopped all charging and the voltage dropped down to about 12.5, fluctuating with inverter load. As soon as I switched off the inverter, the alternator started charging again!

This indicates that the alternator is under full load.

You don't say what size the alternator is?

You don't say whta size the inverter is?

You don't say what size your battery bank is?

You don't say how fast the engine was running?

Also, after charging for a while, the alternator voltage drops down to just over 14.2v. I put this down to some form of temperature compensation by the alternator as it heats up. Any other views?

Many newer alternators have a two stage controller fitted, whioh charges at maximum power and then limits the voltage to ~14.4V.

This will depend on speed of engine, battery size and condition.

[by'eck]

Last weekend, my alternator stopped charging. Having checked the wiring and belt, I took it off the boat to have it tested, and it came back fine.

 

After re-installing it, I started the engine and it was indeed working, charging at a (maybe too) healthy 14.8v

 

When I switched on the inverter however, the alternator stopped all charging and the voltage dropped down to about 12.5, fluctuating with inverter load. As soon as I switched off the inverter, the alternator started charging again! The inverter was switched on whilst I was checking the alternator the first time.

 

The inverter is new and a cheap Chinese pure sine wave unit. As far as I know, it has always done this to the alternator, I just haven't noticed until now as I run on my solar panel when the sun shines!

 

Has anybody experienced this, or can anybody suggest how I might fix it?

 

Also, after charging for a while, the alternator voltage drops down to just over 14.2v. I put this down to some form of temperature compensation by the alternator as it heats up. Any other views?

 

Sounds entirely normal. The alternator will have been working all the time. The voltage across its output will vary with load on the batteries. In this case the inverter and the AC load on that.

 

If you have access to a clamp-on DC current meter on the fat B+ output cable on the alternator, you would see the current increase noticeably as it works harder to supplement the inverters current draw on the batteries.

[Paul Coleman]

Thanks for all the replies - I'll try and answer all of the questions above in one message:

 

blodger -

 

I am sure the alternator is cutting out as the engine RPM rises when I switch on the inverter and drops noticeably when I switch the inverter off. This combined with the voltage readings makes me quite sure it is cutting out.

 

 

Robin J -

 

Alternator Rating: 100A

Inverter: 1000W pure sine but with no loads attached at the time.

Batteries: 3x110Ah leisure in moderate condition + Starter

Engine speed: Exact same effect from idle to 2000 ish (no tacho fitted). Not sure of exact pully ratio but about 3:1.

 

It is a modern alternator, so what you say about their two stage charging makes sense, thankyou.

 

 

by'eck -

 

I understand what your saying, but the load on the batteries is not changing appreciably - maybe a couple of amps to run the inverter electronics, certainly not enough to pull the alternator voltage down two volts.

I don't have a clamp-on ammeter, and my shunt-type volt and ammeter goes haywire when I switch on the inverter.

 

 

One additional thing - when the inverter is powered up, my wired-in volt and ammeter goes haywire, reading about 45 volts and silly amps. I put this down to being an ebay cheapie and used my multimeter instead, but could there be some sort of ripple being generated by the inverter that is mucking about with the alternators ability to measure voltage reliably?

[Gibbo]

One additional thing - when the inverter is powered up, my wired-in volt and ammeter goes haywire, reading about 45 volts and silly amps.

 

I think your inverter is throwing out so much electrical noise that it's stopping everything else working.

[Paul C]

One additional thing - when the inverter is powered up, my wired-in volt and ammeter goes haywire, reading about 45 volts and silly amps. I put this down to being an ebay cheapie and used my multimeter instead, but could there be some sort of ripple being generated by the inverter that is mucking about with the alternators ability to measure voltage reliably?

 

That's worrying - I'd suggest either faulty inverter; faulty voltmeter/ammeter (installation); or some kind of electrical interference between the two.

 

Without the engine running, what are the voltages you read with a multimeter, with and without the inverter on (no loads connected)? And if you connect a sizeable mains powered item to the inverter eg near its rated output, what's the voltage then?

 

Does the inverter get hot?

[RobinJ]

Alternator Rating: 100A

Inverter: 1000W pure sine but with no loads attached at the time.

Batteries: 3x110Ah leisure in moderate condition + Starter

Engine speed: Exact same effect from idle to 2000 ish (no tacho fitted). Not sure of exact pully ratio but about 3:1.

Alternator will develop full power at about 4000 rpm, so at high revs would expect to show resonable charge.

Sine wave inverter will draw a lot of power, but system should be able to handle 100A!

One additional thing - when the inverter is powered up, my wired-in volt and ammeter goes haywire, reading about 45 volts and silly amps. I put this down to being an ebay cheapie and used my multimeter instead, but could there be some sort of ripple being generated by the inverter that is mucking about with the alternators ability to measure voltage reliably?

Ah! Sine wave inverter will generate a lot of RFI, so maybe suppression is needed.

 

Where is the inverter connected (direct to battery)?

Does the alternator go direct to starter, direct to battery?

Where does the feed to instruments come from/where are they in relation to inverter?

Putting suppression capacitors on the instrument feeds will help them, but alternator may be more problematic!

[Paul Coleman]

I think your inverter is throwing out so much electrical noise that it's stopping everything else working.

 

 

 

Ah! Sine wave inverter will generate a lot of RFI, so maybe suppression is needed.

 

Where is the inverter connected (direct to battery)?

Does the alternator go direct to starter, direct to battery?

Where does the feed to instruments come from/where are they in relation to inverter?

Putting suppression capacitors on the instrument feeds will help them, but alternator may be more problematic!

 

It is connected like so:

 

Positive:

Inverter -> fuse -> isolator -> battery

Alternator -> split charge relay -> isolator -> battery

 

Negative:

Inverter -> shunt -> battery

Alternator -> engine block -> shunt -> battery

 

Are there any worthwhile changes to the wiring that might alleviate the problem?

 

Would it be advisable to fit a suppression cap at the inverter itself? My electrical understanding doesn't stretch to RFI, but I do know that capacitors can go bang if abused. What sort of cap / rating would be appropriate?

 

What about ferrite rings, would scattering some of them about the engine hole help?

 

I'm a poor student at the moment, but in a couple of years I'll be able to afford a decent quality inverter. This one is just a stop-gap, so I think I'll live with it for now. If changing the wiring layout or scattering a few caps around would help, then I'll try that!

Edited December 4, 2012 by Paul Coleman

[Gibbo]

If an inverter is throwing out so much RFI that it can shut an alternator regulator down it's highly unlikely that throwing a few caps and ferrites at it will make any difference whatsoever.

 

First thing to do is find out whether it's radiated or conducted. Disconnect the output of the inverter from the boat wiring. Make sure the inverter case is grounded to the hull. Make sure the DC system is grounded to the hull.

 

Try it like that. If it still does it, it's conducted. If it doesn't do it, it's radiated.

 

Come back with those and we can see what we can bodge up.

 

Ah! Sine wave inverter will generate a lot of RFI...

 

No it won't. No more than a normal inverter. In fact, all other things being equal, slightly less.

[smileypete]

Also check the batt terminals, connections, crimps, fuseholders, isolators, for high resistance, including the return connections - look for millivolts of voltage drop using a multimeter when a sizeable DC current being passed though them.

 

Could be the inverter is unstable if there's high resistance somewhere, what make is it?

 

cheers, Pete.

~smpt~