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Engine driven generators


Felshampo
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Hi

I am looking at a boat with an engine driven generator. What is the difference between this and an alternator? Obviously this means you run the engine to run the generator is that a bad thing for the engine. Also is the generator always running when the engine is in use or are you able to uncouple it? Any thoughts would be helpful.

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As Arthur says lots of possibilities but usually when an engine driven generator is mentioned it relates to a unit (that looks like an alternator) but supplies 240v ac

 

It is usually belt driven and runs all the time that the engine is running.

 

The engine will also have one or two alternators for the start batteries and the domestic batteries.

 

An engine driven alternator is not a bad thing as it puts 'load' on the engine which will according to some reduce the likelihood of engine bore glazing but that is completely different ball game.

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There is a boat with a 3 cylinder Gardner that has a 230 volt generator that is coupled via a dog clutch and only engaged while the generator is in use. It also tequires the engine to be run at a constant speed to get the frequency right so can't be used while boating, only when moored.

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It says in the blurb it has a 3.5 kw engine driven generator.

Could be a Travel Power which is basically a big alternator feeding an inverter. Some people have problems with them breaking mounting for some reason.

 

http://fc-marine.co.uk/Electrolux_Travel_Power.html

Edited by ditchcrawler
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We have a 3.5Kw Travel Power, and find it an excellent piece of kit, perfect for operating heavy draw electrical equipment.

 

Although (as already said) it runs off a permanent belt from the engine (the power source is a 340v alternator), the separate control box is switched on or off as required, so there is minimal loading on the engine when it's not required.

 

Ideal inasmuch as it can be used when both cruising or moored, we run a sensitive washing machine, laptops, vacuum cleaner etc from it.

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We have a 3.5Kw Travel Power, and find it an excellent piece of kit, perfect for operating heavy draw electrical equipment.

 

Although (as already said) it runs off a permanent belt from the engine (the power source is a 340v alternator), the separate control box is switched on or off as required, so there is minimal loading on the engine when it's not required.

 

Ideal inasmuch as it can be used when both cruising or moored, we run a sensitive washing machine, laptops, vacuum cleaner etc from it.

So do you mean that when it is switched off it still rotates with the engine but sort of freewheels so it does not slow the engine down and draw any power.

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So do you mean that when it is switched off it still rotates with the engine but sort of freewheels so it does not slow the engine down and draw any power.

 

Yes but as suggested uses minimal power when the unit that controls it is not switched on with a load.

 

BTW the TravelPower if it is such, has a major advantage over a plain AC alternator in that it doesn't need to run at specific revs (1500 or 3000 rpm), this because its output is turned first into DC and then via an inverter in the control box back into AC at the constant 50Hz required by all European appliances. Having said that engine revs may have to be taken well above idle if the full power output is required.

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Yes assuming it is a TravelPower it will be running all the time the engine is, however with the control box switched off the only load is only that from the built in fan on the alternator, belt losses etc. Of course one could remove the belt when the device is not in use but that would be a right faff!

 

We have one and find it very useful. The engine is still likely to have at least one other "normal" alternator as well, but with a TravelPower you also have the possibility of charging the batteries via a mains charger or Combi inverter, which gives a backup in case of 12v alternator failure.

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Yes but as suggested uses minimal power when the unit that controls it is not switched on with a load.

 

BTW the TravelPower if it is such, has a major advantage over a plain AC alternator in that it doesn't need to run at specific revs (1500 or 3000 rpm), this because its output is turned first into DC and then via an inverter in the control box back into AC at the constant 50Hz required by all European appliances. Having said that engine revs may have to be taken well above idle if the full power output is required.

OK. I presume most alternators are DC to charge the batteries, the AC being converted to DC by diodes. So can you get an AC alternator on an engine as it would seem unlikely that any engine runs at a fixed revs. Do genies have an AC alternator? To be honest this was my initial confusion as I didn't know the difference between an alternator and a generator. If the control box is an inverter then does this mean that the TravelPower bit on the engine is the same as an alternator? Edited by Felshampo
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Yes assuming it is a TravelPower it will be running all the time the engine is, however with the control box switched off the only load is only that from the built in fan on the alternator, belt losses etc. Of course one could remove the belt when the device is not in use but that would be a right faff!

 

We have one and find it very useful. The engine is still likely to have at least one other "normal" alternator as well, but with a TravelPower you also have the possibility of charging the batteries via a mains charger or Combi inverter, which gives a backup in case of 12v alternator failure.

 

It seems that the Travel Power is just a high amp DC alternator and inverter. Can the inverter be used without running the engine? If not you might as well use the seperate devices. Also seems daft using a AC charger, from that I presume the Travel Power can't be directly connected to the battery. Another plus for getting seperate devices!

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It seems that the Travel Power is just a high amp DC alternator and inverter. Can the inverter be used without running the engine? If not you might as well use the seperate devices. Also seems daft using a AC charger, from that I presume the Travel Power can't be directly connected to the battery. Another plus for getting seperate devices!

I think you will find the alternator part of the set up runs at a higher voltage than a 12 or 24 volt one, so much lower current. I am not sure if the output is AC or DC

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Travelpower is a conventional alternator (3 phase AC) but rewound to generate a much higher voltage and this makes it substantially more efficient (you could not get 3.5kW out of a 12v alternator of the same size).

The control box rectifies the AC to DC then converts it to 50Hz AC.

The voltage is controlled by varying the rotor current just like a standard alternator.

 

A conventional stand alone generator again uses an alternator but generates the 50Hz directly and so its engine must run at a suitable fixed speed.

 

...........Dave

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Wot Dave said!

 

Our 175A alternator can give about 2kw via the inverter, whereas the near-identical sized TravelPower can give 3.5 kw and runs much cooler than the 175A alternator at the same power, due to its high voltage. Higher voltage to give the same power needs lower current, and it is the current that causes heating in windings and wiring.

 

But in fact we got ours mostly to introduce some redundancy. It gives us an alternative source of mains power should the inverter fail, and an alternative way to charge the batteries (via the Combi) should the 12v alternator fail. Redundancy is good!

Edited by nicknorman
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Wot Dave said!

 

Our 175A alternator can give about 2kw via the inverter, whereas the near-identical sized TravelPower can give 3.5 kw and runs much cooler than the 175A alternator at the same power, due to its high voltage. Higher voltage to give the same power needs lower current, and it is the current that causes heating in windings and wiring.

 

But in fact we got ours mostly to introduce some redundancy. It gives us an alternative source of mains power should the inverter fail, and an alternative way to charge the batteries (via the Combi) should the 12v alternator fail. Redundancy is good!

 

But if you had to choose one or the other, the seperate items I'm guessing would be the one you choose, because it gives you more options.

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But if you had to choose one or the other, the seperate items I'm guessing would be the one you choose, because it gives you more options.

If I understand the question correctly then yes I'd choose a Combi first (or separate inverter and charger) with the TravelPower being a "nice to have" addition, although that presumes the engine had a decent 12v alternator, not some ancient 35A job!

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So there isn't really that much advantage.

 

There is a great advantage in employing a higher voltage alternator. Losses are considerably less. Just compare the size of the alternator on a 3.5 kW TravelPower producing 250-300 volts with that of a 12 volt alternator to give the same power - around 250 amps! If you add inverter losses to turn that into 240 volt AC, more still.

 

Where the 12 volt route makes sense is if you already have large alternators and inverter, giving greater flexibility.

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There is a great advantage in employing a higher voltage alternator. Losses are considerably less. Just compare the size of the alternator on a 3.5 kW TravelPower producing 250-300 volts with that of a 12 volt alternator to give the same power - around 250 amps! If you add inverter losses to turn that into 240 volt AC, more still.

 

Where the 12 volt route makes sense is if you already have large alternators and inverter, giving greater flexibility.

 

Even if you start from scratch, a seperate system makes more sense. A high amp alternator with decent inverter costs roughly the same, but gives you more options, you don't even have too have one alternator, as 2 smaller units can provide the power if you really need a high continuous power without using the batteries.

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