Generator vs Inverter Generator (Genset)

[btrayfors]

After almost 20 years of experience with, first, a small Kubota-powered DC generator (80 amps @ 12VDC maximum) and a small Kubota-powered AC generator (3.5KW @ 120VAC) I have come to believe that a 3.5 to 4.0KW generator is suited only for the smallest of cruising boats.

Here's why:

1. The power factor, earlier mentioned, is critical. Many onboard devices will use more power than you think, based on their ratings.

2. After heating up a bit, or in high ambient temperatures, you won't get anywhere near the rating of the generator for full-time or even long-time loads.

3. A single air conditioner of, say, 16-18,000 BTU will use enough amps so that you are very limited as to what else can be in use simultaneously. Electric hot water heaters, grills, toasters, etc., and even healthy-sized battery chargers will quickly put an excessive load on the small generator, causing it to overheat and/or shut down.

How large is large enough for a 40-50 foot cruising boat? IMHO, at least 8KW is required to do the job. You could squeak by, maybe, with 5.5KW but that's pushing it.

As Captain Bill said, "it's complicated".

And, this Captain Bill is here to tell you that calculating your real needs is a non-trivial exercise.

BTW, Captain Bill #1: in your stead I'd consider adding an efficient, relatively low cost multi-stage charger, like the Iota. They come in many sizes up to 90A, and two identical ones can be chained together to double charging capacity. The size choice would depend on the available power from your generator and the type of battery bank....AGM or flooded (I'd not use the Iota chargers with gelled batteries).

Bill

[Capn Morgan]

I would like to be able to run a dive compressor with my genset. The time has come to chose the size of the genset and I am torn between the 5 and 7 KVa units from Northern Lights. I don't seem to be able to load any more than about 20 amps at 220V so the 5 KVa one should work. However, due to the start load of the compressors, I don't even know if the 7 KVa one will do the job. The dive compressor I have in mind is a Bauer Junior, has anyone got any experience with this combination they would want to share?

Cheers!

[Captain Bill]

Originally Posted by Capn Morgan

I would like to be able to run a dive compressor with my genset. The time has come to chose the size of the genset and I am torn between the 5 and 7 KVa units from Northern Lights. I don't seem to be able to load any more than about 20 amps at 220V so the 5 KVa one should work. However, due to the start load of the compressors, I don't even know if the 7 KVa one will do the job. The dive compressor I have in mind is a Bauer Junior, has anyone got any experience with this combination they would want to share?

Cheers!

I also looked at the Bauer Junior and was concerned over load on my 5KW genset, so I called the factory. They told me I needed a minimum of 7.5 KW. It seems I have two options, get a bigger genset, or buy an inverter that will "fill in" during motor startup. One can buy inverters that automatically sense an overload condition and can deliver power in addition to the genset, drawing power from the batteries for short bursts. This gets you through the startup loads. They are available in 3000 watt units that can support short bursts to 4500-6000 watts depending on the brand. They aren't cheap but they are cheaper than a new genset. If you are starting from scratch you might just want to go with the bigger genset. If you are planning to buy an inverter anyway you might want to go with a smaller genset and inverter combo.

There is also one more alternative. Buy another brand of compressor. Keep in mind I'm not recommending this brand as I have no personal experience with it, but it's now on my list. Coltri compressors are sold under a number of different brands in the US. Depending on what you read, some people swear by them and some people swear at them. What they do have is a startup procedure that essentially starts the motor under little or no load, keeping the amperage requirements below 20 amps on their smallest model(startup, 28 amps full load). This is doable with even a 5 KW genset. If you don't follow the procedure you will overload the genset, but it is an alternative to the big genset. The air output is similar to the Bauer Junior. I personally don't see why a similar procedure would not work on the Bauer, but Bauer does not recommend one, they say get the bigger genset.

[GordMay]

Originally Posted by Captain Bill

I also looked at the Bauer Junior and was concerned over load on my 5KW genset, so I called the factory. They told me I needed a minimum of 7.5 KW ...
... Coltri compressors are sold under a number of different brands in the US. Depending on what you read, some people swear by them and some people swear at them. What they do have is a startup procedure that essentially starts the motor under little or no load, keeping the amperage requirements below 20 amps on their smallest model(startup, 28 amps full load) ...

According to this website, the minimum powerplant recommendation for the 2HP (120VAC) machine is 5kw, while the minimum for the 3HP (230VAC) machines is 7.5kw
➥ Bauer Portable Compressors

I’m surprised that Bauer, apparently, doesn’t fit the Junior with "unloading valves".

[David M]

Originally Posted by Captain Bill

One can buy inverters that automatically sense an overload condition and can deliver power in addition to the genset, drawing power from the batteries for short bursts. This gets you through the startup loads.

You cannot run an inverter as a fill in on the same circuit with a generator because the phase will never match. It's not like DC where you can have multiple power sources on one circuit. With phase matching circuitry it is possible to make two generators work on the same circuit.

[Captain Bill]

Originally Posted by David M

You cannot run an inverter as a fill in on the same circuit with a generator because the phase will never match. It's not like DC where you can have multiple power sources on one circuit. With phase matching circuitry it is possible to make two generators work on the same circuit.

This is usually true, but there are phase matching inverters that can do this. They are a bit more expensive than your common inverter/inverter chargers however. Victron is one vendor that comes to mind. They call the feature "Power Assist". There also seems to be a similar product out of Taiwan called the Combiplus that even has a built in solar charge controller. I've heard lots of good things about Victron but don't know anything about the Combiplus.

Gord,
That info on the web site is recent data that I was not aware of and conflicts with what I was told by the support engineer. If they've put it in writing I suppose someone could hold them to it if it doesn't work. The query I put to the engineer was specifically about the 2hp model.

[Captain Bill]

My appologies for providing erronious information. In a more carefull reading of the combiplus specifications, it appears to integrate with solar controllers, but doesn't actually include one.

[Captain Bill]

Originally Posted by GordMay

According to this website, the minimum powerplant recommendation for the 2HP (120VAC) machine is 5kw, while the minimum for the 3HP (230VAC) machines is 7.5kw
➥ Bauer Portable Compressors

I’m surprised that Bauer, apparently, doesn’t fit the Junior with "unloading valves".

Gord,
I've looked all over the "official" Bauer web sites and cannot find a genset size recommendation. I don't know if Bauer would back the statement on the rayzplace web site. The procedure on the Coltri/Nuvair unit involves opening the moisture drain and the yoke bleed valves before starting the motor and does not involve any special unloading valves. Since the Bauer includes similar valves I don't see why a similar procedure would not allow the Bauer to start with a minimal electrical load. Bauer has not published or endorsed such a procedure as far as I can tell.

[GordMay]

Originally Posted by Captain Bill

This is usually true, but there are phase matching inverters that can do this. They are a bit more expensive than your common inverter/inverter chargers however. Victron is one vendor that comes to mind. They call the feature "Power Assist" ...

Victron’s (Phoenix Multi Plus) “Power Assist” feature can supplement insufficient shore or generator power with battery power.

➥ http://www.victronenergy.com/upload/...paper%20GB.pdf

➥ http://www.victronenergy.com/upload/...0%20-%20EN.pdf

[Capn Morgan]

Hi Bill

I posted something on another thread about this. The idea of helping the g'set with an inverter is quite likely ok, but it also adds to the complexity of the system. I would probably chose not to go this way. Regarding the compressors themselves, someone I know who has been involved with making a living out of them all his life, told me you can't buy better than Bauer, but a Coltri should work well. This unit will start with a 5 KVa generator. What I find disconcerting in all this is that many of these compressors also come powered by petrol (gas) engines which according to their specs are much smaller than the diesel generator. Any idea why?

[Dockhead]

Originally Posted by David M

You cannot run an inverter as a fill in on the same circuit with a generator because the phase will never match. It's not like DC where you can have multiple power sources on one circuit. With phase matching circuitry it is possible to make two generators work on the same circuit.

Not so -- the Victron and Mastervolt charger/inverters which have this feature match the phase and allow you to boost shore or generator power for startup loads. The Victron version is called "power assist".

I think it's much better to use something like this than to size your generator according to your startup loads. If you size for startup loads you end up with an oversized genset which is not working efficiently, is bigger and heavier and more expensive than it needs to be.

[downunder]

Capn morgan

The smaller genset5.5 with Vitron Quattro inverter/charger with 5kw boost for starting should do the job with other side benefits.

Victron Energy - Inverter/chargers - Inverters - Battery Chargers - and more

As Gord May suggested Vitron has the solution. Also check out vitron's generator test link from that page and you will see the solution.

Marine Generator Test - Victron Energy

[atoll]

we run a 3.5 kva 240v belt driven genset off the front of the main engine,gives loads of power when motor sailing at 1600 rpm at 6 knots.

have seen other vesssels with a lay shaft off the main engine,running large compressors and generators this way.

worth thinking about if you have the space?

[Captain Bill]

Originally Posted by Capn Morgan

Hi Bill

I posted something on another thread about this. The idea of helping the g'set with an inverter is quite likely ok, but it also adds to the complexity of the system. I would probably chose not to go this way. Regarding the compressors themselves, someone I know who has been involved with making a living out of them all his life, told me you can't buy better than Bauer, but a Coltri should work well. This unit will start with a 5 KVa generator. What I find disconcerting in all this is that many of these compressors also come powered by petrol (gas) engines which according to their specs are much smaller than the diesel generator. Any idea why?

If you were only buying the inverter to help with compressor startup loads I would completely agree with you with regards to complexity. The reason I'm considering this solution is that I need to replace my current inverter and my undersized battery charger as well as accomodating the startup requirements of a small dive compressor. Since I've had problems with some electrical devices running my current modified sine wave inverter I definitely want a true sine wave inverter. I also want at least a 100 amp battery charger to replace my 40 amp. I can buy such a device from reputable sources for $1300 to $1700. To get a Victron 3000 watt unit with power assist it will cost me $2300-$2600. The delta is now only $800 to $900. I've now solved my problem with compressor startup loads relatively cheaply.

As to why you need a smaller petrol/diesel engine to run a compressor than a diesel to run a generator to run the compressor has to do with losses at each energy form change. That is the compressor engine generates mechanical energy that directy runs the compressor. In the genset you first generate mechanical energy which is changed to electrical energy which then goes to the motor to be turned back into mechanical energy. There is energy loss at each transformation that means you need a much bigger engine to start with in the genset to get the same amount of energy delivered to the compressor shaft.

[capn_billl]

Originally Posted by Capn Morgan

Hi Bill

I posted something on another thread about this. The idea of helping the g'set with an inverter is quite likely ok, but it also adds to the complexity of the system. I would probably chose not to go this way. Regarding the compressors themselves, someone I know who has been involved with making a living out of them all his life, told me you can't buy better than Bauer, but a Coltri should work well. This unit will start with a 5 KVa generator. What I find disconcerting in all this is that many of these compressors also come powered by petrol (gas) engines which according to their specs are much smaller than the diesel generator. Any idea why?

One big reason is the high starting surge current required to start an electric motor under the load of a piston compressor. An engine driven compressor can be evenly matched so the power from the engines pistons corespond exactly to the power required to run the compressors pistons. Running a piston compressor from an electric motor powered from a small generator is pretty much a worst case scenario from a power standpoint.