motor as grid tie generator.... what kind do i need ?

[knighty]

just about to buy a motor to use as a grid tied generator, and thought I better post up and check my options before I do...

I was looking at a large brush-less 3 phase motor, but then I saw a few permanent magnet 3 phase motors and wondered if one of those would be better ?
(max power output will be around the 10kw mark, but maybe a little higher - new freezers being built soon!)

links....
15kw motor, should be big enough (want to keep engine rpm low)
45kw motor, would run cooler.. any down sides?
11kw, cheaper, but would be run near (or at) max power a lot of the time
premenent magnet motor.... will it work, and is it worth the extra?
just nosy with this one... why that shape? is it a PM motor ? all the other ones I've seen that shape are.... but to be fair I havn't seen many!

cost isn't a big deal, I'm a tight git and don;t want to spend more than I have to, but even the expensive motors will pay for themselves quickly so it's not a big deal, I'd rather get the right motor now and be done with it !

[jotec]

It might be worth you posting this on the other (veg) forum as I had some useful replies from Alistair (I think)  on a similar theme. He did seem very knowledgeable about it.
Dick

[noah]

 Standard induction motor/generator (or preferably high efficiency type) are pretty bulletproof and can be run at their maximum indefinately as long as there are no problems with cooling. I take it you are aware of G83 requirements? What is your prime mover?

[guydewdney]

You do NOT want to run the 45kw one - it will overheat (contra-instinctivly).

Read the book 'induction motors as generators' - http://www.amazon.com/Motors-as-Generators-Micro-Hydro-Power/dp/1853396451/ref=sr_1_3?ie=UTF8&qid=1290190871&sr=8-3

Not sure that a PM motor would work - they arnt wired like that afaik.

Just get a motor as close as possible to max power, and the more poles the slower it needs to turn. Connect to grid, spin it very slightly faster than it wants to turn 'free' - ie 3601rpm for a 3600 rpm motor etc.

[Philip R]

Before buying any induction motor, what is the rating and characteristic of the prime mover? Buying too big a machine, you may have mag inrush current issues on start up unless you use a soft starter. Too small, and the machine may run very asynchronously if the prime mover is too big for generator.

Induction motors used as generators require an external source of excitation to magnetise the stator, i.e. the mains(3 phase supply).So the machine will import reactive voltamperes from the mains and suppress the voltage. ( This was a problem with the early windfarms causing voltage control problems on the local networks and the power stations having to provide additional voltamperes through tapping generator transformers and increasing generator rotor excitation.)
This can be compensated for using a 3 phase capacitor bank, but the pf correction will vary with load. Also voltage control problems may occur on open circuit running once excited. Also for asynchronous machines to generate, the prime mover must turn slightly faster than the equivalent mains frequency/ motor speed, as Guy stated ( Please note that figure mentioned was for 60Hz base (3600rpm) not 50Hz (3000rpm 2 pole, 1500rpm 4pole etc.)

Using the permanent magnet generator, will give poor voltage regulation with variable load. Grid tied , this will produce a variable power factor with load, the DNO will not like this either.
 
Talk to one of the drive manufacturers for 3 phase machines, i.e. ABB, Siemens, Control Techniques, Danfoss. Some of their invertor drives can regenerate from an asynchronous generator and control power factor also. However, like an invertor (similar topology) they are not cheap.

[Philip R]

Have a read of Teds G83 article as a seperate topic forum. It is pretty definitive.

[knighty]

it's going to be powered by a 2.5litre diesel engine (perfect for veg)

I'm going to fit a one way clutch to it, so I can spin the motor up and run it for the 3min for the grid tie relay I have to accept the power produced, and then start up the engine and rev it up enough to produce power

once the grid tie relay accepts power and clicks its relay in the spin up power will be removed

(engine can't be started during spin up, and only when grid tie relay is giving the ok)

to start with, I'll run it at a set output, and then I have a plan to increase/decrease the load on the engine as more/less power is needed

if it's all accepted, then great (fingers crossed it will be)

if not, I'll buy a grid tie inverter, rectify the 3 phase output of the motor to DC and feed that into the GTI

I was quoted £5k+change for a control panel to control a generator and sync it into the grid, and "at least" another 5K+change to fit it !!!

I've seen a 17kw 3phase GTI for just under 5k.... so might as well go with that if I have to !

I only want to cancel out the background loads (freezers) and I'm not worried about producing all the power I need..... at a guess 90% is "background" type loads anyway


looks like the 15kw motor linked is the right one for me then..... the 11kw one would probably do, but I might as well buy new and get this done properly to start with.... they do run forever... but for all I know that 11kw one has been out in a field for 10 years

[Philip R]

Knighty,
Thanks for the description. Your detailed description sounds like the basis of a working system.

Later in the text you made the comment "if not, I'll buy a grid tie inverter, rectify the 3 phase output of the motor to DC and feed that into the GTI".

If you are using an induction generator, you cannot easily decouple it electrically from the AC mains system to feed a rectifier/ invertor due to loss of excitation mentioned before, unless you are using capacitors to provide the reactive power with the associated control problems.

[knighty]

will that be easy to do / setup ? (the capacitors)


hopefully it will all be ok and I can avoid the cost of the GTI.... either way it's cheaper than buying an "official" panel to connect the generator to the grid !

[guydewdney]

The regenerative inverters mentioned by Phillip R are a Good Idea - something I have tinkered with.

They often come up on fleabay - at minimal cost. You can alter the rpm of the motor very easily (button pushing). A forum member gave me two 37kw versions capapble of doing this.

Basically - they make the motor run at 49Hz (2940rpm) instead of 50hz (3000 rpm) {note - phillip is right about 3600rpm / 60hz} or 48hz or whatever - and they 'regenerate' the 'excess' back to the mains. Normally used for braking purposes. Super simple. In hindsight - I might have done this instead of the PMG / GTI route I used.

[Philip R]

The capacitors are connected up in delta across the phases.

The sizing is dependant on the degree of correction required, the trouble is the loading of the generator alters the correction required. On open circuit, the machine could over excited.   It might be easier to ditch the capacitors and import the excitation current from the mains.

The operation was to offset your own needs I recall, however, it will effect the system beyond your boundary. Therefore the protection panel should be included in the scheme.

Otherwise you will have to be islanded and use a synchronous generator with voltage control. ( You want to use an induction motor instead!!)

I downloaded an article of the www which I was going to email you. I will have to find it again to be able to link it into this thread, but not tonight.
Cheers Philip.

[knighty]

hmmm.....

I'll give my one way clutch a go and see if the DNO is ok with it.... if not I'll come back for more info ! :-)

[KenB]

Knighty,

Before you spend any money on this project I suggest you contact your electricity supplier first.

I doubt very much if they will allow you to connect any sort of "home built" grid tied induction generator.



Ken

[knighty]

^^  I know.... but I'll give it a try and see what happens.... I'll be under 4kw per phase, and protected by a proper G36/G59 grid tie relay

I could buy a grid tie inverter now, but figure I might as well give the motor a go first...

as far as I see it my choices are....

£40k+ to buy a ready made CHP system...
£10k+ to connect a generator into the grid (without cost of buying generator)
£5k to buy a GTI
£1k to do it myself with an induction motor

might as well start with the cheapest option first, and then buy the GTI inverter later on if I need too, it won't cost me any extra to do it this way

[jotec]

We are all Watling with interest to see how you get on. If you are successfully with the legalities I suspect there will be a number of others who will go the same route - you might be able to set  up as a consultant or even sell the 'parts'.

Dick