An Induction Motor, Stick and a Hornet's Nest

[BioHazard]

So, by extrapolation, the people who ARE doing this are keeping it on the QT. And that's why we can't find info on people who have successfully grid tied.

Bingo. Even if you figure out a way to do it safely, it's still going to have to be approved by some regulatory agency before the power company will ever let you use it. Anything homemade, simply can't be done to code...unless maybe you happen to work for UL.

I've tried asking about this many times, and you are very right about the stick in a hornets nest thing.

[vdubnut62]

Hey Ron,
Here's another little book that I found very helpful on the induction motor thing.

Motors as Generators for Micro-hydro Power
By Nigel Smith
Practical Action Publishing
ISBN 978-1-85339-645-8

a short review in my last post over in this thread:

http://www.microcogen.info/index.php?topic=74.0

Randall

Found it! On it's way from Amazon.
Thanks again.
Ron

Say, just how do the grid tie inverters sense grid voltage/current and switch themselves off?
Sense overcurrent and trip a breaker?
Anyone?

[BioHazard]

I'm not sure how the grid tie inverters work, but, maybe you could use a really small one, just enough to power a relay. Now when the grid goes down your inverter powered relay shuts off.

[mobile_bob]

i too am not sure how the grid tie inverters sense when the grid is down, but i would imagine that there
is a standard they must meet wherein they have so many seconds to disconnect should the grid fail.

if this is the case they can periodically disconnect for a cycle or three just to take a sample and if the grid is ok
reconnect, if the grid is down disconnect.

finding out what that standard is might be tough, however a call to the engineering dept of an inverter manufacture might get
an answer.

once might find there is only a few cycles leeway after a grid blackout for the inverter to disconnect, forcing a sample every few cycles

if this is the case the use of mechanical relays would be precluded with such a sample disconnect/reconnect scheme because they would quickly either wear out or have there contacts burnt. presumably they could use a zero crossing scheme to save the points but the mechanical wear would be rapid i would think.

electronic switching schemes is probably what would be used, along with a zero crossing scheme.

the remaining problem is switching sample disconnect/reconnect speed, it would have to be relatively fast so that the phasing between the genset and grid would not drift out far enough to cause a crash. which would likely mean some sort of added circuitry
to monitor the phase of the line and only reconnect when the phasing between the line and generator are very close to being is sync.

in my thinking that is the big buzz kill for diy grid tie generators, i can think of a few ways to sense and disconnect/reconnect, but can't think of an easy way to sense phasing and reconnect only when the line and genset are in sync.

it looks to be like the odds of connection anywhere close to in sync without some sort of advanced electronics is going to be quite high against that happening.

for the system to work, be automatic and not require human intervention seems like a rather complex project in my opinion, but i have not looked into what types of electronic solutions might be available.

there might be sure a chip that can monitor the line phase, and the gen phase and only allow the electronic switch to reconnect when the two are in phase, such a chip might be cheap as dirt, who knows?

if such a chip exists, then using it with a simple micro controller and a heavy electronic switch (triac?) one might be able to make up
a system that would periodically disconnect, sense for grid voltage and if it is ok then order a reconnect which the phase chip would only allow when the two are in phase, and if the grid was not ok not allow a reconnect until such time that the periodic sampling showed the grid was back online at which time the controller would order a reconnect which would be allowed by the phasing chip only when the grid and gen were back in sync.

now all we need is the sample specifications, how often does the sampling need to take place?, how long after a grid failure does the
system have to disconnect? and where do we find a chip that can take two phasing inputs and only switch when  the two are in phase? or if one does not exist we need someone to either design a circuit that accomplishes this or find an existing schematic.

while i am typing this, perhaps a ups system might have the required electronics to accomplish these goals, it might be that there is enough electronics in one of them to do the sensing , disconnect/reconnect of its system, and do the phase sensing?  if true it might be possible to use one to control an additional external electronic switch heavy enough to do the switching of an induction generator to the grid.

i too have lots of questions, and very few answers

bob g

[mobile_bob]

upon more thought and then finding this pdf

http://www.teriin.org/events/micro/DR-S6-RevA.pdf

it appears as though phasing is not an issue with an induction motor when used as an induction generator?

if that is the case, then using a micro controller to control the process of periodic disconnect/sample/and either remain disconnected
or to allow reconnect/ via a sufficiently heavy electronic switch should be relatively easy to do.

now if it would be up to NEC standards, code approved, insurable, UL or CE approveable, and ok with the power company is another matter entirely.

in any case if one was to do grid tie induction generation, having such a control system at a minimum would be something far better than having no control system at all.

bob g

[mobile_bob]

found the snyc relay, they are pretty neat solution to connecting a generator either to another or to the grid

http://cgi.ebay.com/BASLER-ELECTRIC-BE4-25-SYNC-CHECK-RELAY-NEW-/290432073583?pt=LH_DefaultDomain_0&hash=item439f19bf6f

this allows the two sources to only be connected when they are very close to being in phase,

the only drawback i can see to using one for a grid tie setup as previously discussed is the one million cycle mechanical lifespan of the
relay used,  if it is serviceable this might not be an issue.

in any event one of these units might be handy for an offgrid application where two gensets could be sync and brought online together to gain capacity.  surely one could do this without a syncing relay, but admittedly this would eliminate any chance of train wrecks that are likely if one goofs up or a family member that has no clue how to sync two gensets happens to flip a switch.

at least with this little unit, a flip of a switch would result in the unit taking over control of just exactly when the two units are allowed to be interconnected.

bob g

[Crofter]

http://www.nrel.gov/eis/pdfs/interconnection_standards.pdf     IEEE 1547 Interconnection Standards

A very complicated set of conditions to qualify a device for connection to a utility grid. As far as getting a self made unit tested to qualify, forget it!

No doubt that a person could fab up something that would not go poof when connected and would disconnect if the grid went away. I am sure some of Bobs suggestions would work, but to get the blessings of the Pope you would have to do a heck of a lot more penance than that!

[mobile_bob]

yes that is a pilot duty unit, and i would not suggest using it to close a big mechanical relay/contactor for grid tie work
because it would be quickly beaten to death, that is why i mention some sort of electronic relay, solid state relay.

even then using the sync relays internal mechanical relay even for pilot duty to switch a heavy solid state relay will quickly accumulate
the 1million cycle life of that unit, it would have to be readily replaceable to be of much use.

for an induction generator a syncing relay might not even be needed, however i am not sure that once it loses excitation whether it loses its ability to make power immediately or if it continues making power for a few cycles, if it does and is driven by a lister 6/1
it is conceivable that things could get out of sync in as little as a half cycle if the disconnect happened just as the engine was coming up on compression or just after combustion takes place.  in either event if there is no load i would expect there to be significant phase shift between the generator and the line it is about to be reconnected to.

and yes this is academic, as none of us would ever think of connecting to the grid any non approved gridtied power.

never, ever

bob g

[mobile_bob]

in all fairness, there is a practical application for this project that does not include connection to the utility grid
and would require no code approval, that being in an offgrid installation

if one had a separate building/shed/bunker for his power generation equipment, and was using a single generator
that was sufficient only in size to cover his unscheduled and generally uncontrollable loads, such as lighting and other things
like maybe a puter, tv and the like, he might get by with a small gen producing a kwatt in capacity

beside that small genset he might have a matching small genset, or perhaps a larger one, and when heavy loads are schedule to
be serviced it could be autostarted and with the use of a sync relay be brought online with the first genset to increase capacity while
the heavier load needed serviced, and then after be dropped offline.

while running should the first set be shut down for whatever reason the second if larger could seamlessly carry the load and when the first came back up it could then be automatically reconnected in phase with the first.

while not everyone needs or even wants such a system, the techonology might be useful in certain applications.

bob g

[vdubnut62]

I knew what this was going to turn into, thus my title.

Bob, I'm not sure that a UPS would help at all, I thought that they were just a relay that when it lost line voltage it energized a small inverter to give the computer time to shut down normally. I am probably wrong on that though.

Very simply stated, THEORETICALLY, IF one was to have an induction generator that happened to be grid tied, what would it take to just disconnect the generator if and when grid power fails? No phase sampling, no reconnecting, no anything but disconnect. And stay disconnected. Or at least until it was manually reconnected after mains power was restored. Simple. Well it sounds simple.

Oh according to Bill R's book the  induction motor is not energized until it is brought up to speed, about 50rpm over rated speed,
and the voltage potential is only about 5 volts or so if the start winding is reverse connected and the start and run winding are kept separated until the connection to the grid. He says a "train wreck" is impossible this way. There does happen to be a schematic in the book for this. Once again I'm skeptical. Sorry ,it's the pessimistic a-hole in me. By golly, if anything can be wrecked, then I'm your man.

[LowGear]

Hi mobil-bob,

in all fairness, there is a practical application for this project that does not include connection to the utility grid
and would require no code approval, that being in an offgrid installation

If you're electrics don't meet code and there is no permit then you're probably not going to expect your liability insurance company to pay off if someone gets hurt.  I think you're somewhat liable for a few years after you or your desendents sell as well.  It's a treacherous world we live in these days.  I just put a plug on my bio-diesel rig so it's an appliance rather than hard wired as I first had it installed.  I'm still hanging my buns out in the sun but if I should move then I can just cut the cord so to speak.

Casey

[vdubnut62]

I'm not sure how the grid tie inverters work, but, maybe you could use a really small one, just enough to power a relay. Now when the grid goes down your inverter powered relay shuts off.

Thanks BioHazard, I think you have just made my day.
Ron

[LowGear]

Hi Jens,

Doesn't sound silly to me.  I stopped Mickey Mousing electricals about 20 years ago.  The world just changes too much but then I don't live all the way at the end of the roads.  Survival has a different set of rules but having time and money enough to post on this site kind of puts you a click above that level of life.

Casey

[Ronmar]

An induction motor will be way more forgiving connecting to the grid due to the way it passes energy thru the rotor.  It basically has many small poles, so will sync pretty closely to the grid at first connect.  When it syncs, it will either instantly absorb energy from the grid to speed up, or pass energy to the grid to slow down to align.  By applying torque you are trying to force it out of sync, and this pushes energy onto the grid as the motor "wants" to slow down to sync RPM.

A standard generator is far more tricky to connect as it, in the case of an 1800 RPM generator, only has 4 poles.  Any difference in phase equals a pretty significant rotation angle difference, that must be resolved to get into sync.  Dropping a large generator onto a live buss out of phase usually results in broken couplings, broken mounts, twisted frames and stained shorts... 

To compare grid power to generator power, you need something to measure potential across such as a very low resistance circuit in series with the line.  A comparator circuit looks at both ends of this resistor.  If you are making power, and feeding it to the grid, and the grid is live, the voltage on both sides of the resister should be very similar.  If suddenly the grid goes away, the grid side of the resistor will have less voltage.  The comparitor senses the difference and opens the main contactor isolating the generator.

I seem to recall a discussion over on the Lister Engine Forum along with some O-scope pics from a current transformer on the line of a grid tied induction motor turned generator.  The motor was driven by a 6-1.  The best description was of " hammering current back down the line" as the massive power stroke of the single, made for a very large current spike associated with each ignition event:) 

[vdubnut62]

  The motor was driven by a 6-1.  The best description was of " hammering current back down the line" as the massive power stroke of the single, made for a very large current spike associated with each ignition event:) 

Almost a direct quote from Bill Rogers book.
Now about this comparator circuit.......
Ron