An Induction Motor, Stick and a Hornet's Nest

[vdubnut62]

Anybody ever hit a Hornet's nest with a stick? I figure that's what is about to happen here.
OK, what is the verdict on an induction motor as a generator? Yes I have Bill Roger's book that touches on the subject...BUT
I have found some conflicting information.

It will stop generating when the grid goes down.                   Supposedly loses excitation current?

It won't stop generating when the grid goes down.                Caps keep the core energized.

It will stop generating when overloaded.                              Why? Or is this simply Bovine scatology?

It won't stop generating when the grid goes down. Maybe.      Iron core is still magnetized?
.
It will only start a motor load 1/10 of it's generating capacity.   Why? (see Bovine remark above)

Supposedly they are very easy to grid tie. (I'm skeptical)         Nothing I ever want to do is simple.

Just what is needed to tie one to the grid?                             As in Honda's mini CHP unit, not to make a bunch of power, just slow the meter some. I already have a 12kw backup.

It' OK to tell me I have fallen out of my tree and hit my head.     I'm used to it.
Ron

[LowGear]

Hi Ron,

The meaningful questions.  You're better read than me, as usual, so let me include a couple of base line questions:

     Where is the wiring diagram?

     What three recommended make / models have worked using the above wiring diagram?

     Who is doing this - this afternoon?

Casey

[vdubnut62]

Hi Ron,

The meaningful questions.  You're better read than me, as usual, so let me include a couple of base line questions:

    Where is the wiring diagram?

    What three recommended make / models have worked using the above wiring diagram?

    Who is doing this - this afternoon?

Casey

1. As far as I know, there is no "set" wiring diagram, except for a comment about reversing the start winding leads, therefore my questions.

2. Bill Rogers used a Baldor Super E series 220/240 volt single phase. He also grid tied it to reduce his electric bill, not go off grid. Also apparently any 3 phase unit after about 1975 is good IF you have 3 phase power and possibly be used to make single phase also with less efficiency.

3. hellifIknow---NOT me! Just trying to pick a few brains for later. I know just enough to get in really deep "stuff".
Youtube has a bunch of goobers plugging things into questionable devices.
Ron

[BioHazard]

I've fought with this same question for a while. I always hear induction motors are "easy" to grid tie, but when I start asking about it nobody knows anything or they just start getting mad. With the recent availability of cheap chinese grid tie inverters, I think I'm just going to generate DC and use that.

[mobile_bob]

Bills book covers the application quite well as well as explaining the theory behind it.

if you grid connect a motor it will run at either 1800 nominal or 3600 nominal on 60hz power
actually it will run at either about 1740 or 3540 accounting for what is called "slip"

now while this motor is running if it is belted to an engine it will just motor along until such time
that you speed the engine up and drive the motor up over its syncronous speed by about the same amount
as the slip (about 5% over or 1860 or 3660 or maybe a bit more) at which time the motor stops taking current off
the line and starts to pump power into the line.

if the motor is sufficiently large the engine driving it cannot drive it faster than this generating speed, it will take all the available
hp and convert it into current and pump that current into the grid.

it will be in phase and will stay locked in phase with the grid.

now if you want to do a capacitor excitation or island application (non grid tie) it will work the same as above, however you cannot drive much over 10% of the motor/generator hp rating because the tank circuit formed by the capacitor and the inductance of the motor which is in resonance will go out of resonance and the thing will quit generating.

it is possible to add more capacitance on the fly to drive more than 10% of the nameplate hp rating, however adding and removing the added capacitance fast enough to not only get the job done by adding, but removing it fast enough after the load is removed to protect from voltage spikes. load dump can result in a large increase in voltage if the control system isn't very good.

going back to grid tie, if the grid goes down, in theory the motor generator should quit generating, because there is no excitation source presumably, however if you have a large capacitance somewhere on your side of the line fault it might be enough to keep your generator making power. therein is the problem, while in most cases the thing will simply quit, there is a remote possibility that
it won't quit and that presents a safety issue for a lineman should he be working on the line on your side of the fault.

personally i think grid tie of an induction generator might make sense "if" it is used as part of a cogeneration system and "if" it can
be done safely, to code and approved.

thats about it, as i understand it anyway
what i related is in simple terms, there are much better and more technical explanations available, but i think it all boils down about the same.

bob g

[Ronmar]

The basic nuts and bolts are fairly easy.  Hook motor to engine and apply grid power to motor.  This will spinup the engine.  Once up to speed and "motoring", give engine fuel, compression, spark, whatever and make it create torque.  Once you start increasing the RPM on the motor above it's nameplate rating, it will begin to feed current back into the grid...  The more torque you apply, the more power is fed back into the grid, within the HP and electrical limits of the engine, motor and wiring.  Because the induction motor can be aligned to the incomming frequency at many points around the rotor, you could also start the engine, get the RPM up near the nameplate rating on the motor and then close the grid breaker.  Because the rotor may not be exactly aligned to the freq, there will most likley be a current spike and resultant torque to force the rotor into alignment with the incomming grid frequency.  This would most likley be less than the current drawn by a stationary rotor at startup though.  This is the easy part.  Where it gets tricky is in doing this legally.  Because the grid is not purely resistive but has capacitance, it is possible that once started, the generation process might not stop if grid power is removed.  This could leave you liable if someone working on the lines is injured/killed...  It requies a special circuit called an anti islanding circuit to sense the loss of grid power and sever the grid connection.  Grid tie inverters have this circuitry built into them.  An inverter is probably the easiest way these days to backfeed legally...

[BioHazard]

personally i think grid tie of an induction generator might make sense "if" it is used as part of a cogeneration system and "if" it can be done safely, to code and approved.

That's the dead end I always hit. I have yet to see a single example of this being done "legally", or even a description of how.

On a somewhat unrelated note, if a lineman treats any line like it's dead without first checking with a non-contact voltmeter, or doesn't simply work as if all lines are hot even when he thinks they're off, deserves to get fried. I know they purposely work on high voltage lines while they're live, so there's no reason anybody should get electrocuted even if someone does have an "illegal" generator. Even I use a non contact voltmeter before touching a 120v line, they cost about $10. And my electrical tools have insulated grips...

[Ronmar]

I think complacency and exhaustion play a large role in the fried witicha lineman scenario.  When you have repeated teh same process perhaps a 100,000 times and it has always been off, you might tend to think it will always be off.  Add into that a 36 or 48 hour shift during a major outtage and the little old lady in her bathrobe and rubber boots tapping you on the shoulder every 5 minutes asking when her electric will be back on eventually wears some down to the point of cutting corners...

[LowGear]

Dead Lineman?

I'm not very respectful of 120 AC and laugh when I get tagged.  OK, I don't stand on wet cement floors barefooted and mess around changing out dead receptacles.  I'm pretty cool at the top of an eight foot ladder as well.  And I'm pretty careful about where my left hand is when I'm inside a breaker box.  220 and DC I respect (real men fear nothing) enough to be careful.  So I'm gonna get lose with power line work - No, I don't think so Lucy!  Real men are not idiots either.  I don't think anyone deserves getting hurt but I can certainly see why it happens to amateurs but not pros.

With all the back yard electricians and power generator managers (remember "back yard") around today I can't see a real lineman getting loose at any time.  Here's my question:

Why doesn't the line load that is seen by our 5 - 10 HP motor generator that is seen when the grid goes down get its bearings sucked right off its shaft and grind to a smoking stop in a couple of seconds?  What happens to a generator that is faced with a near infinite load?

Casey

[vdubnut62]

Well, mine smokes a LOT and the lights flicker and eventually go out if I can't get her disconnected quick enough.
Now that's NOT connected to the grid, but to more load than it can handle, 2 refrigerators, deep freeze, 50 gallon water heater,
4 stove burners, 2 ovens, 3 window air conditioner units, 5 ceiling fans, 3 computers, 50 gallon fish tank and a bunch or lights.
Ron

[veggie]

vdubnut62,

Lots of issues.
Sounds like the better way to go is to buy an ST-5 and have done with it.

veggie

[Crofter]

Getting hit with a 180 deg. out of phase connection to the grid is a lot greater for a generator than it merely hitting a dead short. Like the difference between hitting a wall and hitting one that is coming at you at the same speed you are going.

Back feeding? Yes sure, the linesmen hook on grounding clamps and test for voltage but if a good sized generator comes on line backfeeding at a later time  it might overpower those grounding rods they hook on. They will dissipate static or a bit of induced voltage but they wont neutralize a serious feed in. If they have to make a cut in the line or deal with a mess of downed lines they dont need the added problems of someone livening it up in the middle when they have both ends isolated. Every safety rule in the book is written in blood: someone died to put it in there! I dont think Utilities put the provisions on just to spoil our fun.

[LowGear]

Sorry!

I guess my disbelief has crept in.  I think it's a fun thing to think and talk about but I like some pudding in my pie.  Who is actually doing it successfully today?  I ate some crow on the opposing piston engine thread and I'm ready for crow pudding pie on this one too.

Casey

[highwater]

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

[vdubnut62]

Sorry!

I guess my disbelief has crept in.  I think it's a fun thing to think and talk about but I like some pudding in my pie.  Who is actually doing it successfully today?  I ate some crow on the opposing piston engine thread and I'm ready for crow pudding pie on this one too.

Casey

Here's the thing, I don't think that anyone has a damn clue as to how to do this legally. And I do include the Utility companies
in that statement.
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.

Randall thanks for the info, now if I can just find the darn thing.
Ron