Generators and Electric Motors
[Home] - [HELP] - [Forums] - [Groups] - [Classified Ads] - [Subscribe] - [Books] - [Sponsors] -

Go Back   SmokStak > SmokStak® Vintage Electrical Equipment > Generators & Electric Motors General Discussion
Forgot Password? Join Us!

Notices

Generators & Electric Motors General Discussion Antique Generators and Old Electric Motors: Questions and answers about restoring and showing old power generation systems.

Generators & Electric Motors General Discussion

Induction generator from synchronous generator?


this thread has 6 replies and has been viewed 13236 times

Reply
 
Thread Tools Display Modes
  #1  
Old 02-25-2005, 08:57:20 PM
Jim Rankin Jim Rankin is offline
Registered
 
Join Date: Nov 2004
Location: Faunsdale, Alabama USA
Posts: 4,436
Thanks: 71
Thanked 1,185 Times in 967 Posts
Lightbulb Induction generator from synchronous generator?

Maybe someone has tried this or knows about it, so here goes.

I am interested in local power production onto the grid, in our case using biogas from a digester.
One drawback is the high cost and technical difficulties of synchronizing with the power company. The simplest inter-tie allowed is with an induction generator which is excited by the grid, so no problems with synchronization. The generator is turned somewhat faster than synchronous speed so there is positive slip and generates power into the grid as long as the grid is powered. This power grid is an "infinite buss" and gives all the kVAr required to magnetize the generator and accepts all the kVA the generator can produce.

So far so good! When the power goes off, then the induction generator unloads and must be protected from overspeed etc, and even though there may be fuel available and the engine could be run, it cannot generate backup power.

I heard yesterday of running a wound rotor induction motor (WRIM - which is used in larger sizes to control starting torque and running speed) with the wound rotor shorted and then it was in effect a sort of squirrel cage induction motor (SCIM). I have always heard that an AC generator was nothing more than a WRIM with a voltage regulator, attached to a driver like an engine or turbine and turned at a precise speed to get 50 or 60 hz power.

So why couldn't the induction generator be a "normal" AC generator with the rotor current shorted or controlled by a controller when in induction mode and when needed, The grid intertie could be opened, a transfer switch closed, the speed reduced, the voltage regulator switched on and the generator could supply backup power for the facility.

Is this all pie in the sky or does anyone know anything about it.

Jim
Reply With Quote
Sponsored Links
  #2  
Old 02-25-2005, 10:17:43 PM
Raymond Raymond is offline
Registered
 
Join Date: Nov 2004
Location: Zebulon, North Carolina USA
Posts: 467
Thanks: 0
Thanked 20 Times in 19 Posts
Default Re: Induction generator from synchronous generator?

check my posts on induction generators under "generator control" or search for induction generator in this forum. My Grandfather pioneered the use of digester gas fueled generators in 1925 and there are several still in operation today. I order to make use of the power produced, you must recover the heat from the engine jacket as well as the exhaust heat down to about 350 degrees, for the digester or you won't have enough heat. There is only 60 percent power available power due to the power requirement of an extended aeration plant. Power should only be generated on peak. You can't generate cheaper than off peak power even with free fuel. So save gas off peak and run it on peak. This is my specialty. If I can help further email me as there is too much to it to design on a builiten board.
Reply With Quote
  #3  
Old 02-25-2005, 10:24:37 PM
Wayne 440 Wayne 440 is offline
Registered
 
Join Date: Nov 2004
Location: Kentucky
Posts: 7,425
Thanks: 487
Thanked 6,894 Times in 3,214 Posts
Default Re: Induction generator from synchronous generator?

You are essentially describing the function of a gen-set controller in parallel and stand alone operation. As you have probably found, parallel capable controllers are not inexpensive. I think a wound-rotor motor could be used in "induction" mode as generator if the rotor current/voltage could controlled (presumably within the "motor" ratings), but the logistics of designing a controller would be time consuming (expensive). It would probably be more efficent as a synchronous unit and this require the expensive parallel controller. Unless you already own a wound rotor motor, an induction motor is probably cheaper to buy anyway.

Parellel operation of a gen-set with any utility usually costs more in interlock/monitoring/insurance areas than the cost of the gen-set and other infrastructure. A small operation can easily spend more on engineering costs that it would ever be worth.

I think you would be better off in many respects with a DC (your AC can be rectified easily) generator used to power an approved utility interconnect inverter. The engineering for utility interconnect is inherent in the inverter, and relatively easy to implement. If you used a battery bank, your prime mover could be shut down for repair/service etc. without putting you in the dark.

I look forward to hearing more about your project. What is your intended prime mover/kW capacity?
Reply With Quote
  #4  
Old 02-25-2005, 11:26:45 PM
Jim Rankin Jim Rankin is offline
Registered
 
Join Date: Nov 2004
Location: Faunsdale, Alabama USA
Posts: 4,436
Thanks: 71
Thanked 1,185 Times in 967 Posts
Default Re: Induction generator from synchronous generator?

I had always been troubled by the fact that even if you had an induction generator, you would be in the dark and waiting for the power to come back on like everyone else. The only solution seemed to be a backup generator as well as the induction setup.

The other thing is I can find 3 phase motors for an induction generator, but none of them is already bolted onto an engine like the gensets. A coupling arrangement and alignment would be a trick to accomplish though it could be done. I am looking at something which is readily available surplus and simple to install and maintain, but would still provide both peakshaving/parallel operation with the grid and backup power when the grid failed.

This is all a concept except for the fact that I have plenty of digester feedstock on the farm :>) and several engine generator sets already setup for backup use. I am well aware of the difficulty of making any profit selling electricity to the utility especially when we have a low power cost $0.06/kwh retail and a relatively small monthly demand if we could net meter. Even if I could get my power for free (no ownership and maintenence costs), I couldn't afford to spend the kind of money which is usually spent on these setups when they are built with grants by engineers.
Probably why most of these setups haven't persisted, among other reasons.

If I was near any natural gas pipelines, I would probably have more potential for making it fly scrubbing the gas to pipeline quality and forgetting the generator.

I have never thought of dc to ac for an intertie. Would the power quality be a problem for the utility?

Jim
Reply With Quote
  #5  
Old 02-25-2005, 11:38:03 PM
Wayne 440 Wayne 440 is offline
Registered
 
Join Date: Nov 2004
Location: Kentucky
Posts: 7,425
Thanks: 487
Thanked 6,894 Times in 3,214 Posts
Default Re: Induction generator from synchronous generator?

There are many "utility approved" inverters available. Here is a nice one, they are capable of paired operation for 120/240v.

http://www.wholesalesolar.com/produc...er/sw5548.html
Reply With Quote
  #6  
Old 02-26-2005, 12:04:44 AM
Jim McIntyre's Avatar
Jim McIntyre Jim McIntyre is offline
Subscriber
 
Join Date: Nov 2004
Location: Mooresville, Indiana, USA
Posts: 4,562
Thanks: 889
Thanked 2,732 Times in 1,560 Posts
Default Re: Induction generator from synchronous generator?

From what I read, it looks as if WRIMs operate with higher slip, even with zero external resistance, so they would be a tad less efficient than a SCIM.

An old engineering book (1947 Electrical Engineering, by Dawes Vol. 2) describes a "synchronous-induction motor," which it says is fundamentally a wound-rotor slip-ring induction motor. It starts with resistance in the rotor circuit, and then runs as a synchronous machine with a DC controller powering the rotor. The only downside it mentions is low efficiecy.

Not sure about where you live, but in Indiana, the economics of selling power into the grid just don't work out. Even if you operate the cogenerator to produce needed heat, it's still not economically attractive.

BTW, the Dawes book (published by McGraw-Hill) shows up on eBay frequently.
Reply With Quote
  #7  
Old 02-26-2005, 09:14:59 PM
Jeff Conner Jeff Conner is offline
Registered
 
Join Date: Nov 2004
Location: Syracuse, NY
Posts: 170
Thanks: 0
Thanked 132 Times in 27 Posts
Default Re: Induction generator from synchronous generator?

Induction generation is easily done. I have done it at home just for the heck of it. The old electric meters used to stop and go backwards when generating. The new ones don't. Most commercial induction generators are standard squirrel cage induction motors. All you have to do is bring them up to synchronous speed, excite them with nominal voltage through a contactor, and then increase engine speed slightly to take up the slip factor in the motor and monitor the output so as not to overload the windings. Generally only 3-4% over speed is required. The motor will spin the engine if the speed is dropped back to synchronous speed which will erase your electric revenue quickly.

Big mines in the west used to use their ore conveyor motors as induction generators to limit overspeed on downhill conveyors. Engine driven induction generators go way back, but were not popular until about the 1980's when cogeneration became popular. At first the untilities thought induction generators were novelties and didn't really know how to handle them. Then induction generation caught on for both digester and small cogeneration use. The utilities finally put their foot down, and now most every utility will require the same protective relay interface as with a standard self excited AC generator. That makes what should be a simple untility interface, very expensive. Utilities do this not because the induction generator is a danger to the system. They do it to discourage self generation and the loss of revenue it causes for them. There is relatively nothing that can go wrong if the power fails. The engine governor will maintain proper RPM to limit overspeed conditions. You need a very close regulating electronic governor for induction use. You should always take the induction generator off line during a power failure just in case someone should excite the line with a standby generator. The induction generator will place a very low power factor excitation load on the emergency generator zapping much of it's kva capacity unless it is much larger than the induction generator. The characteristic low power factor condition created by the induction generator is sometimes penalized by untilities requiring power factor correction at the generator. This then, also creates a potential for uncontrolled self excitation of the induction generator during a power outage if not disconnected from the grid and the power factor correcting capacitors.

Connecting a standard electric motor to an industrial engine is simple. A stout base is required to maintain alignment. There are flexible couplings that will bolt to the face of an SAE industrial engine flywheel. The center of the coupling has a female elastomeric isolated keyed steel hub that slips over the motor shaft. Normal radial and axial alignment is required, but is not difficult.

Digester gas as an engine fuel is another whole subject. Suffice to say, most healthy digesters produce about 60-65% methane by volume. The rest is inert gases and some real foul stuff that eats out engine bearings and valves. If the digestor gets a little indigestion (and they do), the methane output goes down and the engine suffers more. Fuel gas scrubbers are costly and require maintainance that is not cost effective on small systems. The best thing for large and small engines is to power ventilate the crankcase to draw out the crankcase fumes where most of the corrosive fumes enter the engine as exhaust blowby gases.

It is unfortunate that the utilities take such an adverserial attitude toward induction generation. However, with deregulation and most utilities getting out of generation, it is getting extremely difficult for small self generators to directly participate in selling electric power. Good luck.
Reply With Quote
Reply

Bookmarks


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

F o r u m Jump

Similar Threads Chosen at Random
Thread Thread Starter F o r u m Replies Last Post
Tamper Synchronous generator magneticanomaly Generators & Electric Motors General Discussion 6 09-08-2014 01:30:11 PM
Onan Corp. Alternating Current Synchronous Brushless Generator Model: 88-22040 Ben Clarke Onan Generators 0 01-25-2010 09:47:29 PM
Synchronous Motor = Generator? Jim McIntyre Generators & Electric Motors General Discussion 7 03-14-2005 01:59:55 PM
Synchronous Motor = Generator? K2 Generators & Electric Motors General Discussion 2 10-21-2004 12:46:59 AM
SIGNAL CORPS SYNCHRONOUS GENERATOR bob botsford Generators & Electric Motors General Discussion 0 01-27-2003 09:34:55 PM


Use "Ctrl" mouse wheel to change screen size.
All times are GMT -4. The time now is 07:31:35 PM.

Smokstak and Enginads site search!


All use is subject to our TERMS OF SERVICE
SMOKSTAK® is a Registered Trade Mark - A Community of Antique Engine Enthusiasts
Copyright © 2000 - 2019 by Harry Matthews P.O. Box 5612 - Sarasota, FL 34277