But, as a fine point, 3-phase transformers need to be connected with the correct phasing. Otherwise, any 3-phase induction motors will run backwards. Also, wye- and delta-connected xfmrs are out of phase no matter how either is connected, so they can never be paralleled.
For single-phase power xfmrs, it doesn't matter - unless, as Norm said, there are windings or xfmrs connected in series or parallel. For example, a buck-boost winding or two xfmrs paralleled to provide increased capacity.
Single-phase back-up generators should be wired so that they can never be paralleled with the utility's system, so the phasing of such a generator isn't important.
Doug
While we're off topic, back to my AT&T days, we had remote power stations in places for the L-3 co-axial cable, these were just like in the central offfices, complete with 3-phase AC motors and DC motors on the same shaft as an AC altenator. If the AC failed, the DC motors ran on the 152 Volt batteries I wrote of earlier. One day, in a place far away from where I was working, thank the Lord, the local power company asked to turn off the power for a while to do line work. The powers that be decided it would be ok to just let the plant switch to DC while the power was off, and return to AC when power was restored, that being the normal way the equipment operated. But, when the AC was restored, there were two phases reversed, and the 3-phase motors stopped and tried to reverse direction or something, and the motors started on DC, so they switched back and forth between DC and AC until the protective devices worked and took power off of everything, losing about 5500 toll circuits. Gave a new meaning to the expression "all hell broke loose". Offices hundreds of miles away were rerouting service, as I said we were lucky to be far enough not to get involved, except to the extent that our circuits got busier as the switching machines were routing calls wherever they could find an idle tandem circuit.
Lewis L.
By the way, connecting up generators to run the home in times of a power failure, some idiots parallel the generator with the incoming line, and the pole transformer works backward to put about 20 KV. on lines that are supposed to be dead, and several linemen have been killed this way.
LL
So how is it that I heard you can connect your back-up generator such that if you feed back into the power line .. and your meter runs backward, the power company is supposed to be paying you to provide power to them...??
This is what I've commonly often heard.
Residential meters do not respond to reactive power, so you can consume all of that you want, free. But reactive current does cause some I^2R loss in your home's wiring, which you do pay for.
Doug
::As I remember, about 1964 this story showed up in one of my EE classes. The prof. came to the next lab with a residential meter (single phase, no power factor correction), and demonstrated the meter registered current only, putting power back into the system did not reverse the meter rotation. Seems we were also unable to make the "unplug the meter and plug it back in 180 degrees off" method of cost cutting work. Hope my memories are accurate. Meters are a lot different now, mine has a transmitter to tell the "drive by" meter reader what the reading is.
:Just wandered thru a few google items and it appears meters can be made to run backwards. Gonna have to do some more investigation on this.
:Hi, Rick. Wattmeters and watthour meters measure real power. There is no need for any power factor correction.
:
:Residential meters do not respond to reactive power, so you can consume all of that you want, free. But reactive current does cause some I^2R loss in your home's wiring, which you do pay for.
:Doug
:
:::As I remember, about 1964 this story showed up in one of my EE classes. The prof. came to the next lab with a residential meter (single phase, no power factor correction), and demonstrated the meter registered current only, putting power back into the system did not reverse the meter rotation. Seems we were also unable to make the "unplug the meter and plug it back in 180 degrees off" method of cost cutting work. Hope my memories are accurate. Meters are a lot different now, mine has a transmitter to tell the "drive by" meter reader what the reading is.
::Just wandered thru a few google items and it appears meters can be made to run backwards. Gonna have to do some more investigation on this.
If you reverse just one pair, then a 180-deg offset comes into the picture. So, the wattmeter will compute VIcos(phi +- 180). And yes, the wattmeter will "try" to run backwards - and it would run backwards if it is a "net" meter, which some utilities allow for small customers with generation, particularly renewable (e.g., solar). But meters can be designed to prevent running backwards, and unless they are designed as a "net" meter, they probably won't have the same accuracy in both directions.
Depending on how a wattmeter is hooked up, it will display the power going "into" your house, or the power going "out" of the house. Of course, one is just the negative of the other.
It's analagous to a DC voltmeter. If you hook it up with a negative voltage to the red terminal, the needle will try to deflect backwards. Of course, an AC voltmeter or ammeter won't deflect backwards, because AC is unpolarized - but once you deal with both voltage and current waveforms, as an AC wattmeter does, then all bets are off.
By the way, 3-phase AC power can be measured with just two single-phase wattmeters. You can do a search on "2-wattmeter method." The math can get a little confusing.
But back to your question about installing a residential kWh meter "backwards." The meter is installed by the utility into an approved socket provided by the customer. The socket is such that there really isn't a way to install the meter backwards. You'd have to remove the meter and jigger the internals.
But once the meter reader noticed that the seal was broken, or after strange readings were picked up by the utility's billing computer, there would be big fireworks. Electric utilities call in "theft of service," and try to get felony convictions. They get much uglier than cable TV companies, I suspect.
I've heard of placing magnets on a mechanical meter to slow it down. I don't know if it works. Most new meters are solid state with microprocessors.
Doug
::Thanks Doug, How about the "turn the meter around to make it run backwards"? Can't recall the details, but it seems either way we did it,it ran the same direction. It was one he had grabbed out of a house destroyed by a tornado a few years earlier. Wouldn't let us take it apart.
:
I suppose one would work with the power company, and they would know you had a generator conneceted to the line. You would have to have the current the exact frequency, and phased correctly with the utility. Also, connecting AC generators in parallel is not a matter of hooking up your Honda and running the meter backward. On our aiplanes that paralled supplies, there was an automatic parallel circuit that checked everything and after paralleling tweaked the drives and the fields to balance the resistive and reactive loads. Actually, the two engine jobs don't parallel, they just tie the two systems into one generator if a generator fails, the old guys that did parallel, 727, L1011, had analogue computers and I was a digital guy, so I didn't deal much with paralleling. Actually, I was a radio guy that got into power generating when we got a surplus of radio guys, due to more reliable rado equipment.
Lewis
By connecting a backup generator to the power company grid can cause a hazzard to personnel working trying to restore the loss of power. The generator 120/240 volts can be stepped up to several thousand volts thru the line transformer which can cause injury or worse death to the ones who are out in the snow, ice, rain and lightning storms trying to restore power.
Radiodoc
*************
::
:::By the way, connecting up generators to run the home in times of a power failure, some idiots parallel the generator with the incoming line, and the pole transformer works backward to put about 20 KV. on lines that are supposed to be dead, and several linemen have been killed this way.
:::LL
::
::So how is it that I heard you can connect your back-up generator such that if you feed back into the power line .. and your meter runs backward, the power company is supposed to be paying you to provide power to them...??
::This is what I've commonly often heard.
:
:I suppose one would work with the power company, and they would know you had a generator conneceted to the line. You would have to have the current the exact frequency, and phased correctly with the utility. Also, connecting AC generators in parallel is not a matter of hooking up your Honda and running the meter backward. On our aiplanes that paralled supplies, there was an automatic parallel circuit that checked everything and after paralleling tweaked the drives and the fields to balance the resistive and reactive loads. Actually, the two engine jobs don't parallel, they just tie the two systems into one generator if a generator fails, the old guys that did parallel, 727, L1011, had analogue computers and I was a digital guy, so I didn't deal much with paralleling. Actually, I was a radio guy that got into power generating when we got a surplus of radio guys, due to more reliable rado equipment.
:
:Lewis
:
In the "old" days, a synchroscope was used. When the synchroscope's hand was pointing at 12 o'clock and wasn't revolving, then the two systems would be in phase, and the breaker could be closed. Acutally, the oncoming generator would be running a little fast, so it would pick up load when the breaker was closed - instead of being motorized.
Before a newly installed turbine generator is synchronized for the first time, the phasing and synchronizing controls are checked, double-checked, and tripple checked. A 600-MW generator that is connected out of phase would likely be destroyed.
Doug
:
:I suppose one would work with the power company, and they would know you had a generator conneceted to the line. You would have to have the current the exact frequency, and phased correctly with the utility. Also, connecting AC generators in parallel is not a matter of hooking up your Honda and running the meter backward. On our aiplanes that paralled supplies, there was an automatic parallel circuit that checked everything and after paralleling tweaked the drives and the fields to balance the resistive and reactive loads. Actually, the two engine jobs don't parallel, they just tie the two systems into one generator if a generator fails, the old guys that did parallel, 727, L1011, had analogue computers and I was a digital guy, so I didn't deal much with paralleling. Actually, I was a radio guy that got into power generating when we got a surplus of radio guys, due to more reliable rado equipment.
:
:Lewis
:
But the power company will prescribe all kinds of protective systems that you must install for their safety. Otherwise, they will not allow your generator to run in parallel with their system. This typically winds up being a costly feature and requires careful engineering, and wouldn't be justified for something like my little 5-kW, gasoline-fueled back-up generator.
Many power companies won't go along with running the meter backwards. They will want to install a separate meter for your generated power and pay you, say, 5 cents per kWh for it - and sell you the power you use at maybe 12 cents/kWh. They don't like buying power from you at retail rates, and I can't say as I blame them.
Doug
:
:So how is it that I heard you can connect your back-up generator such that if you feed back into the power line .. and your meter runs backward, the power company is supposed to be paying you to provide power to them...??
:This is what I've commonly often heard.
:
Speaking of phasing alternators, I've read that it's a very tedious job. I've always wondered how they were able to keep alternators phased, since the electric companies do it all the time, and are able to mingle eachother's wires. They've been doing this for 100 years, too, and I wonder how on earth they did it 100 years ago with such accuracy.
T.
It allows switching each of my critical circuits between Line and Generator, while my main breaker to the utility remains closed. It is physically impossible to connect the utility to the generator. This is the kind of switch required by the National Electrical Code.
During an outage, my non-critical circuits remain connected to the utility, so when the utility power is restored, the non-critical lights come back on.
About once every six months, I hook up the generator and exercise it under load.
:If you do want to plug your generator into your whole house, you should disconnect the main breaker. Doing so will prevent power from going to the transformer outside, which might otherwise kill a lineman. Also, unless you wire your generator in at the box, you will only be able to conduct from the generator what the circuit it is plugged into is capable of conducting. If you plug it into a 15 ampere circuit, its breaker will blow if more is consumed. That's probably good in most cases, though, anyway, since most gasoline powered generators that people own aren't that big. The larger permanent jobs, of course, are usually much more powerful. For home use, though, all that you really need is enough to drive a refrigerator, furnace fan (hopefully not the AC), and a few lamps, so 15 amperes should be enough. If you had a 20 ampere generator, I guess you'd have to plug into a 20 ampere outlet, or otherwise wire in at the box.
:
:T.
The Atlanta area has jillions of trees, and about once a year or two, we get an ice storm which devistates the overhead power wiring. Several people I have worked with bought a 6KW or so Home Depot generator, and connected a dryer plug to the output. When the ice strom hits and power fails, they turn off the AC main breaker, and plug the generator into the dryer outlet. By turning off the branch breakers, and turning them on one at a time (heavy loads like fridge first), they can lead a normal life with heat, ice water, tv, computer, stereo, etc, etc. Of course, these guys are radio and electrical techs, so they know how to do this safely. I use a 225 Watt inverter, and get most of the advantages of electricity, who need a fridge when it's below freezing outside? I just have to remember not to run the truck battery down with the inverter.
Speaking of power failures, the wife and I were in Fort Lauderdale one vacation, waiting on a DC-8 to get home, when the power failed for the entire airport. The crew started the auxillary power unit (APU), and the airplane came back to life, complete with air conditioning and everything. A woman passenger demanded to be taken off of "this defective airplane" despite the captain, the agents and everybody trying to tell her that the plane was the *only* thing around that had electricity. She got off of the plane, and we stuck around drinking Delta's booze until the airport got power back and we came home. South Florida is kind of notorious for having power outages as they are not connected to the gird in but one direction, North, so on hot summer days power failures are a daily occurence some times.
Lewis
The plug into the dryer recepticle isn't kosher either. When the plug is pulled, the energized 240-V prongs are exposed. Another concern is that many 240-V dryer circuits are 3-wire rather than 4-wire, so there is not a separate safety ground. The neutral is used as a ground, which is no longer permitted for new installations. It's a particularly bad idea for a generator connection, since loads may not be balanced between the two 120-V legs, causing large neutral currents.
Much better to wire in a transfer switch like this: http://www.gen-tran.com/eshop/10Expand.asp?ProductCode=30310V Installation is easy.
Doug
:
:The Atlanta area has jillions of trees, and about once a year or two, we get an ice storm which devistates the overhead power wiring. Several people I have worked with bought a 6KW or so Home Depot generator, and connected a dryer plug to the output. When the ice strom hits and power fails, they turn off the AC main breaker, and plug the generator into the dryer outlet. By turning off the branch breakers, and turning them on one at a time (heavy loads like fridge first), they can lead a normal life with heat, ice water, tv, computer, stereo, etc, etc. Of course, these guys are radio and electrical techs, so they know how to do this safely.
:
:Lewis
Hey, I didn't say I agreed with the procedure..I used my money to buy a gas fireplace, what with that and the gas grill and my inverter, we can be nice and cozy and fed, without having to worry about a main circuit being the only thing between me and a lawsuit.
Lewis
:
::
::The Atlanta area has jillions of trees, and about once a year or two, we get an ice storm which devistates the overhead power wiring. Several people I have worked with bought a 6KW or so Home Depot generator, and connected a dryer plug to the output. When the ice strom hits and power fails, they turn off the AC main breaker, and plug the generator into the dryer outlet. By turning off the branch breakers, and turning them on one at a time (heavy loads like fridge first), they can lead a normal life with heat, ice water, tv, computer, stereo, etc, etc. Of course, these guys are radio and electrical techs, so they know how to do this safely.
::
::Lewis
T>
When a generator is initially installed, it's phasing is determined by how it is wired and it's direction of rotation. After that, there's really no way to change the phasing.
Another issue is synchronization. In the U.S. electric systems, there will be thousands of synchronous generators, all operating in parallel. For example, most all utility generators east of the Rockies are all running in parallel. Except for very rare fault conditions, it is neigh impossible for any one generator to slip out of synchronization, and power plant operators couldn't do it even if they tried. There isn't really much tedium involved, except for system operators to manage power flows between areas and to keep the system frequency at 60 Hz. Most of that work is controlled by computers.
An analogy is team of 50 horses, all pulling the same plow. If any one horse lets up or pulls harder, the plow might slow down or speed up slightly, but the team still stays together as if in lockstep.
Doug
:Speaking of phasing alternators, I've read that it's a very tedious job. I've always wondered how they were able to keep alternators phased, since the electric companies do it all the time, and are able to mingle eachother's wires. They've been doing this for 100 years, too, and I wonder how on earth they did it 100 years ago with such accuracy.
:
:T.
T.
I watched the Southern Bell guys syncronize the two Diesel generators we had in the central office, the used three light bulbs. As the slave generator was brought up to speed, the bulbs blinked in an A-B-C pattern, which slowed down as the proper speed was reached, and when all were dark, he threw the parallel switch and all lights stayed dark. Then he switched the whole building to Diesel power, and tweaked the governers for equal KVA output. I didn't really want to know too much about the process, they might make me do it sometime, and that thought scared me to death.
Lewis
However, I don't think it would readily show whether the incoming (unloaded) generator is running fast or slow, like a synchroscope does.
With the lamps, the procedure would be to get the two gensets running precisely in synch (the lamps not lighted). Then, with the governor control on the incoming generator, speed it up slightly, which will cause the lamps to alternate light and dark again. Then when the the two generators swing into phase, close the breaker on the incoming generator. This procedure ensures that the incoming generator is running slightly fast, which will cause it to immediately pick up load as soon as the breaker is closed, rather than become motorized.
Doug
Experienced operators will actually close the breaker a few degrees ahead of synch - anticipating the delay time for the breaker to close. They aim for the breaker to close exactly at synch, which then reduces wear and tear on the breaker.
:
:I watched the Southern Bell guys syncronize the two Diesel generators we had in the central office, the used three light bulbs. As the slave generator was brought up to speed, the bulbs blinked in an A-B-C pattern, which slowed down as the proper speed was reached, and when all were dark, he threw the parallel switch and all lights stayed dark. Then he switched the whole building to Diesel power, and tweaked the governers for equal KVA output. I didn't really want to know too much about the process, they might make me do it sometime, and that thought scared me to death.
:Lewis
:
....And, it is very possible for an alternator to get out of phase with others. All it has to do is speed up or slow down from turbine steam variation. If a particular winding is producing negative energy while other similar windings in other alternators are producing positive energy, it'll be out of phase with the others. This will happen if the alternator's rotor isn't in the same position as the others', due to being out of sync. ....That's the thing I'm wondering about. How do they keep everything so precise? Does the simple phenomenon of motoring do this? What if the system somehow gives the turbine just a little too much steam for a moment? The alternator will speed up and get out of phase with the others. How can they so perfectly lock them? Even slight speed differences can make for drastic power losses.
T.
The phase angle between generators varies, too - but not enough to get out of synch.
This all happens without too much human intervention.
Doug
:I assume that you are saying that if an alternator slows down slightly, others will motor it and keep it at speed? ....Otherwise it is very possible that alternators could get out of sync. If the above is happening, though, then I guess I understand how the power company can keep thousands of alternators in sync. once connected. AMAZING!
:
:....And, it is very possible for an alternator to get out of phase with others. All it has to do is speed up or slow down from turbine steam variation. If a particular winding is producing negative energy while other similar windings in other alternators are producing positive energy, it'll be out of phase with the others. This will happen if the alternator's rotor isn't in the same position as the others', due to being out of sync. ....That's the thing I'm wondering about. How do they keep everything so precise? Does the simple phenomenon of motoring do this? What if the system somehow gives the turbine just a little too much steam for a moment? The alternator will speed up and get out of phase with the others. How can they so perfectly lock them? Even slight speed differences can make for drastic power losses.
:
:T.
::T.
Let's take a Boeing 727 electrical system, for example, it has parallel electrical systems and would be a good example of how this stuff works.
First, the generator:
It is a Westinghouse generator weighing 85 pounds. It would fit tightly into a trash can like you have next to your desk. I think its max output is 195 KVA, but I may be wrong. The contactors that it connects to are tested at 175 Amps. The secondary of the transformer in the tester is made of 1/2" copper water pipe, flattened and wrapped with fiberglass tape. It is a brushless generator, with the first field on the back, a 3 phase alternator turing inside that, six diodes on the shaft that connects to the rotating field which rectify the AC, and it rotates in a stator that has the actual windings where the AC is produced. The rotor weight 45 pounds. You can tell the people that work on genertors by their similarity to Popeye in the forearms department.
The generator turns at 6000 RPM. This is provided by a device designed in hell called a constant speed drive (CSD). It is a round device that is bolted on to the drive end of the generator.
There is an accessory gear box on the engine that is driven by the shaft that runs the compressor blades. The fuel pump, hydraulic pump, CSD, and starter connect to this gearbox. The output speed varies greatly with engine speed, thus the need for the CSD. Inside this thing are a governer, a hydraulic motor, and a differentioal gear not unlike what is on the rear axle of a car. Now, this thing works like this: The hydraulic motor is one input of the differential, while the engine is the other. The output turns the generator. The governer is a thing with flyweights, like they used on steam engines 100 years ago. The weights control a vertical shaft, which directs oil to the motor. The motor can run fast or slow in either direction, depending on the governer weights. If the weights are too low, the shaft is positioned to turn the motor in a direction that adds to the engine shaft speed, and produces 6000 RPM output. If the weights are too high, the motor runs the other way and subtracts from the engine to produce 6000 RPM. At cruise, the input shaft is close to 6000 RPM, so the motor doesn't do very much. If the airplane has two engines, it doesn't parallel and works with two separate electrical systems, with provisions to disconnect a generator and combine the two in case of engine or generator failure. So much for two engine airplanes.
Airplanes with more than two engines parallel the systems under normal conditions. The generators still connect to a contactor called a generator breaker, and then to the appropriate bus (AC 1, AC 2, or AC 3). There is another set of contactors connected to the buses, called bus tie breakers. When in parallel, all six breakers are closed, and the airplane has one electrical system with three generators in parallel. You can open the generator breakers and feed external power or auxillary power unit (APU) power into the tie bus and power the airplane buses through the tie breakers on the ground (the APU on a 727 can't be used in flight). I need to run an errand right now, I'll talk more about paralleling later.
For now,
Lewis
::T.
Let's take a Boeing 727 electrical system, for example, it has parallel electrical systems and would be a good example of how this stuff works.
First, the generator:
It is a Westinghouse generator weighing 85 pounds. It would fit tightly into a trash can like you have next to your desk. I think its max output is 195 KVA, but I may be wrong. The contactors that it connects to are tested at 175 Amps. The secondary of the transformer in the tester is made of 1/2" copper water pipe, flattened and wrapped with fiberglass tape. It is a brushless generator, with the first field on the back, a 3 phase alternator turing inside that, six diodes on the shaft that connects to the rotating field which rectify the AC, and it rotates in a stator that has the actual windings where the AC is produced. The rotor weight 45 pounds. You can tell the people that work on genertors by their similarity to Popeye in the forearms department.
The generator turns at 6000 RPM. This is provided by a device designed in hell called a constant speed drive (CSD). It is a round device that is bolted on to the drive end of the generator.
There is an accessory gear box on the engine that is driven by the shaft that runs the compressor blades. The fuel pump, hydraulic pump, CSD, and starter connect to this gearbox. The output speed varies greatly with engine speed, thus the need for the CSD. Inside this thing are a governer, a hydraulic motor, and a differentioal gear not unlike what is on the rear axle of a car. Now, this thing works like this: The hydraulic motor is one input of the differential, while the engine is the other. The output turns the generator. The governer is a thing with flyweights, like they used on steam engines 100 years ago. The weights control a vertical shaft, which directs oil to the motor. The motor can run fast or slow in either direction, depending on the governer weights. If the weights are too low, the shaft is positioned to turn the motor in a direction that adds to the engine shaft speed, and produces 6000 RPM output. If the weights are too high, the motor runs the other way and subtracts from the engine to produce 6000 RPM. At cruise, the input shaft is close to 6000 RPM, so the motor doesn't do very much. If the airplane has two engines, it doesn't parallel and works with two separate electrical systems, with provisions to disconnect a generator and combine the two in case of engine or generator failure. So much for two engine airplanes.
Airplanes with more than two engines parallel the systems under normal conditions. The generators still connect to a contactor called a generator breaker, and then to the appropriate bus (AC 1, AC 2, or AC 3). There is another set of contactors connected to the buses, called bus tie breakers. When in parallel, all six breakers are closed, and the airplane has one electrical system with three generators in parallel. You can open the generator breakers and feed external power or auxillary power unit (APU) power into the tie bus and power the airplane buses through the tie breakers on the ground (the APU on a 727 can't be used in flight). I need to run an errand right now, I'll talk more about paralleling later.
For now,
Lewis
....Or, do they not connect power stations together? Does more than one power station feed a city section at one time, or do switching networks switch to the appropriate one, and never more than one? In other words, if what I already wrote didn't make sense, are power plants ever run in parallel, or are they always run separately, with a network of switches for connecting to other sources if one should ever fail? If power plants are run in parallel, how on earth do they keep the alternators (AC generators) in phase with eachother at all of the different plants? If the alternators in one plant run ever so slightly slower than those in another plant, phase matching won't occur properly, if at all.
Thomas
The phasing is permanently established when a generator is initially installed.
When 3-phase transmission or distribution lines are paralleled, of course the phasing needs to be correct - or there will be a shower of sparks when the lines are energized. But again, once things are initially hooked up, then you're done.
Switches are not installed to allow reversing the phase sequence.
Doug
:......But how do multiple alternating current power generating stations link their wires together and keep all alternators generating in phase?????????? If two power plants connect wires together to feed a city, and both power plants' alternators are not phased exactly the same (each winding producing the same polarity as similar windings at the other power plant), current cancellation will occur.
:
:....Or, do they not connect power stations together? Does more than one power station feed a city section at one time, or do switching networks switch to the appropriate one, and never more than one? In other words, if what I already wrote didn't make sense, are power plants ever run in parallel, or are they always run separately, with a network of switches for connecting to other sources if one should ever fail? If power plants are run in parallel, how on earth do they keep the alternators (AC generators) in phase with eachother at all of the different plants? If the alternators in one plant run ever so slightly slower than those in another plant, phase matching won't occur properly, if at all.
:
:Thomas
(Well, there will be a few degrees difference depending on the flow of power between areas, so it's not precisely simultaneous.)
All interconneced utilities observe the same convention for Phases A, B and C. They are not just arbitrary letters, but the sequence and phasing for the entire electric system.
If, hypothetically, a generator "tried" to slip out of phase during normal operation, there would such massive forces applied to it by the rest of the system that it would probably be blown to bits (unless protective devices came into play). But before that happened, the generator would just naturally pick up or shed load, causing it to stay in phase.
The thing that keeps the system together is its inherent stability. If an interconnected generator "tries" to speed up (for example, by putting more steam through the turbine), it just hogs more of the system load, which counteracts. It's negative feedback.
Doug
Synchronous machines have a separately excited DC field on the rotor. Synchronous motors run a fixed speed, depending on the AC frequency applied to the stator.
Synchronous generators put out an AC frequency which is fixed by the speed of the rotor. Once the generator is paralleled with the system, that rotor cannot change speed. Period.
Doug
T.
It is a Westinghouse generator weighing 85 pounds. It would fit tightly into a trash can like you have next to your desk. I think its max output is 195 KVA, but I may be wrong. The contactors that it connects to are tested at 175 Amps. The secondary of the transformer in the tester is made of 1/2" copper water pipe, flattened and wrapped with fiberglass tape. It is a brushless generator, with the first field on the back, a 3 phase alternator turing inside that, six diodes on the shaft that connects to the rotating field which rectify the AC, and it rotates in a stator that has the actual windings where the AC is produced. The rotor weight 45 pounds. You can tell the people that work on genertors by their similarity to Popeye in the forearms department.
The generator turns at 6000 RPM. This is provided by a device designed in hell called a constant speed drive (CSD). It is a round device that is bolted on to the drive end of the generator.
There is an accessory gear box on the engine that is driven by the shaft that runs the compressor blades. The fuel pump, hydraulic pump, CSD, and starter connect to this gearbox. The output speed varies greatly with engine speed, thus the need for the CSD. Inside this thing are a governer, a hydraulic motor, and a differentioal gear not unlike what is on the rear axle of a car. Now, this thing works like this: The hydraulic motor is one input of the differential, while the engine is the other. The output turns the generator. The governer is a thing with flyweights, like they used on steam engines 100 years ago. The weights control a vertical shaft, which directs oil to the motor. The motor can run fast or slow in either direction, depending on the governer weights. If the weights are too low, the shaft is positioned to turn the motor in a direction that adds to the engine shaft speed, and produces 6000 RPM output. If the weights are too high, the motor runs the other way and subtracts from the engine to produce 6000 RPM. At cruise, the input shaft is close to 6000 RPM, so the motor doesn't do very much. If the airplane has two engines, it doesn't parallel and works with two separate electrical systems, with provisions to disconnect a generator and combine the two in case of engine or generator failure. So much for two engine airplanes.
Airplanes with more than two engines parallel the systems under normal conditions. The generators still connect to a contactor called a generator breaker, and then to the appropriate bus (AC 1, AC 2, or AC 3). There is another set of contactors connected to the buses, called bus tie breakers. When in parallel, all six breakers are closed, and the airplane has one electrical system with three generators in parallel. You can open the generator breakers and feed external power or auxillary power unit (APU) power into the tie bus and power the airplane buses through the tie breakers on the ground (the APU on a 727 can't be used in flight).
On the two-engined planes, the APU, which by the way, is a little gas turbine that burns engine fuel and also provides air for air conditioning on the ground, and makes the avionics guy very popular with guys working on an airplane outside on a summer afternoon in Atlanta, for he is one of the few that could start the APU and get air conditioning. The APU can replace an ailing generator, on a two engine plane, and on the L-1011 could be paralleled into the rest of the electrical system.
Ok, so we have a generator turning at 6,000 RPM, driven by a CSD to keep that constant speed. Remember the little flywheel weights in the CSD. We'll talk about them later. Now, let's go to the Voltage Regulator and the Load Controller (VR) and (LC). The VR is real simple. It is a small box you can hold in one hand, and replaces a monster that used a magnetic amplifier, which must have been designed by the devil himself, or so thinks anyone that's ever had to work on one. The transistor VR has a power tranformer, and a rectifier circuit for itself and another for the generator field. The generator has little magnets in the rotating field, so whenever it is turning it has about a 12 VAC output. This 12V. starts a booting up process when the field switch is closed, something a car alternator cannot do without external power. Up to about 100 Volts, the field transistor is fully on. Then a full wave three phase unfiltered recifier in the VR is producing enough 2400 Hz. output (three phase times full wave equals 6, times 400 Hz. equals 2400 Hz.) to start turning off the field transistor 2400 times a second, producing a square wave output. When the generator is connected to a load, the duty cycle of this square wave varies with the load, to provide a constant 115 Volts. You can see this on a test bench, in real life the square wave looks the same with or without load to me. There is another thing the VR does, it gets an input from a CT, and balances the reactive load, something nobody was ever able to explain exactly how.
There is also a resistive load division, this also uses the output of CTs, and this is the only time one generator knows what the others are doing. The load CT burden resistors (load resistors to us) are connected in series, with the three controller inputs connected across their respective resistor, therefore they are more or less in series also. There is a multivibrator in the load controller that produces a square wave, that is coonnected to output transistors. The outputs are connected to a coil of wire above the rotating flyweights (remember them) in the CSD. The flyweights are magnetized, and the coil attracts or repels them according to the square wave symmetry, and causes the CSD to provide more or less drive to the generator. The symmetry of the square wave is affected greatly by the load on its generator, and less so by the other two generators. In less time than it takes to explain, the generators drive the LCs, the LCs tweak the CSDs, the drive of the generators is adjusted until all three generators share the same load. (Pilot report: #X generator hogs electrical load. Corrective action: Change output transistors in #X load controller. Goof off for a while. Act like you did something brilliant to impress boss.) The VR has an input from the output CT of the generator. This is 90 degrees out of phase with the Voltage. Through a method never quite explained to me, this lowers the field drive, while the CSD increases the shaft drive, and somehow this makes the generator take on more or less of the reactive load. It always worked on the test bench, I never had to work on it, so I don't know very much about it. So now we have three happy generators, sharing the load, connected in parallel. Now, let's talk about how they got thay way, and how they stay that way. All the generators have a Generator Control Unit (GCU). Now days, the VR is inside this guy, and they come in two types, those that work on parallel systems and those that work on independant systems. The independant GCUs check the following. Over or under Voltage and frequency (OV UV OF UF), generator overcurrent (OC), and one I haven't mentioned, differential current protection (DP). There is another set of CTs that live apart from the rest. One half of this pair in in the three wires that connect the generator to airframe ground. The other sits as close as you can get it to the contactor that connects the genetator to the rest of the airplane. These are wired to cancel each other out, so usually the output is more or less zero, regardless of load on the generator. Any time there is an output, whether or not the generator is in use, if a difference of more than a very few Amps is noted, there is a short in the big wires that connect the genetator to the airplane and the field is opened and the generator is isolated. Period. Game, set, match. I once got an attaboy when I got two GCU boxes in the shop, first "generator trips off line", second "bad out of stock" These were digital boxes, and kept a record of what they did in memory. If the idiot mechanic had checked and asked the box what was up, he would have learned, like I did in the shop, that the idiot pilot had reset over 150 DP faults in flight. We grounded the plane and found a clamp holding the generator leads had broken inside of the leading edge of the wing, and the leads were banging up against an important structural member, had worn through the insulation, and were arc welding through the structure of the wing. I took my attaboy down to the break area, combined it with a quarter, and got an awful tasting cup of coffee from the vending machine. Still, I imagine I was having a better afternoon with my awful coffee than the pilot and mechanic were having with thier bosses. So, the GCU can trip off a generator for a shorted or open regulator transistor (OV,UV), Overcurrent (OC), bad
CSD (UF,OF). Let me rest my fingers a while, and I'll talk about generators in parallel, which started this whole rambling of mine.
Lewis
One that's always baffled me is the 3rd brush generator used in older automobiles. Never had one, so I don't know how well they work. I heard that they're not that good.
T.
It is a Westinghouse generator weighing 85 pounds. It would fit tightly into a trash can like you have next to your desk. I think its max output is 195 KVA, but I may be wrong. The contactors that it connects to are tested at 175 Amps. The secondary of the transformer in the tester is made of 1/2" copper water pipe, flattened and wrapped with fiberglass tape. It is a brushless generator, with the first field on the back, a 3 phase alternator turing inside that, six diodes on the shaft that connects to the rotating field which rectify the AC, and it rotates in a stator that has the actual windings where the AC is produced. The rotor weight 45 pounds. You can tell the people that work on genertors by their similarity to Popeye in the forearms department.
The generator turns at 6000 RPM. This is provided by a device designed in hell called a constant speed drive (CSD). It is a round device that is bolted on to the drive end of the generator.
There is an accessory gear box on the engine that is driven by the shaft that runs the compressor blades. The fuel pump, hydraulic pump, CSD, and starter connect to this gearbox. The output speed varies greatly with engine speed, thus the need for the CSD. Inside this thing are a governer, a hydraulic motor, and a differentioal gear not unlike what is on the rear axle of a car. Now, this thing works like this: The hydraulic motor is one input of the differential, while the engine is the other. The output turns the generator. The governer is a thing with flyweights, like they used on steam engines 100 years ago. The weights control a vertical shaft, which directs oil to the motor. The motor can run fast or slow in either direction, depending on the governer weights. If the weights are too low, the shaft is positioned to turn the motor in a direction that adds to the engine shaft speed, and produces 6000 RPM output. If the weights are too high, the motor runs the other way and subtracts from the engine to produce 6000 RPM. At cruise, the input shaft is close to 6000 RPM, so the motor doesn't do very much. If the airplane has two engines, it doesn't parallel and works with two separate electrical systems, with provisions to disconnect a generator and combine the two in case of engine or generator failure. So much for two engine airplanes.
Airplanes with more than two engines parallel the systems under normal conditions. The generators still connect to a contactor called a generator breaker, and then to the appropriate bus (AC 1, AC 2, or AC 3). There is another set of contactors connected to the buses, called bus tie breakers. When in parallel, all six breakers are closed, and the airplane has one electrical system with three generators in parallel. You can open the generator breakers and feed external power or auxillary power unit (APU) power into the tie bus and power the airplane buses through the tie breakers on the ground (the APU on a 727 can't be used in flight).
On the two-engined planes, the APU, which by the way, is a little gas turbine that burns engine fuel and also provides air for air conditioning on the ground, and makes the avionics guy very popular with guys working on an airplane outside on a summer afternoon in Atlanta, for he is one of the few that could start the APU and get air conditioning. The APU can replace an ailing generator, on a two engine plane, and on the L-1011 could be paralleled into the rest of the electrical system.
Ok, so we have a generator turning at 6,000 RPM, driven by a CSD to keep that constant speed. Remember the little flywheel weights in the CSD. We'll talk about them later. Now, let's go to the Voltage Regulator and the Load Controller (VR) and (LC). The VR is real simple. It is a small box you can hold in one hand, and replaces a monster that used a magnetic amplifier, which must have been designed by the devil himself, or so thinks anyone that's ever had to work on one. The transistor VR has a power tranformer, and a rectifier circuit for itself and another for the generator field. The generator has little magnets in the rotating field, so whenever it is turning it has about a 12 VAC output. This 12V. starts a booting up process when the field switch is closed, something a car alternator cannot do without external power. Up to about 100 Volts, the field transistor is fully on. Then a full wave three phase unfiltered recifier in the VR is producing enough 2400 Hz. output (three phase times full wave equals 6, times 400 Hz. equals 2400 Hz.) to start turning off the field transistor 2400 times a second, producing a square wave output. When the generator is connected to a load, the duty cycle of this square wave varies with the load, to provide a constant 115 Volts. You can see this on a test bench, in real life the square wave looks the same with or without load to me. There is another thing the VR does, it gets an input from a CT, and balances the reactive load, something nobody was ever able to explain exactly how.
There is also a resistive load division, this also uses the output of CTs, and this is the only time one generator knows what the others are doing. The load CT burden resistors (load resistors to us) are connected in series, with the three controller inputs connected across their respective resistor, therefore they are more or less in series also. There is a multivibrator in the load controller that produces a square wave, that is coonnected to output transistors. The outputs are connected to a coil of wire above the rotating flyweights (remember them) in the CSD. The flyweights are magnetized, and the coil attracts or repels them according to the square wave symmetry, and causes the CSD to provide more or less drive to the generator. The symmetry of the square wave is affected greatly by the load on its generator, and less so by the other two generators. In less time than it takes to explain, the generators drive the LCs, the LCs tweak the CSDs, the drive of the generators is adjusted until all three generators share the same load. (Pilot report: #X generator hogs electrical load. Corrective action: Change output transistors in #X load controller. Goof off for a while. Act like you did something brilliant to impress boss.) The VR has an input from the output CT of the generator. This is 90 degrees out of phase with the Voltage. Through a method never quite explained to me, this lowers the field drive, while the CSD increases the shaft drive, and somehow this makes the generator take on more or less of the reactive load. It always worked on the test bench, I never had to work on it, so I don't know very much about it. So now we have three happy generators, sharing the load, connected in parallel. Now, let's talk about how they got thay way, and how they stay that way. All the generators have a Generator Control Unit (GCU). Now days, the VR is inside this guy, and they come in two types, those that work on parallel systems and those that work on independant systems. The independant GCUs check the following. Over or under Voltage and frequency (OV UV OF UF), generator overcurrent (OC), and one I haven't mentioned, differential current protection (DP). There is another set of CTs that live apart from the rest. One half of this pair in in the three wires that connect the generator to airframe ground. The other sits as close as you can get it to the contactor that connects the genetator to the rest of the airplane. These are wired to cancel each other out, so usually the output is more or less zero, regardless of load on the generator. Any time there is an output, whether or not the generator is in use, if a difference of more than a very few Amps is noted, there is a short in the big wires that connect the genetator to the airplane and the field is opened and the generator is isolated. Period. Game, set, match. I once got an attaboy when I got two GCU boxes in the shop, first "generator trips off line", second "bad out of stock" These were digital boxes, and kept a record of what they did in memory. If the idiot mechanic had checked and asked the box what was up, he would have learned, like I did in the shop, that the idiot pilot had reset over 150 DP faults in flight. We grounded the plane and found a clamp holding the generator leads had broken inside of the leading edge of the wing, and the leads were banging up against an important structural member, had worn through the insulation, and were arc welding through the structure of the wing. I took my attaboy down to the break area, combined it with a quarter, and got an awful tasting cup of coffee from the vending machine. Still, I imagine I was having a better afternoon with my awful coffee than the pilot and mechanic were having with thier bosses. So, the GCU can trip off a generator for a shorted or open regulator transistor (OV,UV), Overcurrent (OC), bad
CSD (UF,OF). Let me rest my fingers a while, and I'll talk about generators in parallel, which started this whole rambling of mine.
Lewis
You may want to check out:
http://www.allaboutcircuits.com/vol_6/chpt_5/19.html
This may not be exactly off topic but it has to be shifted to the left considerably.
Radiodoc
***********
:This has nothing to do with antique radios or Ian's two current threads.
:
:But, as a fine point, 3-phase transformers need to be connected with the correct phasing. Otherwise, any 3-phase induction motors will run backwards. Also, wye- and delta-connected xfmrs are out of phase no matter how either is connected, so they can never be paralleled.
:
:For single-phase power xfmrs, it doesn't matter - unless, as Norm said, there are windings or xfmrs connected in series or parallel. For example, a buck-boost winding or two xfmrs paralleled to provide increased capacity.
:
:Single-phase back-up generators should be wired so that they can never be paralleled with the utility's system, so the phasing of such a generator isn't important.
:Doug