Removing impedance protection from MOEPED #3

There are not too many turns in it, in reality (despite theory) an asynchronous motor cannot be a generator.Sorry to be the messenger of bad news, but that's it.That's a reason why (I think so) wind-generators coupled to the grid in fact work as motors, dissipating power than producing it as generators.To produce ac, you need a *synchronous generator*, found everywhere from small heads (650 W) to the largest one, 2000 MVA, operated in a nuclear power station.Or, a dc *generator* (not motor).Either with shunt, series or compound excitation.Usually it's shunt.

-- Tzortzakakis Dimitriïs major in electrical engineering, freelance electrician FH von Iraklion-Kreta, freiberuflicher Elektriker dimtzort AT otenet DOT gr Ï "DGoncz 22044-0394" Ýãñáøå óôï ìÞíõìá news:Gkdxd.4212$Qk5.104@lakeread04...

sci.electronics.repair

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Most grid connected large windmills use induction motor/generators as a simple and inexpensive way to generate power. I worked for one of the companies in the Altamont Pass in California who had about 1800 100 KW machines by the time I left. It is quite impressive watching those power meters turn in the 'correct' direction when the machine comes on line.

Earle Rich Mont Vernon, NH

"Dimitrios Tzortzakakis" wrote in message news:cq6i46$eo2$ snipped-for-privacy@usenet.otenet.gr...

Sorry, but you are wrong. Asynchronous induction motors can be used as generators. They have to be driven at a speed higher than the synchronous speed. The winding must be excited from an external source of reactive current (the utility line or capacitor bank will do). They are much harder to regulate/control than synchronous when in an isolated 'island'. But when connected to a grid they can work quite well.

And a DC motor can often be used as a DC generator as well. In fact, many large un-interruptible power systems use DC machines that act as generators to charge/float station batteries normally, yet can instantly change roles to act as motors to drive the AC generator from the batter when the normal power is lost.

daestrom

-- Tzortzakakis Dimitriïs major in electrical engineering, freelance electrician FH von Iraklion-Kreta, freiberuflicher Elektriker dimtzort AT otenet DOT gr Ï "daestrom" Ýãñáøå óôï ìÞíõìá news:ZwCxd.77909$ snipped-for-privacy@twister.nyroc.rr.com...

That's what the theory says anyway.But here we have many problems from these "wind generators" coupled to the grid;lights "flickering";grid instability;many power outages.

Yes, of course but they don't have such great efficiency when working on the opposite role.A generator has the best efficiency when used as a generator, and a motor when used as a motor.

-- Tzortzakakis Dimitri?s major in electrical engineering, freelance electrician FH von Iraklion-Kreta, freiberuflicher Elektriker dimtzort AT otenet DOT gr ? "klasspappa[remove]" ?????? ??? ?????? news:pWBxd.124858$ snipped-for-privacy@newsc.telia.net...

I just run through the book of electrical machines I, one of my old subjects, and it doesn't mention asynchronous generators at all.It only mentions alternators.If one generator needs applying power to it to generate electricity, it loses the purpose of being a generator.The proof of this is our friend's experience;in his post he mentions the asynchronous motor when operated as a generator has an output of only 0.1 V RMS, which is useless for any purpose at all.In no lab have we had a project of converting a motor to a generator;always in motors and generators the apparatus was separate.Talking about asynchronous generators.

"Dimitrios Tzortzakakis" wrote in message news:cq9b7p$ev0$ snipped-for-privacy@usenet.otenet.gr...

Grid instability and 'flickering' is not inherent to a particular machines design. It is more a case of poorly designed grid. When adding any new generation to a grid, proper studies must be done on its impact to the existing grid. You don't just 'slap' another generator onto a grid if it's output is any appreciable percentage of the local load/existing-generation. Regardless of its internal design or prime mover.

You could add a steam turbine synchronous generator of similar size onto that sort of grid and still have instability, 'flickering' and many power outages. Adjust the governor or line-compensation incorrectly and it will drag the local grid all over the place causing all sorts of problem.

Nonsense. A properly designed machine will work equally well in either mode. Have seen many large (300 - 500 kw) machines used for just this sort of thing.

daestrom

"Dimitrios Tzortzakakis" wrote in message news:cq9bu3$hr4$ snipped-for-privacy@usenet.otenet.gr...

Then you need to find another book. An asyncrhonous generator needs an external source for reactive power / excitation. That is not the same as 'applying power to it to generate'. When properly excited, an asyncrhonous generator can supply much more power to the electrical system than it draws in excitation energy. Much like a large synchronous machine needs an external supply to its field to operate, yet it results in a net input of power to the electrical system.

Because 'our friend' didn't supply an external excitation. A synchronous machine with an electromagnet field (not permanent magnets) also outputs very low voltage when spinning with no DC field current. Are you suggesting that synchronous machines are 'useless for any purpose at all'? Of course not. Yet you suggest that an *improperly operated* asyncrhonous machine is proof that all such machines are 'useless'.

Possibly because your lab just didn't bother to explore this area. An async. generator operating in a stand-alone fashion has many drawbacks. The voltage control is difficult unless you have either a wound-rotor machine and control the rotor circuit resistance, or variable capacitance connected to the line terminals.

Because of the torque/slip characteristics of induction machines, the output frequency is also very difficult to control. If the prime mover speed is held constant, the output frequency varies with the amount of real load and the amount of slip (remember in an async. generator, the output frequence is lower than rotor speed. To maintain the output frequency in a stand-alone machine, you must increase the rotor speed as load is applied.

But, if the machine is connected to a 'stiff' grid that approaches the theoretical 'infinite bus', then many of these issues fade. The bus maintains a constant frequency and voltage and the real power output of the machine is controlled by controlling the speed of the prime mover. As speed is increased above the synchronous speed of the machine, more power is supplied to the bus.

Work out how the torque and power vs speed curves of an induction motor behave and simply extrapolate beyond the synch. speed.

daestrom

You know, posting that hyphen at the beginning of your post causes everything below it to be *not quoted* in replies as if it were a signature block.

What didn't get quoted:

Now think about that. I don't care if it's a "generator" or "alternator". My alternator uses 12 V from the battery to suply field magnetization.

The alternator puts out more electrical power than it consumes. Gee, I wonder if that extra energy comes from the mechanical energy driving it.

- Best Regards, Mike

"Dimitrios Tzortzakakis" wrote in message news:cq9bu3$hr4$ snipped-for-privacy@usenet.otenet.gr...

Hardly; you can make a pretty good case that the most efficient generators often DO require some external power for the sake of field excitation, monitoring/control equipment, etc. This initially comes from a bank of batteries, a smaller 'starter' generator where you don't care about efficiency since it'll only be used briefly, etc.

I was talking to a guy here in Oregon whose job is to maintain and monitor a small (a half dozen or so MW, I believe) hydroelectric power plant; one of the points he took pride in was that at _his_ plant he still had enough equipment around that he could get the plant going without any external power whatsoever. He said that many plants have been 'modernized' such that almost all of the control is computerized these days, but the downside is that they require external grid power to get the main generator going at all. I can see that being a defensible engineering choice, although if I had the guy's job I'd also be a little more comfortable knowing I could re-start from an off-grid condition.

---Joel Kolstad

In the industry, we say such a plant is 'capable of a black startup'. As you say, many power plants today are *not* capable of starting up without some external power. During the north-east blackout of 8/4/2003, one of the priorities for the few plants that remained on-line was to supply power to other plants so they could start up again. Many steam plants are not 'black startup' capable as the myriad of circulating pumps and valves have to be running before you can start rolling the turbine. Peaking plants such as gas turbine or diesel often are capable of a 'black startup'.

I can certainly see how a hydro plant would be capable.

daestrom

I am confused about something. Even if a power plant requires external power to start (say, fuel pumps should be operating for some time before water heats up and boils), it would require a lot less power than it produces. Probably many many times less. If so, then a power plant can buy a generator of adequate size, say 1000 kW. It should cost a relatively minuscule amount compared to the cost of the power plant itself.

This is completely analogous to having an electric starter on an engine. A small, cheap piece that can get stuff moving before the main power plant starts up in a self sustained mode.

So, just what is the problem?Have them buy adequate generators and that's all.

Any clarification will be appreciated.

They certainly could, however I think the issue is simply not wanting to devote the money and space to a standby generator that might never be used in the life of the plant. Not to mention maintenance of it, if a diesel backup generator sits doing nothing for 25 years chances are it'll do nothing right when it's needed most. I would still think it a good idea to have one around, but I can see the resons for not having it.

Yes. "Impedance protected" means the motor is mostly inductive at any rotor speed from nominal to stall. If greatly reducing rotor speed doesn't greatly increase load current, then raising speed won't produce much useful power.

A way to think of an induction motor run above synch speed is as a negative resistance. At nominal speed with mechanical load, it draws both inductive current for excitation and "real" or resistive current in phase with the line voltage. The latter is "real" power, some or most of which is converted to mechanical power -- torque * speed. If spun at synch speed, the motor would like like nearly a pure inductance. If spun above synch speed, the "real" current will be out of phase with the applied voltage so it acts as a negative resitance, consuming negative power i.e. pumping power back toward the excitation source that can then be dissipated by other loads.

Hey, everybody. I've been watching patiently for some time now.

You know, I missed the 17 bus from Little River Turnpike the other night because the 29, which pulled to a stop right in front of it, had its banner display out of order. So the 17 figured I was getting on the 29, not stopping anxiously to ask, "Are you by any chance a 17, and do you know if the bus behind you is?" Phhtt. I went and had turkey breast, peppers, and horseradish.

One of the things I check for when I go to little shops like Quizno's is whether they have tap water or carbonated at the fountain. I was in luck. They had carbonated. Cheered, I called Teri on the mobile phone, and we went and set up the repaired fax machine at Mom's after looking at Wild West Tech on the History Channel for five minutes. I normally don't look at television.

Don's reply is the sparkling water on this thread. Of course! If the torque curve isn't steep, the motor isn't stiff, and it'll be a lousy generator. I get it.

Now, electrically, does this inline resistance damp a resonant condition in the parallel L-C "tank"?

Off too bed....

Yours,

Doug Goncz Replikon Research Seven Corners, VA 22044-0394

Once a plant has started up, it certainly does produce more electricity than it uses. A lot more. One plant I've worked at has a gross output of 880MW electric. The 'hotel load' (the power needed to run the plant at full power) is app. 30MW. So the net is 850MW.

But to start up, it needs about 15 - 20MW. And the main turbine is producing zilch. Can you imagine how many diesel generator sets that would take? A nice EMD V-16 can produce about 4MW. So, get about 5 or 6 of those, connect them together with the correct switch-gear and controls and have at it. But you will need this setup about once every 20 years, when a major blackout such as 8/14/2003 occurs. That's a lot of hardware and preventative maintenance for something you only need every 20 years. And 5 or 6 EMD V-16's with 4160V 4000kW output are not 'small, cheap pieces'.

So quite a lot of plants are not 'black startup' capable. The local grid has to be 'up' to supply them with power to enable them to startup. Once they *do* get on-line, it's a different story. But getting there is where they need external power.

Take that nice diesel gen set you recently got. It's output is something like 10kW right? And it just needs a nice 12V battery and electric starter to get going. That's certainly affordable as opposed to a pull-rope. But now imagine it scaled up by a factor of 100 000 so that you have a 1000MW plant. In such a case even the 'cheap pieces' to start it would be very expensive.

Some types of plants are much more easily made black start capable than others. Hydro is perhaps the easiest. But even they need some standby/emergency power for lubrication, gate-controls, cooling and such. Modern gas turbines are probably also pretty easy to black-start. Steam plants are some of the most demanding since they require a lot of pumping power for the feed-water, and the boilers need quite a bit of auxilaries.

Then of course since most plants were built in the era of monopolistic utilities with state regulation, you have to justify your plant's cost to the regulators. The public service commission acknowledges that not *all* the plants in a region need to be black start capable. They would not allow such an 'extravagance' to be past on to the rate payers.

daestrom

Aren't diesel backup generator typically exercised something like every week or so for 15 minutes or somesuch in order to insure that they are ready when actually needed?

Often it is a matter of past-performance. If they fail to start properly, then the test frequency is increased. I've seen it as short as three days on some really troublesome units. But if they perform well, then as infrequent as once a month is common.

In addition to start tests, they are often 'load-tested' as well. The unit is loaded to 100% and run for 3 hours or so to verify it can sustain full load. This is done about quarterly. Or if the unit has been run unloaded too much, they will be load tested in order to bring them up to full temperature and burn out any soot problems. Depends on the unit.

daestrom

Perhaps. "Impedance protected" simply means that the motor has enough impedance so it will not overheat under locked rotor conditions. It's more of a UL term than a technical descriptor. The impedance might be due to stator resistance, rotor impedance and/or loose magnetic coupling between stator and rotor. It is probably mostly reactive because current flowing thru resistive impedance produces heat.