Synchronous information

Our show in Osage Iowa just got a 125 horse Synchronous motor which used to run a large reciprocating air compressor. It is a 240 VAC 3 phase unit with a 125 VDC exciter attached. We have the original motor control panel which is mostly complete.

1) If I could spin this unit at 720rpm and throw a correctly phased 3 phase 208 Witte Diesel into it, (actually does 248 with a 3 PH 7.5 horse motor operating idle at 60 Cycles, 32 amps per leg rated alternator capacity) Could I run this Synchronous unit idle?

2) What would it take to convert this 125 horse motor into a Synchronous generator/ electric horsepower brake? (Safely) It has a flat belt pulley on it now. How would I dispose of the generated power without going back online? How would it be controlled?

Nothing like a couple of simple questions to start ones day.

Lars

What you are doing sounds "do-able" but be sure what you are doing before you do it.

The first part looks pretty "standard." See below.

The second part looks like you need to investigate what the engineers call a "prony brake" which is essentially a "friction brake" encircling a flywheel. A standard pulley can be used for a short time but the brake shoes need to be water cooled. (In essense you're converting mechanical energy into thermal energy in this case.)

One simple version of the prony brake is shown at http://www.tpub.com/machines/8b.htm

Back to item 1. Take a look at any current (i.e. revision 4 or newer) edition of Mark's Mechanical Engineer's Handbook. There is a short section that describes is flowing terms the similarities between a synchronous generator and a synchronous motor (there isn't much difference EXCEPT for the way it deals with POWER FACTOR)

The issue of a synchronous condenser is also covered in which I believe I brushed the edges of earlier in the discussion of induction generators.

As far as the 208 versus 240 voltage issue, by itself this shouldn't affect you too much. Instead, this may be indicative of a difference in delta versus wye windings in the generator motor combination. Offhand I can't tell you which is which but commonly 208 is one form and 240 is the other. One of these is liable to be a "three wire" 3 phase circuit and the other a "four wire" 3 phase circuit (the 4th wire being the "ground." You might have to do some investigating on the grounding necessity and the potential for ground cross currents between the two machines. Depends on the arrangement of the grounded leg of the wye machine. (if it exists at all.)

This sounds like one of those cases where you might want to have some instrumentation handy to see what is actually going on. You're fortunate to get the control panel. The control panel should have some sort of power factor meter. As long as the field is adjusted so power factor is "unity" (which is a function of loading) then you're probably all right.

Take a look at the link below which gives you a "big picture" look at phase, voltage, power factor, and the whole enchalada.

Best regards, Joe


Dispatch Theory & Practice

Power factor meter? Let me guess.. That is the one that is missing. Only meters are ammeters. PF here is supposed to be 1.0 according to the nameplate. Something else I need to have a grasp of yet.

As I recall a 4 ring unit will be a star configuration as in a Winpower 15kw I rebuilt. I do not recall how many slip rings are on the big-un. I think there are only 3 possibly making a delta.

This is all preliminary digging. A lot of questions will have to be answered before anything happens. And it may not be done by me...

So far the only printed information I have located is in my Cyclopedia of Applied Electricity Volume 2. Which is the reason that I want the unit taken care of in the meantime. It is dang well good for something.... May be really interesting in the end.

At the least, perhaps we can belt a stationary steamer to it and demonstrate electrical generation such as is done in Rollag MN at WMSTR.

I plan to have it all mapped out before a wire is connected.

Synshronous motors have a amouster winding(starting winding).This winding is made of copper bars and shorting rings on the outer dia of the rotor.The rotor has two slip rings . This is were the dc is applied to the rotor. The motor is started by first shorting out the slip rings during acceration until about 90 % speed is reached.The shorting resistor is aprox twice the resistance of the rotor .At 90% speed the dc is applied as the shorting resistor is removed.During start up the motor is a revolving transformer at very high frequency. the shorting resietor turns this voltage into current which is disipatted as heat.Once the motor is up to speed it's called instep.Count the number of poles on the rotor and divied into 7200 and that is your syn speed.Say a 20 pole motor into 7200 =360 rpm.the type of motor is used for power factor correction in most plants.A plant with a lot of motors has a high inductive load . the syn . motor is a reactive load. All this is called Vars which we don't want to pay for.

Back when the Atlas Missile defense systems were being installed in silos around the country, they each had a coupkle large engine driven emergency backup generators to supply site power in the event of a failure of utility power. Each of these units had to be tested at some point under a full load and they had the same problem of disposing of the electricity being generated.

All the bright young energetic new electrical engineers scampered around looking for some sort of a dummy load to dump the power into. Each option turned out to be quite expensive to acquire, and once the testing was done it would then become useless surplus equipment.

At one point one of these new bright young energetic electrical engineers came to an old about to retire, self educated electrical engineer, looking for a solution to their vexing problem. After listening to the problem for a while, the old engineer came up with his solution to the problem.

This solution seemed very crude to the young energetic electrical engineer who never-the-less took it back to his engineering department group for discussion. "What does an old goat like that know about these new modern generator plants" was the general opinion expressed, and so the search continued for a solution of disposing of the electricity generated during the generator test.

About a month later a delegation of these new bright young energetic engineers came back to the old about to retire self educated engineer and ask him to repeat his solution to their now very vexing problem all over again. With nothing to loose, and time was quickly running out, the delegation went back to their cubicals and designed a dummy load according to the suggestion that had been given them.

When tested, this system worked just fine. It gave them the ability to increase the load on the generators to be tested and to be able to do it on each phase independently. What was unique was that the system cost very little and was simple to construct.

What they did basically was to make up a grid of welded together concrete type steel re-bar that would fit into a large commercial stock watering tank. The grid could be lowered into the stock watering tank which was full of salty water by use of a rope block and tackle by hand. One of these was made up for each of the three phases. The three stock watering tanks were all grounded together and to earth ground.

With a little salt added to the water in the stock watering tanks to increase the electrical conductivity, the re-bar grids were lowered into the water by the block and tackle's, thus generating a variable pure resistance load for each phase. The current flowing through the water generated heat. The heat was carried away by a flow of water from hoses that kept the steaming stock watering tank's full.

This method was used to acceptance test the emergency backup generators for the Atlas Missile sites scattered around the country. Oh yes, the old self educated about to retire electrical engineer who solved the bright young energetic electrical engineers vexing problem was my father Harry R. Hughes.

So for a dummy load for your syncronous motor/generator, you might consider this idea. Keep in mind that it is necessary to provide non-conductive guide's and stops so the welded re-bar grid can not come in contact the stock watering tank walls, or bottom, and cause a direct short circuit.

When you are done testing, you can sell the stock watering tanks for their original use and the re-bar stock as scrap, or for some other use.

I had an old Thorndike Barnhardt science projects book that described using a petri dish and a couple fishing sinkers with a salt shaker. It was an "adjustable resister" with a homemade induction furnace. I was wondering if that principle could work.

Question. An old locomotive book I have says to saturate a barrel of rainwater until it would no longer accept any salt, (Railroad passenger coach heating) then take a whiz in it thus causing it to take more... Point being, is there an appropriate amount or strength of salt solution for an operation like this? How would one reasonably determine tank volume in something like this? (And now to make it idiot proof. Would not want an enlightening situation.)

Lars

I was not told by my Father how much salt was required. Most water supplies are not pure enough to behave like distilled water, which has a very low conductivity. I would assume that if you could not get enough current flow with the electrode grid all the way submerged, that you would have to add more salt. At least up to the point of maximum salt saturation in the water solution that you mentioned. I would think that it would take a lot of whiz to increase the maximum saturation in a stock watering tank. Maybe a case of beer and a few friends could help in that area.

If all that didn't provide sufficient current flow, then the surface area of the submerged electrode would have to be increased. Once the surface area of the inside of the stock watering tank equaled the surface area of the electrode grid, and the maximum saturation of the water with salt was achieved, that this along with the water flow available would define the ultimate electrical power dissapation capacity of the system.

The electrode grid made out of re-bar would have to have sufficient surface area in contact with the water electrolyte to produce the required current flow. For a smaller test system than I previously described, I would probably use some sort of a stamped out screen material rather than re-bar. It would be lighter, have more surface area per pound of weight than a round re-bar material. In the previously described system they probably used welded together re-bar to make the electrode grid because of the large ammount of current at the power level they were testing at.

As for the lead sinkers in the petrie dish, that would work with small current flows, but the volume of the electrolyte in the dish and the surface area of the sinkers would limit the amount of current you could send through it. Otherwise, what you describe works exactly in the same principle as I described. An electrolytic variable resistor is what it basically is.

Any contaminant in the water will increase its conductivity, but salt is cheap and plentiful and is not otherwise thought of as a hazardous material. None of this discussion says anything about how long the stock watering tank and the electrode grid will last. Ultimately the salt water will corrode the materials and there is the possibility of galvanic corrosion even though the system is operating on alternating current (AC).