Questions on electrical distribution system and motor efficiency

high-voltage

The annual report for my local utility shows that they generate on the order of 7 % more kwh than they get paid for - which I would assume is the order of magnitude of transmission losses. Sure, there may be 5 transformers between your wall plug and the generator, but each one is pretty efficient (99.7% in the larger sizes and even the "pole pig" in your back yard would be in the upper 90's).

Depends on the size and design - again, larger motors are generally more efficient, partly because people care more about efficiency on a 1000 kW motor than on a 0.1 W motor. If you were after maximum efficiency in the range of, oh, say, 10 kW to 100 kW and around 1800 RPM, I suspect that you'd be better off with an AC motor (high 90s).

Like so many questions here on a.e.e, the answer is "it depends". Very low voltage motors ( say 12 or 24 V) are hideously inefficient because brush drop is a big fraction of the terminal voltage. You can't really build a DC commutator motor over about 2500 V or so because you need so many commutator bars to divide the voltage over that the machine becomes uneconomic.

My motor handbook is at the office and my "Standard Handbook for Electrical Engineers 5th Edition" doesn't even try to talk in general about DC machine efficiency, I suspect because it varies by application.

Bill

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What's a more impressive number is the "utilization" efficiency, which is to say the usable output from electrical devices divided by the total energy input at the power plant.

Last data I have (year 2000) puts that at about 17%. This figure reflects all the inefficiencies, boilers, turbines, auxiliaries etc. as well as the inefficiencies at the user's end; incandescent lamps, etc.

In other words, on the average, it takes 100 BTU of coal to deliver 17 BTU of ultimately useful energy.

Bill has answered your question well and BFoelsch has provided an overall coal to useful energy to the user, comparison.

As for DC motors, while high voltage DC machines can be built, a practical limit would be in the 2KV range and an economical limit would be far lower-say 500V. Compared to the typical AC induction motor, a DC motor is costlier to build, less efficient in general (possibly by 3 to 5 % ) and much more difficult to maintain. The commutator/brush system is the main problem and the power winding is fed through these moving contacts. In contrast the induction motor has a stationary power winding and no moving contacts as well as a very simple and rugged rotor. The machine then can be economically built for much higher voltages and powers than DC motors. DC motors are better suited to mobile and many specialty applications but AC machines are the general workhorses. --

Don Kelly snipped-for-privacy@shawcross.ca remove the X to answer

\ All of the previous posters have provided accurate answers to your questions. One thing that was not mentioned however, when discussing the overall efficiency of a power system is the heat loss of the transmission lines themselves.

You mention distribution systems in your post, but if you are including the 5 or more transformers that are part of the overall system, this would include both transmission and distribution.

Some of the transmission lines in certain parts of the country are overloaded and run HOT (100C) which is the boiling point of water and the thermal limit of many of the lines.

Also, at the turn of the previous century, a high voltage dc transmission system was developed using series connected DC motors and generators. It was primitive, but relatively efficient, and was used until the 1930's in Italy.

Look up the Thury System of DC Transmission at:

for more information.

Beachcomber

high-voltage

Cool - someone out there is reading all that Thury stuff I researched.

Bill

The heat loss of transmission lines was included in the loss/efficiency data. Sure many lines run hot , depending on many things including direction of the line, wind, thermal insolation, etc. 100 degrees is tolerable but will result in decreased conductor life. The Thury system died because it was expensive to maintain and it was definitely not "relatively efficient" compared to a corresponding AC line at the same distance, voltage and power transfer . Quote from your reference:" Other Thury systems operating at up to 100 kV DC operated up until the 1930s, but the rotating machinery required high maintenance and had high energy loss."

It served a need at the time but the MG set system was never really viable.

Thanks Don:

Perhaps, I should have said relative to available technology at the time for comparative AC long distance transmission systems. Else why would they invest in the DC technology in the first place?

MG sets were tried on an experimental basis (early 1900's) for substations in Chicago by Mr. Insull's Public Service Company (later Commonwealth Edison) in central city locations where DC was used by elevators. New York, I believe, had exclusively DC power districts in the central city up until the 1950's from the vintage ads I've seen for special radios that would run on this type of current.

MG sets were certainly viable for electric traction systems (such as the Chicago Transit Authority's 600 VDC supply) and the Interurban railroads of the period, and also the New York Transit Authority until well past the mid-century mark, when mercury-arc rectifiers succeeded as replacements.

Beachcomber

Beachcomber

Indeed, I think Don was referring to the problems attended by using MG sets outside their "range of comfort," which is 750 VDC or so.

MG sets, or more likely synchronous converters, were definitely used in all but the most ancient DC systems. The big problem with a true MG set is that it requires two magnetic circuits each capable of handling the full power output of the machine. This makes them very large and very heavy. The synchronous converter requires only a single magnetic circuit that carries only a fraction of the machine's output, which makes them much smaller and lighter. The down side is that the synchronous converter has a galvanic path between the AC and DC sides, whereas the true MG set can provide complete isolation.

I grew up in Buffalo, NY, and in the downtown area, DC, 60 cycle AC and 25 cycle AC were all used about equally until the DC was discontinued in the 1940s. The underground substations were as you suggest all mechanical, although synchronous converters were predominant and thus true motor-generator combinations were rare. The DC was converted from 25 cycle AC, just as it had been done when the first transmission line from Niagara Falls was installed. There were a few substations that contained large battery rooms to even out the peak demand. I am not aware that Buffalo ever had a mercury-arc substation, but many transit systems definitely installed them in the

1930's.

Experimentally to try to get the advantages of higher voltage DC transmission which they could see. However, prior to electronic inverters, the dream was really not feasible, either economically or otherwise. For

10KV at 2KV per machine, there would be 10 stages(up and down) so the efficiency would be leas than 40% excluding line losses, and maintenance costs would be horrible. Even now, DC transmission would not be normally be used for such short, low power links as it would not be cost competitive. I say normally because there are specific cases where it is viable - such as some underwater links, ties between two systems at different frequencies or as a weak tie between two large systems at nominally the same frequency where a weak AC link may hold only for a short time before becoming unstable.

The use of MG sets for specific applications such as those you mention was common and for these applications, was reasonably efficient and economical in comparison to the Thury scheme. My comment about MG sets dealt with them as used in the Thury system. In Central New York there were regions supplied by Edison's original DC system and rather than go in for a wholesale rebuilding of the distribution system, an MG set was an attractive alternative (for a while). Whether they lasted until the 50's - I don't know as the original infrastructure would have been inadequate by then.

Why does it reduce the conductor life?

bud--