From: ems@michael.apple.com (E. Michael Smith)
Newsgroups: sci.energy
Subject: Re: Motor/Generator ???
Date: 24 Dec 92 19:54:09 GMT
Organization: Circle 'C' Shellfish Ranch, Shores-of-the-Pacific, California

In article <1992Dec23.092927.1@pa881a.inland.com> schiewer@pa881a.inland.com (Don Schiewer) writes:
>Can anyone say a few words about motor/generators.

Yes.

They are a very old technology.  They were common in WW II for turning
24-28 VDC into something more like 110-220 AC.  Many were available on
the surplus market for many years.

They are noisy, heavy, durable, and rather simple.

You take a single shaft and wind a motor on one end of it and a
generator on the other end.  The motor is wound to match to supply
you have, the generator to match the demand.  The coupling from
motor to generator is, er, 'highly efficient' ;-) since they share
the same shaft, bearings, housing, ...  Some are made to produce
a combination of outputs from one input and can, thus, replace a
powersupply.

In computer power rooms, the MG set is usually a rather large box
(about the same size as the computer being driven, in many cases,
smaller in others).  The time lag from power fail to power drop
at the mainframe is most often fractional minutes, but that can
seem to be an eternity in computer terms ...

Mostly they are used to filter {spikes, dips, any other transients}
and to convert the frequency to something else (like 400 Hz) that
uses MUCH smaller inductors in the rest of the power conditioning
equipment.

>Are generators more efficient that motor/generators?

By definition, a motor/generator is less efficient than a generartor
alone, since it includes motor losses as well.  If you are asking:
Is the generator portion of an MG set more efficient than the 
generator portion of an {natural gas, gasoline, diesel, hydro, etc.}
powered system, my GUESS would be that they are about the same, being
very similar technology.  If you are asking:  Is the MG set more efficient
than an electric motor coupled to a generator by other means, the answer
is yes!, but not by that much.  I think good MG sets get about 85-90%
efficiency, but the losses in a direct shaft coupled generator are not
very much.  Now if you have a belt drive ;-)

>Would a good motor like those used in R/C cars and planes 
>be a good generator?

Depends on the design of the motor, but most of the time something
optimised for motor use isn't optimised for generator use.

I *THINK* R/C cars use permanent magnet motors that would make good
generators, but you would have to check the particular motor.

Last time I looked, Honeywell still made portable MG sets that were
available in 12 VDC in/ 120 VAC out at about 1 kW.  These were about
the size of a couple of gallon bottle...

The MG set has, as advantages over electronic power conversion 
equipment: ruggedness, simplicity, low cost, abuse resistant,
inherent high filtration of transients, excellent isolation of
input and output power systems.  It has as dissadvantages:  heavy,
noisy, moving parts - so wears out, not very efficient at other 
than full power, those with brush type motors can induce line
noise of their own, they just arn't technologically sexy.

-- 

E. Michael Smith  ems@apple.COM

'Whatever you can do, or dream you can, begin it.  Boldness has
 genius, power and magic in it.'  -  Goethe

I am not responsible nor is anyone else.  Everything is disclaimed.
Newsgroups: sci.energy