backfed ST head

[bschwartz]

Does anyone know what would happen to an ST head if it were backfed (hooked up to) 120/240?
Would it turn functioning as a motor?  Would it smoke? Do nothing?

[mobile_bob]

it will not turn, and yes it would just heat up and smoke

bob g

[Ronmar]

Since it does not posess the startup circuitry of a motor I agree, from a dead stop it will cook untill something smokes.  If it were spinning and excited at the time, it would try to align to the grid power sinewave, and make smoke and lots of torque till it did line up.  If the generator sinewave and grid sinewave were aligned beforehand using a syncroscope or the lightbulb method, the generator would lock into sync with the grid and allow the exchange of energy.  Since it is designd to produce electrical energy, It is generally accepted as being bad to attempt to "motor" a generator...

[WStayton]

Hi guys!

  I remember when I was a kid, my Dad used to use an automotive "generator", before alternators were common/available, driven by a 120 volt ac electric motor to charge batteries (Yea, he was cheap TOO!  <grin>), and, being a typical kid, I delighted in temporarily unplugging the motor and watching as the generator "motored" the whole set-up using battery emf as its driver - I might add NOT with my Dad's knowledge or concurrance, being shy of having my butt paddled!

  So, what is it about a generator that makes it work as a motor - is it that the generator uses brushes to "grab" off pieces of the AC voltage and thus make DC and an alternator uses diodes?

  Remember I'm electrically challenged here!

Regardz,

Wayne Stayton

[mobile_bob]

there are fundamental differences between alnernators and generators

our beloved st generators are not generators, but rather they are alternators

to be a true generator it must have a commutator and brushes,  the thing can then be either a producer of power
or a converter of power (read that a motor if fed DC current),,, the only difference between a dc generator and dc motor
is a slight offset of the field pole pieces and brush positioning (shifted from a bit retarded to a bit advanced or visa versa)
in either case the machine can be optimized to be a good motor or a good generator, or a pretty fair motor/generator such as was
used on briggs and stratton larger hp gasoline engine's, among others.  as a motor generator the optimization was generally in favor
of motor with a hit in efficiency in generation,, mainly because the little engine's didn't need a lot of charging anyways but needed
all the starting torque the motor could deliver.

an alternator can not be a motor generally without some sort of sophisticated switching scheme to energize the field/rotor
as needed to provide for repulsion or attraction to the rotating magnetic field in the stator if it is also fed with AC current.

google converting an alternator to a motor for a better idea of what it takes to make an alternator become a motor.

bob g

[TimSR2]

An ST head that is locked on to the grid will continue to run even if the fuel to the prime mover is cut off. Do not ask me how I know this...   it's rather embarassing

[bschwartz]

Tim, that happened to me once......kind of what prompted my question to begin with.
I didn't yet have a break then make transfer switch, and inadvertently turned the grid power back on before turning the generator power off....... was puzzled when I shut off the fuel rack, and the engine kept turning.

[sailawayrb]

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/motorac.html

...and a good site for learning basic Physics. 

Bob B.