[LMB] TOI, Electricity, and Magnets

anmar Caver anmar.mirza at gmail.com
Thu Aug 27 16:29:57 BST 2020

On Thu, Aug 27, 2020 at 11:18 AM Harvey Fishman <fishman at panix.com> wrote:

> I read the Wikipedia article on asynchronous generators and it seems to
> that it says you must still start with a charged capacitor to initially
> power the field winding.
Good lord Harvey, you have people here who have actual, real life
experience with these things telling you things and yet somehow and
incomplete skim of wikipedia is what you believe?

Since you apparently will only believe what you read on the
internetz instead of from people who have worked with these things and have
extensive education in them: While I explained about self-excitation in
layperson's terms, the articles quoted below say exactly the same thing:

"Self-excitation is made use of most often in DC generators. When a
self-excited generator is started, the initial current in the field winding
is produced by the electromotive force (emf) induced in the armature
winding by the residual magnetic field of the main poles. To sustain
self-excitation, the initial current must reinforce this field. The
additional magnetic flux increases the emf in the armature and,
consequently, the current in the windings of the main poles. Because,
however, of magnetic saturation in the magnetic circuit, the increments in
magnetic flux corresponding to equal increments in current become smaller
as the current builds up. The process of self-excitation continues as long
as the emf in the armature exceeds the voltage drop in the field winding.
At a certain magnitude of the magnetic flux, electric equilibrium is
reached, and there is no further increase in magnetic flux, armature emf,
and excitation current. Self-excitation can be achieved when the value of
the resistance of the field winding does not exceed a certain limit, which
depends on the electric parameters of the generator."

"Modern DC generators with field coils are self-excited generators which
get started with the initial current in the field coils. When generator is
switched off, a small magnetism is developed in rotor iron which induced
electromotive force in the armature due to which current is produced in the
field windings. Initially, weak magnetic field creates less current in the
coil, but to sustain self-excitation, the additional magnetic flux
increases the electromotive force in the rotor, due to which voltage keep
on increasing until the machine takes the full load."

Your very own wikipedia which you did not apparently read completely:
"Self excitation
Modern generators with field coils are usually self-excited; i.e., some of
the power output from the rotor is used to power the field coils. The rotor
iron retains a degree of residual magnetism when the generator is turned
off. The generator is started with no load connected; the initial weak
field induces a weak current in the rotor coils, which in turn creates an
initial field current, increasing the field strength, thus increasing the
induced current in the rotor, and so on in a feedback process until the
machine "builds up" to full voltage."

But by all means, let a five minute self-education via google be more
authoritative than what people who have spent many years learning about and
working with these things. This is the way of the net afterall.

Anmar Mirza EMT, N9ISY, NCRC National Coordinator, RBNC President

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