philkryder said:
On Sep 2, 10:18 am, "daestrom" <daestrom@NO_SPAM_HEREtwcny.rr.com>
wrote:
so, is it correct that there was
"some permanent magnetism"
- but not enough to be what is considered "a permanent" magnet
generator?
This definitional issue of "what is a permanent magnet" and "what is a
permanent magnet generator" might clarify things a bit.
Can you help?
Bascially you have the idea. A 'non-permanent magnet' DC generator has
coils around the stationary pole pieces that will generate a strong magnetic
field for the rotor to rotate within. Because the current through these
coils is DC, and the pole pieces are made from iron, when the current
through the coils is turned off the pole pieces have been turned into
magnets. But the magnetic field is not nearly as strong as when the current
is switched on.
So, just about *every* DC generator has some iron pole pieces that have some
'residual magnetism' in them.
A 'permanent magnet generator' (PMG) does not have any coils around the
stationary pole pieces. Modern ones use rare-earth magnets instead of
simple iron pole pieces. The rare-earth magnets have a stronger magnetic
field than a simple iron magnet (with no coil around it).
The voltage output of a DC generator is a function of the speed that the
wire goes through the magnetic field (RPM x Diameter) and the strength of
the magnetic field. Obviously a stronger magnetic field is often desired to
boost the output voltage (that's why rare-earth magnets are popular in
PMGs). But often *control* is also desirable.
When it comes to *controlling* the output of a generator, there is no easy
way to control the output of a PMG other than changing its speed. With an
electro-magnet style of generator, you can vary the current through the
coils around the pole pieces. This can allow you (or a regulator) to vary
charging current and/or compensate for changes in speed to maintain a
voltage setting.
Most electro-magnet style units depend on the pole pieces having some
residual magnetism for starting up. The field coils are powered from the
machine's output (called 'self-excited'), but when starting there is no
output, so no field coil current, no magnetic field, no output, so no field
coil current, etc. etc. But if there is some small residual magnetism, then
when starting there will be a low (but not zero) output. This will power a
low (but not zero) current in the field coil. This small current in turn
will add its magnetism to the residual magnetism boosting the overall
magnetic field. So the voltage output is a bit higher than before and the
field current rises a bit more and so on. Eventually (well, it happens in a
matter of seconds), the output voltage is up to rating and the regulator
takes over control of the field coil current.
If you disassemble a machine, or it sits idle for a long time, the pole
pieces may lose the small residual magnetism they had. Then when you start
the unit there isn't any output voltage. The solution is to 'flash the
field' by applying some external DC source for a moment or two. This
provides a trickle of current through the field coils to get things started
(or if done while the unit is shutdown, it provides some small residual
magnetism for startup).
(there are a few generators that are 'separately excited', but not common
and not worth discussing here)
daestrom
