Hello Tim,What fond memories this brought back. Many years ago a good friend of mine who worked for the electric company took me into a small unmanned powerhouse, and let me bring up the D.C. exciter first, and then one of the fairly good sized alternators. With lots of instruction he then showed me how to use the synchroscope for phase and speed and put it on line. It took a while, but I got it pretty close. Thanks for the memories. By the way the place was ancient, even then. It's got to be over 100 years old today. Ira.
On 1/19/2012 1:55 AM, [email protected] wrote:
Having worked at Lima Electric (Sadly No longer in Lima Ohio, Factory
Closed) for over 5 years making synchronous alternators, and 3 years in
the Engineering Test Lab I can pass on a little information on alternator
speeds. On a sychronous alternator the rotor operates with DC excitation,
either supplied by slip rings (automotive alternator) or by a excitor
rotot/stator combination.( There is also DC or AC excitation) Lima
electric pioneered alternators without sliprings, this was made possible
by solid state rectifiers.
A synchronous alternator will operate just as well as a motor if voltage
is supplied to the stator. In any case as a motor the rotor locks
exactly to the speed of the rotating magnetic field in the stator.
(unlike an induction motor where torque is produced by slip, synchronous
speed - actual speed of the motor. This can be looked up on the web)
This is what makes a synchronous motor valuable as a clock. Well in any
case in a sychronous motor shaft speed is directly proportional to input
frequency as long as the motor stays in step. (Excessive torque demands
can pull a motor out of synchronous speed) Conversly with a synchronous
alternator. Output frequency is directly proportional to rotor shaft
speed.
Rotors are most commonly made with two, four or six poles for 50 or 60
HZ units. A 2 pole alternator will produce 60 Hz when run at 3600 RPM a
4 pole alternator will produce 60 Hz when run at 1800 RPM and a 6
pole alternator will produce 60 Hz when run at exactly 1200 RPM. A
synchronous motor will run at exactly the same speeds at 60 Hz. 2
Pole=3600 4 Pole=1800 6 Pole=1200 etc. For 50 Hz 2Pole=3000RPM
4Pole=1500RPM and 6Pole= 1000RPM
The formula is RPMsync=120 times Hz divided by the number of poles on
the rotor
For power generation on a grid or any place that more than one
alternator are working together all the alternators must run at exactly
the same speed and must be in phase. If an alternator is not in "sync"
horrible things can happen. The phase voltage and frequency must exactly
match before an alternator is brought on line with the other
alternators. This is where a synchroscope using lamps or meters comes
into use. In our test lab I had two test stands, one with a 250HP
synchrounous motor and the other with a 500HP synchronous motor. The
500HP motor had to be manually synchronized with the incoming line
voltage before closing the main contactor. This involved using a
auxilliary motor to bring the large motor up to synchronous speed. I
then had to manually turn up the excitation voltage to the rotor using a
variac to match the incoming line voltage. A meter would indicate the
voltage phase between the motor (which was acting as a generator) and
the incoming line. The meter would slowly swing between 0volts and
480volts. When the meter read 0 volts I had to close the contactor. If I
was off a little the whole motor would ring like a bell as the rotor was
jerked into step. If I was way off the main fuses to the plant would
blow. (I never did that, thank God!) I did blow a set of 500 amp fuses
though once. In any case for the huge Megawatt alternators in power
plants for the grid it becomes obvious that Frequency control and phase
are very important for proper operation of the grid.
Sorry this got so long. I havn't posted in a long long time and I guess
the verbage just had to vent!
Tim
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