Electronics > Beginners
synchronous motors - Can i have this in English please
Simon:
See attached page 9, it makes no sense, if the slip rings are short circuited how can the exciter put any voltage into the rotor? I assume that the rotor is fed with 3 phase power of opposite polarity to the stator thus mimicking the conditions of a DC fed rotor?
--- Quote ---With this method of starting, the field of the exciter is left de-energised and the
motor is started as a normal wound-rotor induction motor. By selecting the
correct value of starting resistors, the motor can now be made to produce
maximum torque on starting and can therefore start under load.
Once the maximum speed of the induction motor has been reached, the slip
rings are short-circuited and the field of the exciter energised. Once more, a
low frequency alternating torque is superimposed on the induction motor
torque and the machine pulls into synchronism and runs as a synchronous
motor.
--- End quote ---
I think perhaps they mean the short circuit on the slip rings that was never there because they are using starter resistors in series with the rotor is removed and the energiser energises the rotor. Clearly someone was up late writing this having just finished the garbled mess that is the induction motor section of the course.
Benta:
I understand it fully. The induction motor part is clear, and when you're close to synchronism the rotor windings are short circuited to disable the induction part.
At the same time, the starter resistors change purpose and are used to control the exciter. The schematic is a bit convolved, but hey.
ArthurDent:
I have an old single phase repulsion start/induction motor and they are interesting. The switching mechanism is kind of complex but it works fine. Here is a video of a very old version of the motor type with a mechanical switch to go from start to run that pretty well explains the way they work.
Simon:
--- Quote from: Benta on December 28, 2018, 05:15:04 pm ---I understand it fully. The induction motor part is clear, and when you're close to synchronism the rotor windings are short circuited to disable the induction part.
At the same time, the starter resistors change purpose and are used to control the exciter. The schematic is a bit convolved, but hey.
--- End quote ---
No! if you short the rotor winding's by shorting the slip rings you produce an induction motor which is what is use to start the thing not run it. The resistors would be brought to 0 as the motor speeds up making it an approximate squirrel cage motor. at this point the exciter would need to take over. I can only assume they mean short 2 of the slip rings so as to use the entire would rotor coils in the excitation.
Benta:
Actually, it gives you a squirrel cage motor. But it doesn't matter. In synchronism there is no slip, and the squirrel cage is completely out of the picture.
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