Electronics > Beginners
synchronous motors - Can i have this in English please
ahbushnell:
--- Quote from: Simon on December 29, 2018, 05:26:39 pm ---There is a split, if by fork you mean multiple threads in threads please NO!
--- End quote ---
That's a poor joke.
Simon:
I was not aware I was making a joke.
BigMark:
--- Quote from: Simon on December 28, 2018, 04:06:14 pm ---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.
--- End quote ---
Been a while since I studied this subject. Might be time to find my old Brook Crompton motor handbook.
From memory and reading some of your text.
The variable resistors are in a star configuration at one end and attached to the slip rings at the other. On start of the motor the setting on each variable resistor is high and when the motor reaches speed and the resistance setting on each resistor has dropped and eventually reaches zero ohms or therefore no longer has any effect on the motor.
As resistance has dropped to zero the resistors can be taken out of circuit and bypassed by being shorted out at the star point or at the Slip rings using a contactor. The diagram looks to be missing the contactor stage. So when the motor reaches speed the exciter is connected to two slip rings on one side and one slip ring on the other bypassing the resistor starting circuit.
Simon:
correct, they forgot to be a bit more specific about shorting 2 rings only so that an open rotor circuit is still available to be exited. The text was identical to previous material about the starting of induction only motors and left me wondering if someone was getting carried away.
Simon:
So a new bit of confusing syntax. See picture. Is it me or are there 2 definitions of P there? it starts as volts x amps but then becomes volts x amps divided R (Xs).
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