Synchronous Machine Voltage Curves & Torque Relation to Armature Current

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Discussion Overview

The discussion revolves around understanding the relationship between voltage curves and torque in synchronous machines, particularly in the context of armature current. Participants explore theoretical solutions and practical experiments related to synchronous motors, including the effects of field currents and torque on armature currents.

Discussion Character

  • Exploratory
  • Technical explanation
  • Homework-related

Main Points Raised

  • One participant describes their experiments with synchronous motors, noting the relationship between torque, power, and armature current, but expresses confusion over theoretical calculations.
  • Another participant suggests consulting a book on electrical machine theory, specifically recommending works by Krause.
  • A participant provides a conceptual explanation of synchronous motors, likening them to an electromagnet in motion and discussing the importance of phasor representation.
  • It is noted that the rotor's magnetic pole must remain locked to the stator's pole, and slipping can cause the machine to fall out of synchronism.
  • A further suggestion includes a link to a resource that may help clarify concepts related to synchronous generators.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the theoretical aspects of the problem, and multiple viewpoints regarding the understanding of synchronous machines and their operation are presented.

Contextual Notes

Participants mention various characteristics and parameters of the synchronous machine, such as air gap voltages, open and short circuit characteristics, and calculated synchronous inductance values, but do not resolve the underlying assumptions or mathematical steps involved.

Who May Find This Useful

This discussion may be useful for individuals interested in electrical engineering, particularly those studying synchronous machines and their operational principles.

Alesa
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Hello everyone

Im new on the forum and I just discovered what are sync machines...

I have really basic question what I still cannot figure out.. I did some experiments with motor for different field currents and torques. In the result I measured armature currents and did V curves. But next question asks to do find theoretical solution for same problem. As I guess, torque related to the power through sync speed and power related to I armature as I=P/(Va*cos(theta)). However it doesn't work for me and I am pretty lost.

Maybe someone can give me at least direction where and what I should read about.. I have air gap voltages, open and short circuit charachteristics and different input torques and calculated different sync inductance values for open loop voltages. Machine is 400v 0.8kW cos=0.75 50Hz 4 poles 1500rpm.
 
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Get a good book on electrical machine theory. I recommend the ones authored by Krause (Purdue U).
 
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Alesa said:
However it doesn't work for me and I am pretty lost.
The way to think about synchronous motors is an electromagnet in motion being dragged along by the magnetic field of stator ..
A generator is identical except torque is reversed, rotor is held back by stator.

From that mental picture it's a small step to phasor representation of what's going on. Brush up your grade school geometry and dust off your old drafting compass...

We've had a lot of threads on synchronous machines
see if this one is any help.
https://www.physicsforums.com/threads/voltage-and-reactive-power-relationship.876346/#post-5503690
 
The one thing that I would add to Jim's answer is that the magnetic pole on the rotor is always locked to the corresponding pole on the stator; it is not allowed to slip. If the torque is so great that it does slip, the machine falls out of synchronism.
 
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