Synchronous Motor Phasor Diagram

In summary, the conversation is discussing the usage of -Ia for a synchronous motor and +Ia for a synchronous generator, as well as the direction of IaXar in the phasor diagram. The homework equations and attempted solutions are also mentioned, with a request for clarification.
  • #1
jaus tail
615
48
It's not really a problem but I was studying Sync Motors for assignment and didn't understand few things.

1. Homework Statement

http://s1320.photobucket.com/user/ajaytulsiani/media/-%20ia_zps8oujbzrh.jpg.html http://s1320.photobucket.com/user/ajaytulsiani/media/synch_zps3bry0aah.jpg.html
-%20ia_zpsgbz2izmh.jpg
synch_zps3bry0aah.jpg

In the first picture...what is the point of -Ia. I mean when synchronos machine is working as alternator then current is Ia. Now the sync machine is working as motor so direction of Ia is reversed. That is shown in phasor. what is the point of adding a -Ia. And shouldn't power factor be between Ia and Ef instead of -Ia and Ef.

In second picture if you look at third picture, isn't direction of IaXr wrong? Shouldn't it be towards left like the arrow be pointing left so the phasor looks like: Vt = Ef + IaXar?

Homework Equations


For first question...current flows out of armature in case of sync generator while it flows into armature in case of sync motor. Power factor is between load voltage and load current, which is Ia and not -Ia.

For second question equation for motor is Vt = Ef + IaXar. But if you look at second picture, third phasor, then it appears that Ef = Vt + IaXr

The Attempt at a Solution



Well i just tried reading it again and again but didn't understand why IaXar phasor is otherway around?
Any help would be appreciated...

Thanks
 
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  • #2
Hi,
There was a reply here with a link. Is there any reason why the link was removed?
 
  • #3
jaus tail said:
Hi,
There was a reply here with a link. Is there any reason why the link was removed?
Judging by the mentor's memo, the link was advertising, and was unrelated to synchronous motors.
 
  • #4
Let me see if I understand what you are asking: Are you questioning the need to consider -Ia for a motor and +Ia for a generator?

Let's assume the machine is largely resistive, just for clarity. Then however you draw the phasors or label them, the voltage phasors need to be only a few degrees different from the current phasor to comply with being a resistance. Otherwise, if they are almost 180° apart, I guess you'd be illustrating a motor having negative resistance (which, strictly speaking, is what a synchronous generator really is when you have it feeding into the grid).
 
  • #5
Yeah i didn't understand why they've used -Ia for motor and +Ia for generator.
If you look at second picture, the third phasor.
the motor equation is Vt = Ef + IaXar.
Isn't it shown wrong in the picture, the third phasor? Shouldn't IaXar be pointing towards Ef?
 

1. What is a synchronous motor phasor diagram?

A synchronous motor phasor diagram is a graphical representation of the voltage and current phasors in a synchronous motor. It is used to analyze the performance of the motor and determine its power factor and efficiency.

2. How is a synchronous motor phasor diagram different from an induction motor phasor diagram?

A synchronous motor phasor diagram includes a third phasor, known as the excitation or field phasor, which represents the excitation current for the motor's field winding. This phasor is not present in an induction motor phasor diagram.

3. What is the significance of the angle between the stator and rotor phasors in a synchronous motor phasor diagram?

The angle between the stator and rotor phasors, known as the power angle, represents the phase difference between the stator and rotor currents. It is used to determine the motor's torque and power output.

4. How does the power factor affect the synchronous motor phasor diagram?

The power factor, which is the cosine of the power angle, affects the length of the rotor phasor in the diagram. A higher power factor results in a longer rotor phasor, indicating a more efficient motor.

5. What other information can be obtained from a synchronous motor phasor diagram?

In addition to power factor and efficiency, a synchronous motor phasor diagram can also provide information about the motor's voltage regulation, stability, and ability to maintain a constant speed under varying loads.

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