Synchronus Machine Characteristics (AFNL & AFSC)

[cpscdave]

Hi everyone,
I'm a little confused what is going on in our textbook. There are a number of questions that state the table below has the Open Circuit (No Load) and Short Circuit Characteristics of some synchronous machine.
But then the columns don't seem to match that wording (I'd think that open circuit would be first)

I'm wondering if someone can confirm this thought process is right.

If you are looking at a GENERATOR the short circuit filed current (AFSC) will be larger than the no load field current (AFNL) resulting in a short circuit ratio (SCR) less than 1

If you are looking at a MOTOR the AFSC will be SMALLER than the AFNL resulting in a SCR greater than 1?

Does this seem correct?? Or am I way off base here??
Thanks

 

[zoki85]

No, it isn't correct. Short circuit currents are larger than no load currents for motors too

 

[cpscdave]

So then you can never have an SCR > 1?

This textbook is borked it has a couple examples where they calculate the SCR to be > 1

 

[zoki85]

Sync generator and sync motor are in principle same things. Only the direction of the flow of energy is reversed. Therefore, I don't know what they are talking about (or I misinterpreted )

 

[alphy]

for bringing up this confusion with the textbook, it's important to clarify any discrepancies in order to fully understand the material. From my understanding, the table is showing the characteristics of a synchronous machine under two different conditions: open circuit (no load) and short circuit. However, the columns may be labeled in a way that is not intuitive.

To answer your question, you are correct in your thought process. In a generator, the short circuit field current (AFSC) will be larger than the no load field current (AFNL), resulting in a short circuit ratio (SCR) less than 1. This is because when the machine is short circuited, there is a higher amount of current flowing through the field winding, compared to when there is no load.

On the other hand, in a motor, the AFSC will be smaller than the AFNL, resulting in a SCR greater than 1. This is because in a motor, the machine is consuming power, so the field current is not as high as in a generator.

I hope this clarifies any confusion and helps you better understand the characteristics of synchronous machines. If you have any other questions, please don't hesitate to ask.