Synchronous Machine Design

[kevd]

I am curious to know the following, if I had two synchronous generators, both rated equally for kVA and voltage, if they had different sub-transient reactance, say one at 10%, the other at 20%, how would they physically differ in internal configuration/construction or size? Secondly, how does a machine designer modify the sub-transient reactance and what does he/she have to trade elsewhere in machine to achieve this?

 

[rbulsara]

I dont think you can tell any diff in X" by looking at any physical motor or its winding or parts.

I am not a motor designer, but the X" depends on type of winding, pitch of the winding, type of winding material, type of core material (grade of steel), lamination thinkness, exciter charactersitics, etc. and may be more things.

I had a two clients who replaced their synchronous alternators. One replaced them (not knowingly) with units half the X" than the original ones. Other replaced his new units because the 'pitch' was not as specified. The pitch was different because they met the specified X" requirements, but in doing that they altered the pitch. In both cases units look identical to originals and even fit to the same frame and engines...so there you go..

 

[jbartos]

Suggestion: The subtransient reactance Xd" is often given by the amortisseur winding reactance. If the Xd" is smaller the winding is somewhat bigger and vice-versa

 

[kevd]

Thank you both rbulsara and Jbartos.
Makes sense, I am aware that the amortisseur winding has a significant effect. In fact some synchronous motors have no amortisseur winding and this means the X"" is very high so the fault torque is very low.

 

[jbartos]

Suggestion: Reference:
M.G. Say "Altarnating Current Machines," John Wiley & Sons, 1978.
Page 418 Damping
Damping circuit to form "damping cage"