what is the definition of a synchronous motor "hunting" ? Ive looked it up but the web only has the reasons why it happens, for example a variation in load and phase shift on the system, but no definition?

 

A simple definition might well be ....

Hunting is the sustained oscillation of the rotor following a disturbance in a synchronous machine operating on infinite bus bars.
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(Sorry - inserting an actual link to the source scribd page '/doc/36414602/Synchronous-Machine' doesn't work)

I would probably clarify the above somewhat and state that the oscillations are deviations (large or small) about the synchronous rotor speed where the rotor does not lose synchronism altogether.

 

We have had some experiences with sites that make material available, either without attribution or which is under copyright protection. Those sites are blocked.

 

Hunting is the behavior of a feedback control system that for one reason or another is unable to reduce the system error to zero. It works like this:

1. An ERROR value is computed.
2. A correction is applied
3. The system overshoots the mark
4. An ERROR value with the same magnitude but the opposite sign is computed
5. A correction is applied in the opposite direction
6. The system overshoots the mark

Wash, rinse and repeat forever
In non-linear systems this may also be called "limit-cycling". Linear systems cannot produce this behavior.

 

So what are you trying to say once it starts hunting it will carry on hunting? I presumed the soul purpose of the closed loop was to maintain steady-state conditions

 

A control loop in "hunting" condition may or may not be problematic. Modification of the control loop parameters (say by reducing loop gain or increasing damping) will often reduce or stop hunting.

In synchronous machines the rotor speed is essentially 'locked' to the infinite bus frequency. Damper windings are often employed to reduce rotor speed hunting about the synchronous speed.

 

So what are you trying to say once it starts hunting it will carry on hunting? I presumed the soul purpose of the closed loop was to maintain steady-state conditions

soul -- is what the folks who preach are trying to save. Mine is unredeemable!
sole -- the singular or only, as in the "sole purpose"

Driving the system to a steady state condition is indeed the purpose of closed loop control. However, not all systems can be brought to a steady state by any sort of control. This is especially true of the non-linear systems. For a fascinating introduction to non-linear systems check out Strogatz,

http://www.amazon.com/Nonlinear-Dyn...=sr_1_1?s=books&ie=UTF8&qid=1300065614&sr=1-1