Torque input of synchronous machine

[FOIWATER]

I am reading from grainger about transient stability.

I know this will be a amateur question, but...

It says that when a prime mover is turning a synchronous machine, there is a equal and opposite counter torque (electrical counter torque) when the machine rotates at synchronous speed.

This is odd to me, as I would expect an equal and opposite counter torque to stall the machine.

 

[NascentOxygen]

It says that when a prime mover is turning a synchronous machine, there is a equal and opposite counter torque (electrical counter torque) when the machine rotates at synchronous speed.

This is odd to me, as I would expect an equal and opposite counter torque to stall the machine.

I think he is just stating the obvious. That, under stable operation, an equilibrium exists where the applied torque is exactly countered by an equal and opposite electrical torque. (If this weren't so, then the rotor would tend to accelerate or decelerate, until an equilibrium becomes established.)

If the driving torque changes to a new level, then a new equilibrium point must be established such that the new electrical torque exactly counters the new driving torque. (If this new equilibrium can't be established within certain parameters, then, yes, it will fall out of sync.)

It should be clearer if you think of the generator as being "directly" connected into the power network, alongside many others, so that the prime mover torque alone determines whether this machine is generating or motoring.

 

[jim hardy]

This is odd to me, as I would expect an equal and opposite counter torque to stall the machine.

stall? Thought experiment :: as N O said, equal and opposite means there's none left over to accelerate or decelerate the machine's rotating mass.
So ω stays constant , presumably at ω_synchronous

 

[FOIWATER]

got it thanks guys.

This was what I assumed of course, a torque difference tends to change accln to a new steady state speed, but I was having trouble trying to understand that for some reason.

 

[NascentOxygen]

This was what I assumed of course, a torque difference tends to change accln to a new steady state speed, but I was having trouble trying to understand that for some reason.

Well, it won't change to a new speed because its speed is fixed by the frequency of the grid it is directly connected into. What can change is its angle of operation, relative to the rotating field, and as this changes so does its developed torque (it obviously draws more current in doing so). The angle can only slip back so far, though, otherwise it will fall out of sync. There is a limit to everything.

 

[FOIWATER]

Yeah I was referring to torque difference in general. What you are saying makes sense, the reason I asked the question is because I am writing a program that simulates synch. generator swing angles for large disturbances. We are finding critical breaker trip times to limit loss of synch. I have written and tested the program vs IEEE standard buses (14,30) and AEP standards (57,118) and it's working. There was just some issues with the torque that I was wanting to understand better.

Thanks