Synchronous generators

I'm not a hands on electrian, but to give a little info on question (1). others will need to fill in. Synchronous generators where used in South East Queensland Autralia a few years back, when a new power station was being commissioned in the north, resulting in a voltage drop in the South East, which varied with the system load, this was corrected with syncronous generators.

Yes the prime mover was removed, so how did they run up the generator and syncronise with the system, they used a welding machine, I dont know the full details of how the connection was done, there are a number of possiblities. They split the switch yard to isolate it with an adjoining unit, the syncronuus generator was run up to turning gear speed then they where coupled together. Then the adjoining genertor was run up with the syncronised generator following, The adjoining generator was then matched to the system and the switch yard closed up.

Also a gas turbine generator can be used, where after sycronising the turbine is disconnected and shut down. Hope this answers part of the question regarding the prime mover.

Hope I understood your quenstion correct, sorry if I'm wrong.

The answer for your second question about back emf in sinchronous generator is:

yes, there is back emf. It acts so, that the current slows down in the direction of reactance (phase of this drop is -90 degree). We feel it by sinchronous reactance Xs, which is much greater than Ra (resistance of stator coils) and Xa (the stator coils own reactance).

For the third question:

The reactive power doesn't cause stright energy losses but it causes inderect losses.

Because the generator current becomes greater the line resistive losses is rising up.

More of this additional current leads to generator voltage drop. To compensate it the regulator enchances the exitation current, so the exitation losses are increased too.

How can reactive power cause indirect loss? If the generator current increases due to low power factor(i.e. increased reactive power), then wont the resistive line losses also be considered as active power? What I mean is that the losses due to increased current are after all to be considered with the active power handling capability of the machine. Then how do you explain the reactive power capacity?

Ans#1. Excitation cannot be removed-there will be disaster -Salient or Cyl.poles .

You can increase/decrease to make lead/lag PF to suit.

If you do not need the motor to generate torque( do motoring work--) you can run it as a synchronous Condenser.

Naturally--a Bus( so-called Infinite) has to keep the system running!

If the BUS is Very powerful - one low-power Turbine running an alternator connected onto this BUS may be possible to be allowed to be fuel/water/steam cut off and the Alternator will continue to be run by the BUS.

Re #2<In a synchronous motor, is there no back emf generated>

Back EMF is a must have.

<How can it's physical existence> Reverse of Motor theory!

Why cant excitation be removed? Why will there be a disaster?

Question 1

In order for it to become a reluctance motor the rotor field must be short circuited.

This causes short circuit currents to be induced in the rotor windings, and the machine then becomes an induction motor, It is no longer running at synchronous speed because the poles are moving and are no longer polarity defined.

If the field is open circuited i.e. no current flowing in the rotor, there will be no magnetic flux created and no counter EMF generated. hence, the stator winding will destroy itself on overcurrent.

If there is an external prime mover torque source connected, it's not a motor.

Question 2 Of course there is a back EMF either from the short circuit currents or the DC excitation. But as I have mentioned in above, no rotor current, no flux, Ka Boom.

Question 3. Reactive power is still current even though it is not in phase with the voltage. Ohms law says that any current flowing through a resistance creates heat and heat is energy, and in this case energy/power lost.

Oh, I forgot to stipulate the conditions.. this machine MUST be operating on a power system in parallel with another source of power.

One neat thing about synchronous motors, they can use incomming killowatts to generate out going capacitive or inductive reactive power depending on the level of field excitation.

Hope this answers your questions

Snakers.