# Loading a synchronuos generator

1. Aug 3, 2015

### cnh1995

In synchronous generator, field electromagnet rotates and induces voltage in the stator coils. So, when the generator is loaded, it slows down. If the field is stationary and armature is conducting (as in dc generators), there will be "current carrying conductor in a magnetic field", resulting opposite motoring torque, slowing down the generator. But here there is a magnet(rotor) moving in magnetic field(of the stator conductors). How does this slow down the generator?

2. Aug 3, 2015

### Staff: Mentor

It doesn't slow down at all, except very brief small deviations. As the name synchronous suggests, it continues to rotate at the same speed as the grid. It is "phase locked" with the other synchronous generators on the grid.

3. Aug 3, 2015

### cnh1995

I am talking about one in laboratory. It did slow down on loading and we got a load-frequency curve. What slows it down? I mean I can't see where and how I can apply motor effect rule. What happens to Lenz's "oppose the cause" policy?

4. Aug 3, 2015

### Staff: Mentor

In that case, your entire "grid" slowed down, not just the one generator.

Conservation of energy requires that power into the grid, minus losses, must equal power to the load. If you hold mechanical power to the generator contant, then any change in load causes frequency to change until that balance is restored. Ditto if you hold load constant but change the mechanical power driving the generator.

Operation of any synchronous generator supplying power to a grid can be summarized as follows.

1. Rate of change of speed of the generator equals mechanical power minus losses minus electrical power sent to the grid.
2. Rate of change of angle equals generator speed minus actual frequency of the grid as a function of time.
3. Electrical power sent to the grid is proportional to the angle (for small pertubations.)
4. Grid frequency settles to whatever value balances generation minues losses minus loads. i.e. conservation of energy.
Together, those four statements describe the differential equations for a second order damped oscillator. Any change causes frequency to oscillate around the steady state value. The steady state frequency is that which conserves energy.

In the large scale power grid, control systems adjust the mechanical power to all generators to restore frequency to the nominal 60 hz or 50 hz value. In a labratory, or on a small island, frequency can vary a lot.

Does that answer your question?

5. Aug 3, 2015

### cnh1995

Well I understand that the alternator must follow the law of conservation of energy. What I don't understand is "how" it slows down. In a fixed pole rotating armature type generator, motoring torque due to armature current opposes the prime mover and slows it down. Similarly, how could I apply motor effect rule (left hand rule) in synchronous generator?

6. Aug 3, 2015

### Staff: Mentor

You're making it more complicated than necessary, and you're off target.

Neither the prime mover, nor the load are specified here. Each of them will have a slope delta-power/delta-frequency. In general, either slope could be positive, negative, or zero. The steady state change in frequency depends only on the sum of those two slopes. It has nothing to do with the electrical concepts in your train of logic.

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