# Why are salient pole rotors preferred in hydro plants?

1. Jul 12, 2017

### LagCompensator

Hi all,

I tried to post in this post https://www.physicsforums.com/threads/salient-and-non-salient-pole-synchronous-generator.878198/ but I now realise that creating a new thread might have been better.

So...
• Salient rotors are not used in high rotational speed applications, and we must use cylindrical rotors due to mechanical reasons, OK I get that.

• Why are salient rotors the "preferred choice" at low speed applications, such as in hydro power plants? What are the advantages of salient rotors (electrical and/or mechanical), since hydro power plants use salient rotors over cylindrical rotors at low speed applications?
• Are there any benefits of having a non-uniform air gap compared to having an uniform air gap, and hence uniform flux distribution(?)?

Best regards

2. Jul 12, 2017

### Dr.D

If you want to make 60 Hz power (or 50 Hz), with a low speed generator, it takes many, many poles. The construction of so many poles on a round rotor would be very difficult to get enough turns on each pole. Look at a round rotor and see how the turns are laid into the surface of the rotor. Commercial generators almost always use 3600 rpm (or 3000 rpm) with a two pole rotor. Now imagine trying to wind perhaps 20 poles on a round rotor.

3. Jul 12, 2017

### jim hardy

4. Jul 13, 2017

### LagCompensator

Thanks, it makes sense. So basically a cylindrical rotor would be preferred in all cases due to uniform airgap, and hence reduce distortion in the airgap flux? However due to the slow operating speed of i.e. hydro plants we need to use salient rotors, because we would require a lot of poles to get our 50/60Hz waveforms, something that would would not be feasible on a cylindrical rotor.

5. Jul 13, 2017

### Dr.D

Just a small aside here. If we look at the picture Old Jim posted, it is evident that the air gap variation is only a couple of inches or so. Much more extreme variations occur in the salient 4-pole alternators often used with diesel engines for standby power. In that case, four separate pole pieces are usually bolted to a much smaller center piece, with rather large gaps between the pole faces. The air gap variation is often quite large, and yet, these alternators work very well.