# Why are salient pole rotors preferred in hydro plants?

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

## Answers and Replies

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.

LagCompensator, Bavuka and jim hardy
jim hardy
Gold Member
Dearly Missed
Back to basics:

Voltage is in proportion to relative velocity between the conductor and the magnetic field.
In a slow turning water turbine it takes huge diameter to get much velocity.

I think you'd rather assemble this from separate pieces than machine it out of a solid forging.