Magnetostriction

Understanding the origin of the noises made by a machine is important for knowing the machine is in good health. My Powerspout has always made an unhealthy whine a lot of the time, and at the present time when power generation is at its maximum, the noise is particularly concerning.

Listen to the sound:

Although it has made this noise ever since it was commissioned 8 years ago, only recently have I come to understand what its cause is: it is caused by a phenomenon called magnetostriction. 

Magnetostriction is when minuscule changes in dimension occur to the steel cores of the alternator's stator windings as the magnets of the rotor pass over them. The constantly changing magnetic field causes the cores to hum a bit like tuning forks. 

All voltage transformers on the grid network can be heard to have a low pitched hum for the same reason but their hum is based on the mains frequency of 50 Hz; in the case of the Powerspout's alternator the frequency is of the order of 1000 Hz so the hum sounds like a whine. 

I calculate it to be about 1000 Hz from there being 56 magnets, each passing a stator core 20 times per second: so there are 56 x 20 = 1,120 changes in magnetic field every second for each of the 42 poles of the stator; (turbine rpm is 1200 at the present power level of 920 W; 1200 rpm = 20 rps).

The bearings in the Powerspout have run for over 4 years without change and without added grease, and I am particularly listening out for signs they may be coming to the end of their life. The whine of magnetostriction makes this rather difficult but there is a window of opportunity to hear how the bearings are doing when the turbine is shut down and as the rotor comes to rest. 

I only hope I don't miss the sounds of imminent bearing failure if they appear.