Electrical Noise - Sleeve vs Ball Bearing
Electrical Noise - Sleeve vs Ball Bearing
(OP)
We have an application utilizing a PSC motor with a type of triac type speed controller. The application is a motor in a blower, and the speeds run dictate that a ball bearing design should be used. Situation is that if a motor with a ball bearing construction is used, the motor is noisy (low electrical hum - typical of saturation with a speed controller). However, if we change to a rigid sleeve construction (motor is identical electrically), there is no noise. Vibration pickups have confirmed both contructions transmit the same vibration spectrums, but a sound spectrum shows the sleeve bearing does transmit sound differently.
One theory is that there is a current being induced in the shaft. Would a sleeve bearing shield this current, thus creating a "quieter" motor? Or s it more of a dampening effect on transmitting the motor vibration to the blower assembly?
One theory is that there is a current being induced in the shaft. Would a sleeve bearing shield this current, thus creating a "quieter" motor? Or s it more of a dampening effect on transmitting the motor vibration to the blower assembly?





RE: Electrical Noise - Sleeve vs Ball Bearing
The difference in firing gate current levels that exists in QI and QIII does sometimes produce a hum with same frequency as mains frequency. So, if your sound has lots of 50 (or 60) Hz, you can be sure that it is a malfunction in the controller. If you, on the other hand, have a dominating frequency twice the mains (100 or 120 Hz), then you have a purely magnetic/mechanic hum.
Are motors really identical? Even if nameplate data are equal, the electrical characteristics may differ. Especially the leakage inductance, which plays a role when determining di/dt. The di/dt says how fast the holding current is reached and that may influence operation of the triac/motor combination.
A very simple test is to put an incandescent lamp parallel to the motor. It makes the triac well behaved and should eliminate the hum if it is from assymetric triac characteristics. But not if the firing pulses are asymmetrical. Check that with a scope.
The idea that something is induced in the shaft is valid for much larger machines, hundreds of HP. That will not be a problem in machines from FHP to tens of HP. And the current would not be possible to hear. There are many other forces in play that are much more dominant.
Gunnar Englund
www.gke.org
RE: Electrical Noise - Sleeve vs Ball Bearing
Magnetic saturation of the steel is fairly common with PSC motors (though typically easy to avoid), but is more common in variable speed drive applications. Hook up a motor to a dimmer switch type control, turn it down, and you will hit a point where the electrical hum picks up. A simple fix is usually increasing the stack of the motor to decrease the magnetic flux density. The odd thing in this application is the bearing system seems to be dampening 'something'. Agree that induced currents are more typical in larger motors, which is what's troubling with this application.
RE: Electrical Noise - Sleeve vs Ball Bearing
Have you actually changed bearings in one and the same motor? Or are you comparing two individuals with different bearings?
I may not have understood your root problem. Is it the hum or the fact that the hum sounds differently in the two motors?
Gunnar Englund
www.gke.org
RE: Electrical Noise - Sleeve vs Ball Bearing
The situation is application specific, which means all motors, not two individual motors. The question is why is the sleeve bearing constrution sound different than the ball bearing.
RE: Electrical Noise - Sleeve vs Ball Bearing
You have to believe me. I know about these things.
Gunnar Englund
www.gke.org
RE: Electrical Noise - Sleeve vs Ball Bearing
http://w
This link tells all you need to know about triacs, inductive loads, quadrants and saturation.
Gunnar Englund
www.gke.org
RE: Electrical Noise - Sleeve vs Ball Bearing
Back to the root question - why is the sleeve bearing dampening out the noise?
RE: Electrical Noise - Sleeve vs Ball Bearing
I think that you should try the tip with the parallel lamp. It very often makes the controller behave better. It is not a solution, but if you get a better result, you can go back to the controller designer and tell him to control the triggering better.
BTW, is the controller processor-based or using a diac or UJT? Processor-based controllers often perform a lot better than discrete component controllers.
Gunnar Englund
www.gke.org
RE: Electrical Noise - Sleeve vs Ball Bearing