2-phase soft starter motor noise

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I am a pump station contractor, and when starting up a station with two 20 hp, dry-pit submersible, 240 volt, 3-phase sewage pumps, whose Pump Control Panel had 2-phase soft starters for both pump motors, one of them exhibited a nasty-sounding starting noise. The leads were swapped, and the noise stayed with that motor. It was suggested that the starting noise might be an unintended consequense of the 2-phase soft starter, so we ultimately installed a 3-phase soft starter for that motor, which cured the problem. Swapping leads again, the other motor is OK with either the 2- or 3-phase soft starter, but the problem motor makes the noise on the 2-phase, but not on the 3-phase soft starter. It seems to be common knowledge (or common sense) that the 3-phase soft starter is superior and applies its torque more evenly, but I can't seem to find any documentation to the effect that a tendency for motor starting noise is a drawback of 2-phase soft starters. Can anyone steer me to some technical support, or shed any light on this?
 

Jraef

Moderator, OTD
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Electrical Engineer
I am a pump station contractor, and when starting up a station with two 20 hp, dry-pit submersible, 240 volt, 3-phase sewage pumps, whose Pump Control Panel had 2-phase soft starters for both pump motors, one of them exhibited a nasty-sounding starting noise. The leads were swapped, and the noise stayed with that motor. It was suggested that the starting noise might be an unintended consequense of the 2-phase soft starter, so we ultimately installed a 3-phase soft starter for that motor, which cured the problem. Swapping leads again, the other motor is OK with either the 2- or 3-phase soft starter, but the problem motor makes the noise on the 2-phase, but not on the 3-phase soft starter. It seems to be common knowledge (or common sense) that the 3-phase soft starter is superior and applies its torque more evenly, but I can't seem to find any documentation to the effect that a tendency for motor starting noise is a drawback of 2-phase soft starters. Can anyone steer me to some technical support, or shed any light on this?
I wrote a white paper once a long time ago on this, when I worked for a company that made only full 3 phase soft starters and these 2-phase abominations started showing up. That company changed their website and removed access to technical papers unfortunately. But yes, it is a well known side effect. Ramping 2 of the phases while the third is at full voltage inherently creates a severe current imbalance. Unbalanced currents in a 3 phase motor cause what is called "Negative Sequence Currents" to flow in the stator. This then creates a separate magnetic field that is rotating in opposition to the fields that are creating torque. That separate counter rotating field is significantly weaker, but does no useful work and in fact it negates some of the torque being produced normally, resulting in the motor essentially "fighting itself". So for any given unit of current applied to the motor, the motor produces LESS torque than it would with a full three phase SCR control soft starter. The (somewhat warped) theory on why this might be considered "ok" is based on the concept that, hey, if you are using a soft starter, you ALREADY know you want reduced torque so really, this is just a little less but is only noticeable if compared side by side. Using 2-phase soft starting also creates a high DC component in the current flowing to the motor, which makes it vibrate more than it should. One supplier who sells 2-phase soft starters, Siemens, claims to have an "algorithm" for firing the 4 SCRs in such a way so as to reduce that DC component and reduce the vibration. When I worked for them I challenged that concept and they showed me that it did work (from what I saw). If your 2-phase soft starter is not a Siemens however, this will be an issue. But even if it is, the DC component issue is not the whole story.

Purposefully creating unbalanced voltage and therefore Negative Sequence Currents in the motor ALSO has the effect of converting that lost torque directly into heat inside the motor, especially the rotor. The heating of the motor in general is an issue, but on simple centrifugal pumps and fans that accelerate with little load and short ramp time, this is probably tolerable. But 2 phase soft starters should NEVER be used on heavy loads or deep well pumps where it takes a long time for the water to lift to the top of the pipe. The Negative Sequence Currents are rotating at twice the frequency of the rotor, so because it is higher, the flux doesn't penetrate fully and the heating effect becomes more concentrated on the surface of the rotor, then because the overload Relay or protection circuit is based soley on the STATOR current, the effect of this added heating is not seen. This can lead to severe over heating and possibly damaging the rotor bar connections to the end rings.

Unfortunately, that may be the source of your noise. Any minor flaws in the bearings, laminations or frame, even if within tolerance, can amplify vibration to becoming audible, but if the noise is only severe in one of the motors, I'd be concerned that you have already caused a rotor bar separation in that one. The noise may be less noticeable when the motor gets balanced voltage because the inherent torque stresses and vibration from the DC component have been removed, hence it appearing to go away with the 3 phase soft starter. But that is just the symptom going away, not the problem. If a rotor bar has separated, it does not braze itself back on. With all things being nice and smooth, people often live with broken rotor bars for a long time and never realize it, because it just results in minor torque pulsations that will only show up in a vibration spectrum analysis. But that does not mean it is not causing long term problems that will get worse. You might want to order a spare motor to have on hand...
 
2-phase soft starter motor noise

Thanks, Jraef, that's a pretty definitive explanation of the problem. If anyone knows where anything in the literature discusses this motor noise issue on 2-phase soft starters, that would also be very helpful.
My gut feeling is that our motor is OK. The noise was bad enough (about a second-long BRAAP!!! from near the top of the base-mounted vertical motor), that it's probably only been started on the 2-phase starter maybe 20-30 times. On initial startup, when it was noticed, it was found to vary in intensity and duration when the soft start parameters were changed, but never went away. Then, when taken to a motor shop, it only made the noise when the jobsite 2-phase soft starter was brought there, and started a few times. At the motor shop, the noise did not occur with an across-the-line starter or with a VFD. Initially, the station didn't go on-line pending a solution to this problem, then we reverted to an across-the-line starter, with no abnormal noise, and no problems on utility power, until it was found that the on-site generator couldn't start the pumps with ATL starters, due to a 40% voltage dip. Anyhow, with that few starts on the 2-phase RVSS, and the fact that it hasn't gotten worse, and that it sounds fine when started ATL or with its new 3-phase soft starter, I think our motor is OK.
Thanks again.
 

kwired

Electron manager
Location
NE Nebraska
Though it is throwing more money at a problem that maybe could have been similar or even less cost by going with a true three phase soft starter in the first place - would a line reactor help with the problem?
 
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