All,

We have 400A medium voltage (2,300V) SMC-Flex soft starter on a 330 A Motor.
About 1 year ago we noticed a significant pulsating at a consistent frequency (approx. once per second)... never faster or slower only louder or softer.
When the load is removed and the motor is uncoupled the pulsating goes away.
The cables from the SMC to the motor were verified with a Hi-Pot test.
We had a power quality study done.
- Voltage varies from Leg C at 2,354V to Leg A at 2,462
- Harmonics are almost non-existent
- Phasor on leg B was 2 degrees off
- Amperage has a swing of 10 amps while pulsing

The process has no identified issues. The pump impeller is good and new, suction, and discharge are all working as they should be.
The outlet flow matches with the pump curve for amperage and PSI.

The motor is physically moving when examined with a strobe gun at the same pulse rate and gets hotter the more flow increases by opening the valving down stream.

The motor was removed and replaced 2 times and immediately the pulsating returns so we know the motor itself is not the issue. This unit has been in stalled for about 15 years and until last year this had never occurred.

At this point we are not even sure what we could check next. Has anyone seen an issue like this?!?

 

scopegranites,

I apologize if I was unclear. The pulsating motor is actually the motor moving in a counter-rotational pulse. This is causing excessive heat and 10 amp swings. With a strobe gun set to match the RPM you can physically see the motor shaft reversing direction instead of a steading state. I would expect to see this effect with a high 5th harmonic, say above 5%. However I have almost no 5th harmonic on this system according to our power quality metering.

Also the soft start has no issues during the soft start only when in bypass running at full line voltage.

Regards,

 

Is the bypass a contactor? If so, and the bypass contactor is pulled in, then the SMC's electronics shouldn't be involved at all. At that point, it's just as if you were using an across-the-line starter. That seems to point to an issue with the mechanical load or incoming power.