Medium Voltage SMC-Flex Soft Starter - Pulsating Motor

[PLCTech105]

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?!?

 

[Jraef]

I posted this in another forum you asked on, so for everyone else's benefit, here it is again:

Once the bypass contactor closes, the soft start portion of this Soft Starter is no longer involved in any way… UNLESS there is a problem in the motor circuit.

If the motor circuit current begins to exceed the rating of the bypass contactor, the controls will switch BACK to the SCRs in order to current limit, then when the current drops, it will re-close the bypass contactor. Vacuum contactors only move about 1/8 inch, so they don’t make a lot of noise with the door closed, so you may not hear it happening. But that could account for the pulsing. The current rises, the bypass contactor opens, the SCRs current limit, the current drops, the bypass closes again and the current rises again. But because that current rise is so low and slow, it is not being detected as a Short Circuit by the controller. That gives rise to the theory below.

So then the question becomes, why is your motor current surging? You already replaced the motor so that’s not it, but if the motor leads are “leaking”, that could do it. You say that you performed a hi-pot test, but how was that done? A typical hi-pot test just applies a voltage and insures that the insulation dielectric is intact “enough”. But you should be able to also perform what’s called a “tip-up” test where you monitor the rise in voltage AND current with the tester to see that they increase in synch. If the current rises faster than the voltage, you have a small leak. The 10A might be the result of your sampling rate of your meter, but being that the contactor is rated 400A and the motor FLA is already 330A, it wouldn’t have to increase much to exceed the rating of the contactor, meaning the “leak” might only be a few volts, so it might fall into an error margin as “nominal”. That’s why the tip-up test might show it instead.

If you have a way to monitor the motor voltage during these pulsations, you would see the voltage dropping when the SCRs go into current limit. The SMC-Flex monitors only the line voltage, so to see this phenomenon you would need something else monitoring load voltage.

 

[Isaiah]

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?!?

Does this need 1.25% on the motor branch


Sent from my iPhone using Tapatalk

 

[Jraef]

Does this need 1.25% on the motor branch


Sent from my iPhone using Tapatalk

If you are referring to the motor lead conductor sizing, yes, you always need that.