Soft start issue

[mohpower]

Hi everyone,

As I understand the problem, the soft-starters, motors, and pumps start and run normally. Intermittently, as the pump is turned off the Soft-Start trips on a fault code for "faulty power module" with a secondary possible problem of "faulty unit".

The system uses a 600:120 V PT and a 120 V to 24 Vdc converter to power the starter.
There are two (alternating) identical pumps both experiencing the same symptoms without any apparent pattern.
About 12 feet above the sewage pumps are symmetrical pinch valves for each pump, so there is a possibility of some reverse flow of the motor and perhaps some back feed as a result.




I've never seen a situation like this where the problem is so random.

The big question, is the power supply or the soft-starter affected by reverse flow of liquid in the pipes?
Have intermittent problems like this come up in the past?
Should the starter be replaced based on this symptom?

Please provide any suggestions you may have or forward on to help me dig out a solution. All you help is appreciated

thank you

 

[gepman]

Does your softstart have a softstop? If so how is it configured?

It might also be helpful to have the brand and model number since different brands and models have different features.

 

[jraef]

Ditto on gepman's requests, but to pique your interest a little, yes it is possible on at least two fronts.

If you are using the pump control / decel function and your initial torque drop is too low, flow can reverse instantaneously and if the check valve is slow, may backspin the motor at a super-synchronous speed. But because the motor is still being excited by the soft starter decel function, it will regenerate. Then depending upon your soft starter's design, the voltage present on the load side may trigger the Shorted SCR detection circuit.

Either that, or the reverse power creates a current spike and therefore a di/dt situation at the SCRs that causes one or more of them to self commutate (fire on their own) at the wrong time, which is being detected in a similar manner by the soft starter. The solution then would be to make sure the initial decel torque setting is high enough to maintain pressure in excess of the head and bring it down slowly enough for the check valve to actuate before flow reverses.

If you are not using the decel/pump control option, residual magnetism in the motor's windings may be doing the same thing if flow cause the motor to backspin, but this is less likely unless you have some extreme head pressure where the flow would reverse within milliseconds.