Problems With Toshiba/mototronics Mvc3 Soft Starter

Hi there, this is my first time posting here, but not my first time to visit this board. I have gained a lot of insight into drive and soft start theory and operation by reading the postings left here over the past few years.

Now I have a problem of my own, one with a Toshiba/Motortronics built JKSSS+/MVC3 soft starter. The starter drives a 1750 HP 2300V 3600 RPM motor, which drives a pump used to cut coke out of process drums at the refinery using high pressure water. The reason for the use of the soft starter is to limit the affects of water hammering on start up of the system, as water is moved hundreds of feet from ground level up to the top of coker cutting derrick. To date, we have two soft starters installed on two 1750 HP motors. The system setup has proven to work on both pumping systems, but not without problems.

In the case of one of the pumping systems, we have experineced erratic trips on the soft start system, with the controller reporting a short circuit trip running the startup operation of the soft starter. Sometimes the soft starter will successfully accelerate and startup the motor for 3 to 4 successful attempts, and the it will fail to startup the next time. When soft starter problems occur, we place the starter in bypass and start the motor DOL. We are able to still do this within the system, as we have a three-way motor operated bypass valve installed on the line, which will slowly fill up the line and bypass a portion of the water flow back to the holding tank. Once the line is filled with water, a limit switch in the line is made, which tells the valve to go to full flow. This is used as opposed to using soft start on the motor. The valve currently is use today does not suffer from leakage issues, thus the valve does not affect soft start usage during startup when the valve is effectively disabled from the system. Thus we can switch between the two system setups, but it requires disabling the three-way valve before operating soft start, as the more reliable mode of operation today is still to use the three-way bypass valve.

I have performed maintenace checks with our plant technicians, following the troubleshooting procedures outlined in the maintenance manual, as well have had a Toshiba service engineer come on site to take a look at the equipment. Static checks have been performed on the SCR stacks (2300V, 600A sized), as well as having made fiber optic cable connections verified as properly connected and output signals from the controller boards verfied as being within tolerance. None of the SCR stacks were shorted, with resistance values falling within tolerances. We had experienced failed startup soft starts and successful soft starts with a device setting of 50% initial voltage with 500% FLA current limit, and a 20 second acceleration time on the motor. Note, I only requested Toshiba field service after I was able to simulate reoccuring trips during system startup before asking for assistance. During his visit, he found nothing wrong with the physical equipment or with the SCR components. We were able to successfully run a soft start, with water slowly filling the line with no water hammering occurring, and the motor was able to successfully accelerate out of the 20 second soft start without tripping.

Also, the motor and feeder cable systems have also been checked. The motor is just over one year old, being in service since May 2007. The motor was specifically built to handle loads at reduced voltages and for a longer acceleration time. At the time the motor was installed, the feeder cable was meggered and a PdMA MCE test was performed. The cable passed the meggering, and the PdMA test indicated no resistive imbalance on the cable or insulation problems. And when the starter is put into manual bypass and the motor starts DOL, we have not experienced a short circuit trip from the starter (blowing one of the main line power fuses).

The only thing on the system setup that is out of place is voltage imbalance measured per phase by the digital controller. There is a 2 to 3 percent imbalance between minimum and maximum values, but nothing high enough to cause a current imbalance trip. Could this cause a reported short circuit trip on the soft starter? Or could it be a load issue with the pump system? Each time the pump starts, there is a varying amount of water left in the piping, so I assume that the pump is pushing against varying pressure every time the pump starts. Could this affect soft starter performance? Could it be a failed controller board as well? We have replaced the programmable control panel before, but not the main controller boards before. As it has been discussed previously with Motortronics MVC3 products (which is the Toshiba device that we are pretty much using), the device firmware changes often. Could this be a problem that has been fixed in a later firmware update, and I just do not know about it?

As the performance of the soft starter has remained erratic, we have kept using mechanical means to slowly fill up the piping and avoid water hammering. The ultimate goal is to still get away from the use of this three-way valve, as this is a costly annual maintenance expense to pull and shop the valve, and operate without the three-way valve as we have done with the other cutting water pump system.

If any of you could provide some insight or informaiton from past experiences that may be able to help me out, I would greatly appreciate your advice. Thanks.

Andrew