Challenge

The customer’s original drive was a 1992 vintage Saftronics DC drive that was part of a system purchased through an auction or plant closing as the system was well-used and showed signs of multiple previous owners. The field supply initially went out and damaged the field of the rewound DC motor.  The customer had previously utilized another group to service the drive and their skillset consisted of AC drive familiarity, but very little DC knowledge- a rare commodity. This service provider replaced the field supply with a brand new Emerson model. The challenge with this solution is that while the Emerson field supply is good, it is akin to putting mag wheels on a Pinto. I personally would have utilized a Carotron field supply because of its robustness and simplicity. The Emerson unit was set 33% higher than needed, the interlocks for field loss where dead-ended in the spaghetti-like jumble of wires and the tuning was incorrect. As a side note, when tuning field supplies, generally the integral rate is high versus the proportional gain. In this case, the field was tuned exceptionally hot.

B&D Technologies has vast knowledge and solution capabilities for the plastics industry and because of this, the Baldor motor plant refers customers to us.  This particular customer, a plastics manufacturing plant, had lost several 300HP DC motors on their key extrusion line and they desperately needed to know why the motors were failing.  We immediately dispatched team members of our in-house Applications Engineering Group (AEG) to provide a complete diagnosis and quickly get this customer’s facility back online.

Background and Diagnosis

To add to the bedlam, the motor thermostat and other protection measures, including the forced ventilation of the motor (the main motor blower), were dead-ended in the wiring jumble (see “Photo 1” below for reference).

The field failures all showed damage on the entire field winding. This indicates that too much current caused the failure. It is unclear whether the weakness came from the blower, thermostat or field current maximum— or a combination of all these— caused the failures.

Bottom line, plant production was priority and the system needed to be rapidly fixed.

Solution

Our AEG worked to identify a prompt action plan. The AEG originally looked at the ABB EP version of the DCS800 drive because it was a pre-built solution complete with control transformer, contactor and fuses. However, the lead time was a day too long, so we decided to build the solution ourselves as it would take just two days (16 hours) to complete.

While the drive and other components were being delivered, we created a new schematic from an understanding of the Saftronics design. Saftronics, like many other United States based DC drive companies, prefer to use the DC side (loop) contactor instead of the European version of the AC contactor. The AEG prefers the AC contactor approach based on complete isolation of the thyristors when the drive is disabled.  This approach minimizes leakage and allows you to easily add safety measures if the situation requires it.  We also added a beefed-up control transformer, line reactor, 900A fuses on the input side, as well as one on the positive armature lead (non-regen design).

 

Process

Working quickly, on day one we gutted the system, finished the schematic and started installing the new components (see “Photo 2” below for reference). We used a stud welder and created anchors for large hardware including the reactor, drive, contactor and transformer— saving time over traditional installation methods. We used flexible MCM250 cables in parallel with crimped ends instead of lugs.

Day two consisted of finishing the control wiring and adding the interlocks for the blower, hydraulic pump and motor thermostat. These components were carefully installed as a failure of any one of these components would create a drive shut down. The field for this size DCS800 was on board and fixed at 6.2 amps. See “Photo 3” below for reference.

We began commissioning around 2:00 p.m. on the second day. DC drive commissioning is significantly faster than it used to be. The parts of the commissioning process are still the same—field tuning, armature tuning, I/O configuration, feedback device calibration, speed controller and field weakening. Since the drive was non-regen without external feedback, two of the sections were bypassed based on being non-applicable. Alas, the drive ran without an issue and maintained a perfect field and tight speed control based on dozens of step-response tests.

DC Extruder Drive Building Process

Opinion

Over confidence knows no boundaries in this business and it is easy to get in over your head. The key to success is honesty. B&D Technologies has strong customer loyalty because we display fairness and honesty rather than smooth talk and whizzy tricks.

The technicians from the original service company should have looked more closely at the failures and the interlocks. And if they needed help, the folks at Emerson would have provided someone to properly tune the field. Instead, the customer incurred the cost of the first incorrect servicing and two rewinds while experiencing complete production shut-down—to the tune of roughly $100,000.  This cost summary does not include the expense for diagnosing the root problem and deploying the correct solution— which included replacing the entire drive.

Last Words

The ABB Authorized Value Provider program highly resonates with customers who know what it means. To distill all of the information into a single word is “trust”. ABB trusts B&D Technologies and our Applications Engineering Group will always do what is right for our mutual customers.

C. Tolbert DOE