Case Study – The GBS Group Provides Valuable Insight Into Damaged Propulsion Motor Repair Saving Time and Added Expense
Military Sealift Command and Maersk Line Limited have Predictive Maintenance programs in place to perform electrical testing of large DC propulsion motors. During the testing it was reported the armature and stator had problems and needed to be replaced. The report stated armature banding and Armature coil insulation appeared damaged due to overheating and AC drop test performed showed three bad field coils. The recommendation was to remove the motor and have it overhauled in the shop.
The propulsion motor is a General Electric 800 HP DC Electric Motor, Model 42G977. The propulsion motor is located in shaft alley located below the waterline. The only way to remove this motor from the ship is a hull cut. The motor weighs 41,300 Lbs. and does not fit thru the existing deck penetrations. To reduce the time in the shipyard, it was determined that GBS would replace the bad field coil and armature while in place. The GBS Group had responsibility to ensure the motor was properly overhauled and also to ensure the ship schedule was met (Owner Representative).
The repair of large AC and DC motors is often required to be done onsite vice removal to dedicated motor shops for repair. One of the technical challenges for in-place repairs is to fully understand the problem associated with the motor or control system and develop a plan to restore full operation in a timely and cost effective manner. We have the technical know-how and past performance assessing electric motor and control systems and determining the required course of action to fully restore operation. We are able to assist in key decisions that have added value to the repair process.
The GBS Group provided shipyard and subcontractor oversight for the interference removal and initial motor disassembly. Once the motor was split, a thorough inspection of the motor armature and field coils was accomplished. Testing indicated the field coils were connected in a series/parallel configuration. This caused the initial test results to be inaccurate. The field coils were separated and retested. During testing it was determined that the number 5 field coil was shorted, but all remaining field coils were satisfactory. GBS proceeded to have the armature cleaned and inspected. The armature banding, that was originally reported bad, was in good shape and not loose or damaged. A full set of electrical tests were performed on the armature and all tests were sat.
Based on accurate data from testing and visual inspections of the opened motor, a meeting was held with operator (Maersk Line Limited) and Owner (MSC) to discuss repair options. It was GBS recommendation to replace the damaged field coil in place and not remove or replace armature. All parties agreed this was the proper course of action, as it would correct all conditions with motor and reduce repair time and costs. All repairs were accomplished and motor fully tested. Repair time was reduced from an estimated 5 weeks to less than 3 weeks.