Motor Testing Proves Successful
EP Editorial Staff | November 1, 2003
At its primary location in Indianapolis, IN, Allison Transmission, part of General Motors Corp., uses the Total Motor Maintenance (TMM) concept every day from motor inventory and delivery to testing and reliability of motors.
Quality network planned maintenance
Allison Transmission follows the General Motors North American (GMNA) United Auto Workers Quality Network Planned Maintenance (QNPM) process. This program provides a common process and consistent structure to ensure that equipment, machinery, tools, and facilities operate in a safe manner and are available to competitively produce the required products to meet customer needs.
Operating principles that define the direction the QNPM common process takes were referenced throughout the TMM planning and implementation process to ensure that all activities focused on accomplishing these objectives:
• Provide on-going support and direction at the plant, division, and GMNA levels.
• Ensure that manufacturing is the champion and owner of the planned maintenance program.
• Create opportunities for all employees to participate in the process
• Implement the operator involvement concept
• Pursue proactive maintenance
• Achieve world-class performance in safety, quality, throughput, and cost
• Support continuous improvement
There are 12 interdependent elements in planned maintenance that are integral to a successful process. Each element contributes to and provides support for the others. The linked elements, in total, provide the base for the Planned Maintenance Process (Fig. 1).
Commodity management is the term that Allison Transmission uses for the partnership program with its primary motor supplier. Key results from the program include improved quality of service and reduced operating and inventory costs. Allison spare inventoried motors are kept at the supplier’s warehouse. The supplier meets monthly with Allison personnel and reports on purchases, replacements, delivery time, and hard and soft savings (Fig. 2).
Using motor circuit analysis (MCA) as one of the technologies (along with infrared, vibration, ultrasonics, etc.) in the motor program, Allison can more accurately serve its customers’ needs and expectations. A technician, even with limited experience, can test a motor in minutes prior to removing it and sending it to a supplier’s motor repair shop.
Root cause analysis plays a large role in evaluating the motors with both internal MCA testing and on the supplier’s end. Upon completion of the motor repair, the supplier provides Allison with a Repair Report and a Reason for Repair Report. If the fault is due to contamination, a sample of the contamination found inside the stator windings is collected by the supplier and passed on to Allison’s technology department for lab analysis. All of this information assists the company in resolving the root cause of the motor problem and failures.
This partnership with the motor repair shop has proved to be effective. Allison can call 24 hours a day, seven days a week and have a stored motor delivered and on its dock within two hours. The response time has been invaluable in planning production schedules. Allison also has access to the motor supplier subject matter experts. As a result, the supplier is considered part of our reliability toolbox.
In the end, the motor supplier answers to Allison Transmission’s commodity management team, which includes the QNPM representative, electricians from the motor shop and reliability department, the spare parts team, maintenance supervisors, and individuals from the finance department.
Allison Transmission’s motor program is a crucial component within operations. With MCA, motors that have problems can be tested to confirm the fault before being removed and sent out for repair. If a motor problem is not found, the electrician helps the service technician find a root cause. Motors that are difficult to install are tested prior to calling machine repair personnel for installation. Motors in the supplier’s warehouse are audited on a quarterly basis with an MCA test.
Some test routes have been established because of repetitive motor failures, and these motors are tested and trended monthly as part of the MCA process. Motors with pumps are tested prior to rebuilding the pump in order to determine if it is more economical to replace the motor-pump combination than to rebuild it.
Motor circuit analyzers
After attending a motor circuit analysis seminar at the 2001 GM QNPM Symposium, Terry Bowen, Allison Transmission QNPM co-champion, believed the company could benefit from implementing an MCA program in its technology department. Prior to purchasing the ALL-TEST motor circuit analyzers from BJM Corp., Old Saybrook, CT, analyzing motors involved a lot of guesswork.
Occasionally, motors would be sent to a supplier without a complete problem diagnosis. After testing by the supplier, a report would be sent back indicating that no problems were found. Now with the MCA program in operation, Allison sees more uptime on machinery and a decrease in the “no problem found” reports.
Approximately 50 Allison skilled trades personnel are being trained in the application and use of MCA instruments via an internal eight-hour course. The trades involved in the training are electricians, powerhouse stationary engineers, and air conditioning and maintenance supervisors.
Motor problems identified
Motor stator faults found by using MCA vary from turn-to-turn, phase-to-phase, coil-to-coil, ground faults, and rotor faults. Rotor faults, which are more common in 4160 V motors than 480 V motors, include broken rotor bars, eccentricity, and casting voids. Looking at the phase angle and current frequency on the MCA unit can identify stator faults. By comparing the winding resistance of each phase to one another, high resistance connections can be seen.
Ground faults can be seen by the insulation-to-ground test. By comparing the impedance and the inductance readings to each other, contamination can be observed and can range from coolant fluid, oil, and water to overloaded windings.
The contamination on servo motors will start showing its ill effects months prior to failure. The general trend is that there will be service calls indicating an overcurrent condition on the panel. After tracking work orders through the Allison computerized maintenance management system, the overcurrent fault will most likely appear more frequently, requiring a work order to change servo motors.
Area planners have received communication alerting them to the overcurrent condition and how it can be detected before a servo motor has completely failed. Compared to a reactive course of action, planned maintenance provides for cost avoidance. A clean dip and a bake from the motor shop are cheaper and more efficient than a complete rewind.
The applicable cost avoidance spreadsheet is sequentially shared across the QNPM network according to the following:
• An MCA work order is dispatched
• An electrician responds to the motor site
• An MCA test is conducted and analyzed and a determination is made
• An action plan is implemented.
For example, if a servo motor tests good using MCA, a root cause investigation is initiated to check for other causes of the fault such as a blown fuse, SCR, drive, cable, or connecter to the motor. If a cable is replaced, a cost comparison between proactive and reactive maintenance is documented based on maintenance history (Table 1).
Allison Transmission prefers proactive to reactive maintenance, particularly from a financial perspective. As shown in Fig. 3, the total cost savings at Allison attributable to the MCA program in 2002 was $307,664.
Single phase testing
When testing three-phase motors, the MCA unit works well when performing comparisons between windings. But what about testing single phase?
Allison uses dc motors, which have a set of field windings (two wires) and the interpoles and armature (two wires) for many applications. The engineering test department uses eddy current dynamometers in order to put a simulated load on all manufactured transmissions for testing purposes, which also have two sets of windings with just two wires.
How are these two wire devices compared? First an MCA test is performed on the winding, then the information is stored in the database along with the nameplate information to identify like motors. Finally, the winding with problems is compared to like windings to reveal problems.
MCA in action
Case Study 1: Infrared Thermography (IR). An electrician running a predictive IR route noticed a hot motor. The motor was a 7.5 hp coolant pump in a group of five identical machines.
A work order was submitted for a motor circuit analysis to be conducted and subsequently the MCA was completed and analyzed showing no problems with the motor. A work order for vibration analysis was written, and the results determined that the temperature was being driven up due to a bearing fault. The coolant pump was replaced and the temperature was in line with the group of machines.
This particular machine is a machining center for transmission cases. When a coolant pump motor fails, historically there would be a loss of production and possibly an assembly operation shut down.
Case Study 2: MCA vs DMM and insulation-to-ground test. An electrician running a predictive IR route noticed a hot 5 hp motor on a machine with 4 drill heads that performs a drilling operation. The MCA was performed and analyzed, and the impedance and inductance readings, clearly not in parallel, were compared. The results showed the motor windings were contaminated.
Impedance or inductance cannot be seen with a digital multimeter (DMM) or an insulation-to-ground tester. Both the resistance and the insulation-to-ground test were good. The motor was sent for repairs as this model was not available in the warehouse. MCA was performed to determine the reason why the motor had this contamination. The motor shop did a full autopsy on the motor and, after cracking open the end bells, it was obvious that the problem was fluid in the windings.
The unknown liquid was poured into a sample bottle. The motor shop did extensive repairs on the windings, and also applied an epoxy seal to the area after determining the liquid was a mix of coolant and hydraulic oil. The motor was returned and installed in less than 24 hrs. This machine drills a series of holes on the carrier for the transmission. If the machine had run to complete failure, it would have shut down the assembly line. Order time on a new motor was estimated at 3 days.
Case Study 3: #8 Air Compressor, 4160 V, 1000 hp. On June 18, 2003, the power house tradesmen provided data to the reliability department for review and clarification of MCA readings on the 4160-V, 1000-hp motor on the #8 air compressor. A resistive unbalance of 84.5 percent was found.
The motor was tested at the motor control center then at the motor connection lugs. The bad connection at the lugs was found and corrected, reducing the unbalance to 0.17 percent. This case again showed that MCA is useful, as the 4160-V connections at the compressor did not have to be taken apart and put back together. The motor did not have to be removed and sent to the motor shop supplier. This saved the cost of an unnecessary motor repair and the loss of compressed air for some of the production machines.
MCA has made a definite impact at Allison. With the NFPA 70E personal protective equipment issues approaching, off line motor circuit analysis continues to be valuable and safe. The motor world will now be viewed differently from the days of just using a multimeter and an insulation-to-ground tester. Allison Transmission believes and trusts systems that consistently contribute to proactive maintenance. MT
Information supplied by Dave Humphrey, a journeymen electrician with General Motors Allison Transmission, Indianapolis, IN
|Conduct MCA test||Remove old servo motor|
|Replace cable||Send out for repair|
|Re-test motor||Install new motor (have not gotten to root cause yet)|
|Labor man-hours – 6||Labor man-hours – 15|
|Machine downtime hours – 4||Machine downtime hours – 8|
|Loss of production hours – 4||Loss of production hours – 8|
|Cost of cable||Cost of servo motor|
|Problem solved||Problem not solved|