Testers prevent catastrophic failures of metro traction motors

The British Columbia Rapid Transit Co Ltd operates the SkyTrain light transit system in which fully automated trains run on two lines serving the Vancouver metropolitan area. Built for the Expo 86 World's Fair, it has since become the world's longest, automated light rapid transit metro. The system uses the same family of linear induction motor-driven Bombardier trains as the Scarborough RT line in Toronto, the Putra LRT in Kuala Lumpur, Malaysia, and the JFK AirTrain in New York.

Recently the Rapid Transit Company purchased a Baker Instruments AWA 2.2 winding analyser to facilitate predictive testing of the SkyTrain traction motors. These LIM (linear induction motor) units are AC induction motors with the primary mounted on the vehicle (light rail) and the secondary on the track. The LIM is a single-sided, axial type design. It is common to refer to the primary at the LIM; however, to obtain the performance needed the motor includes the secondary.

The core of the primary is made of high-grade electric steel laminations. These steel structures limit the thrust and vertical force in relation to the vehicle. To maintain the correct gap between the running primary and reaction rail, the height of the primary is made adjustable by eccentric cams. In addition, to the closed underside, rubber scrapers are attached at each end of the core to prevent any mechanical damage to the core due to debris on the reaction rail.

Three-phase windings consist of form-wound coils insulated with a polyester-based insulation system with a silastic coating. The insulation system is rated class H. The LIM temperature is monitored continuously and appropriate action is taken should the temperature exceed the current documented limits. The application of forced-air cooling via two high-speed cooling fans is used to maintain the LIM operating temperature at or below a nominal 90degC. The LIM typically operates at an 80 per cent duty cycle with 20 seconds' dwell time at stations.

Currently 500 LIMS have been tested, revealing that 24 of them had poor insulation quality and required service and overhaul. This early detection of insulation malfunction allowed time for corrective action before significant damage could occur. Since these LIMs were found prior to failure the cost of repair was significantly less.

On average a complete rewind of one unit is approximately C$40,000.00. For all 24 units this would have been at least C$960,000.00. As these failing motors were identified prior to severe damage occurring, the need for major overhaul was avoided and the cost of repair per unit reduced to about C$7,000.00 - which represents a significant saving over the potential costs.

The early detection of the insulation weakness also prevented in-service failures that would have caused significant problems to both the public and The Rapid Transit Company.

Muamer Dedovic-Deda, the Vehicle Technical Analyst at the Company, said: "By using the Baker AWA 2.2 we prevented 24 catastrophic failures and most importantly saved approximately C$792,000.00 of taxpayers' money. We hope and expect to get another 10 years of good service from these LIM's, now that they have been repaired. Since this was our first programme of testing with the AWA 2.2, future testing will enable performance trending and the ability to predict the likelihood of any more potential problems."