AC Motor 1,250kw failure of die cast aluminum rotor “design C”
Dave Sexsmith
Rotating Equipment | Winder EMST | Electric Motors Sales | Business Start-ups
Repair process: Extruded aluminum bar and shorting ring replacement.
Analysis
This rotor bar configuration is clearly designed to enhance the deep bar effect. Why designed that way? Because it’s a design C motor. Designed for high starting torque at normal starting current. It will be less tolerant of stalling and repetitive starting. And more likely to be “rotor limited” as opposed to “stator limited” meaning more likely rotor is first item damaged thermally during starting abuse.
Note: A minimum number of starts is critical and under stalled conditions as this becomes destructive. The rotor condition is indicative of this type of failure.
Procedure
Documentation
1. Metallurgical: Aluminum Alloy Analysis Test.
2. Laser scan for CAD file.
3. Dissolve the aluminum in a hot tank of caustic soda.
Welding protocols and notes
4. Lincoln electrode #1100 is the closest match to the Aluminum Alloy Analysis Test. Also, it has great conductivity and low sensitivity to hot cracking.
5. Remove aluminum oxidization prior to welding prep.
6. 100 % argon gas is usually the gas of choice but mixtures like 50/50 argon/helium can be helpful for welding thick sections. Make sure the polarity is connected properly. mig welding aluminum requires reverse polarity.
7. Hot Cracking is the formation of shrinkage cracks during the solidification of a metal. Preventing hot cracking can be as simple as choosing a filler metal that has a weld metal chemistry with a lower crack sensitivity. Stringent design protocols must be used to prevent hot cracking by using the appropriate joint design.
8. Rotor requires pre-heat prior to welding of 250°F to prevent cold cracking by reducing the speed of cooling. Note: Use welding consumables that are low in hydrogen to minimize the hydrogen that is diffused into the weld.
9. Center shorting ring of rotor joining must encompass entire area with welded aluminum and join all surfaces of extruded aluminum bars.
10. Reheat to 250°F and cool in oven for 24 hours.
11. Machine center and end shorting rings 20 thou less than rotor laminations.
FAT
12. Test rotor with growler noting any deflection in current.
13. Test rotor with 2,000-amp induction test and thermal scan.
14. Balance rotor to G 2.5 specification.
15. Perform open rotor test noting any deflection.
16. Locked rotor current compared to same make and model.
17. Perform full-load run test.
18. Broken Rotor Bar Detection by Power and Current Based FFT Analysis.
Motor Rewinder
6 年Hi Dave, what would be the chances of motor failure due to die cast rotor defects.Recently i encountered a problem with a marathon made water pump of 15 HP. Winding was fried despite of all cautions took during rewind. On second chance i replaced it with another rotor and it works well. There was no defects seen on the original one except for some color changes due to high temperature. Even it drawn normal no load current. FLA was 3 amps above ratings.
Unaited Energy Engineering services
6 年Great job
Havy duty machining - Erbil
6 年Good job sir ...excellent work