Electricity + Control May 2016

DRIVES, MOTORS + SWITCHGEAR

Some rotor failures

Figure 4: Typical double cage rotor bar configurations.

In Figure 4 , the solid copper and the double cage copper/brass configurations are only a few of the possible arrangements, square, oblong, rectangular or other sections are possible. Generally, the outer cage in a double cage configuration is a brass alloy; other al- loys with a suitable resistance (conductivity) can and are used at the motor designers’ discretion.

Image 6: Aluminium rotor stalled Image 7: Alternative view of Image 6. overheating.

Image 8: Overheating of connections. showing lamination damage.

Image 9: Fractured rotor bar.

Image 2: Image 3: Double cage with deep bar inner cage. Double cage.

Image 4: Typical double cage Image 5: rotor punching.

Image 10: Fractured rotor bars. Image 11: Fractured rotor bars.

Deep bar single cage.

Cast aluminium rotor bars can be practically any profile as the bars are cast into the slots and will fill any shape punched into the lamina- tions. It is important that the rotor bars are tight in the slot otherwise they will vibrate, resulting in work hardening and premature failure. Some rotors, generally only in larger ratings motors, have the bars wedged with two opposite wedges under the bar to ensure a tight fit.

Image 12: Core burning due Image 13: End-ring burnt/sheared to broken rotor bar. off.

Material

Melting Point

Density Kg/dm 3

Coefficient of expansion a/ 10 – 6

% Conductivity

Silver

100

Aluminium 658

2,6 8,4

23,8 18,5

55

Brass

900

23 – 29 Dependent on alloy

Copper

1083

8,8

16,5

94

Image 14: End ring sheared off. Image 15: View of rotor.

Table 1: Relative properties of common materials.

May ‘16 Electricity+Control

31