Avoiding torsional failures in induction motors

Torsional problems can be insidious in that they often go undetected with standard package instrumentation until a catastrophic failure occurs. Fundamentally, these failures are a result of interaction between the torsional critical speeds (natural frequencies) of the system and excitation energy produced by the drive system and load.

(Figure 1: Torsional telemetry system components. Figure 2: Excitation of a torsional critical speed)

In most cases, these issues are best resolved by shifting the critical speeds such that the offending excitation can no longer excite them. This is the primary goal of a torsional analysis during the design stage. However, once a machine is built and put into service, the changes necessary to affect a significant shift in the critical speeds can sometimes be difficult, and be costly to implement.

In addition, for some designs, the required duty cycle will necessitate occasional excitation of torsional critical speeds, even when their placement has been optimized. Further, the rotating structural components of typical induction motors have the potential to aggravate torsional critical speeds and shaft stress behavior.

In systems without a gearbox, the typically installed lateral shaft or casing vibration probes are not sensitive to torsional vibration. In most cases, specialized instrumentation must be used to capture this type of data. One common method involves the use of a strain gage telemetry system, as shown in Figure 1.

Several other methods of obtaining this data exist, including the use of a torsiograph or encoder on the outboard end of the motor or driven equipment. However the data is obtained, the calculated torsional critical speeds can potentially be confirmed during startup or shutdown events, as depicted in Figure 2. In this illustration, a torsional critical speed is evident as it is traversed by relatively low-level energy generated at several orders of running speed. This figure illustrates the concept that the train may need to tolerate some level of critical speed excitation during operations involving a speed change, even if the subject mode is not excited at nominal speed.

In addition to common mechanical excitation sources (including the driven equipment), motors and variable frequency drives also produce excitation energy that can interact with the critical speeds. The type of data shown in Figure 2 can be used to assess the damping present in the system (by observing the amplification factor of the response peaks). Additionally, dynamic torque or stress levels obtained from the test data can be compared to rotor-dynamic predictions.

(Justin R. Hollingsworth is Principal Engineer for the Rotating Machinery Dynamics Section in Southwest Research Institute’s (SwRI) Mechanical Engineering Division) 

More in the May/June 2014 issue of Turbomachinery International