Torsional Analyses of Variable Frequency Drives

Paper No. 60

Torsional Analyses of Variable Frequency Drives F. R. Szenasi, ASD Applications in Utility Power Plants, EPRI/PEAC Seminar, Reno, Nevada, February 21-23, 1990. 

One of the major improvements in efficiency of plant operation has been the use of variable frequency driven motors so that the motor speed can be adjusted to maximum efficiency in the system. Due to the increased operating speed range and additional excitation mechanisms, variable frequency motors require special consideration when analyzed for torsional vibrations. The torsional analysis performed for motors utilizing variable frequency drive (VFD) controllers includes the evaluation of the system response to dynamic torques by both the electrical excitation at harmonics of the electrical frequency resulting from the variable frequency drive and the mechanical excitation at the operating speed. The electric excitation frequencies of concern include the fundamental electrical frequency, and the 6th and 12th orders of electrical frequency.

The torsional response characteristics of rotating equipment should be analyzed and evaluated to ensure the system’s reliability. Severe torsional vibrations often occur with the only indication of a problem being gear noise or coupling wear. Excessive torsional vibrations can result in gear wear, gear tooth failures, key failures, shrink fit slippage and broken shafts in severe cases. Specifications such as API 617, API 618 and U.S. MIL STD 167 provide guidelines and criteria for evaluating system torsional response characteristics. The Shock and Vibration Handbook [1], Nestorides [2], Ker Wilson [3] and Rotordynamics of Machinery [4] provide references on torsional analysis procedures. The two major analysis techniques commonly used are the Holzer Method and the Eigenvector-Eigenvalue procedure (Modal Superposition Method).

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