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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