ArticleVibration Considerations In VFD Driven Systems
by Ron Eshleman
This paper introduces the reader to VFD drives with a focus on how their introduction affects the vibration of rotating equipment. It begins with an introduction to variable frequency drives, explains briefly their major components, how they work, and what electrical frequencies (harmonics) are usually introduced when a VFD is employed. The tendency for VFD drives to excite system natural frequencies and cause resonance problems is mentioned. The author finishes the paper with two case histories where VFD drives played a major role in the vibration problems experienced by the rotating machinery.
Variable frequency drives (VFDs) used to control the speed of alternating current induction motors have become the preferred mechanism for controlling the speed of industrial equipment. These VFDs have replaced DC drive systems in many applications where speed and torque control are essential. However, these less expensive units do not come without a price — the potential for serious vibration excitations.
The majority of AC drives produce pulse width modulated current (Fig. 1) that simulates the alternating current power at variable frequencies. Pulse width modulated drives produce variable frequency output by rectifying (converting to DC) sinusoidal utility power and then inverting it at the desired frequency to AC (Figure 2). A logic circuit and software control the inverter to provide variable voltage and frequency required to run an induction motor at variable speed. The output wave of a pulse width modulated drive is not the sinusoidal waveform that an AC induction motor would normally encounter but a series of constant amplitude pulses (Fig. 1).
The amplitude of each pulse is the DC bus voltage of the drive. The pulse width depends on the desired output voltage. The wider pulses produce higher average output voltages. Strings of positive and negative voltages at a given frequency determines the frequency of the output — an approximate sine wave. The current waveform will contain high amplitude harmonics at the switching frequency .
The six-step inverter uses six silicon-controlled rectifiers (SCRs) to change the power from DC to AC. Firing of the SCRs is controlled electronically to produce a stepped waveform (Fig. 2) that simulates a sinusoidal variable-frequency waveform. The six-step inverter causes 5th, 7th, 11th, 13th harmonics on the current waveform and 6th, 12th, 18th harmonics on the output torque. These harmonics can be sizable amplitude (Table 1). Therefore torsional resonance is a possible problem on any system. Some controllers go to a 12-step inverter to eliminate this excitation problem .
Mechanical systems have many natural frequencies that when paired with an exciter will result in a condition of resonance. The harmonics in the power supplied by a VFD can excite torsional and lateral resonances in the drive system, machine structural resonances, and resonances in piping and ducting. These conditions are best illustrated with an interference diagram showing frequency against speed. However, this diagram does not provide any knowledge about severity. A computer-generated Lund Diagram, Figure 3, shows damping at the various resonant points . Otherwise field data must be acquired to determine severity.”
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