Article
Variable Frequency Drives: Are They Heroes Or Villains?

by Harlow C. Hall

ABSTRACT

This paper presents the topic of variable frequency drives from the viewpoint of electrical design with the goal of minimizing damaging harmonics. The concept of impedance as the combination of resistance, capacitance & inductance is described. Different circuit designs are discussed along with how they affect the development of harmonics. The three different types of harmonics are described and the negative effects of each. Tolerances for harmonic amplitudes in current spectra are given. Factors that affect harmonic amplitudes are presented such as circuit design, impedance, machine speed, temperature & cable length. The concept of standing waves or voltage reflection within transmission lines is discussed and how matching motor impedance with transmission line impedance minimizes them. The switching frequency or pulse width modulation rate (PWM) is defined and how a change to this parameter affects both heat buildup & standing wave formation. The paper concludes with general recommendations for VFD system design that should minimize the formation of harmonics.

PREVIEW

“Process engineers consider variable-speed drives, especially AC variable-speed drives, a boon. Some would like to replace all cylinder and constant-speed drives with variable frequency drives (VFDs, or AC servos). Simply tie an AC induction motor to a VFD for infinitely variable speed. Add a process computer, and the process is under control.

It is a fact that variable-speed drives optimize processing parameters, thereby saving money. It is better to operate a motor at one-half speed than at full- speed with constant starts and stops. Operation at half speed allows a smooth flow of material. In addition, the number of times a motor can be started and stopped is finite, and it is cheaper and more efficient to operate a motor continuously at a reduced speed.

Variables-speed drives have unique problems, however, that present challenges to maintenance personnel. The objective of this article is to describe some of the problems and ways to monitor and minimize them. It is important to have some knowledge of basic electrical theory, linear and nonlinear circuits, harmonics in power conversion, harmonic amplitudes and limits, design parameters, and current analysis.

Basic Electrical Theory
Three forces must be overcome if current is to flow in an AC circuit: resistance, capacitance, and inductance. The amplitude of the AC voltage in the circuit with respect to time at a given frequency is a sine wave. The amplitude of current flow with respect to time as a function of resistance is also a sine wave. This sine wave is in phase with the voltage sine wave. The amplitude of the current flow with respect to time as a function of capacitance is a sine wave that leads the voltage by 90°. The amplitude of current flow with respect to time as a function of inductance is a sine wave that lags the voltage by 90°. The relationships are shown in Figure 1.

Linear and Nonlinear Circuits and Loads
The two classes of electrical circuits and loads are linear and nonlinear. In a linear circuit the current varies in proportion to the voltage to maintain a sinusoidal waveform. This is not the case with a nonlinear circuit in which the three forces due to resistance, capacitance, and inductance can vary independently of each other. As a result the current waveform is not sinusoidal and harmonics form. The combination of resistance, capacitance, and inductance is called impedance.”

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