Variable-speed drives gain favor as a way to minimize energy costs.
Harmonics And Power Quality
Power quality, including harmonics often associated with variable-frequency drives, can be an issue in facilities using large amounts of energy. Harmonics are deviations from the sinusoidal fundamental ac line voltage and current. Most electrical power in North America is delivered at 60 Hz, and harmonic frequencies operate at multiples of that. So, in a 60-Hz system, the second harmonic is 120 Hz, the third 180 Hz, and so on.
Harmonics distort the sinusoidal fundamental current and voltage. Higher harmonic amplitudes mean greater electrical-waveform distortion, which can ultimately damage equipment and lead to failure.
Unlike an ac motor operating across the power line, current drawn from a distribution transformer feeding a typical ac drive is far from sinusoidal. This is because the drive takes current from the transformer only at certain times during its cycle. This converts ac line voltage to a fixed dc voltage within the drive. The drive then pulse-width modulates the fixed dc voltage into variable-frequency voltage for the motor. The ac-to-dc conversion generates harmonics. Current flows during part of the cycle but is off during other parts of the cycle, creating an odd-looking current waveform. Distorted current creates distorted voltages.
Adhering to IEEE 519 standard helps ensure power quality by limiting the maximum current distortion caused by nonlinear loads. There are several methods recommended to reduce line-current harmonics created by drives. For instance, adding line reactors or passive filters reduces current harmonics. But under some conditions they also reduce the dc bus voltage within the drive in full-speed, full-load conditions. This means the drive cannot provide full power to the motor, limiting the motor's power output to about 95% of nameplate rating. Multipulse drives are other solution. No derating is necessary and the drives are generally less expensive than other mitigation methods.
For information on drives and energy efficiency, visit: ab.com/drives/energy_savings/index.html
For information on Rockwell Automation's PowerFlex 400, a drive developed specifically for fan and pump applications, visit: ab.com/drives/powerflex/400/index.html