Rockwell Automation has released a compact, high-horsepower electric drive that brings a host of advantages, even to traditionally DC drive-powered applications.
The Allen Bradley LPM 20 AC drive is a PWM (pulse-width-modulated) liquid-cooled drive that provides sensorless vector and general purpose Volts-per-Hertz regulation for high-power applications. Featuring an active-rectifier with input filter, the LPM 20 meets IEEE519 standards and CE standards for Europe for line harmonics. Full line regeneration capability produces a braking system by pulling power back into the AC line.
As a benefit of the LPM 20's synchronous rectifier front ends, IGBTs will allow power to flow into or out of the drives. This is beneficial in that it allows an AC drive to absorb power from the application and put it back on the AC line with the same level of efficiency as it exhibits when motoring. This one feature allows AC drives to be used in almost any application, which was previously solved with a DC drive.
A synchronous rectifier can also regulate the voltage level on the DC bus. The 'buck boost' nature of the topology allows voltage levels on the DC bus to be higher than the peak of the AC line. During line sags, brownouts and other low voltage conditions, the synchronous rectifier can maintain the DC bus at its rated voltage and thus provide full output voltage to the motor under almost all low line conditions. A 480 V motor can be run on a 380 or 480 V supply, delivering full torque across the speed range. For a 380 V supply, the output to the motor can be a maximum of 480 V at maximum speed.
The current waveforms produced by a synchronous rectifier are regulated to be sine waves. Thus the current harmonics can be regulated to meet IEEE519, with no additional modifications. Current harmonics are less than 5%, while voltage harmonics in the range of 0,5 to 2% are typical at the drive.
Just as critical as it is to keep the DC bus above an acceptable voltage level, it is also possible to be too high. A synchronous rectifier will regulate the DC bus and lower the voltage if it rises above the desired set point during regeneration. Repetitive line spikes will increase the DC bus voltage during the spike, however the synchronous rectifier will reduce the level as soon as the spike has passed, thus preventing the ratcheting up of the DC bus voltage level with each spike. The LC filter formed by the rectifier and the DC bus capacitors will also limit the transient voltage excursions.
The input line currents are regulated by the synchronous rectifier, permitting a predetermined power factor to be set by the controls. This power factor is usually set to unity to maximise the unit's current draw; however, it is possible to dynamically change the power factor to meet the user's desire to have an improved plant power.
Each of these benefits have varying degrees of importance to a user, but with a good synchronous rectifier, the user always has the option to avoid issues and improve performance. With a fix front end, options are limited and there may be no solution for some problems that arise in the field.
LPM 20 drives are built on the Allen Bradley Powerflex 700 AC drive regulator. The LPM 20 drive is mounted in a IP 20 enclosure. Specific chiller options and water-to-water or water-to-air cooling options can be configured to meet specific application needs. The LPM 20 has a displacement power factor of 30,99, with a line frequency of 50/60 Hz (±2 Hz) and carrier frequency of 2 kHz standard (software-selectable).
For more information contact Jeff Sandison, marketing communications manager, Rockwell Automation, 011 654 9700, jksandison@ra.rockwell.com
| Tel: | +27 11 998 1000 |
| Email: | mjunius@rockwellautomation.com |
| www: | www.rockwellautomation.co.za |
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