Best practice in the application of variable speed drives can be considered from the macro level – from supply authority to consumer; and from the micro level – from drive controller to motor.
SA Instrumentation and Control was fortunate to catch up with Skibinski at the recent Rockwell Automation University in Johannesburg. Gary Skibinski, Ph.D., is a consulting engineer who specialises in this field and consults to the drives business of Rockwell Automation. He has authored and co-authored numerous papers on the design of motors and drives and is quoted in patent applications and drives text books.
His papers cover such subjects as:
* Universal induction motor model with low-to-high frequency response characteristics.
* Determination of parameters in the universal induction motor model.
* Zero-switching-loss inverters for high-power applications.
* Effective utilisation of surge protection devices.
Skibinski has been honoured with best paper awards and sits on technical committees or working groups of NEMA and IEC relating to variable speed drives.
A story of two parts
Skibinski explained that best practice in the installation of variable speed drives has two perspectives: the one being the impact of the drive on its supply network and the other being the installation between drive controller and motor.
From a supply perspective, one of the problems with the application of variable speed AC drives is that they impose 5th degree and 7th degree harmonics onto their supply networks. When large numbers of such motors are all contributing to these harmonics this becomes a problem for supply authorities and other consumers.
Drive to supply authority
IEEE519 ‘IEEE Recommended practices and requirements for harmonic control in electrical power systems’ was developed to address this problem. It does not specify limits for individual equipment, but for individual consumers.
There is no absolute best practice regarding the impact that a consumer has on its supply network, but as a rule of thumb supply authorities and consumers are working towards the goal of no more than 5% current total harmonic distortion (THD) being imposed back onto the supply network.
The level of current THD can be reduced in several ways. Passive LC filters can be installed at the point of supply, active filters can be applied, inverters with higher pulse rates can be used or active switching front ends can be used.
Skibinski quotes typical figures for different drives: A standard 6-pulse drive can impose up to 39% THD at 0,9 power factor (PF) on its supply network, a 12 pulse inverter around 9% at 0,95 PF and an 18-pulse device between 3,5 and 4,5% at 0,99 PF.
Active switching front ends (AFE) can reduce the level of distortion further – to between 2,5 and 3,5% THD at 1.0 PF, or can even be configured for a leading power factor.
Drive to motor
While there is still much debate about how best to achieve the spirit of IEEE519 the recommended practices for the installation of drive controller and motor are far better understood and documented.
To help SA Instrumentation and Control readers, Skibinski has given the publication permission to share Rockwell’s ‘Wiring and grounding guidelines for pulse width modulated (PWM) AC drives’
The guide has chapters on:
* Cable selection.
* Impact of power supply configuration (supply trans-former windings) on drive performance and safety.
* Grounding (earthing) schemes for safety and noise reduction.
* Installation practices.
* The reflected wave phenomenon.
* Electromagnetic interference.
…and is well worth a read before installing that next AC drive.
For the complete article visit http://instrumentation.co.za/+c8842 and for more information contact Jeff Sandison, Rockwell Automation, +27 (0)11 654 9700, jksandison@ra.rockwell.com, www.rockwellautomation.co.za
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