EMI from drives: How to control high-frequency noise of adjustable speed drives
Adjustable speed drives (ASDs) generate disruptive high frequency electrical noise, also known as electromagnetic interference (EMI) and radio frequency interference (RFI). ASDs that use the widely used insulated gate bipolar transistor (IGBT) create the most electrical noise. EMI/RFI issues resulting from the ultra-fast high frequency switching of the IGBT must be addressed.
The Frank Bartos June 2008 Control Engineering article
by providing an improved grounding path.
Radiated or conducted stray voltages that are emitted generate high frequency nuisance ground currents once they enter physical earth or related open paths. There are three main reasons such ground currents must be controlled, rather than mitigated into earth.
1. Safety . High frequency ground currents cause potential arcing and sparking at points of physical earth. Safety is a primary concern in the mining industry and other hazardous processing environments where these issues are unacceptable with regard to personnel. Depending on the environment, arcing and sparking at ground can cause severe damage to equipment. High frequency ground currents can also severely affect livestock animals in the vicinity of operating ASDs.
2. Premature motor failure . High frequency ground currents generate motor bearing currents, which cause motor bearing fluting or pitting. The bearing damage causes premature motor failure. No motor manufacturer warranties motor bearings against failures. See the causes motor failures pie graph.
|Pie graph shows that bearing and stator are main causes of motor failure. Source: Control Engineering , Zero Ground , and PDMA Corp .|
3. Interference across frequencies outside the range of active limiting devices. When interference occurs outside the frequency range of an active limiting device, the high frequency switching of IGBTs will continue to interfere with sensitive electronic devices (such as sensors, metal detectors, transducers, PLCs, and other devices) and adjacent ASD system cells.
An alternative exists to the standard practice of mitigating the effects of high frequency ground currents by redundant grounding to physical earth. Consider that, once introduced to physical earth, stray currents can migrate in any direction and re-enter the ground grids via multiple grounding paths. Technology can create an alternate high frequency low impedance ground path as a way to contain, control and redirect these high frequency ground currents away from physical earth and from hazardous locations. The technology reduces high frequency ground currents to near zero by redirecting them back on an extremely low impedance path to the common point at its’ source. The nuisance energy chooses to remain on this desired path.
Inner and outer components form the system assembly. The inner component takes into account the means by which these high frequency and common mode voltage energies move about: capacitively coupled, inductively coupled, radiated, induced galvanically, and conducted. Parallel motor conductors are, in essence, a long capacitor (insulator, conductor, insulator, conductor, insulator, and conductor).
Any one of the motor conductors could be viewed as an uncoiled inductor with inductive coupling to each. The high frequency energy can be radiated as though an antenna. This energy also is conducted down one conductor. Implementing standard grounding practices, these high frequency transients stray not only phase-to-phase, but phase to ground, and everywhere within the system. It must be noted by inherent inverter design these transients are not coupled to a neutral point common mode.
The inner component incorporates a much thicker cross-linked polyethylene (XLPE) insulation on the parallel motor conductors which assists in reducing crosstalk between the conductors. Each of the phase conductors, now ground wrapped, are cabled in-lay with an overall PVC jacket to form a poly-phase cable.
The outer component incorporates mechanical crush strength, a high frequency low impedance path and a high current low frequency conductive path with an overall jacket. A tinned copper braid is woven over a flexible bronze helical core allowing for physical flexibility and EMI/RFI shielding. (See table below on shielding effectiveness.) PVC or zero halogen polyurethane compound with specific properties jackets the braided core. The outer component termination device is a standard off-the-shelf fitting, which has the desired mechanical and electrical properties needed for various applications.
|Outer component shielding effectiveness|
|Source: Control Engineering and Zero Ground|
How does it work?
The system assembly is terminated using a simple termination method. Phase conductors terminate, as designed, on the load side and line side of the ASD. The high frequency path and the low frequency high current path must continue uninterrupted from the motor through the ASD and terminate at the power source neutral point, and then be directed to physical earth. These nuisance energies are now viewed and confirmed as minimal when measured at physical earth. (See before and after graphic.)
|Source: Control Engineering and Zero Ground|
Before and after measurements show how the Zero Ground High Frequency
Quieting disruptive electrical noise
Safety : This passive system offers safety and reliability by containing and directing the high frequency and common mode transients, thereby inhibiting nuisance common mode currents, and directing hard fault currents away from hazardous locations on the bronze helical core, to be safely dealt with. The process of inhibiting these stray energies from peripheral devices and the earth system benefits personnel and adjacent equipment.
Premature motor failure : By containing, controlling and redirecting high frequency ground currents within its low impedance ground path, the passive system reduces motor bearing discharge which extends motor life.
Interference, including EMI/RFI : This passive system covers all load impedances and is not frequency dependent. The system contains, controls, and redirects high frequency electrical noise by containing and mitigating stray capacitance, capacitively coupled energy, inductively coupled energy, and common mode voltage. It also minimizes corona inception voltage (CIV) in motor cable leads, eliminates motor frame voltage to ground, and minimizes crosstalk between adjacent cables. In addition it eliminates stray voltage (CMV) and thereby inhibiting stray currents (CMC) from flowing in PE equipment grounding circuit(s).
Zero Ground High Frequency Extraction System (HFES)
The patented Zero Ground High Frequency Extraction System (HFES) and assembly method reduce high frequency ground currents to near zero.
|Zero Ground High Frequency Extraction System (HFES) is said to reduce high-frequency ground currents to near zero.|
In a recent installation, Dan Coolidge, engineering manager of The Electri-Flex Co ., set up a comparison on two newly installed identical drive systems on his manufacturing floor. Coolidge specified HFES on one system and had the other system wired conventionally.
“Oscilloscope readings taken after installation and at repeated intervals verify that zero currents are going to ground on the system with HFES. Our vibration analyst is already hearing bearing wear on the identical drive system without HFES, where ground currents are measuring approximately 1,500 milliamps,” notes Coolidge.
Addressing the entire spectrum of issues resulting from high frequency ground currents, HFES increases system mean-time-before-failure (MTBF) and reduces costly unplanned downtime. HFES is used on the input and output of the drive, and supports distances over 150 meters on each cable assembly segment. When installed correctly per specification, this passive HFES system requires no regular maintenance and has no routinely replaceable parts.
HFES complies with NEC 250 and CSA Section 10 with many installations to date. Many of leading drive and motor manufacturers have evaluated and approved HFES. Earlier this year, the
for use in all surface and sub-surface mines in this country.
For more on this topic, also see: System reduces VFD ground currents .
Richard E. Jacky is engineering manager, Bob Hopkins is VP engineering, Mark Panko is VP sales, all with Zero Ground LLC .
– Edited by Mark T. Hoske , editor in chief
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