Patented system solution provides a more effective grounding method.
Zero Ground LCC, Waukegan, IL, USA
Technology and the electrical/electronics industry have changed at a dizzying pace. Among these many changes is the growing use of motors with adjustable speed drives (ASDs) or variable frequency drives (VFDs). An unforeseen consequence of using these efficient new motors is the production of high frequency ground currents. These ground currents can and often do affect sensitive devices, people, and animals—as well as causing costly unplanned downtime in ASD/ VFD applications. When these problems arise, technicians may try a trial-and-error approach such as retrofitting I/O or EMI filters. Too often, these attempts prove ineffective and overly costly in terms of both time and dollars wasted.
Analyzing the Problem
ASD/VFD technology today relies on very fast switching power semiconductor devices. ASD/VFD manufacturers have turned to insulated-gate bipolar transistors (IGBTs) as the semiconductor of choice. This preference arises from characteristics of IGBTs including cooler operating components, smaller footprints, quieter operating systems, and smoother turning of motors—all of which add up to major dollar savings by end users. One significant disadvantage with IGBTs is the rapid turn-on and turn-off time.
Specifically, a nuisance transient voltage occurs at the juncture point of the turn-off/turn-on. As the speed of switching and the DC bus voltage increase, the high frequency transients increase as well. The faster the IGBT switches and the higher the DC bus voltage, the greater the increase in high frequency transients. These high frequency (transients) ground currents often affect sensitive devices (in all areas of industry), people, and even animals—for example, on dairy farms. Eventually, these high frequency ground currents lead to the degradation of dielectric insulation systems (motors, transformers, and wiring) and to motor bearing fluting. Also, interference can stray into other systems/devices—metal detectors, card readers, servo amplifiers, proximity sensors, communication hubs, robotic controls, wireless communications, and low voltage control wiring (0-1 mA, 4-20 mA, 0-10 Vdc, etc.). Other vulnerable equipment includes machine control protocols (Devicenet, Profibus, Modbus ) and video/security systems, machine vision systems, and flow sensors.
The Traditional and Non-Traditional Approaches
The conventional wisdom and standard practice of grounding electrical circuits to “physical earth” for safety (per the National Electrical Safety Code) began over 100 years ago. The past few decades have brought technological innovations such as the VFDs mentioned above. Equipments such as UPS’s, servos, and computer-controlled systems typically generate high frequency ground currents, which when grounded to “physical earth” can affect other sensitive electrical/electronic equipment. These troublesome high frequency ground currents must be dealt with since they cannot be eliminated.
Still, these high frequency ground currents can be contained, directed, and controlled. A new conduit system provides a more effective ground that reduces high frequency ground currents to zero, reduces motor bearing currents, extends motor life, and minimizes crosstalk between adjacent cables. This new approach also contains stray capacitance, inductively couples energy, directs fault currents away from hazardous locations, and eliminates both motor-frame and VFD-frame voltage to ground. This highly effective grounding, in turn, reduces unplanned downtime and increases mean-time-between-failure (MTBF).
The approach allows grounding techniques to “catch up” to the recent changes in today’s high-speed electronic systems. For example, by replacing a standard wiring system in a VFD cell, the new system reduced 1,538 milliamps to zero. Zero was defined as five places to the right of the decimal point (Figures 2 and 3).
This system provides unsurpassed performance in reducing EMI/RFI caused by PWM or high frequency power supplies. The system has been engineered and designed as a complete wiring system to be installed between a source and an adjustable speed drive controller and an electric motor (Figure 1), or in any wiring scheme in which high frequency voltages might stray. Patented cable/conduit components, coupled with a patented installation method, create a very low impedance return path for the high frequency ground current energy.
Unlike conventional VFD wiring systems that are grounded to earth, the new system assures grounding and safety although in a very different manner. The system provides ground via the cable system, not the components. No filters are needed to reduce ground currents to zero. Most drive manufacturers recommend that cable lengths should not exceed 50 feet. This grounding solution has worked successfully with cable lengths exceeding 300 feet.
The Need for Adequate Grounding
Today, many manufacturing/business facilities are operating with deficient plant electrical distribution, and facility managers may be unaware of the problems. Many plants have older, outdated (over ten years old) electrical systems that need to be upgraded. Often, a once effective electrical system has been altered by changes over time. The addition of new machinery, the installation of switching power supplies or sensitive equipment, changes in electrical service, and utility power spikes and surges will bring about changes in the in-house grid. Overall, there is a significant probability that electrical distribution systems installed a decade ago now harbor major inadequacies.
Meeting all requirements of National Electric Code Sec. 250 should not be difficult. Unfortunately, many systems that were initially grounded correctly are no longer in compliance. For example, many cities have replaced aged metallic water supply meter mains with PVC. These mains, which were once effective in providing grounding, are now virtually useless since PVC is not a conductor. Bonds, where two different types of metal join, corrode. Aged grounds can and will corrode over time. Some cities have already moved to correct these problems.
This grounding system specifically targets applications in which three-phase and single- phase motors are utilized and any application in which EMI/RFI problems are a concern. Likely sites for this new system include galvanic detectors sensing nuisance alarms, and ultrasonic transducers detecting inaccurate information from liquid flow in piping systems. Currently, installed applications can be found at dairy farms, clean rooms/HVAC, hospitals (i.e.: surgery, intensive care), and marinas—places where damaging leakage currents are unacceptable. For new or retrofit installations, some recommended applications would include: mining, water filtration/waste water treatment, farms, pulp and paper mills, glass, and electro-plating. Industrial applications using automation/robotics where AC servos are being utilized are in special need of such protection because sensors determine where servos will be positioned, and noise can result in erroneous feedback to the sensors. Other likely candidates for installation of this system are surface/subsurface vessels fitted with onboard switching power supplies.
In addition to safety and interference issues, the reduction in high frequency ground currents achieved via this system can have a significant impact on motor life. Recent industry testing has verified a significant reduction in motor bearing currents, thus extending motor life. It is possible to run a motor on mains-supplied power for years without incident. Similarly, it is possible to run a motor with VFD for significant energy savings with no untoward occurrences. Unfortunately, adding another VFD in the circuit of this same application can be disastrous if proper precautions are not taken. The motor bearing could fail in a matter of months. Bearing fluting is caused when an electrical discharge occurs between the bearing inner or outer races and the rotating balls. Since a minimum of 41% of motor failures are bearing related, using this system to reduce motor bearing currents could have a significant impact on efficiency and economy.
Developed in 1998, this system solution had been in testing and development with multiple site installations, until its 2005 market introduction. At present, there are system solutions offered for low voltage (600 V) with ratings of 10 HP or less at 33 amps, 50 HP or less at 70 amps, 125 HP or less at 205 amps, and 250 HP or less at 410 Amps. The system has been used for the safe wiring of all input and output connections found in large switching power supplies. All cables and conduits in these systems are UL listed materials, and these systems meet all requirements of NEC 336 Power and Control Cable Type TC, of NEC 350 Liquid Tight Flexible Metal Conduit Type LFMC, and of NEC 250 grounding. The cables are rated for 2000V AC RMS and are dielectric tested (12 kV – 22 kV). Other than the cable/conduit, these systems utilize all standard off-the-shelf industry fittings and components. These systems received U.S. patents in 2005 for the components and installation method, respectively.
Mark Panko can be reached at [email protected] or www.zero-ground.com.