Driving Out the Noise

To prevent degraded performance and limit disturbance to nearby equipment, the inherent interference generated by AC drives needs to be minimized. Frank Bartos explains how.

They save energy and optimize automation systems, but adjustable-speed drives (ASDs) – also known as variable speed drives (VSDs) or variable frequency drives (VFDs) – inherently produce high frequency electrical noise. Fast switching transistors producing large voltage changes in the drive’s power section are the prime source of the electrical noise orelectromagnetic interference (EMI).

EMI refers to any interference with normal operation of equipment (and drives) due to passage of abnormal energy along cable connections (conducted noise) or via radiated-wave reception (radiated noise). Conducted EMI has a spectrum from 150 kHz up to 30 MHz, while radiated EMI ranges from 30 MHzto 1 GHz.

Though the focus here is on AC drives, DC drives also are affected. Radio-frequency interference (RFI) is an associated disturbance that affects communication equipment; it is generally considered a subset of EMI. While EMI/RFI frequencies are above the audible range, its effects may be heard oncommunications devices.

EMI must be mitigated in the design and installation of ASDs to prevent degraded performance or harm to the drive itself, as well as to limit propagation ofinterference to nearby equipment and facilities.

Electromagnetic compatibility (EMC) is an overall approach that addresses EMI mitigation and immunity for electrical/electronic equipment. An holistic power drive system (PDS) solution that includes drives, motors, enclosures, and cabling is recommended in the main international EMCstandard covering drives (IEC 61300-3).

First & second
Two drive installation environments are defined in IEC 61300-3. Residential areas (first environment) have stricter EMC requirements than industrial facilities (second environment). A PDS connected to a public low voltage power network (first environment) requires EMI filters, notes Mark Kenyon, ProductMarketing Manager for low voltage drives at ABB.

“It’s also recommended to use filters in industrial situations (second environment) if there are potential victims, i.e. susceptible equipment, in theneighborhood,” he says.

EMI filters attenuate conducted disturbances in a line-connecting-point by grounding the disturbances to earth. A standard feature on most new drives is a second environment EMI filter, using ferrite cores (or rings) and a resistance-capacitance (RC) network, says Kenyon. “Additional first environment filters installed internal or external to the drive are offeredas options in some drives,” he says.

At Siemens, EMI/RFI effects receive full attention during adjustable-speed drive development. Drives are tested in a sound chamber to ensure that electric noise emission to the environment is minimized and the drives are unaffected by the environment, explains Wolfgang Hilmer, Manager of Drives Technology. Testing focuses on the power module where fastswitching, insulated-gate bipolar transistors (IGBTs)that generate EMI are located.

Filtering is an important defense against EMI/ RFI. An EMI filter is standard on Siemens drives to meet EMC for industrial applications. Smaller drives generally have a built-in EMI filter. “On larger drives, the filter often is a separate add-on unit. It traps electrical noise going out to the line,”says Hilmer.

Devil in the details Reducing radiated disturbances can be a case of “the devil is in the details”, since various parts of the power drive system must be shielded against radiated emissions. Enclosures, cabling/wiring, and system installation receive prime attention. Provisions cited byABB include use of:

• Enclosures with unpainted, non-corroding surfaces at points where plates, doors, and other metal parts make contact, as well as conductive gaskets used in doors and covers

• Unpainted installation plates firmly bonded to a common ground point, which ensure all separate metal items have a single path to ground

• Special high frequency (HF) cable entries for HF grounding of power cable shields

• Conductive gaskets for HF grounding of control cable shield

• Shielded power and control cables, each separately routed

• Grounding of the entire PDS installation

At Siemens, testing has found that grounding the drive enclosure is very important for controlling EMI, particularly for large, enclosed units. “A basic way to get rid of electrical noise is route it to groundvia low resistance paths,” says Wolfgang Hilmer.

Other basic EMC provisions cited by Hilmer include use of grounding straps between doors and the enclosure. The use of ground wire on its own is not sufficient because it offers a high resistance path to electrical noise sources. “We use braided straps to ground doors to the frame. The large surface area can effectivelyconduct high-frequency noise,” he says

In fact, all power drive system elements – drive, motor, and cables – should be grounded. “The rest of the system is as important as the drive itself,” Hilmer adds. This includes detailed attention to drive installation to complement the manufacturer’s efforts to minimizeEMI/RFI emission, he explains.

Setting standards
The applicable international standard, IEC 61800-3, specifies four EMI limit categories related to drive output and residential or industrial environment. It also covers test methods for ASDs. European standard EN 55011 covers similar requirements forindustrial and scientific/medical RF equipment.

Adjustable-speed drives must comply with EMC standards if used in Europe and elsewhere, but currently no US standards exist that directly and completely cover EMC of drives. The closest is Federal Communications Commission (FCC) rules and regulations, Part 15. However, it exempts industrial equipment, such as drives, unless they cause objectionable interference, which then requires the drivemanufacturer to fix the problem.

Exempting drives from FCC Part 15 comes from a lack of testing procedure specified for use with drives. According to ABB’s Kenyon, “It’s difficult to define a test to comply with the specification, as drives don’t have standard type or length of cable for supplyingpower to the motor.

Exempting drives from FCC Part 15 comes from a lack of testing procedure specified for use with drives. According to ABB’s Kenyon, “It’s difficult to define a test to comply with the specification, as drives don’t have standard type or length of cable for supplyingpower to the motor.

Most manufacturers include EMC filtering as a standard feature to meet growing demand for single-design global drive products and attract OEM export business. The cost of adding a filter offsets greater logistical problems of having to offer different drive productsfor North America and elsewhere, notes ABB.

Siemens also advocates a single EMC design for its adjustablespeed drives regardless of where they’re built. This happens despite less concern for EMC directives in the US, explains Hilmer. Features such as NEC requirements or circuit breaker and fusedesign differences (IEC vs. NEMA) are adjusted locally.

Danfoss Drives likewise notes the growing international adoption of new harmonized IEC standards, such as IEC 61800-3 for EMC of drives. Jørn Landkildehus, Manager EMC, Reliability & Functional safety at Danfoss, mentions the new European EMC Directive 2004/108/EC as another standard that adds clarification to EMC requirements. It requires following documented good engineering practices in EMC areas such as emissions mitigation, coupling andradiation, and equipment immunity.

“This resonates with our efforts to increase awareness of the installation’s impact on the EMI performance of drives,” says Landkildehus. Guidelines and training sessions are in place at Danfoss to educate employees and customers on the correctinstallation techniques of AC drive systems.

Optimize filter size
Yaskawa Electric America (YEA) notes that electrical noise generated by a specific drive topology is fixed. “However, we can implement filters within the drive to reduce its propagation and effect on external electrical environments,” says Dr Mahesh Swamy, ChiefEngineer for R&D.

An EMI filter (internal or external to the drive) is used to attenuate conducted interference, depending on the level of EMI reduction desired, while shielding of cables is an obvious choice toattenuate radiated EMI.

However, Swamy is quick to point out that shielded cables can increase conducted EMI due to presence of a low impedance path for ground currents from conductors to the grounded shield. He suggests careful optimization to prevent exacerbating groundcurrents.

Advanced drive design also offers benefits. Yaskawa’s threelevel AC drive topology introduced in 2003 – while not directly lowering EMI levels – simplifies common-mode and normal-mode filtering. This is said to ultimately reduce conducted and radiated EMI. “Filter size is reduced significantly due to smaller step size in the associated common-mode voltage when operating at outputsexceeding 230-240 V,” says Swamy.

Also, the three-level design has an output carrier frequency spectrum inherently higher than the operating frequency (typically 2:1). “This helps reduce the size of the normal-mode sine wave filter,” he states. The smaller filter developed by YEA is reportedlyvery effective in reducing conducted EMI.

Another advanced AC drive topology introduced by YEA is the matrix converter – a direct AC-to-AC, four-quadrant converter that comes without a bulky DC link circuit. YEA’s matrix converter also enables smaller step size in the common-mode filter. Moreover, the architecture lends itself to developing a combination normal/ common-mode filter, which tests have shown to significantly reducethe size of the input EMI filter, according to Swamy

A line-ground capacitor is used in all input EMI filters. “This is notorious for introducing large ground currents. Integrating the common-mode and normal-mode filter in the matrix converter can reduce the EMI filter’s input line-ground capacitor by a factor of 10,” adds Swamy. “This is very significant in alleviating problemsof using aggressive EMI filtering methods.”

Balancing performance
Baldor Electric concurs that high frequency noise generation is anundesired result of newer power-switching devices that enable high-performance AC drive designs. “Proper balance must be achievedin drive design that provides the needed switching transitions toimprove efficiency and drive/motor performance, whilelimiting EMI that these fast transitions tend to cause,”says, Phil Camp, Product Specialist.

A line reactor in the input converter at the point where the drive connects to the power system helps limit EMI propagation to the plant system. However, an isolation transformer can be an even better choice in lieu of a line reactor, according to Camp. Transformers provide better EMI reduction out to the supply, while lowering line current crest factors that lessen stress onthe converter, he explains.

Baldor likewise considers EMI filters essential in the drive package. “Filters should be designed to aid in mitigating both differential (line-to-line) and common mode (line-to-earth) noise,” states Camp. Another consideration applies specifically to higher power drives that use phase-controlled devices (such as SCRs) to helppre-charge bus capacitors during drive start-up.

pre-charge bus capacitors during drive start-up. Noise emissions of the drive will be reduced if the phase-controlled devices are only used for the pre-charge, but not if used to regulate busvoltage during normal operation, notes Camp.

Design of the output inverter can also reduce drive-generated EMI. “Choice of output switches should take into account proper selection of device transconductance so thatswitching of the device is controllable by the gate current,” says Camp.

“The free-wheeling (inverse) diode paired with each switch should have a ‘soft’ turn-off as well as low reverse recovery specifications. Gate drivers should be designed to switch thedevices in a way that limits rate of voltage change (dv/dt) at the drive output.”

This requires careful trade-off between noise reduction from slower switching and more switching losses in the inverter section due to longer on/off transition of the powerdevices.

Software algorithms using space-vector, pulse-width modulation (PWM) technology further help reduce EMI by minimizing the output devices’ switching transitions. Another software design consideration makes use of “minimum pulse-width blanking” – a technique where pulses shorter than a specific on-time period (for example, 5 μs) are eliminated,explains Camp.

Residential proximity
Siemens’ Wolfgang Hilmer points out that AC drives increasingly find application near residential areas, such as in HVAC (Heating, Ventilation and Air Conditioning) for buildings and municipal water plants. That raises further issues for EMI mitigation,requiring that drive installation follows strict EMC guidelines.

In contrast, industrial facilities nearly always include a transformer that typically blocks the drive’s EMI/RFI from reaching outside the plant. Also, there is more capacity tocounteract radiated EMI compared to residential facilities.

Small, internal filters often suffice to meet EMI regulations for industrial drives, notes Yaskawa Electric. However, to meet IEC standard EMC regulations for residentialapplications, including hospitals, external and perhaps dual-stage filters may be needed.

Danfoss Drives verifies that EMC “requirements in the domestic environment are the toughest to meet.” With HVAC applications being one of the company’s traditional markets, Danfoss designs its drives with EMC in mind, starting from the concept stage, according to Landkildehus. This has led to development of state-of-the-art EMI/RFI filtersthat provide reliable and economical EMC mitigation.

Built into the drives, these filters are designed for long motor cables (typically, 150 m shielded and 300 m unshielded). “To accommodate this requirement, the design has to be optimized as well from a thermal management perspective. Built-in filters offer the advantage of integrating their thermal design into the general thermal design of the drive,”says Landkildehus.

With a growing trend toward global drive designs it’s expected that more adjustablespeed drives will comply with international EMC standards and thereby offer more “silentrunning” characteristics.