Focus on ELECTROMAGNETIC INTERFERENCE: Reducing EMI from 3-phase drives

Drive current variations due to PWM switching are the root cause of EMI from 3-phase drives. Reducing them reduces interference.


Electromagnetic interference (EMI) consists of unwanted electrical signals emanating from a piece of electrical or electronic equipment that interferes with the operation of other equipment. EMI can be conducted or radiated. Generally, EMI appears at frequencies higher than the normal frequencies of currents involved in the radiating equipment’s operation (baseband).
For example, the normal frequencies involved in operation of a 3-phase motor/drive set are in the tens or hundreds of Hertz. Within the drive, however, switching frequencies of pulse-width-modulated (PWM) inverters are likely to be in the tens of kiloHertz or higher. In addition, inductive and capacitive effects can lead to ringing at radio frequencies induced by rapid changes in current as IGBTs in inverter final stages rapidly switch on and off. While the 3-phase baseband currents carry power between the inverter and motor, switching transients should really stay within the electromagnetically shielded drive enclosure, and radio frequency ringing should ideally not occur at all.
Yet, these unwanted signals can and do escape from the drive enclosure. Unless drive designers take steps to suppress these signals, they will escape at levels high enough to disturb nearby equipment, and become an EMI nuisance.
Conducted EMI consists of interfering signals conducted along cables and wires into equipment directly connected to the drive. Conducted EMI signals can travel along high-voltage cables carrying phased voltages, along neutral return lines, through ostensibly grounded cable and chassis shields, and even through ground buses themselves.
Radiated EMI travels as an electromagnetic wave similar to a radio wave from the EMI source to circuits and equipment that may be completely disconnected electrically.
In all cases, an EMI signal’s strength relates directly to the rate of change (first time derivative) of the inducing current. For sinusoidal currents, it is proportional to peak current and frequency by Faraday’s Law . Furthermore, Fourier’s Theorem says that any periodic signal can be decomposed into a series of sinusoids at harmonics of the base frequency, so higher harmonics of rapidly changing currents (such as the switching currents in PWM inverters) cause the most problem.
Conventional inverter technology, where large currents must be switched very rapidly, generates large EMI signals. To suppress them, drive designers incorporate expensive suppression measures, such as heavy shielding and EMI suppression filters.
This paper shows how the G7 Drive technology by Yaskawa mitigates EMI issues associated with industrial and commercial application of PWM inverters for high-power motor drives. It does so by drastically reducing the strength of EMI signals at the generating source: the inverter final stage.
The 480V G7 Drive adopts a Neutral-Point Clamped (NPC) 3-level inverter technology. The basic behavior of the NPC technology has significant advantages over a conventional 2-level inverter. In particular, 3-level inverters have smaller output voltage steps that reduce problems related to surge voltages at the motor terminals, motor shaft voltage and bearing current, leakage current, as well as others. Reducing these currents at the source reduces the need for bulky and expensive EMI suppression later on.
When the cable between the inverter and motor is long, voltages at the motor terminals are higher than those at the inverter terminals due to the steep voltage transient and distributed inductance-capacitance combination of the cable. Since the voltage step of the 3-level inverter is one-half that of the two-level inverter, the peak voltage at the motor terminal is significantly lower than for a 2-level inverter.
As Figure 1a shows, voltage in an inverter final stage can swing up to twice the input voltage when a step voltage is applied to an L-C resonant circuit. This is exactly the situation when fast-acting IGBTs in a drive final drive an inductive motor load.

2-level inverters produce large spikes

3-level inverters produce smaller spikes

Figure 1: Incorporating 3-level invertors in a drive’s final stage significantly reduces voltage spikes.

The overshoot magnitude of E adds to the original voltage E , making the peak value as high as 2 E .
Contrast this with the situation in Figure 1b, the voltage jump in a 3-level NPC inverter is 0.5 E , which is added to the original voltage of E , resulting in the peak value of 1.5 E .
High dv/dt of the common-mode voltage causes leakage currents to flow from the conductors of the cable and motor windings to the ground through the parasitic capacitances in these components. These leakage currents create noise problems to equipment installed near the inverter. It is also strongly related to the radiated EMI noise level.
Thanks to the smaller common-mode voltage steps, 3-level inverter leakage current is much smaller than that in a 2-level inverter. Smaller steps mean proportionately reduced EMI generation. Reduced EMI generation makes it possible to achieve better EMI performance with less radical suppression efforts. Coupled with additional benefits of improved insulation life and reduced bearing wear (see “

How switching transients reduce motor life

” elsewhere in this newsletter) make the G7 Drive Series a superior solution for high-power motor installations.
For more on EMI issues relating to variable speed drives, see “ Silence of the Drives ” by consulting editor Frank J. Bartos, P.E. in the June 2008 issue of Control Engineering . You can also download this article in podcast form here .

No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
Sister act: Building on their father's legacy, a new generation moves Bales Metal Surface Solutions forward; Meet the 2015 Engineering Leaders Under 40
2015 Mid-Year Report: Manufacturing's newest tool: In a digital age, digits will play a key role in the plant of the future; Ethernet certification; Mitigate harmonics; World class maintenance
2015 Lubrication Guide: Green and gold in lubrication: Environmentally friendly fluids and sealing systems offer a new perspective
Drilling for Big Data: Managing the flow of information; Big data drilldown series: Challenge and opportunity; OT to IT: Creating a circle of improvement; Industry loses best workers, again
Pipeline vulnerabilities? Securing hydrocarbon transit; Predictive analytics hit the mainstream; Dirty pipelines decrease flow, production—pig your line; Ensuring pipeline physical and cyber security
Cyber security attack: The threat is real; Hacking O&G control systems: Understanding the cyber risk; The active cyber defense cycle
Designing positive-energy buildings; Ensuring power quality; Complying with NFPA 110; Minimizing arc flash hazards
Building high availability into industrial computers; Of key metrics and myth busting; The truth about five common VFD myths
New industrial buildings: Greener, cleaner, leaner; New building designs for industry; Take a new look at absorption cooling; Offshored jobs start to come back

Annual Salary Survey

After almost a decade of uncertainty, the confidence of plant floor managers is soaring. Even with a number of challenges and while implementing new technologies, there is a renewed sense of optimism among plant managers about their business and their future.

The respondents to the 2014 Plant Engineering Salary Survey come from throughout the U.S. and serve a variety of industries, but they are uniform in their optimism about manufacturing. This year’s survey found 79% consider manufacturing a secure career. That’s up from 75% in 2013 and significantly higher than the 63% figure when Plant Engineering first started asking that question a decade ago.

Read more: 2014 Salary Survey: Confidence rises amid the challenges

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.