This month, DrivesMag asked North American process chemical plant designers and facility managers about their facilities and process lines. The answers to the question: What communication method do you use in your plants/processes? return some interesting insights into automation and control networks and the systems that define their work. Users were able to choose more than one answer and were given the choice of "other", but rarely selected it.

This research has a confidence factor of 90% and a confidence interval of 10%.

An electrician explains the basic operation of an ABB VFD.

Siemens boasts about its automation offer and how it increases productivity and energy efficiency in this promotional video.

Siemens explains how to upgrade old machines with energy efficient motors and drives in this promotional video.

Highlights from their answers:

• "Drives are a must have in all HVAC projects. The real issue is that customers become familiar with one brand or product."
• "HVAC systems all have them. The issue is who, not what."
• "The market seems to believe in them now. When we're convinced of short payback we don't resist."
• "We know we need them. We listen to the market for brand preference."

Data to follow in subsequent posts.

ABB and Technicon explain the science of energy savings with AC drives.

Magnetek, Inc., announces the addition of Bob Peddycoart as Business Development Manager of Radio Controls and Mark Sullivan as a Regional Business Development Manager for Magnetek’s Material Handling business.

Bob Peddycoart has more than 15 years of sales experience in the mobile hydraulic market. after working at Altec, Hetronic and Cervis.  His initial exposure to radio began while he was stationed in Augsburg, Germany, working in signals intelligence for the U.S. military. 

Mark Sullivan joins Magnetek as a Regional Business Development Manager responsible for representing Magnetek’s material handling products in Southeastern Ohio, Western Pennsylvania, West Virginia and Western New York, with 25 years of experience in the overhead crane industry, with a background in installation, service, modernization and project management.

By Adalberto Jose Rossa – Formerly of WEG Automacao, now with Infineon - with contributions from Mark Zawadzki from International Paper

Read the first in this series here.

Introduction

This is the second part of a series of three technical articles, which studies currents in installations with typical variable frequency drives (VFDs) and three-phase AC Induction or Permanent Magnet motors (PM motors).

In the first part the circuit modeling and main parameters influencing the stray currents were presented.

In this second part the measurement strategies for CM currents are presented as well the main effects of the common mode currents in the drive components and installations.

Measuring stray currents

Inspired by a Lord Kelvin quote, "If you cannot measure it, you cannot improve it", we must start by measuring the CM currents to be able to evaluate the system performance and the mitigation actions suggested in item 6.

Considering the typical waveforms of the CM currents showed in the previous item, the frequency components in it, a high bandwidth current probe is required. A minimum bandwidth of 5 MHz is suggested. Besides, in order to be able to embrace up to 3 conductors, a high loop area probe must be used.

There are commercial air-cored flexible Rogowski coil current sensors [1] available which fulfill these requirements. For example, the CWT series from Power Electronics Measurements Ltd (www.pemuk.com) with measuring range from 300 mA to 300 kA and frequency range from 0.1 Hz to 16 MHz.

The current waveforms can be viewed on a digital storage oscilloscope with minimum 100 MHz bandwidth and 500 MS/s sampling rate. Example: Tektronix TPS2014, exceeds these values.

When measuring the CM currents the fundamental VFD frequency must be varied in the entire operating range to check for worst-case values of CM currents. Normally the biggest CM peak current occurs at a low fundamental frequency and, consequently, very low VFD output voltage when the 3 upper or the 3 lower IGBTs are turning on almost simultaneously. This is the main reason why normally conducted high-frequency disturbances measured using a LISN and a receiver show the highest amplitudes at low frequency, typically in the range from 0.5 to 3 Hz.

Measurement tips: The points for measurements are shown in figure 6 for a typical PDS installation with a shielded motor cable, used here as reference.

The waveform at (C) shows the total CM current flowing from the VFD and should ideally return through motor cable shield and/or ground cable conductor (E).

So measurements in (C) and (E) should be ideally equal.

As the motor frame is normally grounded at point (II), a ground loop from VFD ground point (I) to motor ground point (II) is created (see figure 6). The magnetic field created by currents flowing through U, V and W conductors can induce low frequency currents on this loop. This phenomenon is not rare and makes measuring currents at (E) and (F) harder. These induced low frequency currents will appear added to the return currents from VFD output. The measurement at (B), here embracing the three ground cables, will remove the low frequency induced currents in measurements.

Measurements at (A) can show both the return CM currents to the VFD and the currents Òimported from lineÓ by VFD internal filter capacitors connected at line inputs (R, S and T) to ground or dc bus to ground. Normally VFDs have these caps to reduce Radio-Frequency emissions.

Simultaneously measuring points (A) and (C) with 2 current probes can show details of how much CM current flows from the line and the relation with (C). Besides the amplitude, the frequency components of these currents are important and related to emissions and risk of interference.

The current amplitudes measured at point (A) should be as low as possible to avoid conducted and radiated RF emission from VFD.

Measurements at other points, like the load shaft and shaft grounding, can be performed as well, but will depend on the availability of specialized measuring equipment.

Figure 6 – Basic measuring points of CM currents in motor drive system

(a) CH1: VFD input CM current (R, S, T lines) CH3: VFD output CM current (U, V, W lines)	(b) CH1: current at VFD grounding conductors CH3: VFD output CM current (U, V, W lines)
Figure 7 – Examples of CM current measurements with short non-shielded motor cables (5 meters) Line voltage: 575 VAC VFD/Motor: 350 HP/575 V

Effects on motors and Installations

Electromagnetic Interference (EMI):
The PDS can interfere with other equipment by two types of generated RFI disturbances: conducted and radiated. Referring to the simplified drive system diagram in figure 2, part of the CM current flowing through the supply cables (i_g4) can create voltage distortions and possible disturbances in equipment fed from these power lines. The CM currents flowing in the grounding system (i_g2 and i_g4) create electromagnetic fields and consequently radiate energy through these. The radiated energy will be bigger as loop areas 1 and 2 (figure 6) increase or when i_g2 and i_g4 increase. The conducted and radiated RFI interference levels and frequency ranges are specified in EMC standards as EN 61800-3 [3].

Life-time reduction of motor bearings: The discharge current pulses through the partially oil insulated film between bearing balls and race can cause erosion, commonly referred to as fluting or Electric Discharge Machining (EDM) [5]. This phenomenon is well known, but the effects are very difficult to predict. It depends on many variables like, for example, the motor operating speed. Some preventive actions at motor side can be taken, like insulating motor bearing on non-driven end, or on both ends and short circuiting the rotor and the motor frame by means of a sliding brush [6]. These measures are more common to be adopted for bigger motor frames. The reduction of VFD output CM current amplitude and frequency spectrum is also effective, as well as the electrical installation conditions, like the grounding system, in improving the bearing life.

VFD trip by ground fault protection or overcurrent protection:
The CM currents are added to the motor current and can cause the activation of the output overcurrent protection or the ground fault protection of the VFDs. The CM peak currents do not vary linearly with motor power and the trip is more likely to occur in low power PDS. The motor cable length also greatly influences the risk of trip. The VFD ground fault protection in general purpose drives is basically a current transformer (CT) which embraces the 3 input lines (R, S, T), or the 2 dc lines (+ and -) or the three output lines (U, V, W). So, these CTs sense the CM current directly in each part of the circuit. The CT located at the three output lines (U, V, W), is the most sensible, detecting the complete CM current. Normally the protection strategy is based on a rectifier/comparator at CT secondary with light filtering due to the fast response needed at output short-circuit to ground condition. This leads to a higher sensibility and risk of false activation. External CM current monitors and monitoring insulation devices sometimes used in the line supply side of the PDS are less susceptible to trigger by CM currents due to the filters in their circuits.

REFERENCES:

1. David E. Shepard, and Donald W. Yauch: "An Overview of Rogowski Coil Current Sensing Technology", LEM DynAmp Inc.
2. Technical Specification IEC 60034-25: Rotating electrical machines – Part 25: Guidance for the design and performance of a.c. motors specifically designed for converter supply
3. BS EN 61800-3:2004 - Adjustable speed electrical power drive systems. EMC requirements and specific test methods
4. Patrick J. Link, P.E.: "Minimizing Electric Bearing Currents in Adjustable Speed Drive Systems", Pulp and Paper Industry Technical Conference, 1998. Conference Record of 1998 Annual
5. Jay Erdman, Russel J. Kerkman, Dave Schlegel, and Gary Skibinski: "Effect of PWM Inverters on AC Motor Bearing Currents and Shaft Voltages", IEEE APEC Conference Dallas. TX March, 1995
6. Weg Technical Guide: "Induction motors fed by PWM frequency inverters", www.weg.net

Danfoss is kicking off their third annual "EnVisioneer of the Year" award competition and invites entries for consideration.

The competition recognizes U.S. end use customers, building owners and original equipment manufacturers (OEMs) that have introduced a new product, opened a new facility or invested in a building or system upgrade in the past 18 months using Danfoss products or solutions to realize significant energy and/or environmental savings.

A distinguished panel of judges representing different disciplines in the heating, ventilation, air-conditioning and refrigeration (HVACR) industry will review all applications and select the winner.

As part of their recognition, a donation will be made to Rebuilding Together in honor of the selected 2012 EnVisioneer of the Year. Rebuilding Together is the nation’s leading nonprofit working to preserve affordable home-ownership and revitalize communities. Their network of more than 200 affiliates provides extensive renovations and modifications to the homes of low-income Americans at no cost to the homeowners, making the homes safer, more accessible and more energy efficient.

The award will be presented in Chicago as part of AHR EXPO 2012.

Last year’s winner, MultiStack LLC, installed two MultiStack chillers at Valley High School, Las Vegas, Nevada.

By switching to the magnetic levitation-powered chillers and a central plant control system, the 45-year-old school reduced its energy consumption by 70 percent, from 1.5 kW/ton to .443 kW/ton.

For more information about the EnVisioneer of the Year competition and to submit an entry, please visit http://envisioneering.danfoss.com/About/EnVisioneering+Award.htm, or contact Lisa Tryson, director, corporate communications and public relations, at This e-mail address is being protected from spambots. You need JavaScript enabled to view it or 410-513-1142.

Interested participants may enter the competition by submitting an application no later than January 12, 2012.

Vacon announced today that it will establish an R&D center and lab for high-power AC drives in the Raleigh-Durham area, North Carolina, in the USA, adding emphasis on research in its U.S. operations and offering new job opportunities.

"We want to be closer to research centers, universities and existing power electronic clusters. This is why we will place our R&D center and lab for high-power AC drives in the Raleigh-Durham area. We will start active recruitment immediately," says Dan Isaksson, Vice President - R&D.

"We believe that focusing 100% on AC drives is how we can best grow. Delivering superior products and services requires competence and focus. The global high-power AC drives market is growing, and this is definitely a growth opportunity for Vacon."

Headquartered in Finland, Vacon started its U.S. operations since the company acquired the AC drives business of TB Wood's in 2007. The new Vacon, Inc. manufacturing facility was completed in 2009 in Chambersburg, PA, and in 2010 it received LEED Gold Certification, established by the U.S. Green Building Council.