The European integrated motors and drives (IMD) market has recently recorded consistently high growth rates, except in 2009 when it was affected by the economic recession. During that year, the European IMD market witnessed greater decline in demand (about 14%), compared to the global IMD market (about 13%).

However, with numerous drivers and fewer addressable challenges, the European IMD market is ready to bounce back. Frost & Sullivan anticipates a cumulative annual growth rate of 12.1% from 2010 to 2017.

“The demand for high efficiency, along with the need to reduce energy consumption, is set to attract investments in IMD solutions,” notes Frost & Sullivan Research Analyst Ramasubramanian Natarajan. “Heightened knowledge about their potential benefits will extend the implementation of IMDs across an extensive range of industrial applications.”

IMDs are expected to gain preference over stand-alone motors and drives over the long-term. This, however, will depend on anticipated technological advancements in functionality and the availability of the technology at an affordable cost.

“The optimal compatibility of the variable frequency drive (VFD) with the motor in an IMD ensures efficient performance, with efficiency levels exceeding 90%,” remarks Ramasubramanian. “This also makes IMD units easier to deploy than procuring motor and drive as two separate components and then combining them to achieve desired performance. It also reduces lag time and increases productivity.”

While these are positive signs, the immediate challenge for IMD manufacturers will be to scale down high initial costs and project product benefits more clearly to end-users. Another issue has been the technical inability, so far, to develop IMDs for higher power ratings.

“Due to technological limitations, above a certain point, the physical size of the product makes the integration of motor and drives lose its meaning,” explains Ramasubramanian. “While VFD solutions are in position to meet customer demands for higher power rating applications, IMD solutions are not perceived to be cost-effective at high power levels, thereby limiting the overall growth potential of the market.”

Technological advances and their availability across a range of power ratings will lead to the deeper penetration of IMDs into a wider range of applications. This, in turn, will boost customer acceptance of the technology. Competitive price levels will also contribute to encouraging demand for integrated motors and drives across key end-user industry segments.

According to Frost & Sullivan, the European Market for Integrated Motors and Drives market earned revenues of $285.3 million in 2010 and estimates this to reach $632.8 million in 2017. The research covers AC, DC, servo and stepper integrated motors and drives.

If you are interested in more information on this study, please send an email with your contact details to Anna Zanchi, Corporate Communications, at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Eaton's new C441 Ethernet series of communications cards allow customers to select from Ethernet/IP, Modbus TCP, HTTP web services and Modbus RTU communication protocols in a single card. The innovation is designed to help industrial customers, machinery OEMs and panel builders with flexible communication options to configure, control and monitor their systems.

Integral web services provide an easy-to-use web-based graphical user interface (GUI) to make it easier to recognize potential problems. Using a laptop or smart device, customers can drill down to a given load by simply entering an IP address into their web browser. With four levels of access, the cards ensure only those with credentials have access to critical or sensitive functions.

An Eaton representative explained, “Integral web services allow for configuration, control, monitoring and diagnostics. Also, the flexibility in communications allows OEM customers to add value to their equipment with simple and desirable features, like supervision and control.”

The C441 is compatible with Eaton C440, XTOE and C441 electronic motor protection relays, S611 soft starters, and can be used as stand-alone input/output (IO). Now, the suite of protocols available for Eaton motor protection and soft starter solutions includes PROFIBUS, Modbus TCP and RTU, DeviceNet, and Ethernet IP.

Problems can be addressed in real time with intelligent monitoring readouts from starters and onboard available four digital inputs (120 volts alternating current or 24 volts direct current) and two discrete relay outputs. With a dual port switch, the Ethernet card allows for easy daisy chaining and reliable ring configurations. Additionally, customers have the ability to use one network for control and another for monitoring – through the Modbus serial protocol used in parallel with the Ethernet network –allowing redundant communications and greater reliability with a single card.

Since the introduction of variable speed electronic controls for DC and AC motors, users have noted a troubling problem; voltage potential is sometimes induced on the shaft of the motor.  The resulting current can discharge through gearbox bearings, through the motor bearings, or through the encoder or tachometer attached to the motor.  This discharge causes the premature failure of motor bearings and/or encoder bearings. This paper discusses the phenomenon of encoder and motor damage due to motor shaft currents through encoders, and proposes several remedies and guidelines .

Problem

Both AC and DC motors controlled by electronic speed controls can experience “shaft currents”, where voltage is induced on the motor shaft, and the resulting current attempts to discharge to ground, or circulate through the motor through any conductive, contacting surface.  Several excellent papers have been published on the topic, including Rockwell’s 3/11/02 “Inverter-Driven Induction Motors Shaft and Bearing Current Solutions” (1), “Don’t Lose Your Bearings” (2) by A. Muetze and A. Binder, IEEE 1077-2618/06, and “Mitigating Stray Currents in AC Drives Installations”(3) by Adalberto José Rossa.  These shaft currents can discharge through arcing contact between the bearings and bearing races, causing the characteristic fluting damage to the races.  This paper does not examine the causes of shaft currents, only the resulting impact on motor/encoder combinations. 

The discharge of motor shaft currents can damage encoder bearings, and improper encoder installation and design can cause shaft current discharges that damage motor bearings.  Solving these problems is essential to avoiding premature motor bearing failure and/or encoder bearing failure.

Background

As detailed in the shaft grounding papers listed (1,2,3), any current path to ground, or return path between the shaft and motor frame can be highly undesirable.  This is especially true when the current path is combined with the typical vector duty motor configurations shipped today, with a non-isolated bearing on the drive end and one insulated/isolated bearing on the non-drive end. (Figure 1)  If the encoder creates a path to ground and/or the motor frame, current may circulate through the non-isolated drive-end motor bearing, and it can discharge through the encoder bearings. (Figure 2)

Industrial encoders were originally designed as stand-alone devices that were coupled to the motor or load (Solid Shaft/Coupled).  As magnetic technology was introduced, and miniaturization and hardening of optical designs progressed, encoder manufacturers began to offer encoders for direct motor mount.  These designs fell into two broad categories:  Modular, and Hollow Shaft.  Modular encoders are typically mounted to a machined C-face on the motor, and the rotor is mounted to the shaft.  Hollow Shaft encoders are mounted to and supported by the motor shaft and secured by an antirotation arm or tether.  Solid shaft styles are also still in wide use.

Solid Shaft Encoders:
Standard solid shaft encoders (also called coupled) are not electrically isolated: The metal encoder shaft is linked to the motor shaft with a conductive coupling; current is carried through the encoder bearings to the frame, and the encoder is solidly bolted to a surface that is typically connected to the motor frame, such as a flange or foot.  Electrical outputs are often connected via metallic grounded conduit, and some encoder designs do not isolate circuit ground from the encoder housing.  This means shaft currents can quickly end the life of motor (or encoder) bearings. (Figure 3)

Because the encoder can mount in the typical grounding brush mounting location on an older motor, some solid shaft encoders offer shaft grounding kit options.  Because they are mounted on the non-drive end of the motor shaft, they have the potential to create a circulating current path through ground (Figure 4), which can damage the uninsulated motor bearing as previously outlined.

Modular Encoders:
Modular encoders (also called pancake or C-face mount) are inherently isolated electrically by their design and do prevent shaft current discharge.  Although the encoder stator/housing is grounded to the motor frame, the rotor is electrically isolated from the sensor mounted in the stator by the air gap between the sensor and the rotor.  Usually this gap is at least 0.010” [0.25mm], sufficient to electrically isolate the motor shaft’s relatively low voltage potential from the housing.

However, modular encoders may offer shaft grounding brush options, which create a path to ground, potentially endangering the uninsulated drive motor bearing in a motor with a single isolated bearing on the non-drive end.

Hollow Shaft Encoders:
Hollow shaft encoders (also called tethered) are not inherently electrically isolated; the typical metal hollow shaft directly grips the motor shaft; current can be carried through the encoder bearings to the frame, and the antirotation arm forms the return path to the motor housing.  As in solid shaft designs, some manufacturers connected or capacitor-coupled circuit ground to the housing. Moreover, some industrial encoder connectors feature metallic conduit adapters, offering a third path to ground through the encoder bearings.

Encoder manufacturers recognized the problem of antirotation arms early on, and added insulating washers to the antirotation arm mounting kit.  However, many installers were (and still are) unclear on the purpose of these insulators and arm, and hoping for a more a sturdy mounting, they directly bolted the antirotation arm to the encoder, and shaft currents were free to flow once more.  Encoder manufacturers responded with insulating nylon sleeves or coatings in their shaft assemblies to eliminate the need to isolate the arm with plastic washers.  Users then discovered that the shaft sleeve plastics flow easily under clamping pressure, and do not have the same coefficient of expansion as the steel motor shaft.  With the temperature cycling characteristics of vector duty motors, hollow shaft encoders with nylon sleeve designs have been slipping off motor shafts in ever-increasing numbers.

Hollow shaft encoder bearings may also be destroyed by motor shaft current discharge just like motor bearings(3).  Most encoder bearings are less than 1/10 the size of motor bearings.  This makes encoder bearings much more vulnerable to damage from motor shaft currents.  Fluting and the corresponding bearing destruction may occur within days or months.  Many users have reported large-scale hollow shaft encoder failures when shaft currents were unchecked. 

Adding to the woes created by hollow shaft encoders, users began to demand shaft grounding kits. In some cases, the users hoped that the grounding brush would eliminate both the motor bearing damage and the hollow shaft encoder bearing failures they had experienced.  While shaft grounding brushes on the non-drive end can, in some cases, protect the encoder bearings, they do create a path for destruction of an un-insulated motor drive-end bearing in a motor with a single non-drive end isolated bearing.  Many customers now prohibit the use of shaft grounding kits on modern motors.

Application Guidelines:

The first guideline is a simple one: Apply shaft grounding wisely:

• Don’t add shaft grounding brushes to the same end of a motor as the insulated or isolated bearing when the other motor bearing is un-insulated.  The resulting circulating current can cause premature motor drive-end bearing failure.
• If the motor does not have any insulated or isolated bearings or the motor has two insulated bearings, shaft grounding kits may be applied at the encoder or drive end with no ill effects.

Second, follow encoder manufacturers’ guidelines for installation:

• If the hollow shaft encoder antirotation arm is provided with insulating washers, use them.  For solid shaft encoders, select an insulated coupling that isolates the encoder shaft from the motor shaft.

Finally, select an encoder that by its construction methods prevents shaft current discharge through the encoder without special installation concerns:

• The encoder should prevent discharge to ground through encoder signal connections and conduit adapters, as well as prevent circulating currents through the encoder frame and bearings.
• Consider encoders that are inherently isolated, and therefore prevent motor and encoder damage from shaft currents through the encoder, like those from Avtron Industrial Automation, Inc., author of this technical article.

References

(1) “Inverter-Driven Induction Motors Shaft and Bearing Current Solutions”, Rockwell Automation Industry White Paper, 3/11/02
(2) “Don’t Lose Your Bearings”, A. Muetze and A. Binder IEEE 1077-2618/06
(3) “Mitigating Stray Currents in AC Drives Installations” (Parts 1,2 & 3), Adalberto José Rossa, Drivesmag.com: 10 November 2011-09 January 2012

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.

By Adalberto José Rossa - WEG Automacao, with special thanks to Mark Zawadzki from International Paper for the text revision and valuable ideas and suggestions.

This technical article studies the stray currents which appear in installations with typical variable frequency drives (VFDs) and three-phase AC Induction or Permanent Magnet motors (PM motors). These currents flow in electrical power cables through stray capacitances to the electrical ground system.

Part 2 will discuss the stray current measuring strategies and the effects of the common mode currents and part 3 will deal the counter actions to mitigate the undesired effects. The entire series will be featured, exclusively, on http://www.drivesmag.com in the System Design and Engineering Category between now and January 2012.

The undesired main effects on drive systems are the following:

• electromagnetic interference (EMI) in other equipment fed from the same power line or close to the drive system components, VFD, motor or motor cables;
• premature damage of the motor bearings, gearbox bearings or machine bearings, when part of the stray currents flow through them;
• VFD trip by ground fault protection or overcurrent at output.

In the first part the most common VFD topology, with IGBTs as active components, is investigated and the author's intention here is to shed some light on the basic phenomenon.

Figure 1 – PDS simplified diagram

 

Current paths and stray currents as system related parameters

To learn more about the subject it is necessary to understand the basic phenomenon and related parameters. We can start from a basic circuit relation for the common mode values:

(1)

It is well known that general purpose inverters create a common mode voltage v_CM. The v_CM waveform is influenced by the PWM strategy, the dc bus voltage and the switching frequency. We will, however, take a detailed look at the electrical current transitory behavior every time that one IGBT turns ON.

The circuit in figure 2 shows i_CM current paths and a simplified single-phase circuit for CM current study. The cable/motor model used to represent z_CM is composed of R_S, L_S, as series impedance and C_g as the total stray capacitance to ground. The VFD CM output current is represented by i_go in the single phase model. See the "Cable/Motor CM Model" in figure 2.

Figure 2 – Common mode circuit and CM current paths

 

The CM current waveform can be predicted exploring the cable/motor circuit RLC series circuit model [1] subjected to a step voltage when T1 or T2 is turned on [2]. The circuit has a natural oscillation frequency w_n (rad/s) or f_n (Hz):

(2)
(3)
Typical circuit values lead to the well known under damped transient response, with damping factor z<1.
(4)
The transient current i_go in figure 2 will have a waveform similar to that shown in figure 3 when upper IGBT (T1) is turned on.

 

Figure 3 – i_go Waveform when upper IGBT (T1) is turned on

 

The current i_GO as a function of time is represented by:

(5)

 

where a is the damping coefficient, also known as neper frequency or attenuation, and defined as:
(6)

 

and z_o is the circuit characteristic impedance:
(7)
The i_go peak current is influenced by the line phase voltage peak and z_o.

When the lower IGBT (T2) turns on, the i_go waveform presents a similar behavior, but the initial current peak is negative.

Figure 4 shows some real examples of measured CM currents for longer cables.

In real circuits the applied pulse voltage is not a perfect step, but has a rise time t_r, or a change rate dV/dt. The relation between t_r and circuit model component values can influence the peak current waveform. If t_r is longer than ½ of the natural oscillation period T_n (8) the leakage current decreases considerably [1].
(8)
(9)

(a) 100 meter shielded motor cable Scales: horizontal: 20 ms/div vertical: 5A/div ; Ipk-max= 10 Amps IRMS= 1.5Amps (switching frequency= 5 kHz)	(b) 500 meter shielded motor cable Scales: horizontal: 20 ms/div vertical: 10A/div Ipk-max= 21Amps IRMS= 5.6Amps(switching frequency= 5 kHz)
Figure 4 – Examples of CM current measurements with shielded motor cables Line voltage: 575 VAC VFD/Motor: 10 HP/575 V

Based on the author's experience, the relation in (8) may be satisfied only for low power VFDs with short motor cables. LetÕs analyze the field measurements for a 350HP/575V VFD and 5 meter motor cable length shown in figure 7(b). In this case T_n@ 2 ms, so for t_r ³ 1 ms a significant CM current reduction would be accomplished, when compared to a perfect step voltage. This is hard to get in reality, considering that the typical t_r for modern IGBTs is normally in the range of 0.1 to 0.5 ms [4]. It is, however, possible to get some control of t_r during VFD design. More details are provided below.

In reference [2] a more complex equation for is developed also considering the effect of the pulse voltage dV/dt.

Table 1 presents some simulation values based on series RLC circuit model. The RLC values are adjusted considering a 300 meter shielded motor cable. For this cable length no difference was observed varying the rise time from 100 to 500 ns. This can be predicted since T_n@ 18.2 ms is so much greater than the largest possible t_r value.
The pulse voltage amplitude, however, influences the maximum peak current as can be seen from simulation #1: V_P= 470 V and #2: V_P= 376 V .

The maximum peak current is directly proportional to the pulse voltage amplitude as predicted by equation (5). Therefore, the CM currents for long motor cables are basically determined by cable/motor parameters and line voltage.

Table 2 presents some simulation results based on series RLC circuit with component values adjusted to a 6 meter shielded motor cable. Differently from the situation with longer cables, the initial peak current is influenced by the voltage pulse rise time. In simulation #3 the maximum peak current is approximately 5 Amps for 100 ns rise time and ca. 2.8 Amps in simulation #4 for 300 ns rise time. The line voltage also has an influence on initial CM current peak value, as can be seen from simulation #5.

Hence, for short cables, the CM currents are determined in practical applications by three parameters: cable component values (RLC), line voltage and voltage pulse rise time.

The PWM pulse pattern can also influence peak current values, especially when pulses are reapplied before the transient period is extinguished. This is more likely to occur with longer motor cables, since damping lowers (a decreases) as motor cable length increases. This effect can vary from VFD to VFD and operating conditions like switching frequency, V/f curve, voltage boost, etc.

Table 1 – Simulations with RLC circuit model for 300 m shielded motor cable 10 HP motor

 

Simulation	Circuit	Pulse	CM
#1	RS=10 W LS= 300mH Cg= 28 nF (fn@ 55 kHz; Tn= 18,2 ms; z@ 0.04;a@ 16.67x103)	Amplitude= 470 V Rise time= 100 to 500ns Scales: Vertical: 2 Amps/div Horizontal: 50 ms/div
#2		Amplitude= 376 V Rise time= 100 to 500 ns Scales: Vertical: 1 Amp/div Horizontal: 50 ms/div

Table 2 – Simulations w/RLC circuit model for 6 m shielded motor cable and 20 HP motor

 

Simulation	Circuit	Pulse	CM
#3	RS=10 W LS= 6mH Cg= 1 nF (fn@ 2 MHz; Tn= 0.5 ms; z@ 0.06;a @ 833x103)	Amplitude= 470 V Rise time= 100 ns Scales: Vertical: 1 Amp/div Horizontal: 1 ms/div
#4		Amplitude= 470 V Rise time= 300 ns Scales: Vertical: 1 Amp/div Horizontal: 1 ms/div
#5		Amplitude= 376 V Rise time= 300 ns Scales: Vertical: 1 Amp/div Horizontal: 1 ms/div

Notes on IGBT switching speed

• Many (negative) things are said about modern, fast IGBTs and their high dV/dt, with negative effects in VFD RF emissions and other adverse effects, like the life-time reduction of bearings.
• Fact: modern IGBTs switch faster than IGBTs from older generations and their switching (turning off) speed is difficult to control [3], [4], and [5]. The good news is that these IGBTs can have reasonable switching speed control when turning on, through gate resistor and/or gate-emitter capacitor tuning. The turn on speed is important when CM currents are analyzed.
• As real life tends to be a little bit more complicated than it should, the IGBT V_CE at turn on normally shows two different dV/dt values, as shown in figure 5, which also shows a dependence on the dV/dt values to gate resistor and gate-emitter capacitance values. The gate turn on voltage level also influences the dV_CE/dt at turn on. The DV_CE1 as showed in figure 5 has a value which will depend on dc bus stray inductance and IGBT di_C/dt at turn on [3], [5].
• VFD designers should keep limited dV/dt at turn on during design and circuit component selection. There is also a trade-off between dV/dt and IGBT losses that must be optimized.

(a) w/RG= 2,2 W; CGE= 39 nF Time scale: 100 ns/div Measured values from figure (CH4): dVCE/dton(1)= 1,5 kV/ms dVCE/dton(2)= 2,4 kV/ms	(b) w/RG= 2,7 W; CGE= 100 nF Time scale: 250 ns/div Measured values from figure (CH4): dVCE/dton(1)= 0,72 kV/ms dVCE/dton(2)= 1,4 kV/ms
CH1: gate-driver input voltage (+15 V commands the gate-driver to turn on the IGBT) (10 V/div) CH2: gate to emitter voltage (10 V/div) CH3: VFD output current (collector current after turn on) (200 Amps/div) CH4: IGBT VCE (200 V/div) Figure 5 – 900 Amps/1200 V IGBT waveforms at turn on at different values of RG and CGE

Terms and definitions

IGBT: Insulated Gate Bipolar Transistor is the main power electronic switch used for low voltage AC drives today.
V_CE: IGBT collector to emitter voltage. Normally shows DC link voltage at IGBT turn-off and a low value, called saturation voltage in the range of 2 V, when turned on.
VFD: considered here is the most common power topology for general purpose three-phase low voltage motor control. A simplified schematic diagram appears in figure 1, with three-phase diode uncontrolled input bridge (D1 to D6), line reactors (L_L) or dc bus inductors (L_d+, L_d-), dc bus filter electrolytic capacitor bank (C_dc) and three-phase IGBT output bridge (T1 to T6) with anti-parallel free-wheeling diodes (D7 to D12).
Power Drive System (PDS): composed of the VFD, related accessories like metallic panels, line reactors, fuses, motors and interconnecting cables.
PWM: Pulse Width Modulation here referred to the VFD output voltage
Switching Frequency: IGBTs switching rate
Motor shielded cable: here are considered the cable types and installations according to the IEC 60034-25, with both shield ends, at VFD side and at motor side, connected to the ground.
Line supply: in this work the solid neutral grounding TT or TN three-phase system is considered, although neutral resistor grounding system should have similar behavior.
Common Mode voltage (v_CM): sometimes called zero sequence voltage, is the sum of the output voltages in respect to ground, which is not zero for PWM inverters, when comparing to an ideal three phase voltage with sinusoidal wave form, where the neutral voltage measured against the ground is zero Volts. In comparison the typical three-phase sinusoidal shape voltage sources the neutral voltage in respect to system safety ground is zero.
Common Mode currents (CM - i_CM): the stray currents in VFD installations are commonly referred to as zero sequence currents or common mode currents (CM currents). The term CM currents will be adopted here.
EMI: Electromagnetic Interference, considered here as electromagnetic interference from PDS on other electrical equipment. On site electronic equipment, like computers, programmable logic controllers (PLC) and sensors are more susceptible to radio frequency interference (RFI). Normally the more important for practical cases with electronic equipment, as computers, programmable logic controllers (PLCs) and sensors is the radio frequency interference (RFI).
RFI Filter: Radio Frequency Interference Filter, which can be added externally as a component or included inside the VFD cabinet (built-in RFI filter). The latter is more common in modern VFDs. Its purpose is to reduce the emissions of the PDS according to the limits from established standards, like EN 61800-3, therefore reducing the risk of the VFD system interfering with other equipment.
LISN: Line Impedance Stabilizing network. Standard line impedance defined by IEC standards for conducted emission measurements.

References:

1. Satoshi Ogasawara, Hirofumi Akagi: ÒModeling and Damping of High-Frequency Leakage Currents in PWM Inverter-Fed AC Motor Drive Systems, Industry Applications Conference, 1995. Thirtieth IAS Annual Meeting, IAS '95
2. Annette Muetze, "Scaling Issues for Common-Mode Chokes to Mitigate Ground Currents in Inverter-Based Drive Systems", IEEE Transactions on Industry Applications,
3. Volume: 45 , Issue: 1 , January/February 2009
4. Andreas Volke, Michael Hornkamp: "IGBT Modules, Technologies, Driver and Application", 1^st Edition, 2011, Infineon Technologies AG
5. "Application Note AN2003-03: "Switching behavior and optimal driving of IGBT3 modules", 2003, Infineon Technologies AG
6. Reinhard Herzer, Arendt Wintrich: "IGBT Gate Drive Technologies – Principles and Applications", Semikron, PCIM Tutorial 2009
7. M. BŠ§ler, P.Kanschat, F.Umbach, C. Schaeffer: "1200V IGBT4 -High Power- a new Technology Generation with Optimized Characteristics for High Current Modules", PCIM Europe 2006
8. Piotr Luniewski, Uwe Jansen, Michael Hornkamp: "Dynamic Voltage Rise Control, the Most Efficient Way to Control Turn – off Switching Behavior of IGBT Transistors", Pelincec 2005

 

Increased discussion about global warming and energy dependency is leading to massive push toward energy-efficient systems. And electric motors account for about 60% of the total industrial power use in Europe.

Slowing down a motor to its optimum speed could lead to significant energy savings and help in reducing carbon footprints. Carbon emissions are directly linked to climate change (global warming), which demands strong reduction in energy usage. The road to greater energy efficiency starts with highly efficient electric drive systems. There is a simple business sense for being more energy efficient as it has a direct impact on our investments. Lower the energy consumption, lower are the energy bills, reducing the need for energy production, contributing to lower carbon emissions. Reducing energy waste leads to decline in energy spent. Most businesses can account their contribution in balancing global warming by cutting down their energy consumption. Usage of electric drives can reduce the overall energy consumption of a plant by up to 50%, depending on how efficiently the system is implemented.

Global Warming – Statistics and Impacts:

Frost & Sullivan estimates that in the current scenario, about 25,000 million tons of CO2 is released by the industries across Europe and despite several measures this has projected an increase year on year. The need for energy has been rising steadily and as per International Energy Agency’s (IEA’s) publication, if the current policies were to be in effect in the future, the emission would increase by 120% in 2050, while oil demand would rise by 65%. The effect of global warming has already started to show adverse effects on the environment. Numerous long-term changes have already been recorded for example, increase in global average air and ocean temperatures, meltdown of Arctic and Antarctic snow and ice, leading to increase in the sea level.

Global Warming – Mitigation and Measures:

The Industrial sector is responsible for consumption of about 41.6% of the total electricity produced in Europe.

Recent years have witnessed an influx of regulations applied to all manufacturing industries across Europe. In 2001, the climate change levy was introduced, followed by climate change agreements. The European Union Emission Trading System (EU ETS) came into effect from 2005, followed by the environmental permitting regime in 2008. The most recent, Carbon Reduction Commitment Energy Efficiency Scheme is now being widely implemented across Europe, starting from April 2010. The new set of motor efficiency regulations namely IE1, IE2 and IE3 is another big step in controlling carbon emissions. All the motors entering the market on or after 16 June 2011 must comply with IE2 standards. From 1 January 2015, motors of power rating between 7.5 kW to 375 kW being placed in the market must comply to IE3 efficiency standards or IE2 standards if they are equipped with variable speed control. From 1January 2017, motors of power rating between 0.75 kW to 375 kW being placed in the market must comply to IE3 efficiency standards or IE2 standards if they are equipped with variable speed control.

Chart1: Energy Consumption: Per cent Distribution by Application Sectors (Europe), 2010

Undoubtedly, the biggest impact on energy savings can come from applying variable speed drives to many of the electric motors used throughout industries across Europe. Realizing this, Carbon Trust – an independent non-profit organization set up by the UK Government- has taken a large step in controlling CO2 emissions through its Big Business Refit program, which encourages industries across UK to invest and implement newer and more environmental-friendly technologies like variable frequency drives and energy efficient electric motors, by extending interest-free loans from £3,000 to £500,000. In the first half of 2009, Carbon Trust extended interest-free loans to hundreds of SMEs to equip their businesses with the latest energy-saving technology. Carbon Trust claims that they are saving an average of £14,000 each on their annual energy bills.

Variable Frequency Drive:
A variable frequency drive (VFD) allows a motor’s speed to be varied electrically, instead of mechanical means. The insulated gate bipolar transistor technology, which creates the variable voltage and frequency to control a motor’s speed, technically, has much greater efficiency and offers wide flexibility of operation. Apart from saving a great deal on electrical power consumption, it also offers soft-start capability, where-in the motor is slowly ramped up to the desired speed, instead of being thrown abruptly upon switching on. This reduces the mechanical stress on the motor and increases its life. The motor’s speed can be easily adapted to changing process conditions using a variable frequency drive, thereby achieving greater process control. Optimizing the speed of a motor using a VFD can result in enormous energy conservation, reducing the cost of recurring investments.

Usage of VFDs can also contribute in the reduction of other secondary cost cutting factors. For example, heating a business premise consumes significant amount of energy. It is estimated that for every 1oC of extra heat increase, there is an increase in the energy consumption by 8%, thereby adding 8% to the energy bills. Employing several electric drives, spread over the premises, can contribute to overall heating of the premises, reducing the energy required for HVAC devices.

Adopting Green Technology:
The implementation level of VFDs across European industries is estimated to be only about 45% to 50%, by 2010. This has resulted in a reduction of CO2 emissions by 6,000 million tons per annum. With increasing presence of VFDs in diverse markets across Europe, further reductions in CO2 emission is inevitable. The return of investments for today’s advanced drives is less than a year. This makes the usage of VFDs relatively economical and enables companies to be environmentally responsible, thereby making the technology sustainable.

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This is first in a DrivesMag Exclusive series of articles on the business of drives from global research company Frost & Sullivan.

In some drive applications, several motors are connected to the load at various locations. If the motors are mechanically coupled together through the driven machinery, load sharing must be considered. In this type of application, torque can be transmitted from one of the motors through the mechanical elements of the driven machine to the location of the other motor or motors. If too much torque is transmitted from one motor to other parts of the machine, that motor will be overloaded.

This article will discuss only applications that can be handled by several motors connected to one adjustable frequency drive. We will describe how the drive controls the speed of all of the motors while the motors share the load. A future article will deal with applications that involve load sharing among several adjustable frequency drives each connected to a single motor.

Examples Of Load Sharing Applications

The following examples will illustrate some of the types of applications that involve load sharing.

Conveyors

The power needed to drive a conveyor is used to overcome friction that is distributed over the entire length of the conveyor. If several small motors are used rather than one large one, power is transmitted electrically to the point of use, rather than mechanically, permitting smaller mechanical components to be used.

Roller Tables

Roller tables are conveyors consisting of a number of independent rollers. The rollers convey a load consisting of objects that are in contact with several rollers at a time as shown in Figure 1. Each driven roller is coupled to its own motor.

Figure 1 Roller Table

Other Examples of Load Sharing Applications

Lime kilns log debarking drums and other large rotating structures that are driven through ring gears are sometimes driven through more than one ring gear. In some applications of this type, more than one motor is used to drive a single ring gear.

Inherent Load Sharing With AC Motors

The type of load sharing discussed in this article is based on the inherent load sharing capability of AC motors. If two or more identical AC motors are mechanically coupled together and powered with the same frequency and voltage, they will inherently share the load. Identical motors powered with the same frequency and voltage have identical speed-torque curves. Since the mechanical connection forces the motors to operate at the same speed, the motors will operate at identical points on their speed-torque curves as shown in Figure 2.

Figure 2 AC Motors With Inherent Load Sharing

Some short-term speed variation may be possible among motors connected to a common load if the mechanical system is not perfectly rigid. A speed differential between two motors can be sustained only until the mechanical play between them is taken up. The take-up of mechanical play would include "winding up" or compressing any springiness in the interconnecting mechanical system. Until the slack is taken up, the speed differential between two motors causes an imbalance in load sharing between them. Once the slack is taken up, the motors will again evenly share the load.

It is possible to have load sharing between mechanically coupled motors of different horsepower ratings. To make load sharing possible, the motors must have similar speed-torque curves when percent of rated torque is plotted against speed. For example, assume that the level of torque marked with the dotted line in Figure 2 represents 75% of rated torque rather than 75 lbs.-ft. Assume that the top curve is for a 60 Hp motor and the bottom curve is for a 40 Hp motor. For a 60 Hp, 1800 RPM motor, full load torque is about 180 lbs.-ft. and 75% of rated torque is about 135 lbs.-ft. For a 40 Hp, 1800 RPM motor, full load torque is about 120 lbs.-ft. and 75% of rated torque is about 90 lbs.-ft. If both motors are operating at 1770 RPM, they will each be operating at 75% of their respective full load torque ratings. If the required load torque increases or decreases, the motors will share the load and develop equal percentages of their respective full load torque ratings.

For any given load variation, a NEMA design D, high slip motor will exhibit more speed variation than a standard NEMA design B motor. Figure 3 shows an expanded view of typical speed-torque curves for NEMA B and NEMA D motors. At full load, the NEMA B curve shows a speed of 1760 RPM with 40 RPM or 2.2% slip. The NEMA D curve shows a speed of 1710 RPM with 90 RPM or 5% slip at full load.

Assume that two motors are sharing a load with each motor operating at 80% of its rated load. If the motors are NEMA B, the operating speed will be a little higher, but in either case, two motors of the same design will operate at the same speed if they are operating at the same load.

Assume that load on one motor increases to 90% of rated and that the load on the other motor decreases to 70% of rated. If design D motors are used, the speed of the more heavily loaded motor will decrease by 9 RPM while the speed of the lightly loaded motor will increase by 9 RPM. If design B motors are used, each motor would change speed by 4 RPM in response to the same load change. Because a given load change causes a larger difference in speed, any slack that appears in the mechanical system will be more quickly taken up if NEMA D motors are used.

Figure 3 Effect of Load Change with NEMA D vs. NEMA B Motors

To look at this another way, assume that the mechanical system will permit only 18 RPM of speed differential between two motors as shown in Figure 4. If the speed of each motor increases or decreases by 9 RPM, the torque developed by the NEMA D motors will vary by only ±10% while the torque will vary by ±22% if NEMA B motors are used. From either viewpoint, high slip motors share the load more evenly than standard motors.

Figure 4 Effect of Speed Change with NEMA D vs. NEMA B Motors

Multiple Motors Connected to One Adjustable Frequency Controller

The inherent load sharing capability of AC motors can be easily utilized when motors are powered by an adjustable frequency drive. An adjustable frequency drive can be used to provide power to any number of motors. The motors are simply connected in parallel so that each motor receives the same voltage and frequency. If the motors are mechanically coupled together, they will share the load as described above. The motors must be coupled together by a mechanical connection that is capable of transmitting torque without causing damage due to torque variations among the various sections of the machine. Motors that are coupled together by contact with a web or strand of material can not be connected to a common controller. Such applications require an individual controller for each motor so that the individual section speeds and the web tension can be closely regulated.

Sizing the Adjustable Frequency Controller

The adjustable frequency controllers must be selected based on motor current requirements not motor horsepower. The sum of the normal operating currents of the connected motors must not exceed the normal output current rating of the controller. The maximum short term overload current requirement of the motors must not exceed the short term overload current capability of the controller.

In many multiple motor applications, there is no provision for connecting and disconnecting individual motors. The motors must be started and stopped as a group by starting and stopping the controller. In applications where individual motors are started independently, the controller must be sized so that the short term overload current capability of the controller is sufficient to supply the maximum inrush current requirement. For starting one motor after N motors are already running, the short term overload current rating of the controller must be at least N X Motor Full Load Current plus 1 X Motor Locked Rotor Current. A motor’s locked rotor current is typically about 6 times the full load current.

When high slip motors are used, it is important to remember that high slip motors require more overload current to provide a given level of overload torque. While a NEMA design B motor will provide close to 150% torque for 150% overload current, a NEMA design D motor may require significantly more current to produce the same torque.

Motor Protection

The electronic motor protection provided by an adjustable frequency controller can not provide adequate protection in a multiple motor application. Conventional overload and short circuit protection must be provided for each individual motor. The individual motor protection is usually arranged to shut down the controller. In some applications, it may be possible to arrange the motor protection to disconnect an individual motor and allow the remaining motors to assume the total load. Be sure to check the controller instructions to be sure that the controller will operate properly when part of the load is disconnected.

Individual Motor Adjustments

Since all of the motors that are connected to a single controller receive the same voltage and frequency, there is no means of adjusting the speed or torque of an individual motor. Adjusting the output frequency of the controller changes the speeds of all of the motors as a group. No other adjustment is available. In the past, variable transformers or series inductors have sometimes been used to provide voltage adjustments for individual motors. Now that small adjustable frequency drives are economically available, individual controllers should be used in applications requiring adjustments for individual motors.

- Chuck Cowie, DrivesMag contributor.

Imagine one motor and one drive, but two, separately controlled independent shafts. Such is the premise of John Casey's (Independent Engineering Concepts) new idea for a rotary electric machine with non-coaxial rotors.

_(Mock-up)

The motor would include a housing assembly, at least one set of stator windings, and at least two rotors with axes of rotation that are non-coaxial. The rotors would be mechanically decoupled from one another, but electromagnetic coupling would allow the rotors to rotate at the same or different speeds and direction.