EVOLUTION OF MOTOR AND VARIABLE FREQUENCY DRIVE
`TECHNOLOGY
`
`Dale E. Roethemeyer, U.S. Electrical Motors Division of Emerson Electric
`David R. Yankaskas, Control Techniques Drives~ Emerson Electric
`
`ABSTRACT
`
`The purpose of this paper is not to describe in detail the operation and construction of variable frequency
`drives (VFD's) but to make the reader aware of specific market and product trends effecting the evolution
`and use of the motor drive combination. In addition, two specific application examples using standard AC
`drives in two different industries will be explored giving the reader a practical insight into energy
`conservation.
`
`INTRODUCTION
`
`Electric motors have long been and will continue to be the general workhorse of industry providing an
`efficient and reliable transfer ofpower for industrial and commercial applications. Specifically, the fixed
`speed three-phase squirrel cage induction motor has been defined as the general workhorse of industry.
`The motor speed depends solely on the number of stator poles and the frequency of the incoming voltage
`supply. In relation to the benefits of transferring electrical energy to mechanical energy the squirrel-eage
`induction motor is rugged and reliable requiring minimum maintenance at a very reasonable cost. Over
`the years a great many devices have been applied to change the speed of the motor through mechanical or
`electrical means. Until the advent of the AC variable frequency drive in the late 1950's, mechanical
`devices remained the predominant way to control the speed of the motor. But, early AC variable frequency
`drives (VFD) were bulky~ unreliable in critical applications and very expensive compared to the
`alternatives at the time.
`
`With the passage of the Energy Policy Act of 1992, there is an increasing need for energy conservation
`and efficient use of energy enabling commercial and industrial facilities to minimize production costs,
`increase profits and stay competitive. This energy act states specifically that certain motors manufactured
`after October 24~ 1997 must meet the new energy standards found in the National Electrical
`Manufacturers Association (NEMA) standard MG...1-1987, Table 12-6C or Table 12-10 in MG-1-1993.
`Certain motors can be defined as any NEMA designed, general purpose, T frame squirrel-eage induction
`motor under NEMA Designs A and B, through 200 hp., single speed, foot mounted and polyphase
`operating at 230/460V at 60 HZ. Exempt from this legisla!ion are DC motors and the special and definite
`purpose AC motors.
`
`rfechnology has made great strides in inlproving the AC variable frequency drive to present day standards.
`Variable speed drives (VFD's) together with motors have emerged throughout industry as the popular
`approach to improve process control, product quality, reduce energy consumption and expand automation
`and diagnostics. Yet, energy efficiency motors and inverter duty motors are actually two separate product
`offerings serving two distinct applications. It is essential to understand these differences to insure a
`successful motor/VFD installation. This paper will attempt to introduce the reader to better understand
`the evolution of these products within the dynamics of an emerging energy conscious marketplace.
`
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`LIBERTY EXHIBIT 1026, Page 1
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`

`

`THE MARKET
`
`Nationally, there are more than one billion motors in the U.S. with 40 million of these motors in
`industrial and manufacturing operations consuming 70% of the total electrical energy in a typical
`manufacturing facility. According to Paul Scheihing, Motor Challenge Program Manager for The
`Department Of Energy, "Nationally, U.S. industry spends more than $30 billion annually on motor
`systems energy cost." 1 Overthe past fifteen years the use of VFD's with NEMA squirrel-cage induction
`motors has increased dramatically in the industrial and the commercial market segments. For example,
`during the mid eighties the petroleum and chemical industry critically viewed VFD's with a cautious eye.
`Today, this industry views the VFD as just another standard component within the system designers
`toolbox. Each year we see the effects of an increasing market as more and more industries realize the
`advantages to VFD's.
`
`Figure 1 segments the use of electrical energy in the U.S. by application. From this chart we can conclude
`rather quickly which applications where one can achieve energy savings with a motor drive combination;
`the pumping, blowers/fans and the HVAC segments. As we discussed earlier, traditionally the motivating
`force to apply VFD's in the past was to obtain speed control comparable to a mechanical or DC drive. As
`the marketplace moves forward with regard to the aforementioned Energy Policy Act and we see the
`increased emphasis on energy conservation the use of higher performance AC standard drives will
`continue to grow exponentially. In fact, we see the standard AC drives market as a whole growing at a
`10% annual compound growth rate from today to the year 2000. With the DC drive market declining at a
`4% compound growth rate. The drives market shown in Figure 2 defines that 52% of the standard AC
`drives market is in the 7.5 through the 200 hp. motor relationship. Note, this is the bulk of the market
`that is also impacted by the EPACTlegislation. While figure 3 takes the next step breaking out the AC
`standard drives market by specific application. Once again, we see the air moving and pump applications
`offering the largest application segments for the standard variable torque AC drive technology.
`
`Variable torque applications like HVAC and pumping fit well with the analogy of an individual driving a
`car on an interstate at 60 miles per hour. Let's assume the engine under the hood is running at a constant
`RPM maintaining the 60 mph speed. Instead of the driver varying the pressure applied to the accelerator
`to change the speed of the vehicle, the driver in this analogy will slow the vehicle down by applying the
`brake pedal to create the desired resistance against the engine. As you could image this is a very
`inefficient approach, not only in energy consumption in miles per gallon but the accelerated wear
`on all of the component parts.
`
`In a traditional commercial fan/blower application the rpm of the applied motor is fixed while the cubic
`feet per minute (cfrn) is varied through the use of dampers. The damper is very similar to the brake pedal,
`creating the resistance against the output of the motor. For example, if we look at a typical applied HVAC
`system, 850/0 of the time the percent of air flow needed to operate a commercial system is between
`40% and 85% of the air flow required. Maximum air flow or 100% of design air flow required for a
`system is only needed 2% of the time. In other words, 98% of the time the air flow can be varied through
`the use of a variable frequency drive while still maintaining system efficiency.(figure 4)2
`
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`LIBERTY EXHIBIT 1026, Page 2
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`

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`With an energy efficient motor the user receives a premium motor manufactured specifically to lower
`energy consumption . Energy efficient motors commonly use low loss steel laminations along with a high
`degree of copper content thereby developing a motor with much lower thermal rise levels. The insulation
`system on an inverter grade motor has a more robust insulation system which usually consists of not only
`the energy efficiency motor construction attributes but other key features which includes insulated magnet
`wire, additional taping and/or lacing on the stator end turns as well as an improved stator insulation on
`end turns, slots and between phases. Plus extra cycles and/or additional coatings of varnish or VIP on the
`stator itself. Figure 5 clearly states that as industrial users applied standard motors in the past with 1,200
`or above Hz PWM drives we have grown accustomed to a half life from a standard motor. Now with the
`increase of IGBT technology and increased KHz switching speed we can readily see the effects of
`insulation life of a standard motor compared to "Inverter Grade" motors. When retrofitting an existing
`system and upgrading to IGBT technology it is essential to consider the impact such drive technology will
`have on the life of the system.) We strongly feel our "Inverter Grade" motor demand will increase at a
`pace greater·than the drive industry growth rate once the marketplace realizes the benefits of increased
`motor life through ""Inverter Grade" motor construction.
`
`AC DRIVE EVOLUTION
`
`The VFD is used to create a controlled frequency AC wave form to the AC induction motor. By changing
`the frequency of the AC wave fornl the speed of the AC motor is changed. It is equally important to
`control the flux density in the motor to maintain torque producing capabilities throughout the speed range.
`The flux density is controlled by maintaining the volts/hertz ratio supplied to the motor. The torque is
`essentially proportional to the volts/hertz ratio squared. In other words, if the frequency or voltage is
`changed without changing the other , the torque characteristics change as a square function.
`
`Development
`
`Drive Usage
`
`1958
`
`Solid state power devices develop
`known as SCR's.
`
`DC drives become available.
`
`Early 1960's
`
`The cost effectiveness of SCR's improves.
`
`Late 1960's
`
`using digital
`Analog control
`control and firing circuitry.
`
`The 1970's
`
`Development of large scale integrated circuit
`(LSI) technology.
`
`Pre 1985
`
`SCR's/GTO's using six step technology.
`Drives are large, bulky and expensive.
`
`DC SCR drives readily available
`for industrial applications.
`Performance and understanding of
`these applications improves.
`
`Development of phase locked loops
`for synchronization improves line
`noise immunity allowing DC
`drives to operate better.
`
`Custom integrated circuitry
`improves the reliability and cost
`of current circuitry.
`
`Marginal acceptance a larger
`acceptance in certain industries
`like petro/chern and textile.
`
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`LIBERTY EXHIBIT 1026, Page 3
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`

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`1985..... 1989..
`
`Bi-polar PWM technology, smaller more
`economical drives evolve.
`
`1987..89.. 1990
`present
`
`IGBT technology, smaller drive packages
`with micro drives for smaller hp motors.
`Switching frequency becomes ultrasonic.
`
`A greater acceptance among users.
`There is still an issue with noise.
`Inverter grade motors in
`development along with field
`testing.
`
`A much larger acceptance. Micro
`drives have actually become
`commodities. Inverter grade
`motors are launched in 1990.
`
`future....
`
`Total motor drive compatibility. Systems sold
`as one. Energy efficiency across all industries
`and energy users. Motor development will
`parallel non-sinusoidal drive development.
`
`The micro VFD will be mounted
`in the motor conduit box totally
`unexposed to the elements.
`
`As you can see VFD technology has improved due to power devices and the microcomputer. Early
`VFD's used silicon controlled rectifiers (SCRs) or gate tum off thyristors (GTOs) and were either
`variable voltage inverter (VVI) or current source inverter (CSI) drives. As technology improved the
`pulse width modulated (PWM) drive was introduced. Originally the PWMdrive used Darlington bi-polar
`transistors as power devices. However, with the introduction of the insulated gated bi-polar transistor
`(IGBT) in the late 1980's vast improvements in the design of AC VFD's resulted.
`
`Vector controlled drives were also introduced around the time as IGBT's became more readily available.
`They are similar to the PWM VFD except they use a more sophisticated level of control logic. The basic
`principal is to model the motor's electrical performance inside the controller. This allows the controller to
`perfectly match the motor performance to the load requirement. Vector controlled drives provide higher
`dynamic performance. There are many types today with both direct and indirect methods of control. Some
`of them are stator-flux control, rotor-flux-oriented control, magnetizing-flux-oriented control and etc.
`Vector controlled··drives are currently making an impact on the total DC drive market. By no means is the
`DC market dead nor will it be in the near future, but as the vector controlled drive becomes more accepted
`it will replace certain DC drive applications.
`
`Costs of VFD's have come down dramatically primarily from the improved technology of power devices,
`micro computer advancements and improved manufacturing techniques. For example, a 10 horsepower
`drive manufactured in the early 1980's was large and bulky. Today that same drive can be held in the
`palm of your hand at approximately one third the cost.
`
`APPLICATION CONSIDERATIONS
`
`Probably the first question that should be asked is what type of application are you considering for a VFD.
`There are basically three types: constant torque, variable torque and constant horsepower applications. It
`must be pointed out that constant and variable torque loads are the dominate two types of applications.
`
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`LIBERTY EXHIBIT 1026, Page 4
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`

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`A constant torque load is one in which the torque required is independent of the speed. The torque can be
`100% throughout the speed range or vary. Examples of constant torque loads are conveyors, rotary lobe
`pumps, positive displacement pumps and compressors, punch presses, wire drawing machines, paper
`machines and printing presses. Considerations when applying VFD's to these type loads will be starting
`torque requirements, speed regulation, torque response and close loop capabilities.
`
`Variable torque loads are applications where the torque required is proportional to the speed. This is
`where the basic affinity laws apply and as a result energy savings. The volume is directly proportional to
`speed, pressure is proportional to the square of the speed and power is proportional to the cube of the
`speed. Typical applications are most pumps, fans and specific HVAC applications which fall under the
`definition as centrifugal loads. When a VFD is applied to a centrifugal load the horsepower drawn from
`the AC lines very nearly follows the centrifugal load curve. Since the functional relationship of
`horsepower {or power) to speed is cubic, the energy required drops almost cubically as the speed is
`reduced. Most centrifugal loads today use some mechanical means to vary the volume, be it valves,
`dampers.. etc. The obvious reason to use a VFD for centrifugal loads is the potential energy to be saved.
`In the majority of most applications the energy saved will offset the initial cost and justify a suitable
`payback not to mention the ability to achieve precise control.
`
`Constant horsepower loads are applications where torque varies inversely with speed. These types of loads
`require high torque .3t low speeds and low torque at high speeds. Typical applications are lathes and metal
`cutting tools operating over wide speed ranges. Some extruders, mixers and center driven winders can
`also be constant horsepower type applications.
`
`ACTUAL APPLICATIONS
`
`R.A. Miller Hardwood Company: North Tonawanda, N.Y.
`
`R.A. Miller operates six kilns to dry out rough-cut lumber and to steam inject precise levels of moisture
`into the process wood to prevent splitting, cracking and checking. Prior to installing VFD's and energy
`efficient motors, R.A. Miller used three to five motors in each kiln to circulate air to dry the wood. The
`motors were controlled manually by turning individual circuit breakers on and off whenever the humidity
`levels were unsuitable and dampers were used to regulate the required air flow. As you could image, this
`resulted in uneven airflow, wasted energy, labor and wood.
`
`An initial payback analysis was presented to R.A. Miller by the local utility and the VFD manufacturer.
`Initial estimates were as follows:
`
`Installation Cost
`Utility Rebate
`Actual Cost
`Projected Yearly Savings
`Expected Simple Payback
`
`$ 46,286
`$(18,560)
`$ 27,726
`$ 29,088
`11 months
`
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`LIBERTY EXHIBIT 1026, Page 5
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`Actual Results were:
`
`First year energy savings exceeded $40,000
`Increased operating hours
`Reduced demand charges
`
`Improved product quality due to tighter process control
`A realized net waste reduction of 40/0
`A net realized payback each month
`
`By applying VFD's and energy efficient motors R.A. Miller realized significant savings. For
`instance, at 90% speed (air flow) the energy consumed was 250/0 less than at full motor speed (100%)
`with damper control. At 60% speed (air flow) the VFD's consumed 20% to 25% of the energy compared
`to 67% energy consumption before at full speed with damper control. In conclusion, the total energy
`savings varied from 25% to 50% from the original control scheme which consisted of running the motors
`at a fixed speed and damper modulation.
`
`In addition to the energy savings R.A. Miller realized real payback in improved product quality
`and a more precise flow pattern that resulted in much tighter quality control in the drying process.
`This enhancement in tighter control resulted in improved humidity control which intum raised
`the quality of the produced hardwood. R.A. Miller realized less checking and splitting in their product
`for shipments. This is an excellent example where a customer has experienced the benefits we discussed
`earlier in the introduction of this paper, improved process control, increased product quality and a net
`realized reduction in energy consulnption.
`
`First Interstate Plaza: San Diego, Ca.
`
`First Interstate is a multi- storied office building in the San Diego area. They are managed by the
`Compass Management and Leasing Company. The building has approximately 465,000 square feet of
`office and parking space. This retrofit project was proposed to the customer by the local utility company,
`a VFD manufacturer and a major chiller/air handler company.
`
`This retrofit proposal included replacing the lighting fixtures throughout the building and applying VFD's
`to twenty-two air handlers, four garage fans and two 700 ton chillers. From the VFD standpoint the
`customer wanted to replace their conventional on/off air flow system with a constant air flow system. In
`addition, the VFD's on the garage fans would allow better removal of carbon monoxide from the
`underground parking area.
`
`Estimated project cost for the mechanical portion (air handlers and chillers) was $478,000 with a rebate of
`$140,000 from the local utility. Payback for the mechanical portion is approximately three years. The
`expected annual dollar savings on this project is 30%. Estimated annual units of energy savings is 2.66
`million kilowatts including replacing the lighting fixtures.
`
`Even though actual savings have not been tabulated by the utility at the writing of this paper, savings were
`guaranteed at a minimum of 30%. With three separate companies cooperating together toward a common
`goal of realized savings the customer is seeing a substantial improvement in building operation. Just based
`on the conservative energy savings estimate the building owner will be able to reflect back to the tenants a
`lower rent which will in turn increase his overall building occupancy rate due to more competitive rates.
`Not to mention the unrealized benefit of less wear and tear on the mechanical equipment that will extend
`overall life expectancy.
`
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`LIBERTY EXHIBIT 1026, Page 6
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`

`

`CONCLUSION
`
`Motor and VFD manufacturers are continually striving to improve overall performance of their products.
`This paper has attempted to give a survey of the past, present and future developments and trends in AC
`induction motors and VFD's. The three-phase squirrel-cage induction motor will remain the dominant
`machine for industrial drives. In the future the marketplace will see a distinct increase in vector controlled
`drives as well as a continued but a gradual decline for DC drives.
`
`there will be a continuos search for new materials which will combine thermal
`There is no doubt
`conductivity, electrical insulation and mechanical strength to enhance the reliability of motors and drives
`in the future. This will result in a revolution in the packaging of integrated power modules providing a
`complete controller for micro drives. In the near future we feel the market will see an integration of power
`electronics into the motor. Such a compact design will provide industries with a complete motor and
`drive package as one unit. With regard to power electronic devices, the major area the market will see
`future development and expansion wiUbeinthe quantity produced and an increase in the power rating of
`the power modules. Power tnodules wiU.rangefrommuItiple-device modules to smart devices to power
`integrated circuits or intelligent power modules. The continued trend of lower costs through improved
`manufacturability will significantly contribute to the future development of intelligent drive systems.
`
`REFERENCES
`
`Paul E. Scheihing, ~'Motor Challenge Overview: Keeping Production High While
`Bringing Costs OO\\ln" Turning Point, Department Of Energy, Winter 1994.
`
`2
`
`3
`
`Chris O'Brien, ~"Energy-EfficientMotor Drive Systems in Commercial Buildings: A
`Profitable EnvironmentalOpportunity"Energy Efficient Electric Motor Systems
`National Conference February 9-10, 1993.
`
`Tom E. Dale~ "'Adjustable Speed
`Electrical Motors~ April·1993.
`
`Applications" Energy Savings Strategies U.S.
`
`OTHER REFERENCES
`
`Professor P. Vas P.H.D.,D.Sc. C. Eng., FIEE~ Dr. W. Drury Ph.D., C. Eng., FlEE
`~IoFuture Developluents and Trends In Electrical Machines and Drives." 1994
`
`547
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`LIBERTY EXHIBIT 1026, Page 7
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`

`

`

`T
`
`YHPRATING
`DRIVES
`
`HP
`
`mB
`
`1
`
`> 750 HP
`14%
`
`1 - 5HP
`
`51 - 200 H
`27%
`
`705 - 50 H
`25%
`
`FIGURE 2
`
`U1
`
`~l
`
`O
`
`LIBERTY EXHIBIT 1026, Page 9
`
`

`

`

`

`Profile of 'y,ical AirfliowReq:'uiire'lflents
`
`Hottest day
`
`~
`
`"\j
`
`....:.:.:.:.:.:.:.:.:.:.:-~
`
`.'.
`
`I·~I
`
`Percent of Annual
`Operatina Hours
`12
`
`Coldest
`
`10
`
`8 6
`
`4 2
`
`o
`
`25
`
`30
`
`35 40
`
`75
`70
`65
`55 60
`50
`45
`rcent Maximum Air Flow NeBde
`
`80
`
`85
`
`90
`
`95
`
`100
`
`U1~
`
`Source: York Applied Systems
`
`FIGURE 4
`
`LIBERTY EXHIBIT 1026, Page 11
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`

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`