1454

7.10 Variable-Speed Drives

A. BRODGESELL (1970) R. D. BUCHANAN, J. B. RISHEL (1985)

B. G. LIPTÁK, R. H. OSMAN (1995) I. H. GIBSON (2005)

Types of Drives: A. Mechanical drivesB. Hydraulic, hydroviscous, and fluid coupling drivesC. Magnetic particle, fluid, and eddy current drivesD. Variable voltageE. Variable-speed DC, thyristor DCF. Variable-frequency AC

F1. Voltage source, pulse-width modulated (PWM)F2. Current sourceF3. Load commutated inverter (LCI) on synchronous motorF4. Power recovery versions of F1, F2, or F3

Abbreviations Used: ASCI—Autosequentially commutated current-fed inverterGTO—Gate-turn-off thyristorIGBT—Insulated gate bipolar transistorLCI—Load commutated inverterPU—Per unitPWM—Pulse width modulatedRMS—Root mean squareSCR—Silicon-controlled rectifierVFD—Variable-frequency driveVVVF—Variable-voltage variable-frequency drive

Size Range: A. 1 to 100 hp (0.75–75 kW)B. Up to 4000 (up to 3MW) C. Air-cooled up to 900 hp (670 kW); water-cooled up to 18,000 hp (13.5MW)D. 10 to 100 hp (7.5–75 kW)E. From under 1 to 500 hp (to 375 kW)

F1. From under 1 to 500 hp (to 375 kW)F2. From under 100 to over 1000 hp (75 to 750 kW)F3. From 100 to 20,000 hp (75 kW to 15MW); generally above 1000 hp

(750kW)

Speed Turndown: A. 6:1B. 3:1 to 40:1C. 5:1 to 10:1D. LowE. 10:1 with standard and 100:1 with tachometer feedbackF. 3:1

Efficiency at 70% of Full A. 50%Speed: B. 55%

C. 58%D. 52%E. 80%F. 78%

Inaccuracy of Speed 2 to 5% in standard (can do better with feedforward) and 0.1 to 1% with tachometerControl: feedback designs

MSY101

MCC VVVF

Flow sheet symbol

© 2006 by Béla Lipták

7.10 Variable-Speed Drives 1455

Variable Speed Drive Costs E. For a 200 hp (150 kW) motor, a single converter, and thyristor DC drive: (Excluding the Motor): $13,000 F1 and F2. For a 200 hp (150 kW) induction motor: $20,000

F3. $200/hp ($270/kW) at around 1000 hp (750 kW) size and $100/hp ($130/kW)at around 5000 hp (3.75MW) size

(For the costs of other sizes and designs, see Tables 7.10r and 7.10u.)

Partial List of Suppliers: ABB Inc. Automation Technologies Drives and Motors (www.abb-drives.com)Allen-Bradley Div. of Rockwell Automation (www.ab.com/drives)ASIRobicon Inc. (www.asirobicon.com)Baldor Electric Co. (www.baldor.com)Cutler-Hammer Div. of Eaton Corp. (www.eatonelectrical.com)Danfoss Inc. North America Motion Controls (www.namc.danfoss.com)Emerson Control Techniques (www.emersonct.com/index.htm)Eurotherm Drives (www.eurothermdrives.com)General Electric/GE-Fuji Electric Co. (www.geindustrial.com/cwc/home)Hitachi Ltd. (www.hitachi.us)Lenze Corp. (www.lenzeusa.com)Mitsubishi Electric Corp. (www.meau.com)Omron IDM (www.idmcontrols.com)Reliance Electric Div. of Rockwell Automation (www.reliance.com)Robicon Corp. (www.robicon.com/products/acdrives/index.html)Saftronics Inc. (www.saftronics.com)Siemens Energy & Automation (www.sea.siemens.com/drives/default.html)Square D Div. of Schneider Electric (www.squared.com)TB Woods Inc. (www.tbwoods.com)TECO-Westinghouse Motor Company (www.tecowestinghouse.com/Products/

drives.html)Telemechanique Div. of Schneider Electric (www.modicon.com/Default.htm)Toshiba America (www.tic.toshiba.com/products.php)Unico Inc. (www.unicous.com)US Drives Inc. (www.usdrivesinc.com)Yaskawa Electric America Inc. (www.magnetekdrives.com)

INTRODUCTION

This section begins with a brief explanation of the role ofvariable-speed drives in improving the efficiency of trans-porting materials. After that, the different variable-speeddrive designs are described, starting first with the older elec-tromechanical designs and then proceeding to the more mod-ern electrical and solid-state drives.

TRANSPORTATION EFFICIENCY

Constant or variable-speed drives can be applied to the trans-portation of liquids and gases. The amount of discharge pres-sure needed in these transportation systems is a function ofthe flow rate demanded by the process, the load. This rela-tionship between the required flow and pressure is called thesystem curve. This curve rises with load (Figure 7.10a), whilethe pump curve drops with increasing flow. When constantspeed drives are used, the pump, fan, or compressor curve isfixed, and therefore it cannot coincide with the system curveat more than one point.

As shown in Figure 7.10a, a control valve or damper needsto be introduced to burn up (waste) the excess energy, whichshould not have been introduced in the first place. Variable-speed drives eliminate this waste by shifting the pump or fancurve to cross the system curve, thereby eliminating waste byintroducing only as much transportation energy as is neededto meet the load (Figure 7.10a). This subject is covered inChapter 8, where the control of variable-speed pumps, com-pressors, fans, and turbines is covered in some detail.

CHARACTERISTICS OF VARIABLE-SPEED DRIVES

All drives for centrifugal pumps, regardless of type, musthave certain characteristics to make them acceptable for oper-ating centrifugal pumps. Some of these characteristics are asfollows:

1. Broad speed turndown without damage to the motoror variable speed drive. The operating speed range formost variable speed pumps is from 50 to 100% of thesynchronous speed. However, through misadjustment

© 2006 by Béla Lipták

1456 Regulators and Final Control Elements

or momentary system dynamics, the controls may causethe pump to run at zero speed. The drive must be ableto function with zero pump speed without damage oractuation of circuit protection devices such as thermaloverloads for a significant period of time.

2. Control repeatability, so that fluid system controls canachieve desired pump speeds at all loads.

3. Reliability. The variable-speed drive and motor mustbe designed to fit field operating conditions that varyappreciably in centrifugal pump applications. Ambienttemperature in the field is often outside the “nominal”rating of the motor. Ambient temperature of the driveelectronics may also be other than anticipated by thedesigner.

4. Serviceability. The personnel normally employed formaintenance of motors and pumping equipment mustbe capable of servicing the electronic, electrical, ormechanical equipment involved on each specific appli-cation.

ELECTROMECHANICAL DRIVES

Electromechanical drives include those that utilize an elec-trical motor to drive a mechanical, speed-changing devicethat is connected to the centrifugal pump. These include thefollowing:

1. V-belt drives2. Hydraulic couplings3. Hydroviscous couplings4. Eddy current couplings

There are a great number of methods for speed adjustmentof rotating machines. These methods fall into two categories:speed adjustment of the prime mover and speed adjustment

through a transmission connecting the driver to the drivenmachine.

Within each of these two groups several degrees of sophis-tication are possible, ranging from manually actuated step-wise speed changes to continuously variable automatic speedchangers. Each method of speed control offers certain advan-tages and disadvantages (Table 7.10b) that must be weighedagainst the design criterion to permit a proper selection.

Mechanical Variable-Speed Drives

Stepped Speed Control Mechanical methods of speedadjustment offer a number of gear and pulley devices for bothstepped and continuously variable-speed control. Steppedspeed control methods provide setting a number of speedsvery accurately. However, they are not readily adaptable toautomatic process control.

The stepped pulley system shown in Figure 7.10c is oneof the earliest methods of speed adjustment. Its advantagesare low cost and simplicity, but belt slippage contributes toinefficiency, high maintenance, and reduced speed control.Two factors to be considered in the design of a stepped pulleysystem are the proper ratio of pulley diameters to obtain thedesired speeds and proper pulley dimensions to maintain belttension for all positions.

Pulley dimensions for a system such as in Figure 7.10ccan be determined from the relationships in Equations 7.10(1)and 7.10(2).

7.10(1)

7.10(2)

The variables in the above equations are defined inFigure 7.10c.

FIG. 7.10aWhen pumping with a constant speed drive, energy is wasted, as the unnecessarily introduced pumping energy is burned up in a flowcontrol valve. In contrast, when the flow rate is reduced by lowering the speed, such energy is not introduced in the first place.

Head orpressure

100% speed

67% speed

P1

P2

F2 F1

System curvewhen valve wastes

excess energy

Operating pointwith constant speed

drive

Actual syste

m curve

Operatingpoint withvariable speeddrive

Flow

Speed, % Flow, %Horsepowerrequired, %

10090807060504030

10090807060504030

1007351342213

63

π2 41 1

1 12

( )( )

R rR r

d+ +

−=

π2 42 2

2 22

2

1

2

2

( )( )

R rR r

d

S

S

R

r+ +

−=

© 2006 by Béla Lipták

7.10 Variable-Speed Drives 1457

Gear transmissions offer high efficiency of power trans-mission at precise stepped speed control. For speed changesof gear train drives, either clutches or brakes are required orthe change must be made at rest. Epicyclic gears, such asplanetary gears, offer the most compact unit, operating qual-ity, and high efficiency. However, auxiliary clutches andbrakes are required, and the cost is therefore higher than thatof other gear drives. The complexity of control arrangementsincreases rapidly with the number of speeds required, makingthe planetary gear drive impractical above four or five speeds.

Continuously Variable-Speed Drives The cone pulley sys-tems in Figure 7.10d is a natural evolution of the steppedpulley system shown in Figure 7.10c. Cone pulleys aredesigned similarly to stepped pulleys. A series of diametersis calculated equidistant on the pulley axis. The diameter endpoints are joined to form the pulley contour.

The cone pulley is inherently inefficient, because contactsurface speed varies across the belt, causing slippage. Beltsmust be kept narrow to reduce slippage and wear, and thusthe capacity for power transmission is reduced. Belt guidesat the pulleys are required to hold the belt in position. Theseguides must move simultaneously for speed changes.

Variable-Pitch Pulleys Cone pulleys were the forerunnersof the more sophisticated variable-pitch pulley systems,which permit continuous, automatic speed adjustments overwide ranges. Speed adjustment in all of these systems isobtained by means of sliding cone face pulleys or sheaves,whose effective diameter can be changed.

A simple variable-pitch sheave is shown in Figure 7.10e.Two flanges are mounted on a threaded hub. The flanges areset to the desired spacing and locked in place with a set-screw.Speed adjustments are accomplished by changing the flangespacing, producing in effect a pulley of different diameter.This method is very economical, but it requires stationary

TABLE 7.10bFeatures and Applications of Various Rotary Drives

Type of Drive

Service and Feature Electric MotorEddy Current or

Magnetic CouplingsMechanical Stepped-Speed Transmissions

Continuously VariableMechanical Drives

HydraulicDrives

Very wide range of speeds �

Few speed steps with remote control �

Few speed steps with local and manual output � �

Very high output power at variable speed � �

Vibration at load or driver �

Small or moderate load with narrow speed range �

Accurate speed control � � �

Shock loads, frequent overloads �

Speed reversal required � � �

FIG. 7.10cEarly method of speed adjustment: the stepped pulley system.

R2 R1

r2 r1

d

Driver pulley speedS1 = constant

Driven pulley speedS2 = S1 ( R

r )

FIG. 7.10dContinuous variation of speed can be provided by the cone pulleysystem.

Beltguides

Driver pulley

Driven pulley

d

R1

r1

© 2006 by Béla Lipták

1458 Regulators and Final Control Elements

speed adjustment and special adjustable motor bases to main-tain belt tension.

In place of the adjustable motor base, a spring-loadedflat-face idler pulley can be used to maintain belt tensionwhen center-to-center distance must be held constant. Align-ment of driving and driven sheaves is also critical, becausebelt wear will be severe with poor alignment. Also, speedadjustment with this drive is limited because the set-screwmust engage a flat surface.

A number of designs are available with in-motion speedadjustment. The designs vary from manual speed adjustment,through a crank or handwheel, to automatic actuators. Gen-erally these drives use one sheave with adjustable pitch andone spring-loaded sheave that automatically adjusts itself tomaintain belt tension (Figure 7.10f).

Speed adjustment is obtained by means of a mechanicallinkage that moves one of the flanges of the driving sheave.The opposite flange on the driven sheave is spring loadedand moves to maintain belt tension and alignment. Radialmotion of the belt along the flange faces in response to speedchanges is facilitated by the rotary motion of the sheaves.When speed adjustments are made at rest, the belt cannotmove radially and the adjusting linkage may be damaged.On automatic actuators, an interlock should be provided tovent air off the diaphragm whenever the drive is stopped andinhibit speed changes at rest.

Variable-pitch sheave drives are available as packageunits including motor. Speed adjustment ranges of 4:1 arecommon, but higher ranges to 10:1 are possible. Horsepowerranges to 100 hp (75 kW) are also possible. For very narrowspeed ranges, drives to 300 hp (225 kW) are available.

Hydraulic Variable-Speed Drives

In hydraulic variable-speed drives, a pump with fixed orvariable displacement drives a hydraulic motor, which itselfcan have fixed or variable displacement. The pump is drivenby an electric motor at fixed speed. Output speed is controlledby changing pump or motor displacements.

Pumps and motors are virtually identical, differing onlyin the location of the power input and output. Several designsof variable-displacement units are possible, including axialpiston, radial piston, and vane types. An axial piston unit isshown in Figure 7.10g.

Of course, combinations of pumps and motors with vari-able or fixed displacement are possible for speed control.Pumps with variable displacement combined with motorswith fixed displacement yield a variable-horsepower, fixed-torque drive. Motor speed reversal is possible by reversingthe pump stroke.

Combinations of fixed-displacement pump with variable-displacement motors produce drives with fixed horsepowerand variable torque. Speed reversal in this combination, how-ever, requires the use of a valve to reverse the supply andreturn connections at the motor. A variable-displacementpump in conjunction with a variable-displacement motor hascharacteristics intermediate between the two previously dis-cussed combinations. Range of speed control, however, iswidest for this combination.

Pumps and motors with fixed displacement can be usedfor speed control by controlling the amount of fluid deliveredto the motor. This can be accomplished by means of a pres-sure relief valve on the pump discharge and throttling valvein the oil line to the motor. Motor reversal can also be accom-plished by means of a four-way valve, reversing the oil supply

FIG. 7.10eAutomatic speed adjustments over wide ranges can be providedby the variable-pitch sheave.

Setscrew

Hub

Movableflanges

FIG. 7.10fThe main components of a continuously adjustable mechanicalspeed transmission system.

Fixedflange

Driver

Belt

Loadingspring

Load

Springloadedflange

Fixedflange

Controllinkage

Adjustingplate

Fulcrum

AdjustableflangeBelt

© 2006 by Béla Lipták

7.10 Variable-Speed Drives 1459

and return lines at the motor. These methods, however, arecomparatively inefficient and are not used with automaticcontrol.

Hydraulic drives are available as pump-motor packages,or the motor can be mounted remotely and connected to thepump by hydraulic tubing. Displacement is adjustable throughpush-pull rods or handwheels, and both methods are adaptablefor operation by hydraulic (usually integral) and pneumaticactuators or by electric motors for automatic speed control.Speed ranges are adjustable up to about 40:1 at power ratingsup to 4000 hp (3 MW).

Several attributes of hydraulic variable-speed drives areclear advantages over the other types of drives discussed.Hydraulic drives offer the fastest response in acceleration,deceleration, and speed reversal. They are generally bettersuited than other types of drives for high shock loads, fre-quent speed-step changes, and reversals.

A disadvantage associated with hydraulic drives is fre-quent fluid leaks. Besides requiring maintenance, leakage ofhydraulic fluid creates safety hazards that can preclude theiruse. Nonflammable hydraulic fluids are available, but thechoice of compatible fluid and equipment may be limited. Foroffshore applications, once-through hydraulic systems aresometimes used. These are normally water-based fluids withlow-marine-toxicity additives.

Hydraulic systems normally operate at pressures of1500–3000 psi (10–20 MPa), but higher pressure equipmentis available and can save weight. To avoid cavitation damage,it is usual to maintain the entire return system at a slightpositive pressure in the order of 70 psi (500 kPa).

Fluid Couplings Fluid couplings consist of two couplinghalves, one driven by a standard electrical motor and the otherconnected to the centrifugal pump. Oil is circulated in thecoupling to regulate pump speed. A splitter or diverter assem-bly applies the oil to the coupling or bypasses it around thecoupling. The pump shaft speed increases as the amount ofoil supplied to the coupling is increased, thus producing asimple control system for varying pump speed.

The fluid coupling has historically been one of the mostpopular variable-speed devices for centrifugal pumps becauseof its relatively low first cost, high reliability, and ease of

maintenance. It is a slip-type device, like the eddy currentcoupling, and has a lower efficiency curve than the variablefrequency, direct current, and wound rotor regenerative typesof variable speed drives.

Hydroviscous Drives Hydroviscous drives are similar tofluid couplings in configuration and operation. Instead of anoil-filled coupling, one or more disk assemblies are used,with driving and driven members pressed together by oilpressure. Increasing the oil pressure increases the pumpspeed; likewise, reducing the oil pressure reduces the pumpspeed.

The hydroviscous drive is, therefore, a slip-type device,like the eddy current coupling, and has a lower efficiencycurve than most electrical-type drives. This drive oftenrequires a more complex control system than other drivesbecause of variations in oil pressure caused by oil viscosityor temperature.

Magnetic Variable-Speed Drives

Eddy Current Couplings The eddy current coupling wasfor many years the most acceptable drive for centrifugalpumps, particularly for large vertical turbine pump applica-tions. It is rugged in design and available in both horizontaland vertical configurations. The eddy current coupling usesa standard, constant-speed motor that drives the eddy currentcoupling, which consists of a drum connected to the electricmotor and electromagnetic pole-type rotor assembly, whichis connected to the centrifugal pump, inside of and free fromthe drum.

The speed control system regulates the amount of fluxthat exists between the drum and rotor assemblies. Theamount of slip or speed difference between motor and pumpincreases and decreases with the flux density in the coupling.Being a slip-type, variable-speed device, the eddy currentcoupling has a lower efficiency than the variable frequency,direct current, and wound rotor regenerative types of variablespeed drives.

In most electrical machinery, eddy currents are detrimen-tal to operating efficiency, and great pains are taken to elim-inate them. In the eddy current coupling, however, these

FIG. 7.10gAxial piston pump provides the variable displacement in this hydraulic variable-speed drive.

Rota

tion

Rota

tion

StrokeZero stroke

Stroke

© 2006 by Béla Lipták

1460 Regulators and Final Control Elements

currents are harnessed and are the basis for the operation ofthe coupling. The eddy current coupling is a nonfrictiondevice where input energy is transferred to the output througha magnetic field.

The eddy current coupling consists of a rotating magnetassembly separated from a rotating ring or drum by an airgap. In addition, a coil is wound onto the magnet assembly(Figure 7.10h), or, on larger units, the coil is stationary onthe coupling frame.

There is no mechanical contact between the magnets anddrum. When the magnet assembly is rotated, the drumremains stationary until a DC current is applied to the coil.Relative motion between magnet assembly and drum pro-duces eddy currents in the drum, whose magnetic fieldattracts the magnet assembly. Attraction between the twomagnetic fields causes the drum to follow the rotation of themagnet assembly. The attraction between the two rotatingmembers is determined by the strength of the coil’s magneticfield and by the difference in speed between the two mem-bers. Thus, by controlling the coil excitation, the amount ofslip and, hence, the output speed can be controlled.

Slip and Slip Loss Being a slip device, the eddy currentcoupling must of necessity develop slip and reject the slippower in the form of heat. The amount of slip loss can bedetermined from Equation 7.10(3):

7.10(3)

wherePs = slip loss powerPL = load powerSs = slip speed (rpm)S0 = output speed (rpm)

Slip devices always generate heat, so eddy current cou-plings are either air or water cooled. Small air-cooled units

below 5 hp (4 kW) can be designed to dissipate all of therated power. Air-cooled units above 300 hp (225 kW) in sizecan dissipate only about 25% of the rated power capacity.For this reason, air-cooled units are not recommended wherecooling capacity would be greater than about 20% of ratedpower.

Water-cooled units are designed to dissipate the ratedhorsepower continuously. In addition, water-cooled units showa small efficiency advantage over air-cooled ones, particularlyon “water-in-the-gap” types, where the water contributesslightly to the torque capability. Generally, water-cooledunits are preferred to air-cooled types except when lack ofcoolant precludes their use or where very low slip losses areencountered.

Eddy current couplings require only a low percentage oftransmitted power for excitation. Typically, a 3 hp (2.2 kW)unit will require 50 W of excitation, while a 12,000 hp (9 MW)unit will require 20 kW. Eddy current couplings are readilyadaptable to silicon-controlled rectifier (SCR) (Section 7.2),providing speed control within 1% accuracy, over 10–100%load change.

Size and Efficiency Efficiency of the eddy current cou-pling is acceptable in the large sizes and at full speed torque,as will be shown later. At lower speeds, efficiency dropsconsiderably, while efficiencies as high as 95% are possibleat full excitation and torque. Because there is no contactbetween the input and output shafts of the coupling, theunit will not transmit vibrations. On prime movers thatexhibit some torsional vibration, the use of an eddy currentcoupling can be of advantage because it will attenuate thesevibrations.

Integral combinations of motor, coupling, and excitationare available in small sizes of 1 hp (0.75 kW) or less. Air-cooled couplings range in size up to 900 hp (670 kW), butthe larger sizes are practical only where a relatively low cool-ing capacity is required. Liquid-cooled units can be as largeas 18,000 hp (13.5 MW) capacity. Speed-control units rangefrom simple open-loop control to precise, automatic closed-loop control with tachometer-generator speed feedback.

Magnetic Particle Coupling The magnetic particle couplingoffers another solution to adjustable speed drives. Basicallythe coupling consists of two concentric cylinders separated byan air gap and a stationary excitation coil surrounding thecylinders. Ferromagnetic particles fill the gap between theconcentric cylinders.

When a controlled amount of current is used to energizethe coil, the particles form chains along the magnetic lines offlux connecting the cylinder surfaces. The shear resistance ofthe magnetic particles is proportional to the coil excitation andprovides the basis for power transmission from input to output.

The output torque of this unit is always equal to inputtorque, regardless of speed. This fact allows the output torqueto be set at standstill by controlling the coil excitation. When-ever the torque capacity of the coupling is exceeded, slip will

FIG. 7.10hThe main components of an eddy current coupling.

Rotation

Coil

Rotating fieldmagnetic lines

Eddy currentsgenerated

Airgap

Rotating ringor drum

Magnetassembly

P PS

Ss Ls=0

© 2006 by Béla Lipták

7.10 Variable-Speed Drives 1461

result with accompanying heat liberation. Whenever the cou-pling is used for speed control, selection of a coupling withadequate cooling capacity is of great importance. Air-cooledunits are available with large heat dissipation capacity; how-ever, water cooling is more effective and therefore preferred.

Magnetic particle couplings can be used effectively forspeed control, because they can provide a constant torqueoutput independent of speed. However, a closed-looped con-trol system is required for effective speed control in order tocancel the effect of changing torque on slip speed.

Magnetic Fluid Clutches The magnetic fluid clutch is sim-ilar in operating principle to the magnetic particle coupling,but either a magnetic dry powder or magnetic powder sus-pended in a lubricant is utilized. A typical disc-type magneticfluid clutch is shown in Figure 7.10i.

While the operating principle is similar to that of mag-netic particle coupling, the clutch can be used only for low-power applications in the order of a few horsepower.Recharging of the clutch due to fluid deterioration is a definitedisadvantage; however, recharging can be fairly easilyaccomplished during routine maintenance.

VARIABLE VOLTAGE

One of the least desirable drives for centrifugal pumps is thevariable voltage drive utilizing NEMA D, high-slip, inductionmotors. This is because of their poor efficiency, limited speedturndown, and need for special motors. This drive is no longerin common use.

POLE-CHANGING AC MOTORS

AC motors, while not readily usable for continuous speedcontrol, can be made reversing or furnished with several fixedspeeds. The squirrel cage induction motor is best suited fromthe standpoint of reliability, and it can be furnished with upto four fixed motor speeds.

Speed changes are accomplished through motor startercontacts that reconnect the windings to yield a different num-ber of poles. In the resulting pole motor, the sections of thestator winding are interconnected through the motor starterto provide different motor speeds. On separate windingmotors, a different winding with the required number of polesis energized for each motor speed. The starter contactors,however, must be interlocked to prevent two or more contac-tors from closing simultaneously. In current practice, morethan two-speed designs are unusual, and variable-frequencydrives have become common.

SOLID-STATE VARIABLE-SPEED DRIVES

In the past three decades, the use of solid-state variable-speeddrives has increased dramatically. There are several reasonsfor this trend. The energy shortages of the 70s were a pow-erful incentive to find new ways to save energy. Because athird of all the electric power generated in the United Statesis consumed by AC motors, a major focus was on means ofcutting the power consumed by those motors.

The main use of the AC motor is as a prime mover forpumps and fans. The traditional practice of flow control wasto use a throttling valve or damper. These methods are noto-riously wasteful (Figure 7.10a). Efficient flow control can beachieved by changing the speed of the pump or fan andthereby eliminating the valve entirely, but with an AC motorthat could only be achieved (while maintaining efficiency)by two-speed motors, rotary frequency changers, or invertermotor drives.

The increase in the cost of electric power brought thepayback period down to the point where the use of AC drivesbecame cost-effective. During this same period, technologi-cal advances improved the performance, increased the reli-ability and efficiency, and lowered the cost of AC and DCsolid-state motor drives. The main developments that causedthis rapid advance were the improvements in semiconductorswitching devices, large-scale integrated circuits, and themicroprocessor.

The engineers designing the drives immediately adoptedthese advances and incorporated them into their products. Asa result, the price of small AC variable-speed drives (below 20hp/15 kW) has dropped by about a factor of three. The com-bined effect of better and cheaper drives and the need to saveenergy is responsible for rapid growth of the solid-state driveindustry.

Underlying Semiconductor Technology

All solid-state variable-speed drives are based on semicon-ductor switching devices. The important ones are the rectifierdiode, the power transistor, the thyristor, the gate-turn-offthyristor (GTO), and the insulated gate bipolar transistor(IGBT). All except the diode have the property of being

FIG. 7.10i Disc-type magnetic fluid clutch.

Magneticfluid

Driver

Coil

Load

© 2006 by Béla Lipták

1462 Regulators and Final Control Elements

controlled by a low power signal that changes their state fromblocking to conduction.

Thyristors can be turned on by gate control but not turnedoff at the gate. The GTO, transistor, and IGBT have both turn-on and turn-off capability. The ability of these devices to handlehigher currents and voltages has improved steadily, with theresult that drives could be built for ever-larger power ratings.

Table 7.10j illustrates the primary switching propertiesof devices available in production quantities in 1992. Today,drives are available that operate directly at 7200 V, althoughthe most common drive voltages in the United States are 230and 460 V, three-phase.

Although the thyristor was the first semiconductor switchused in drives, it has been supplanted in the power range below500 hp (370 kW) by power transistor- and IGBT-based drives.The GTO has been used at all drive power levels, but it too isbeing displaced by the transistor and IGBT. GTO-based drivesare primarily limited to drives above 1 MW and those in whichspace and weight is at a premium, such as propulsion systemdrives. Due to the voltage limitations of the semiconductorswitches, transistor drives are available at 600 V and below.

Drive Circuit Topologies

All drives have an input stage that converts the fixed voltageand constant-frequency AC mains to DC. If the input devicesare controllable, like thyristors, then the DC output of this stageis adjustable. If the input devices are diodes, which conductwhenever forward biased, then the output DC is a fixed pro-portion of the input AC voltage. After the input stage, the detailsof the circuit depend on the type of motor being controlled.

The simplest power circuit applies to DC motors, becausetheir speed may be controlled by adjusting the DC armaturevoltage. AC drives are more complex, because the outputvoltage must be adjustable-frequency and adjustable-voltageAC. This necessitates another stage of power conversion toinvert the DC from the input stage into AC for the motor. InAC drives the DC power from the input stage is filtered bya bank of electrolytic capacitors or an inductor.

Variable-Speed DC Motors

Direct current motors lend themselves extremely well to speedcontrol, as demonstrated by the variety of speed-load andload-voltage characteristics obtainable by means of parallel,

series, or separate excitation. A typical set of speed-load char-acteristics for shunt, series, and compound motors is shownin Figure 7.10k.

The inherent speed regulation, or constancy of speedunder varying load conditions, of the shunt motor is showngraphically in the diagram. The speed change of the motoris only 5% for a load change from zero to full load. Althoughthe methods of speed control are applicable to all types ofDC motors, the superior speed regulation of the shunt motoraccounts for its wider use on control applications.

The speed of a DC motor is a function of armaturevoltage, current, and resistance; the physical construction ofthe motor; and the magnetic flux produced by the field wind-ing. The equation relating these variables to speed is:

7.10(4)

whereS = motor speedV = armature terminal voltageI = armature currentR = armature resistanceΦ = magnetic field fluxK = a constant for each motor depending on physical

design

Three methods of speed control are suggested by Equa-tion 7.10(4): adjustment of field flux Φ, adjustment of arma-ture voltage V, and adjustment of armature resistance R.

Field Flux Adjustment The first of these, adjustment of fieldflux, involves varying the field current. This can be accom-plished either by means of a field rheostat or by the currentbeing controlled electronically by a set-point signal throughan energy-throttling device such as an SCR. Open-loop control

TABLE 7.10jComparison of Semiconductor Switches

Device TypeBlocking Voltage

Current Capability Switching Time

ThyristorGTO thyristorTransistorIGBT

6000 V4500 V1400 V1400 V

5000 A1000 A 500 A 400 A

50–300 µsec 10–50 µsec 3–10 µsec100–300 nsec

FIG. 7.10kSpeed-load characteristics of DC motors.

Series motor

Compoundmotor

25 50 75 100

50

25

75

100

Perc

ent o

f rat

ed lo

ad

Percent of rated speed

Shuntmotor

SV IR

K= −

Φ

© 2006 by Béla Lipták

7.10 Variable-Speed Drives 1463

should be utilized only when the load is constant and precisespeed control is not essential.

Closed-loop control is generally used in conjunction withelectronic devices and offers better control than open loop,but at higher cost (Figure 7.10l). Either type of controlrequires a measurement of the controlled speed; in closed-loop control the measured speed is compared with the setpoint, and the field current is adjusted automatically to bringthe difference to zero.

For a discussion of speed sensors refer to Section 7.19of the Process Measurement volume of this handbook. Inopen-loop control a speed readout is required to permit setadjustments by the operator for the desired speed.

Adjustment of field flux yields a motor with constanthorsepower. The allowable armature current is approximatelylimited to the motor rating in order to prevent overheating.The effects of changing flux and changing speed effectivelycancel each other so that the allowable output horsepower—the product of armature current and induced armaturevoltage—is constant. Torque, however, varies directly withfield flux, and therefore this type of speed control is suitedfor applications involving increased torque at reduced speeds.The speed range possible with field flux adjustment isapproximately 4:1.

Armature Voltage Adjustment Armature voltage controlcan also be used to control motor speed. With armature volt-age control, the change in speed from zero to full load isalmost entirely due to the full-load armature resistance drop,and this speed change is independent of the no-load speed.

Consider two motors with identical speed changes from zeroto fully loaded, but one operating at 100 rpm and the otherat 1000 rpm at zero load. In terms of percent speed change,a 10 rpm variation may be unacceptable for the lower-speedmotor but may be insignificant for the higher-speed motor.

One method used for armature voltage control is the use ofa motor-generator set to supply a controlled voltage to the motorwhose speed is to be regulated. An AC motor drives a DCgenerator whose output voltage is variable and supplies thevariable-speed motor armature. An obvious disadvantage of thismethod is the initial investment in three full-size machines; thismethod is so versatile, however, that it is often used.

Armature voltage, of course, can also be controlled elec-tronically by one of the devices discussed in Section 7.2.These are more commonly used for speed control due to thelower investment and greater efficiency as compared to themotor-generator set. In the motor with controlled armaturevoltage, both the allowable armature current and field fluxremain constant. The driver therefore has a constant torqueoutput, as opposed to the constant horsepower output of thefield-controlled motor.

The armature voltage method of speed control yields speedranges in the order of 10:1. By combining field flux control andarmature voltage control, speed ranges of 40:1 are obtainable.The base speed of the motor is set at full armature voltage andfull field flux; speeds above base are obtained by field fluxcontrol, speeds below base by armature voltage control. Speedranges greater than 40:1 are obtainable through the addition ofspecial motor windings and SCR controls.

Armature Resistance Adjustment Adjustment of armatureresistance is another method of speed control suggested byEquation 7.10(4), and it can be used to obtain reduced speeds.An external, variable resistance is inserted into the armaturecircuit (Figure 7.10m). Speed regulation with this methodand its variants is very poor, however, and this type of speedcontrol is not commonly used. An added disadvantage of thearmature resistance method is the decrease in efficiency dueto the power consumption in the resistor.

Starting Circuits DC motors must be protected from highin-rush currents during starting. In Figure 7.10n, the startingcurrent through the armature is limited by resistors R1, R2,

FIG. 7.10lSCR-based motor speed controls can either be open or closedloop.

Tachometergenerator

Speedcontrolled

motor

Speedcontrolled

motor

SCR SCR Tachometergenerator

SCRgate

circuit

SCRgate

circuit

Speedsetting

potentiometer

Speedreadout

Automaticcontroller

SpeedreadoutManual

speedset point

Open loopSCR motor speed control

Closed loopSCR motor speed control

Integralunit

Integralunit

FIG. 7.10mShunt motor is controlled by varying the armature resistance.

Linevoltage

Motorfield

winding

Externalresistor

Motorarmature

© 2006 by Béla Lipták

1464 Regulators and Final Control Elements

and R3. When the start button is depressed momentarily, relayM and timer delay relays TD1, TD2, and TD3 are energized.The two M contacts close instantaneously, providing powerto the motor. After a time delay, contact TD1-1 closes, shunt-ing resistor R1. Contacts TD2-1 and TD3-1 close succes-sively until the armature is directly on the line. The numberof resistors is set by the torque and current limitations of themotor and the desired smoothness of startup.

Braking, Control, and Reversing The motor will also act asa generator, converting mechanical energy to electrical energy.This feature can be utilized to advantage where rapid stoppingof the motor is required. In this configuration, when power isdisconnected from the motor, a resistor is automatically con-nected across the armature and the mechanical energy of therotating member is dissipated as heat in the resistor.

DC motors can be run in reverse rotation by changing thepolarity of the armature voltage. This can be accomplished bymeans of two contactors—one forward, one reverse. Thesemust be either mechanically or electrically interlocked toinhibit closing both contactors simultaneously and to preventapplying reverse line voltage to the motor prematurely.

Normally, DC power for operation of the motors is notreadily available. Rectifier tubes and solid-state devices suchas SCRs are used extensively to convert the incoming powerto DC and simultaneously to throttle the current or voltagedelivered to the motor in response to an external controlsignal. A discussion of these devices is given in Section 7.2.

Pump Drives Direct current motors have become more pop-ular for pump drives because of the development of silicon-controlled rectifiers. The SCRs and direct current motor haveresulted in a variable-speed drive of modest first cost andhigh efficiency. The significant objection to direct current

motors has been maintenance of commutators and brushes.Direct current motors have been used on elevators in publicbuildings for many years.

Unlike centrifugal pumps, elevator loads are of the con-stant torque type. Brush life varies from 6 weeks to 18 monthson elevator applications. At present, no extensive experienceis available on brush and commutator wear and maintenanceon centrifugal pump applications in the 1–100 hp (75 kW)range. If trained maintenance personnel are available at thepoint of application, the direct current drive can provide ahighly efficient, variable-speed drive for centrifugal pumps.Cost of brushes and brush maintenance must be included inany economic evaluation. At the present, direct currentmotors are not as readily available as AC induction motors.

Thyristor DC Drives

The thyristor DC drive is the oldest form of solid-state driveand is still a viable choice, although the drawbacks of theDC motor, including high cost, size, and sparking, have notbeen overcome. These drives can be built over a wide powerrange, but due to difficulties in commutation in the machinethey are usually limited to less than 700 VDC output. Thy-ristor DC drives are available off the shelf with or withoutthe motor from fractional to 500 hp (375 kW). Fractionalhorsepower drives typically are powered from 240 VAC sin-gle phase, while the range above 5 hp (3.7 kW) is furnishedfor 240 or 480 VAC three-phase.

The behavior of the shunt DC motor makes it relativelyeasy to control. Torque is proportional to the product ofcurrent and flux, while voltage is proportional to the productof speed and flux. Figure 7.10o is a block diagram of a singleconverter thyristor DC drive suitable for two-quadrant oper-ation. The thyristor converter is equipped with a fast currentregulator, which effectively controls the motor torque.Enclosing the current loop is a speed regulator that amplifiesthe speed error and generates the current reference for theinner current loop. Speed feedback is provided by a tachom-eter for precision (0.1–1% accuracy and 100:1 turndown) or

FIG. 7.10nThe starting circuit of a nonreversing DC motor.

TD1

M

TD2

TD3

M

M

TD1-I TD2-I TD3-I

Motorarmature Motor

field M R1 R2 R3

Overloadsensing

relay

Line voltage −+

Motorstarter

relay coilStart

button

Stopbutton

Time delay relays

Overloadrelay

contact

FIG. 7.10oSingle converter thyristor DC drive.

Fieldcontroller

DCmotor

Current regulatorand phase control

3 fAC

fixedvoltage

© 2006 by Béla Lipták

7.10 Variable-Speed Drives 1465

by the motor terminal voltage for less demanding applica-tions (2–5% accuracy and 10:1 turndown).

Because the converter can produce current in one direc-tion only, a fast-reversing drive requires a second converterin inverse parallel to the first (see Figure 7.10p). Only oneconverter at a time is in conduction, depending on the desireddirection of the motor torque. Although it is also possible tocontrol direction with reversing contactors in the armaturecircuit, nowadays that feature is done with dual convertersor field reversing.

Field-reversing drives use the property of the machinethat the torque direction is controlled jointly by armaturecurrent direction and flux polarity. For applications in whichthe fastest reversing is not necessary, a small dual converteron the field is an economical approach to reversing driveapplications (see Figure 7.10q).

Approximate costs of thyristor DC drives are listed inTable 7.10r. These costs include only the power conversionequipment, not the motor. Although the cost of the DC driveelectronics is less than that of the AC variable-frequency

drive, when the much lower cost of the AC motor is included,an AC drive and motor package usually costs less than theDC package of the same rating. This is almost always trueif a weather-protected motor or explosionproof motor isrequired.

Variable-Frequency Drives

Often called adjustable frequency, these drives convert alter-nating current to direct current and back to alternating currentat frequencies from 0 to 120 Hz. Of solid-state construction,these drives provide a highly reliable means of varying pumpspeed. They have wire-to-shaft efficiencies as high as 95%at full speed and 70–75% efficiencies at 40% speed whendriving new, high-efficiency motors. The motors are standardinduction types, found in stock in most major cities. (Loadcommutated inverter, or LCI, is applied to synchronousmotors.)

The primary objections to these drives were their highfirst cost and complex designs. The development of powertransistors and other electronic advances has reduced the costof these drives, and the ongoing training of field personnel inelectrical and electronic service has made variable frequencymore acceptable in fields that have, traditionally, usedmechanical, variable-speed drives. Variable-frequency drivesare available up to 5000 hp with broad speed turndown ranges.

FIG. 7.10pDual converter thyristor DC drive.

DCmotor

Fieldsupply

Current regulator, phase controland bank selection logic

3 fAC

fixedvoltage

FIG. 7.10qThree-phase field-reversing thyristor DC drive.

3 fAC

fixedvoltage

DCmotor

Reversingfield

controller

Current regulator andphase control

TABLE 7.10r2004 Cost of Thyristor DC Drives

Power Rating Single Converter Dual Converter

100 hp (75 kW) $5,000 $6,500

200 hp (150 kW) $7,000 $8,000

500 hp (375 kW) $15,000 $18,000

1000 hp (750 kW) $20,000 $27,000

© 2006 by Béla Lipták

1466 Regulators and Final Control Elements

For the AC variable-frequency drive, the power electron-ics is more complex than for the DC drive, but the advantagesof the AC motor over the DC are so pronounced that ACdrives are now more economical in most applications andwill ultimately supplant DC drives. More sophisticatedapproaches such as the use of field-oriented control havepermitted AC drives to match the performance of DC drives.

There are two basic approaches to AC drives, which aredistinguished on the basis of how the DC power from theinput stage is handled. If the DC link filter is a capacitor andthe inverter output consists of pulses of voltage from the link,then it is a voltage-fed drive. This category includes all driveswith diode bridge inputs, such as pulse-width modulated(PWM) drives.

On the other hand, if the link filter is an inductor and theinverter output consists of pulses of current, then it is acurrent-fed drive. In this group we find the conventionalautosequentially commutated current-fed inverter (ASCI),the load commutated inverter, and almost all other inductionmotor drives that operate at 2.3 kV and above. It is worthnoting that different types of drives generally use differentswitching devices, as the different properties lend themselvesto certain types of circuits.

Transistors and IGBTs are used in PWM drives; thyristorsare used in DC drives, ASCIs, and LCIs. GTOs can be usedin any of the drive circuits. These four types of drives (DC,PWM, ASCIs, and LCIs) dominate the solid-state variable-speed drive market. There is no one “best” drive; all of themhave a different mix of features such that one needs to definethe application requirements carefully in order to choose thebest drive for the particular service.

Pulse-Width Modulated Drives Figure 7.10s illustrates thepower circuit of a PWM drive using transistors. It is a verysimple circuit, with an input diode bridge, fixed voltage DClink, and six output switches. Control of the output voltageand frequency is exercised entirely by the modulation strat-egy for the transistors. As the output voltage waveform indi-cates, the DC link voltage is applied to the motor in shortpulses by turning the transistors on and off.

The duration of the pulses and their spacing controls theharmonic spectrum of the output. The objective is to have alarge fundamental and as small an amount of harmonics aspossible. The inverter determines the amplitude and fre-quency of the motor voltage, but the motor current is deter-mined by the motor parameters and the load on the motor.Modern PWM drives switch rapidly (greater than 1 kHz fortransistors; greater than 10 kHz for IGBTs), so that the low-order harmonics are essentially eliminated and a smooth sinu-soidal motor current is obtained.

The best modulation strategies require very complexcombinatorial logic or fast real-time computer processing,features that are now readily available in PWM drives butwere not when they first appeared on the market. TransistorPWM drives are available in fractional through about 500 hp(400 kW) at rated input voltages of 120, 240, 480, and 600VAC; GTO PWM drives are available in even higher voltageand power ratings.

Current Source Variable-Frequency Drives The basic powercircuit of the current source inverter is shown in Figure 7.10t.The input conversion is performed by a thyristor bridge. Thisis controlled with a current regulator to function as an adjustable

FIG. 7.10sPulse-width modulated inverter drive using transistors.

3 fAC

fixedvoltage

Voltage controlled oscillatorpulse width modulator

base drivers

Inductionmotor

Transistor poles IGBT poleOutput voltage from PWM inverter

IGBT: Insulated gate bipolar transistor

© 2006 by Béla Lipták

7.10 Variable-Speed Drives 1467

current source. The voltage ripple of the converter is filtered bythe choke in the DC link so that a smooth current flows intothe output inverter stage. This current is steered in sequenceinto the motor windings by the thyristors so that the motorreceives a trapezodial current waveform.

Because the thyristors cannot be turned off by gate control,commutation is forced by the energy stored in the capacitors.This energy is exchanged between the machine inductance andthe capacitors. It is prevented from discharging into themachine by the diodes in series with the thyristors. In this casethe inverter establishes the motor current, but the motor voltageis determined by its parameters and load.

Unlike the PWM inverter, the output amplitude is con-trolled by the current regulator on the input converter, and theoutput stage controls only the frequency. Because this type ofdrive is constructed from thyristors, it can be economicallybuilt in very large power ratings. A cost comparison of ASCIand PWM variable-frequency drives is shown in Table 7.10u.Once again, cost of the machine is not included.

Load Commutated Inverters Yet another common type ofdrive is the load commutated inverter, whose power circuit isshown in Figure 7.10v. This is also a current-fed topology, asevidenced by the similarity to Figure 7.10t. The input converteris a thyristor bridge equipped to function as a controlled currentsource. Smooth DC current flows through the choke into thethyristor inverter, where it is switched into the motor windings.But in this case commutation is performed by the machine.

By operating the machine at a leading power factor, thethyristors are naturally commutated because an off-going

device experiences reverse voltage from the machine. How-ever, a synchronous machine is required because an inductionmotor cannot be operated at a leading power factor. Althoughit is possible to build LCIs over a wide power range—100–20,000 hp (75 kW–15MW)—they are generally used inapplications above 1000 hp (750 kW), where the cost differ-ential between a synchronous and induction motor narrows.The cost of a medium voltage LCI (excluding motor andexciter) ranges from $150 per hp ($200 per kW) down to $75per hp ($100 per kW), as the rating increases from 1000 to5000 hp (750 kW to 3.7 MW).

Drives with Power Regeneration The ability to receive me-chanical energy from the load and return it to the AC line isinherent in the ASCI and LCI, and the DC dual converter. The

FIG. 7.10tAutosequentially commutated current-fed inverter (ASCI).

Inductionmotor

Voltage regulatorvariable frequency

oscillatorCurrent regulator

phase control

3 fAC

fixedvoltage

Motor current from current-source inverter

TABLE 7.10u2004 Cost of Variable-Frequency Induction Motor Drives

Power RatingPWM Including

Reactor Current-Fed ASCI

10 hp (7.5 kW)20 hp (15 kW)50 hp (37 kW)100 hp (75 kW)200 hp (150 kW)500 hp (375 kW)1000 hp (750 kW)

$1,700 $2,500$5,300$7,500$11,000

$8,000$12,000$18,000$32,000

Note: The reactor price adds 50–90% to the base price of thevariable-frequency drive electronics.

© 2006 by Béla Lipták

1468 Regulators and Final Control Elements

PWM drive requires an additional input stage to achieve thisproperty. This capability is essential for a four-quadrant drive.

The wound rotor motor has been used for many years tovary the shaft speeds of cranes, machine tools, and similarheavy equipment. The energy loss due to heating of second-ary resistance in these earlier applications has been recoveredthrough the use of rectifiers and inverters and has resulted inefficient drives for centrifugal pumps. The higher cost of thewound rotor motor and drive makes this design cost-effectiveonly for larger pumps in the range of 500–10,000 hp (375 kW–7.5MW).

Comparison of Solid-State Drives

The specific requirements of the application determine whichtype of drive is best; they all have somewhat different prop-erties. It is difficult to make quantitative comparisons becausedrives of the same type may behave differently due to detaildesign differences such as size of suppressors and rating ofdevices. Some general comments follow.

Input Power Factor The diode bridge input configurationslike PWM drives usually have near-unity displacement powerfactor, regardless of speed and load. However, the total powerfactor can be much lower due to input harmonic currents ifthere is inadequate inductance in the DC link or on the lineside of the bridge. Drives with an input thyristor converter likeDC drives, ASCIs, and LCIs have an input displacement powerfactor proportional to PU speed (for DC drives) or PU speedtimes motor power factor in the case of current-fed AC drives.

Physical Size and Weight The DC drive is the most com-pact design as it has fewest components; PWM drives areslightly larger; and the current-fed units are the largest andheaviest due to the magnetic components.

Immunity to Shorts and Ground Faults The current-fed de-signs (ASCI and LCI) have inherent immunity to load shortsand ground faults due to their current regulators and high-impedance link chokes. PWM drives rely on detecting suchfaults and quickly turning off the transistors. DC drives arefrequently equipped with transformers to isolate the drivefrom a ground-referenced AC source.

Input Harmonic Currents All solid-state motor drives drawharmonic currents from the source. The harmonic spectrumof all the six-pulse drives that were mentioned results in atleast 30% total harmonic current. The LCI, ASCI, and DCdrives that have a highly inductive DC link draw odd har-monics excluding those divisible by three. The magnitude ofthe currents is approximately 1/h times fundamental whereh is the harmonic number.

The same harmonics are present in the input currents ofa diode bridge, but it is harder to quantify the current distortionbecause that depends on the values of the AC line reactanceand link choke, if present. For low inductance PWM designs,the total harmonic current can easily exceed 50%.

Motor Effects All VFDs have a nonsinusoidal output. Thisresults in a higher RMS motor current for the same motor outputas compared to line (sinusoidal) operation. Consequently, moremotor heating occurs, necessitating careful motor selection orde-rating.

The extra current varies from 3% for LCI and ASCI drivesdown to well under 1% for PWM drives. However, theextremely high amplitude and high frequency modulation ofPWM drives raise eddy current losses and rotor losses in themachine. A rule of thumb is that the total motor losses increaseby 5–15% (compared to line operation) when used with adrive. In the case of DC drives, the output voltage is not smoothbut is rich in harmonics. However, the armature inductance in

FIG. 7.10vLoad commutated inverter (LCI).

Exciter

Synchronousmotor

Current regulatorphase control

Machine convertercontrol gate drives

Typicallyseries

devices3 f AC

fixedvoltage

Machinecurrent

© 2006 by Béla Lipták

7.10 Variable-Speed Drives 1469

modern DC motors filters this so that the current is acceptablysmooth and no motor de-rating is necessary.

Efficiency of Solid-State Drives

All the drives mentioned here have efficiencies at full speedand full load of 95% or more. Drives rated at higher powerhave slightly higher efficiencies than do lower power unitsbecause the fixed-power-consumption components, such ascontrol circuits and cooling fans, constitute a larger propor-tion of the smaller power rating. This is also true for highervoltage ratings, because the conduction losses of the semi-conductors arise from a nearly constant forward voltage drop.

The focus of efficiency should be on the energy savingsdue to the use of the drive (as compared to throttling devices)rather than small differences between types of drives. Further-more, there are no industry standards on efficiency measure-ment or calculation for drives, which means that precisioncomparisons between manufacturers’ claims are not verymeaningful. To some extent there is a trade-off between effi-ciency and protective circuits, as components like suppressorsand line reactors degrade efficiency but improve the reliabilityof the drive.

Recent VFD Developments

Over the past 40 years, since the VFD was first developed,the weight of the drive electronics has dropped rapidly. Thefirst Danfoss VLT5 2 hp (1.5 kW) drive weighed 128 lb(58 kg). The current equivalent VLT5000 weighs just 17.6 lb(8 kg), a sevenfold reduction.

Current-generation equipment offers serial communicationcapability (ModbusPlus, LONWorks, Profibus, Device-Net,etc.) and is frequently provided with PID control capabilitywith 4–20 mA I/O, providing access to a variety of internalparameters.

When the drive motors are fitted with an encoder, positionfeedback allows a whole series of individual drives to besynchronized, matched in angular position, or adjusted asthough they had a mechanical gearbox between them.

For asynchronous motors, normally applies,where (T) is the torque developed, IL is the rotor current, andφ is the air gap flux of the machine. To optimize torque fromthe motor, the air gap flux of the machine ( ) shouldbe kept constant (Figure 7.10w). This means that if the linefrequency (f) is changed, the line voltage (V) must be changedproportionally.

For heavy starts (screw conveyors) and to optimize thestalling torque, an extra (start) voltage (Vo) is required. Whenloaded and in the low speed range (f < 10 Hz), the voltageloss is clearly seen on the active resistance of the statorwinding (particularly in small motors), leading to a specificweakening of the air gap flux (φ).

When operating above normal supply frequency (manysmall four-pole motors are capable of operation up to two-pole speeds), the torque available becomes inversely propor-

tional to speed, and the motor is limited to design power,regardless of speed.

EFFICIENCY OF VARIABLE-SPEED DRIVES AND PUMPS

The following three efficiencies should be evaluated for anyapplication of variable-speed pumps:

1. Wire-to-shaft efficiency of the variable-speed driveand the motor. Obviously, this is the ratio of the usefulenergy applied to the pump shaft divided by the energyapplied to the drive-motor combination.

2. Pump efficiency.3. Wire-to-water efficiency of the drive, motor, and

pump. This is the ratio of the useful energy applied tothe water divided by the energy applied to the pump-motor-drive combination.

Following is a list of terms that will be used to describethe efficiencies of various pump-motor-drive combinations.All of the energy terms are in horsepower to simplify thisdiscussion; they can be converted into watts (HP = 746 w).

Chp = Energy input to an electrical drive Ce = Efficiency of an electrical drive Ihp = Energy input to an electric motor Me = Efficiency of an electric motorDhp = Energy input to a mechanical-type drive De = Efficiency of a mechanical-type drivePhp = Energy input to a centrifugal pump Pe = Efficiency of a centrifugal pumpWhp = Water horsepower or useful energy applied to

the water Ws = Wire-to-shaft efficiency (often called line-to-

shaft efficiency) Ww = Wire-to-water efficiency

Figure 7.10x describes the configuration of an electrical-type variable-speed drive and pump.

T IL~ φ ×

φ ~ /V f

FIG. 7.10wThe motor torque is optimized by keeping the ratio of line voltageto line frequency constant.

0

40

60

80

120

100

20

0 20 40 60 80 100% Frequency

% Vo

ltage

Voltage v. Frequency

V n/f n = constant

Field weakening range

© 2006 by Béla Lipták

1470 Regulators and Final Control Elements

Following are the equations for this combination. Theefficiencies are entered into these equations as fractions.

7.10(5)

7.10(6)

7.10(7)

7.10(8)

7.10(9)

7.10(10)

Figure 7.10y describes the configuration of the electro-mechanical drive and pump along with the various energyterms that are applicable to this combination.

The following equations are similar but not necessarilyequal to those for electrical drives. Equations 7.10(5) and

7.10(6), for water horsepower and pump brake horsepower,are the same.

Dhp = mechanical drive input horsepower

7.10(11)

7.10(12)

Ws = wire-to-shaft efficiency

= Me × De × 100% 7.10(13)

7.10(14)

Evaluation of VSD Efficiencies

The above equations demonstrate that the wire-to-shaft effi-ciency for any drive-motor combination is dependent uponthe efficiency of the electric motor and the drive, whether thedrive is electrical or mechanical. As shown in Equations7.10(9) and 7.10(13), the efficiency of both the motor andthe drive must be determined to achieve the true wire-to-shaftefficiency. These component efficiencies should be deter-mined throughout the anticipated operating speed range, notjust at full-speed condition.

The high-efficiency electric motor has created the opportu-nity for achieving higher wire-to-shaft efficiencies for variable-speed drives, but it is imperative that equivalent motors be usedwhen comparing the efficiency of one type of drive with that ofa different type. If high-efficiency motors are used, their effi-ciencies should have been secured from tests conducted inaccordance with NEMA Standard MG1-12.53a, which is basedon Institute of Electrical and Electronics Engineers (IEEE) Stan-dard 112, Method B.

Figure 7.10z provides a general comparison of the effi-ciencies of various types of variable-speed drives for centrif-ugal pumps. The curves shown in this figure should not beused for energy calculations for a specific application. Rather,efficiencies for drives and motors under consideration shouldbe certified by the manufacturers of that equipment.

As shown in Figure 7.10z, most electric drives are moreefficient than electromechanical drives at reduced speeds. Theexception is the variable-voltage electric-type drive, which isgenerally less efficient than mechanical drives. The mechan-ical drives are usually less efficient than the electric drives,because most of them utilize slip between the input andoutput shafts of the mechanical drive. This results in a sliploss that is directly in proportion to the amount of slip. Theequation for slip loss is

7.10(15)

FIG. 7.10xThe configuration of a pump provided with an electrical variable-speed drive.

FIG. 7.10yThe configuration of a pump provided with an electomechanricalvariable-speed drive.

Power supplyChp

Electricdrive

Ihp Php

Whp

Whp water horsepower

GPM pumpTDH in ftof water

=

= × ( ))3960

Php pump brake horsepowerWhpPe

= =

Ihp motor input horsepowerPhpMe

= =

Chp energy input toelectrical driveIhpCe

= =

Ws wire-to-shaft efficiency

PhpChp

Ce M

=

= × = ×100%ee × 100%

WW wire-to-water efficiency

WhpChp

Ce Me Pe

=

= = × × × 1100%

Ihp

Dhp

Php

Whp

= PhpDe

sameas the electric motor shaft horsepowe( rr)

Ihp motor input horsepowerDhpMe

= =

Ww wire-to-water efficiency

Me De Pe

== × × × 100%

sliphorsepower lossslip rpm Php

output rpm= ×

© 2006 by Béla Lipták

7.10 Variable-Speed Drives 1471

The wire-to-shaft efficiency of the electromechanicaldrive is

7.10(16)

Circulation losses are those losses incurred by the con-stant rotation of the input shaft, along with energy for specificuses such as the loss imparted by oil pumps to circulate oilin fluid couplings and hydroviscous drives. Circulation lossesmust always be stated by mechanical drive manufacturers.

The overall system efficiency index (SEI) of a variable-speed pump installation is determined as follows:

7.10(17)

whereEp = Pump efficiencyEm = Motor efficiencyEv = Variable-speed drive efficiencyEu = Efficiency of utilization = (Qa)(Ha)/(Qd)(Hd)Ha = Actual headHd = Design headQa = Actual flowQd = Design flow

CONCLUSIONS

The above brief descriptions provide general information onthe various types of variable-speed drives. Actual selectionof the drive for a specific application requires an evaluationof (1) wire-to-shaft efficiency, (2) first cost, (3) reliability,(4) serviceability, (5) maintenance costs, (6) need for specialmotor, (7) speed range, and (8) control repeatability.

No variable-speed drive should be selected for a centrif-ugal pump without careful evaluation of the process loaditself to determine the feasibility of variable speed. This,along with consideration for the eight factors listed above,should result in the selection of the optimum drive. For moredetails on pump applications and pump controls, refer toChapter 8.

Bibliography

Barnes, M., Practical Variable Speed Drives and Power Electronics, Oxford:Newnes, 2003.

Blaschke, F., “The Principle of Field Orientation as Applied to the NewTransvektor Closed-Loop Control System for Rotating FieldMachines,” Siemens Review, May 1972.

Bose, B. K. (Ed.), Adjustable Speed AC Drive Systems, New York: IEEEPress, 1981.

Bose, B. K., Power Electronics and AC Drives, Englewood Cliffs, NJ:Prentice Hall, 1986.

Bower, J. R., “The Economics of Variable-Speed Pumping with Speed-Changing Devices,” Pump World.

Brumbach, M. E., Electronic Variable Speed Drives, Clifton Park, NY:Delmar Learning, 2001.

Doll, T. R., “Making the Proper Choice of Adjustable-Speed Drives,” Chem-ical Engineering, August 9, 1982.

Fischer, K. A., and Leigh, D. J., “Using Pumps for Flow Control,” Instrumentsand Control Systems, March 1983.

Gabriel, R., Leonhard, W., and Nordby, C., “Field-Oriented Control of aStandard AC Motor Using Microprocessors,” IEEE Transactions onIndustry Applications, March/April 1980.

Gottliebson, M., “Explore the Use of Variable Speed Water Booster Pumps,”Water and Wastes Engineering, May 1978.

Gray, C. B., Electrical Machines and Drive Systems, Essex, England: Long-man Scientific and Technical, 1989.

Hall, J., “Motor Drives Keep Pace with Changing Technology,” Instrumentsand Control Systems, September 1982.

The Hydraulic Institute, “Pump Life Cycle Costs: A Guide to LLC Analysis,”2005 (www.pumps.org).

The Hydraulic Institute, “Variable Speed Pumping: A Guide to SuccessfulApplications,” 2005 (www.pumps.org).

IEEE, Adjustable Speed Drives: Self-Study Course, New York: IEEE, 1998.

FIG. 7.10zThe wire-to-shaft efficiencies of various variable-speed centrifugalpump drives.

90

80

70

60

50

40

30

20

Perc

ent o

f wire

-to-

shaf

teffi

cien

cies

0 20 40 60 80 100Percent speed

Electrical drivetypesF4 - Energy recoveryE - Direct currentF2 - Current source variable frequencyF1 - Voltage source variable frequency

Electromechanicaldrive types

TypesC - Eddy currentB - Hydraulic or hydroviscousA - Mechanical

Variable voltageusing nema D,high slip motors(type D)

WsPhpIhp

Php Me

Phpslip rpm

output rpmP

=

= × ×

× +

100%

hhp circulation losses

Php Meinput rpm

ou

+

= × × 100%

ttput rpmPhp circulation losses× +

SEI Ep Em Ev Eu= × × × −( )10 6

© 2006 by Béla Lipták

1472 Regulators and Final Control Elements

Irvine, G., and Gibson I. H., “The Use of Variable Frequency Drives as aFinal Control Element in the Petroleum Industry,” 2000 IEEE IndustryApplications Society Annual Meeting, World Conference on IndustrialApplications of Electrical Energy, Rome, October 2000.

ISA, Motion Systems & Control, Online Collection of Technical Papers,2003.

Janki, C., “What’s New in Motors and Motor Controls?” Instruments andControl Systems, November 1979.

Jarc, D. A., and Robechek, J. D., “Static Induction Motor Drive Capabilitiesfor the Petroleum Industry,” IEEE Conference, September 8–10, 1980.

Leonhard, W., Control of Electrical Drives, New York: Springer-Verlag,1985.

Liu, T., “Controlling Pipeline Pumps for Energy Efficiency,” InTech, June1979.

Merritt, R., “Energy Saving Devices for AC Motors,” Instruments and Con-trol Systems, March 1980.

Merritt, R., “What’s Happening with Pumps?” Instruments and ControlSystems, September 1980.

Morgan, R. A., “Large AC Motor Drives Gain Attention,” Instruments andControl Systems, February 1981.

“Motor Controllers,” Measurements and Control, February 1986.Murphy, J. M. D., and Turnbull, F. G., Power Electronic Control of AC

Motors, Oxford: Pergamon Press, 1988.

Polka, D., Motors & Drives: A Practical Technology Guide, ISA, 2003. Pollack, J. J., “Adjustable Rate Pumping,” Plant Engineering and Mainte-

nance Conference/East, December 10, 1979.Rajashekara, K., Kawamura, A., and Matsuse, K., Sensorless Control of AC

Motor Drives: Speed and Position Sensorless Operation, New York:IEEE, 1996.

Rishel, J. B., “Water System Head Analysis,” Plant Engineering, October 1977.Schauder, C. D., Choo, F. H., and Roberts, M. T., “High-Performance Torque

Controlled Induction Motor Drives,” IEEE-IAS Conference Record,1982 Annual Meeting, pp. 563–568.

Schroeder, E. G., “Choose Variable Speed Devices for Pump and Fan Effi-ciency,” InTech, September 1980.

Sen, P. C., Thyristor DC Drives, New York: John Wiley & Sons, 1981.Solar, D. N., “Applications of Inverters,” IEEE Conference, April 14, 1980.Spitzer, D. W., “Variable Speed Drives: Principles and Applications for

Energy Cost Savings,” ISA, 2003.Stefanides, E. J., “Electronic Drives Give Energy-Efficient Flow Control,”

Design News, October 6, 1980.Stewart, R. J., “Applying Adjustable Speed AC Drives,” Instruments and

Control Systems, September 1982.Wasko, C. R., “500 Hp 120 Hz Current-Fed Field-Oriented Control Inverter

for Fuel Pump Test Stand,” IEEE-IAS Conference Record, 1986Annual Meeting, pp. 314–320.

© 2006 by Béla Lipták