1077-2618/09/$26.00©2009 IEEE

For power density and energy savingsin industrial applications

THE PRESENT AND FUTURE MARKET

for motors places high value on power

density, operating efficiency, reliability,

variable speed

operation, and low cost. Perma-

nent magnet (PM) motors are

now able to meet these market

expectations. Compared with the prolific induction motor,

PM motors provide the attributes of efficiency and reliability

and also have the additional advantages of higher power

density (power per mass or volume), superior power factor

(low current), low rotor temperature, and synchronous

operation. Advancement in mag-

net technologies allows operation

at higher temperatures without

permanent magnetization loss.

PM motors are now economically viable because of the

availability of rare-earth magnets, such as neodymium iron

boron, at lower prices. Performance comparisons among

induction motors, surface PM, and salient pole PM motors,Digital Object Identifier 10.1109/MIAS.2009.934443

B Y M I C H A E L J . M E L F I ,S T E V E E V O N , & R O B B I E M C E L V E E N

© ARTVILLE

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including various configurations of PMmachines, are presented in this article.

Three-phase induction motors havebeen widely recognized as the workhorseof industrial applications in the pulpand paper/forest products industry. Overthe past 30 years, there have been cleartrends in motor utilization that demandhigher power density and increasedenergy efficiency.

In many industry applications,motor size and inertia are critical.Motors with high-power density canoffer a performance advantage inapplications such as paper machines.However, high-power density cannotcompromise reliability and efficiency.

Induction motors have been able toincrementally improve energy effi-ciency to satisfy the past requirements. Both mandated(legislated) efficiency [1] [Electric Policy Act (EPACT)]and voluntary levels [2] [National Electric ManufacturingAssociation (NEMA) Premium] have been provided usingproducts derived from general-purpose induction motors.Another method considered for enhanced induction motorefficiency is using cast copper rotors in place of aluminumfor medium-size ratings [3].

As further improvements to energy efficiency aredesired, along with lower noise and variable-speed operat-ing capability, other technologies beyond simple inductionmotors should be considered. Because of dramatic im-provements in magnetic and thermal properties of PMmaterials over the past 20 years, along with considerablecost reduction, synchronous PM motors represent viablealternatives. PM motors have long been recognized as pro-viding higher efficiency than induction motors, but limita-tions in terms of motor control, as well as magnet materiallimitations (performance and cost), have severely restrictedtheir use. PM motors are now available to cover a signifi-cant portion of medium horsepower applications in thepulp and paper industry.

PM Motor Technology

PM MaterialsPMs have been widely used in motion control motors formany years. Over the past 40 years, the magnet materialshave gone through substantial changes with regard toeverything from basic chemistry to cost. Today, there arematerials available to allow much more power-dense,higher efficiency PM motors.

Before the 1970s, PM motor designs typically includedeither Alnico or ceramic/ferrite magnet materials. Althoughceramic magnets are quite economical, the motor air-gapflux densities that can be achieved with these magnets donot come close to the levels commonly achieved with induc-tion motors. Although Alnico magnets can achieve higherflux densities, their poor resistance to demagnetization lim-its their use in motors.

The rare-earth PM materials were introduced first withSamarium Cobalt products, both SmCo5 and Sm2Co17

types. These were available since 1970 and provided the

first opportunity to achieve motor air-gap flux levels comparable with induc-tion motors. These magnets have out-standing thermal capabilities both interms of minimal variation in flux withtemperature and with regard to maxi-mum temperature capability. However,the cost has remained high, limitingtheir use to niche applications.

The introduction of neodymiumiron boron (Nd2Fe14B) magnets inthe 1980s provided the promise ofsubstantially lower cost comparedwith samarium cobalt magnets. Theinitial Nd2Fe14B magnet materialshad very limited temperature capa-bilities, with high susceptibility todemagnetization above 120 �C. Overthe course of the past 15 years, the

material properties have been improved such that somegrades can be used to 180 �C (see Figure 1). In addition,the magnetic field capabilities have been furtheradvanced, and equally important, the magnet costs havecome down significantly. Furthermore, magnets havebeen developed that can be either injection or compres-sion molded in place, allowing substantial rotor lamina-tion design freedom.

PM Motor Primary Electromagnetic ConfigurationsThere are many possible rotor configurations for PMmotors. All are synchronously operating machines butwith various other characteristics. Examples of some of thepossible characteristics are possible across the line startingor the development of saliency-based torque (in additionto the magnet torque).

Figures 2–7 show typical electromagnetic (lamination)configurations for PM motors and a typical inductionmotor. As can be seen, there is a wide range of possibleconfigurations that can be employed in the design of PMmotors with the ultimate selection often being dependenton systems issues and manufacturing preferences.

In Figure 2, the rotor can be seen to have magnets onthe outer surface of the rotor. Inside of the magnets is a

1

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100

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160

180

200

Aln

ico

Cer

amic

Sm

2Co 1

7

Nd 2

Fe 1

4B(c

. 199

0)

Nd 2

Fe 1

4B(c

. 200

5)

Max

. Ope

ratin

g T

empe

ratu

re (

°C) + + +

Maximum magnet-operating temperature.

THE MOTORRATING WAS

IDENTICAL WITHTHE INDUCTION

MOTOR WITH THEEXCEPTION THAT

THE BASEFREQUENCYWAS 120 Hz.

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rotor magnetic structure that has cir-cumferential symmetry. Because of thissymmetry, this rotor can be said to haveno saliency, i.e., no preferred/easy direc-tion of magnetization. All motor torqueis produced because of the interaction ofstator current with magnet flux.

In contrast with Figure 2, it can beseen in Figure 3 that, while the magnetsare still located at the rotor outer diame-ter (OD), the teeth that are in betweeneach adjacent magnet create a dissym-metry or saliency. This saliency allowsthis motor to produce a torque compo-nent (reluctance torque) due to the samemechanism as employed in a synchro-nous reluctance motor. In addition, because of the rotor sali-ency, the inductance of the stator has a different value asaligned with the rotor quadrature (q) axis, compared withthat of the direct (d) axis. The ratio of the q-axis inductance(Lq) to the d-axis inductance (Ld ) is called the saliency ratioand is one figure of merit for salient rotor PM machines.

With the rotor construction seen in Figure 4, the mag-nets have been placed in the interior of the rotor magneticstructure. This again results in rotor saliency and a ratio ofLq to Ld that is greater than one. This configuration alsoprovides the opportunity to use simple rectangular blockmagnets, which would be lower cost than the arc segmentmagnets seen in Figures 2 and 3.

The rotor of Figure 5 takes the single layer, single fluxbarrier, and interior PM design of Figure 4 and extends itto multiple layers. Doing so can provide a means to create

higher saliency ratios than the single-layer design. However, if constructedfrom magnet blocks, the rotor assem-bly can be unusually complex. Anotheroption is to use either compression- orinjection-molded magnets for a multi-layer assembly such as this.

Figure 6 shows another single-layerinterior PM design, but this time, witha squirrel-cage rotor winding built intothe rotor. This provides a means to do afull-frequency line start of this type ofmotor, allowing it to be run across theline, rather than just on inverter power.

Figure 7 shows the cross section ofan induction motor. The rotor does not

have a specific direct and quadrature axis that is fixed rela-tive to the rotor. Rather, the flux actually rotates at slipfrequency within the rotor. If one were to consider a specificinstant in time, there would not be substantial saliency inthis rotor. There are rotor constructions with a combinationof saliency, no magnets, and a squirrel cage (synchronousreluctance motors), but those are not considered here.

PM Motor System ImplicationsAn obvious difference between PM and induction motorsis that the magnetic field is there all the time for the PMmachine, whereas an induction motor field can reduce(when the motor is unpowered) to a small residual level.This has implications for any potential overhauling loadsthat the motor might be coupled to. If the motor was dis-connected from the line, there could still be a substantial

4

d

q

+

Interior PM motor with rotor saliency via a single-flux barrier.

5

d

q

+

Interior PM motor with multiple-flux barrier rotor saliency.

3

dq

+

Surface PM motor with teeth between magnets to createrotor saliency.

2

q

d

+

Surface PM motor with no rotor saliency.

THE NONSALIENTPM ROTOR

CONSTRUCTIONPROVIDES HIGHEFFICIENCY AND

HIGH-TORQUEDENSITY.

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voltage at the motor leads if such anoverhauling load caused rotation of thePM motor. Depending on the speed ofrotation, it is possible to have a hazard-ous level of voltage present on an elec-trically unconnected motor. This couldrequire special maintenance proceduresto provide workers’ safety. This is notdifferent from the case for dc brush-type PM motors or motion-control PMbrushless motors. However, the indus-try using PM machines needs to befamiliar with these characteristics.

When running a motor across a rangeof speeds, the transition from below toabove base speed requires a transitionfrom constant flux to field weakening. Motors operatedstrictly at below base speed do not go through this transition.In an induction motor run on adjustable frequency, thetransition is a matter of the volts per hertz ratio (V/Hz),changing from a fixed value to a decreasing level.

For PM motors, the excitation from the magnets is con-stant, so field weakening is less of a natural phenomenon.Because of the constant magnet excitation, the PM motor hasan open-circuit internally generated voltage, which is a linearfunction of speed throughout the entire operating range

E ¼ p

23 Xr 3 Kpm, (1)

where E is the peak phase voltage; p, the number of poles;Xr, the rotor rotational frequency in rad/s; and Kpm, themagnet flux linkage in Weber.

As a result of the constant magnet flux, operation of thePM motor at above base speed requires that the stator cur-rent has a phase relationship to the rotor position thatallows it to reduce the air-gap flux. This can be thought ofas having negative current in the d axis [5]. With PMmotors that have no rotor saliency (Figure 2), the statorcurrent would be strictly in the q axis at below base-speedoperation. As the motor speed moves beyond the basespeed, to keep terminal voltage constant, there is a need toprovide negative d-axis current, effectively field weakeningthe air-gap flux. The further above base speed the motor isrun, the more negative d-axis current is required to counterthe magnet flux. As a result, the total stator current (vectorsum of the d- and q-axis components) increases without acommensurate increase in output power. Eventually, thenegative d-axis current is too high to sustain.

As opposed to the nonsalient design, a salient pole typeof PM motor (Figures 3–6) does not derive all of its torquefrom the magnet flux. Instead, it achieves a portion of its tor-que via magnetic reluctance, similar to that for a synchro-nous reluctance motor. One advantage of developing torquevia both magnet flux and rotor saliency is that there is lessnegative d-axis current needed to counter the magnet flux athigh speeds. This additional design freedom to proportionthe relative torque contributions of the rotor saliency andmagnets generally allows the salient pole PM motors to bedesigned for greater operation at above base speed [6].

Another system-level issue for PM machines is related tothis operation at above base speed. Although the inverter

can control the stator currents toweaken the magnet flux via negatived-axis current, there is an issue thatoccurs if the inverter were to shut downwhile operating at above base speed. Insuch a case, the motor terminal voltagewould rise to the full value predictedby (1). While running at above basespeed, the motor terminal (line–line)voltage could have a peak value thatexceeds the rated inverter dc-bus volt-age. The freewheeling diodes of theinverter output stage would then forman uncontrolled diode rectifier connect-ing the PM motor to the dc bus. Thehigh-motor terminal voltage would be

rectified and put power onto the dc bus, resulting in a rapidrise in the dc-bus voltage level. A scope trace showing thiseffect is seen in Figure 8, where the bus voltage rose from618 to 796 Vdc, with about half of that rise occurring in 10ms. For this system, the 796-V level was acceptable, but aninverter trip at higher speeds could have damaged theinverter components.

Another advantage that salient-pole PM motors offercompared with nonsalient designs is the opportunity forself-sensing of the rotor position. This can allow a high-bandwidth speed and torque performance without the needfor a speed or position-sensing device such as an encoder orresolver. By taking advantage of the fact that the statorwinding inductance can be quite different in the d and qaxes, the salient-pole PM motor can essentially provide itsown rotor position feedback information.

THE SALIENT-POLEPM MOTOR CAN

ESSENTIALLYPROVIDE ITS OWNROTOR POSITION

FEEDBACKINFORMATION.

7Induction motor.

6

qd

+

Interior PM motor with rotor saliency via a single-flux barrierand a squirrel-cage winding for starting.

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Comparative PerformanceInduction/PMIt can be seen in Figures 9 and 10 thatPM motors provide an opportunity toextend efficiency to a level beyond thatdefined by EPACT and NEMA Pre-mium. In addition to the improvedefficiency at rated load seen for the PMmotors in Figure 9, there are also ad-vantages with regard to more lightlyloaded efficiency [Figure 10(a)]. Be-cause of the reduced no-load current, asa result of the magnets providing the required flux, the PMmotor efficiency stays quite high even at fairly light loads.This same feature of having the flux provided by PMs alsoprovides very high-power factor, especially noticeable atlight loads [Figure 10(b)].

Comparative Test DataThe motor used for the comparative testing of inductionand PM rotors is a laminated NEMA 250 frame, totally

enclosed fan-cooled (TEFC) motor. Itshould be noted that the motor is not astandard NEMA cast iron-frame motorbut is manufactured for a wide range ofvariable-speed applications. Figure 11shows a picture of the surface PM testmotor. It is interesting to note that thedc-load motor that it is coupled to isboth larger in size and has to be inter-nally ventilated to achieve the samerating as the TEFC PM motor.

The rating chosen for the test was75 hp at 1,800 r/min base speed. For this testing, three mo-tors were manufactured using identical stator laminations

9

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96

98

1 10 100 1,000hp

% E

ffici

ency

PM

Premium EfficiencyIEEE 841

Energy Efficient

Efficiencies of low voltage, TEFC, 1,800 r/min motors.

10

92

93

94

95

96

97

98

0 20 40 60 80 100 120

% Load

0 20 40 60 80 100 120

% Load

% E

ffici

ency

Energy Efficient

Premium EfficiencyPM

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60

65

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75

80

85

90

95

% P

ower

Fac

tor

Energy Efficient

Premium EfficiencyPM

(a)

(b)

Typical partial-load (a) efficiencies and (b) power factorsof 75 Hp, TEFC, 1,800 r/min motors.

dcPM

11A 75-hp, TEFC PM motor (on the right) coupled to a dc-load motor.

8

dc BusVoltage

Trig?

178 V796 V

40.60%

M 100 ms ACh1Ch1 5.00 V 2.80 VCh2 2.00 VCh4 40.0 V

TeK PreVu

dcdddddddd BusssssssVoVoVoVoVoVoltll agaggagagagaggeeeeeee

Trig?

178 V796666666 V

40.60%

M 1MMMMMMMM 00 ms ACh1Ch1ChChCh1ChChChChCh1h1Ch1hChhhh1CCh1Chh1h1ChChhh1hh1hCChCCCChh 5.05 05 0005 000000 V0 V00 V000000000 2.80 VVVVVVVVVVVVVVVCh2CCCCCC 2.00 VCh4Ch4hh 40.00 0 V

TeK PreVu

Increase in dc-bus voltage due to inverter trip while at7,200 r/min on a 5,000 r/min base-speed PM motor trace 4:dc-bus volts, 40 V/Div.

THE RATINGCHOSEN FOR THETEST WAS 75 hp AT1,800 r/min BASE

SPEED.

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and core lengths. Every effort was made to minimize the var-iations in cooling while optimizing the performance of eachmotor. Since these motors were design for variable-speedoperation, all the test data are with the motors powered by apulsewidth modulation (PWM) inverter. After the testingof the surface PM motor resulted in a large gain in efficiency,it was decided to test the salient pole PM design at a yethigher torque rating, corresponding to the torque of a100-hp motor. The following sections will present the resultof each motor and a summary to compare the overall results.

Induction Motor Test DataThe first test is for an induction motor with a cast alumi-num rotor. To minimize material variations, the motorwas pushed to 75 hp. Typically, this motor would be ratedat 60 hp. Since the insulation system of this motor is classH (180 �C), the motor is capable of operation with a classH temperature rise (125 �C).

The frame size of the motor is FL2586, which is a NEMA250-frame shaft height, and is not designed to conform tothe NEMA rating per frame size table. A 250-frame NEMA,TEFC motor would have a maximum of 20 hp in this frame.The induction motor was wound as a four-pole motor with a60-Hz base frequency. The motor was dynamometer testedon a 100 hp, dc dynamometer per IEEE 112 Standard TestProcedure for Polyphase Induction Motors and Generators methodB. The test results for this motor can be found in Table 1.

Surface PM Motor Test DataThe second motor to be tested was the surface PM motor.The motor rating was identical with the induction motor

with the exception that the base frequency was 120 Hz.The decision was made in the development process towind this motor as an eight-pole motor. The surface PMeight-pole design had both performance and cost advan-tages over the surface PM four-pole design. The statorlamination was identical with the induction motor lami-nation and was such it could be wound for an eight-polesurface PM stator.

The stator core length and frame size were identicalwith the induction motor. The same fan, fan cover, andbrackets were used for this motor. The motor was also oper-ated on a PWM drive with different software for the PMmotor. As with the induction motor, the rating was 75 hpat 1,800 r/min. Figure 12 shows a picture of the surfacePM rotor.

The surface PM motor was load tested per IEEE 112method B (as modified for an inverter-fed PM motor) onthe same 100-hp dc dynamometer. The test results can befound in Table 2.

Salient Pole PM Motor Test DataThe final motor to be tested was a salient-pole PM design.Similar to the induction motor, this motor was designed asa four-pole motor. The motor actually used the same statorand a different winding than the induction motor. The fan,fan cover, and brackets were also identical with those usedin the induction and surface PM motor. Figure 13 shows a

12Surface PM rotor.

13Salient pole PM rotor.

TABLE 1. INDUCTION MOTOR-PERFORMANCEDATA.

Horsepower (heat run) 75.5 hp

Volts 459 V

Base frequency 60 Hz

Full-load amperes 92.3 A

Full-load speed 1768 r/min

Full-load efficiency 93.6%

Full-load power factor 82.0%

Full-load torque 224 lb/ft

Total motor losses 3.88 kW

Temperature rise by resistance 111.5 �C

TABLE 2. SURFACE PM MOTOR-PERFORMANCEDATA.

Horsepower (heat run) 75.4 hp

Volts 405 V

Base frequency 120 Hz

Full-load amperes 85 A

Full-load speed 1,800 r/min

Full-load efficiency 96.2%

Full-load power factor 98.1%

Full-load torque 220 lb/ft

Total motor losses 2.23 kW

Temperature rise by resistance 70.7 �C

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picture of the salient-pole PM rotor. The salient-pole PMmotor was also load tested per IEEE 112 method B. Testresults can be found in Table 3.

As seen in Table 3, this motor was also tested at thenext higher rating of 100 hp or a 33% increase in load.This increased load capability was met while still remain-ing within a class B temperature rise.

Comparison of Test ResultsIt can immediately be seen that a PM motor can provide amore efficient, higher power factor, cooler running motorfor this 75-hp, 1,800-r/min rating. As an alternative tosimply increasing efficiency and reducing temperaturerise, it can also be seen (through the salient pole PM testresults in Table 3) that a substantial rating increase is alsopossible. Test results comparing the three motors at a75-hp rating can be found in Table 4.

Comparative Power DensityThe data previously shown in Table 3gives an idea of the opportunity forincreased rating capability of a givensize of motor that is designed forPM rather than induction. The 33%rating increase is fairly typical insuch a comparison.

In Tables 5 and 6, a slightly dif-ferent approach is taken intocomparison with regard to ratingcapability or power density. In Ta-ble 5, it can be seen that, while aNEMA induction motor in a 256-Tframe would be rated 20 hp at1,800 r/min, a 250-T frame PMmotor can achieve a fivefold in-crease in this rating to 100 hp.Similarly, Table 5 also shows thatfor a 100-hp typical NEMA induc-tion motor, a 405-T frame wouldbe used, resulting in about twice

TABLE 4. COMPARISON OF TEST RESULTS.

Frame Type Laminated Steel

Rotor Type Induction Surface PM Salient Pole PM

Enclosure TEBC TEBC TEBC

Horsepower 75.5 hp 75.4 hp 75.0 hp

Volts 459 v 405 v 395 v

Full load speed 1,768 r/min 1,800 r/min 1,800 r/min

Base frequency 60 Hz 120 Hz 60 Hz

Full-load current 92.3 A 85.0 A 90.2 A

Full-load power factor 82.0% 98.1% 93.2%

Losses 3.88 kW 2.23 kW 1.85 kW

Full-load efficiency 93.6% 96.2% 96.8%

Full load torque 224 lb/ft 220 lb/ft 219 lb/ft

Temp rise 111.5 �C 70.7 �C 89.9 �C

TABLE 5. ENCLOSED MOTOR COMPARISON.

Frame Type Cast Iron Laminated Steel

Rotor Type Induction Surface PM Salient Pole PM

Enclosure TEFC TEFC TEBC TEBC

Hp @ 1,800 r/min 20 100 100 100

NEMA frame size 256 T 405 T 250 250

Weight 325 lb 1,160 lb 532 lb 532 lb

lb/hp 16.25 11.60 5.32 5.32

Full-load current 25.5 A 115 A 117 A 119 A

Full-load power factor 78.9% 86.4% 90.3% 93.4%

Full-load losses 1.116 kW 4.381 kW 4.12 kW 2.396 kW

Full-load efficiency 93.0% 94.5% 94.8% 96.9%

Rotor inertia 2.42 lb=ft2 26.1 lb=ft2 4.9 lb=ft2 4.9 lb=ft2

Temp Rise 66 �C 74 �C 120 �C 77.6 �C

TABLE 3. SALIENT POLE PM MOTOR-PERFORMANCEDATA.

Hp (heat run) 75.0 hp 100 hp

Volts 395 V 400 V

Base frequency 60 Hz 60 Hz

Full-load amperes 90.2 A 119 A

Full-load speed 1,800 r/min 1,800 r/min

Full-load efficiency 96.8% 96.9%

Full-load power factor 93.2% 93.4%

Full-load torque 219 lb/ft 292 lb/ft

Total motor losses 1.85 kW 2.40 kW

Temperature rise byresistance

59.9 �C 77.6 �C

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the weight of the equivalent rating supplied as a PMmotor in a 250-T frame.

Table 6 demonstrates a similarly dramatic improve-ment, this time with open motors rather than the enclosedmotors shown in Table 5. Table 6 also uses different horse-power and frame sizes for the comparison to demonstratethe scalability of the gains that can be made.

It can also be seen in these comparisons that theimproved power density is achieved without a sacrifice ofenergy efficiency, power factor, or temperature rise.

ConclusionsIt is clear that a premium will continue to be placed onenergy efficiency in motors. The circumstances aroundglobal warming and the availability of future oil suppliesonly increase the focus on electric motors as key elementsin the efficient utilization of energy resources. PMs offeran important tool in the quest for cost-effective ways tofurther increase motor efficiencies. The characteristics ofPM motors are sufficiently distinct from that of inductionmotors that users need to understand their operation toensure successful applications.

The nonsalient PM rotor construction provides highefficiency and high-torque density, whereas the salient-pole construction offers some distinct advantages, espe-cially with regard to system issues.

Full flexibility to use other than 50 or 60 Hz base fre-quencies to further optimize performance obviouslyrequires application with an inverter. Once the decision ismade to use an inverter, the variable-frequency degree offreedom, with regard to pole selection, works in favor ofthe PM motor design.

The improving performance to cost relationship of PMmaterials is getting to the point where the increased powerdensity of the PM motor can be large enough to offset thecost of the magnets. While the authors would not suggestthat PM motors are going to replace the widespread usageof induction motors, the PM motors provide an interest-ing alternative, especially in the case where an inverter is

to be used in the application. It is for those inverter-fedapplications that the first usage is expected in the pulpand paper industry.

RecommendationsAt this time, it would be prudent for NEMA and IEEEstandards and application guides to be revised to reflectthe application of PM motors. Areas for revision mayinclude starting performance plus integrated behaviorwith adjustable frequency controls. Standards and Appli-cation Guides, which should be evaluated for revision toprovide PM machine considerations, include NEMAMG1, IEEE 841, and IEEE 1349. Application guidelinesfor the use of machines without any slip would also beanother area to consider.

References[1] ‘‘EPACT legislation,’’ Congressional Rec., Jan. 1994.

[2] Motors and Generators, NEMA Standards Publication MG1-2003, Para-

graph 12.60.

[3] J. Malinowski, J. McCormick, and K. Dunn, ‘‘Advances in construc-

tion techniques of ac induction motors—Preparation for super-

premium efficiency levels,’’ in Proc. IEEE PCIC Conf., Sept. 2003,

pp. 1665–1670.

[4] Standard Specifications for Permanent Magnet Materials, MMPA Standard

100.

[5] N. Bianchi and T. Jahns, ‘‘Design analysis and control of interior PM

synchronous machines,’’ in Proc. IEEE Annu. Meeting, IAS Tutorial

Notes, Oct. 2004, pp. 2.1–2.22.

[6] R. F. Schiferl and T. A. Lipo, ‘‘Power capability of salient pole perma-

nent magnet synchronous motors in variable speed drive applications,’’

IEEE Trans. Ind. Appl., vol. 26, pp. 115–123, Jan./Feb. 1990.

Michael J. Melfi is with Baldor-Reliance in Richmond, Ohio.Steve Evon (stevon@baldor.com) and Robbie McElveen arewith Baldor-Reliance in Greenville, South Carolina. Melfiand Evon are Senior Members of the IEEE. McElveen is aMember of the IEEE. This article first appeared as ‘‘Perma-nent Magnet Motors for Energy Savings and Power Densityin Industrial Applications’’ at the 2008 Paper and PulpIndustry Conference.

TABLE 6. OPEN ENCLOSURE MOTOR COMPARISON.

Frame Type Cast Iron Laminated Frame

Rotor Type Induction Induction Salient Pole PM

Enclosure ODP ODP DPFV DPFV

Hp @ 1,800 r/min 5 50 50 50

NEMA frame size 184 T 326 T 210 180

Weight 69 lb 639 lb 363 lb 230 lb

lb/hp 13.8 12.8 7.3 4.6

Full-load current 6.4 A 59.2 A 60.6 A 56 A

Full-load power factor 83.1% 83.8% 85.7% 92.6%

Full-load losses 0.504 kW 2.21 kW 4.03 kW 2.32 kW

Full-load efficiency 88.1% 94.4% 90.2% 94.1%

Rotor inertia 0.3 lb=ft2 4.9 lb=ft2 2.32 lb=ft2 0.8 lb=ft2

Temp rise 69 �C 76 �C 79 �C 74 �C

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