Comparing AC Induction With Permanent Magnet Motors In .

increase the total weight. The values shownare only looking at the active parts and don’tinclude housing, bearings, etc.Performance and Efficiency: The constraintsof the induction motor FEA simulation wereset to run the motor at its maximum efficiencyat all torque-speed points. This resulted inthe induction motor being run at its maximumvoltage condition. Doing this had the effect ofputting the induction motor in the best lightwith respect to the comparison to the PMACmotor outlined in a later section1.The following chart shows the intermittent (redline) and continuous (blue line) output torquefor the FEA generated induction motor:Figure 9: Intermittent and continuous power output of FEAoptimized vehicle-duty IM motor. (Graph source: ParkerHannifin)The torque and power of the induction motor,as indicated in Figure 8 and Figure 9, fallrapidly at the base speed. As a result, thepeak and continuous power values for theinduction motor just meet the design criteria.One of the next areas of performance toexamine is the efficiency of the IM. The nextgraph shows the efficiency map of the motor:Figure 8: Intermittent and continuous torque output of FEAoptimized vehicle-duty IM motor. (Graph source: ParkerHannifin)The next chart shows the intermittent andpeak power output of the same motor:Figure 10: Efficiency map of FEA optimized vehicle-duty IMmotor. (Graph source: Parker Hannifin)When comparing this to the PMAC motor(shown later), the induction motor has lowerefficiency across the entire operating region.This is due to the fact that the induction motorneeds to create both the rotor magnetic fieldand the stator magnetic field, as described in Copyright 2013 to the present, Parker Hannifin CorporationPage 6

the Operation section. Both magnetic fieldsare created from the circulation of currentthrough copper; this means there are I2Rlosses – that is the current squared multipliedby the resistance. This loss is present on boththe rotor and the stator in order to produce amagnetic field.The IM only has copper bars on the rotor andas a result, the motor can produce low torquevalues at high speed with very high efficiency.This is clearly seen in the figure. At highspeed and low torque both the stator field andthe rotor field can be very small, and hence,the magnetic losses are very low.Cost: The cost of the induction motor isperhaps the strongest benefit. Traditional IMmotors are readily available and have largeglobal usage. Vehicle rated motors are not asavailable and have some higher costs due tomore strenuous testing and environmentalrequirements. The active materials, however,remain the same between the two.Construction: Permanent magnet (PM) motorshave not been around nearly as long as IMs.There are a couple variations of PM motors –brushed DC and brushless AC.Brushed DC motors are readily available andhave been built for a long time. They arefound in everything from small toys toindustrial equipment. DC motors have copperwindings on the rotor and magnets in thestator. They are not a typical choice forvehicle applications – though they are found.Brushless PMAC motors are built with copperwindingswrappedaroundindividuallaminations. These copper wire assembliesmake up the diameter of the stator.The following diagram shows the activecomponents of a brushless PMAC motor:The following table outlines the cost of theactive materials of the IM as a percentagewhat a comparable PMAC motor might cost(discussed later). It is assumed that thePMAC motor equals 100%:Figure 11a: Internal permanent magnet motor active parts.(Drawing source: Infolytical Corporation)Table 4: Cost of active components within a vehicle dutyinduction motor relative to a comparable performing PMACmotor.Table 4 shows that the costs of the activecomponents of the IM motor sum to about26% less than the equivalent PMAC motor.The primary reason for the lower cost is dueto copper bars on the rotor instead ofmagnets.2. Permanent Magnet MotorsFigure 11b: Magnetic flux paths for a PMAC motor. (Drawingsource: Parker Hannifin)There are many different configurations of therotor for PMAC motors. But, one can see thatthe magnets are positioned within the rotorcore (laminations) in a similar manner as theIM. The advantage is that the magnet has itsown permanent magnetic field and does not Copyright 2013 to the present, Parker Hannifin CorporationPage 7

require any additional current to generate thefield. This characteristic is the reason formany of the PMAC advantages.Figure 12 shows the entire motor assemblyfor a PMAC motor.Table 5: Active dimensions of vehicle duty PMAC motor.The next table sums the entire weight of theactive materials in the PMAC motor:Table 6: Total weight of active components within a vehicleduty PMAC motorAs with the IM, there will be additional weightlength added to the PMAC motor whenadding a housing, bearings, cooling, etc.Figure 12: Brushless PMAC motor full assembly, cutaway(Drawing source: Infolytica Corporation).Operation: Brushless PMAC motors work on asimilar principle as the IM, however, there isan energy savings because the magnets havea permanent field at the rotor, where IMsrequire the electronics to push additionalenergy into the copper bars of the rotor togenerate the field.Performance and Efficiency: The followingtwo figures outline the torque and powercapabilities of the brushless PMAC motor.Continuous torque is shown in blue andintermittent torque in red.The motor controller is connected to the motorand pumps voltage and current into thecopper windings. As the voltage and currentchange in the windings, so also are the northand south poles. They switched from onestator “tooth” to the other, and the rotor isattracted to the moving magnetic stator field.This causes the torque and rotation of thePMAC motor.Size: The brushless PMAC motor is verycompact due to the magnets in the rotor. Thereduction of both mass and volume is anattractive feature when space is a premium.Recall from Table 1 the specifications for thevehicle. The following table outlines theoverall size for the active components of aPMAC motor that meets the vehiclerequirements.Figure 13: Intermittent and continuous torque output of FEAoptimized vehicle-duty PMAC motor. (Graph source: ParkerHannifin)Continuous power is shown in blue andintermittent power in red. Copyright 2013 to the present, Parker Hannifin CorporationPage 8

However, it was mentioned that IM did have ahigh region of efficiency at low torque andhigh speed. When examining the Figure 14,the PMAC motor has significant losses in thissame region ( 60Nm & 4000rpm) becausethe rotating field from the rotor magnetsproduces losses in the stator.Figure 13: Intermittent and continuous power output of FEAoptimized vehicle-duty PMAC motor. (Graph source: ParkerHannifin)This motor maintains a very flat torque profilewith speed. Because of this, the peak andcontinuous power exceed the original targetvalues by significant margins.Cost: The components used in the PMACmotor are very similar to those used in theinduction motor: Copper wire, statorlaminations and rotor laminations. However,instead of the copper bars in the rotor, thePMAC motor has permanent magnets. Thesemagnets are made of rare earth materials thatare more expensive than copper. This addscost to the motor, but is the responsiblecomponent for the reduced size and addedefficiency.The efficiency plot of the PMAC motor isanother aspect that adds value. The followinggraph shows efficiency over the operatingregion:Table 7: Cost of active components relative to total within avehicle duty PMAC motor.2. Summary of IM and PMAC motorsThe last several sections discussed thedifferences in performance, efficiency, costand construction of both IM and PMACmotors.Figure 14: Efficiency map of FEA optimized vehicle-dutyPMAC motor. (Graph source: Parker Hannifin)The following table summarizes the size andweights:The graphic shows that the PMAC motor ishighly efficient over a significant portion of theoperating region – much more so than the IMmotor.The added efficiency is due to the constantmagnetic field being present in the magnets.This eliminates the I2R losses that penalizedthe IM.Table 8: Summary of IM and PMAC weights, size and volume. Copyright 2013 to the present, Parker Hannifin CorporationPage 9

The IM is a bit shorter, but is significantlyheavier and it consumes nearly twice thevolume of the PMAC.The followingperformance:tablesummarizestheTable 9: Summary of IM and PMAC performance relative to thespecification.Finally, the cost comparison between the IMand the PMAC:though the IM is the logical choice due to thelow initial cost – assuming it met the specs.However, taking a closer look at how thecustomer plans to use the motor can shedsome additional light on each motor.Before examining a full operation cycle, a fewpoints will be examined more closely: Hightorque, low speed (Table 11), Mid torque, midspeed (Table 12) and low torque, high speed(Table 13).Low Speed, High Torque: The first item thatstands out at Operating Point #1 is that theresistive losses of the induction motor isabout 3 times higher than the brushlessmotor.Table 10: Summary of IM and PMAC costAt first glance, we notice the following:1. IM takes up more space than PMAC2. PMAC is lighter than IM3. Both IM and PMAC meet performance4. Maximum efficiency is very close5. IM costs 26% lessIf the size of the motor itself was not aconcern, both motors on the surface seem tomeet the specifications. But, the InductionMotor costs less.Initial conclusion: Induction motor wins due tocost.However, what happens when each of thesemotors are placed into the drive cycle of thevehicle?Value-in-use: The Drive Cycle1. Importance of the operation cyclesTheprevioussectionexplainedthedifferences of the induction motor and thePMAC motor. On the surface, it looked asTable 11: Operating point 1: low speed, high torque and lossesfor IM and PMAC motors4.This is caused by the fact that enough currentneeds to be supplied in order to create boththe rotor and the stator magnetic fields. Sincethere is no field created by a magnet, veryhigh currents can be delivered to theinduction motor without causing thelamination material to saturate. The inductionmotor has no problem producing the peaktorque with minimal magnetic saturation;however, due to the extreme losses, theinduction motor cannot produce the peaktorque for very long or the motor will overheat.If a long duration peak torque is required fromthe induction motor the motor will need toincrease in size.Mid Speed, Mid Torque: Table 12 shows thatthe brushless motor losses are extremely lowin the middle region of the speed torque Copyright 2013 to the present, Parker Hannifin CorporationPage 10

curve. This contributes to the motor’s veryhigh efficiency.heat that must be dissipated by the coolingsystem.If the vehicle has a battery powering themotor, the losses are an expense that mustbe examined. These losses directly impacthow long the motor can run given a fixedamount of energy in a battery pack.Table 12: Operating point 2: Mid speed, mid torque and lossesfor IM and PMAC motors4.This point improves the IM’s efficiency quitesubstantially. Still, the IM has twice as manylosses as the PMAC – and losses meanadditional heat that needs to be delivered tothe cooling system.High Speed, Low Torque: Table 13 showswhere the induction motor shines.Drive Cycle: Next, the IM and PMAC motorFEA models were used to simulate theperformance of a full electric vehicle.The magnetic FEA tool was used todetermine the torque produced by the motorand the magnetic losses experienced by eachmotor at each point in time. Interpolationbetween FEA solutions was performedbetween torque and speed operating pointsthat were close to each other in order toreduce the calculation time.Three different drive cycles were examined:“City”, “Rural”, and “Highway”.Table 13: Operating point 3: High speed, low torque andlosses for IM and PMAC motors4At low values of torque and high values ofspeed the induction motor outperforms thePMAC motor. The very low flux density inboth the stator and the rotor will keep the totallosses to a minimum at this point.There is one more noteworthy take away fromthese operating points: rotor losses. Theinduction motor has significantly higher rotorlosses than the PMAC motor – almost 50times larger in Point #1. Most of these lossesfrom the induction rotor are derived from theresistive losses (I2R) in the copper bars. Littlecan be done to reduce these losses. ThePMAC motor, however, can be designed tohave ultra low rotor losses and thus be moreefficient at more operating points.These three different drive cycles were basedon data collected from three real life drivingscenarios. The drive cycles are set such thateither the IM or PMAC motor can execute thecycle. Also the vehicle is simulated with asingle-speed fixed ratio transmission. Table14 outlines the vehicle data used:Table 14: Vehicle data for a Class 2 or 3 delivery vanThis vehicle is representative of light duty,Class 2 or 3, sized delivery van.Drive Cycle - City: The city drive cycle ispresented in Figure 15. The average speedis less than 7 MPH and the vehicle starts andstops are very frequent.These three operation points show that wherethe motor is operated can significantly impactthe losses in the motor and consequently the Copyright 2013 to the present, Parker Hannifin CorporationPage 11

Figure 15: City drive cycle. (Graph source: Parker Hannifin)This is typical of city driving in moderatetraffic. For city diving simulations, this 440second segment is repeated for a duration ofone hour. Table 15 contains the result ofrunning the drive cycle for both motortechnologies.stator at high torque. In addition, the vehiclewith the induction motor consumed slightlymore energy, 2.85 kW*hr/hr versus 2.82kW*hr/hr, as compared to vehicle with thePMAC motor. This slight difference was dueto the small increase in vehicle weight due tothe heavier induction motor. This indicatesthat there is a penalty for carrying extraweight; however, the penalty is small forlarger vehicles that carry heavy loads.Drive Cycle - Rural: The Rural drive cycle ispresented in Figure 16. The average speed isabout 30mph, and vehicle starts and stopsare less frequent.Figure 16: Rural drive cycle. (Graph source: Parker Hannifin)Table 15: Losses and battery energy used on 1hr drive usingthe city cycle dataIt was assumed that all kinetic energy of thevehicle was captured and sent to the batteryduring every speed reduction, minus themotor losses and mechanical losses duringregeneration. The values in the table arekW*hr of energy used for every hour ofdriving.This is typical of driving in and around a ruralneighborhood. For rural driving simulations,this 1070 second driving segment is repeatedfor a duration of one hour. Table 16 containsthe results for the two motors executing therural drive cycle.In general, the Induction motor lossesconsumed 34.6% of the total battery energy,whereas the PMAC motor consumed only17.3% of the battery energy.The total energy used for the one hour citydrive was 27.8% less for the PMAC motor. Orstated another way, the PMAC motor wouldbe able to propel the vehicle 27% furtherthan the same vehicle on same route with anIM.The biggest detriment to the induction motoris the very high resistive losses in the motorTable 16: Losses and battery energy used on 1hr drive usingthe rural cycle dataThe total battery usage is very close whencomparing the two motors. The biggestconsumers of battery energy in the vehicleare rolling friction and aerodynamics forces.The motors consumed less energy because Copyright 2013 to the present, Parker Hannifin CorporationPage 12

the starting and stopping was less frequentand the average speed was higher.Drive Cycle - Highway: The Highway drivecycle is presented in Figure 17. The averagespeed is about 57 MPH. The vehicle stopsare very infrequent; however, speedadjustments are frequent and gradual.Figure 17: Highway drive cycle. (Graph source: ParkerHannifin)This is typical of driving on a highway withmoderate traffic. For the highway simulations,this 1070 second driving segment is repeatedfor an equivalent of one hour of driving inthese conditions. Table 17 contains theresults of the two virtual motors executing thehighway drive cycle.Table 18: Summary of battery energy used on 1hr drive usingeach of the three cyclesIn the case of both the Rural and Highwaycycles, each motor seems to use about thesame amount of battery energy. On thosetwo cycles, there is not a significant amount ofdifference between the IM and PMAC motor –only 1%.This was observed in spite of the apparentadvantage PMAC efficiency map (Figure 14)suggested over the IM map (Figure 10). Thisbecomes more obvious once one realizes thatthe vehicle losses consume the most energyat elevated speeds.The contrast between the city cycle and theother two is quite significant. The City Cyclefavors the PMAC motor by a great margin.The PMAC motor consumes significantly lessenergy than the IM. If fact, the PMAC motorwould allow 28% more drive time under thesame conditions.This has significant impact on the batterypack costs. If a vehicle is to be operated overan eight-hour shift, the battery pack requiredfor a truck with an induction motor or a PMACmotor is quite different.Table 18: Battery pack size required and associated cost foreach motor technologyTable 17: Losses and battery energy used on 1hr drive usingthe highway cycle dataLike the rural cycle the total battery usagewas nearly the same for the two motors.Again, this is due to the vehicle itself beingthe largest consumer of the battery power.Drive Cycle – Summary: The following tablesummarizes the three drive cycles results.Table 18 shows the difference in battery packneeds for both the PMAC motor and IM.These two battery packs would allow thevehicle to make it through an 8-hour shift.The OEM could reduce costs by 2,280 if thepack size was reduced and the PMAC motorwas used. The difference in pack costs wou

forward with an electric hybrid program, there are two major choices for electric motors: induction motors (IM) and permanent magnet AC (PMAC) motors. With that in mind, a finite element program is used in order to compare the performance of the two types of motor designs – the IM and PMAC motor. The results presented in the