Permanent Magnet Motors in Direct-Drive Applications

Toachieve systems design goals, where motors are involved, of increasedefficiency and improved power density, along with lower noise and variablespeed operating capability, technologies beyond induction motors should beconsidered. Permanent magnet (PM) motors have long been recognized as providinghigher efficiencies than comparable induction motors. However, limitations interms of motor control, as well as magnet material performance and cost, haveseverely restricted their use. However, dramatic improvements in magnetic andthermal properties of PM materials over the past 20 years have led to thedevelopment of synchronous PM motors that are now viable alternatives. Figures1 & 2 show typical efficiencies and power factors for various motor types[1].

Anotherinnovation to consider in your systems design project is laminated frame motortechnology. Laminated frame motors consist of a stack of laminationspermanently riveted under controlled pressure. The cast iron outer frame iseliminated, allowing more room for active (torque-producing) magnetic material.

Aparticular advantage of this construction is that the air used to cool themotor is in direct contact with the electrical steel. There is no thermalresistance path as that which exists in a traditional cast iron frame withcontact to the stator lams. The heat transfer mechanism in a cast iron framemotor is highly dependent upon the stator to frame fit. Laminated frameconstruction eliminates this issue.

Inrecent years, industry drivers have forced the development of an optimized,finned, laminated motor design. To improve the cooling and increase powerdensity, fins have been added to the exterior of the stator laminations. Theaddition of the optimized cooling fins increases the surface area available forheat dissipation. The result is improved heat transfer and a power increase of20-25 percent is typical for a given lamination diameter and core length.

Itis this improved cooling method, along with the higher efficiency and powerfactor achieved with the PM technology that allows for increased power densityin these motor designs. Power density is the key for being able to match theheight restriction of the existing gearbox.

RetrofitCase Study

Thiscase study involves the retrofit of an existing cooling tower constructed in1986 at Clemson University in South Carolina. The existing tower had:

Thetower is comprised of two identical cells. For this study, one cell was retrofittedwith the new slow speed PM motor and VFD while the other was left intact asoriginally constructed. This allows for a direct comparison of the two fandrive solutions. PM Motor image shows the PM motor installed in place of thegearbox.

Priorto the installation, the current being drawn by the two original inductionmotors was measured with the fans running at full speed. An ammeter was usedand the current was measured to be 47A, rms on both induction motors. As theinduction motors are identical, this is a good indication that both cells wereoperating under the same load conditions. After the PM motor and VFD installation was complete, the current was again re-checked and found to beonly 41A for the PM motor. The induction motor on the original, identical,tower was still drawing 40 47A.

Fromthis data, it was determined that both cells were running at less than fullload and that the load should be increased on each cell. To this end, the pitchof the blades on each fan was increased to 12 degrees. This change of pitchcaused the fans to draw more air, thus increasing the load on each motor.Further, the increased air flow improved the effectiveness of the overall towerperformance. Again, power measurements were made and a third party testingservice was engaged to verify the manufacturer's results.

Forthe final blade pitch, 4.5 kW less power consumption was observed on the cellwith the PM motor installed. The PM motor solution requires less input powerfor each load point (blade pitch).

ElectricalConsiderations

In addition to the PM motor design features alreadydetailed, another challenge of this application was that the PM motor had to berun sensorless. There was no room to install a speed feedback device, such asan encoder or resolver, and still meet the height restriction of the existinggearbox. In this harsh environment, a feedback device would be a liability asfar as reliability is concerned. Therefore, a sensorless PM control scheme was developedto satisfy the requirements of this application.

Several things had to be considered when forming thisalgorithm. One challenge was the inertia of the fan. This was taken intoaccount to prevent the motor from falling out of synchronism when starting andchanging speeds. Figure 3 shows a portion of a typical start from rest. Notethe smooth acceleration and low starting current required. A typical 480-Vinduction motor started across the line would draw 347A [2], compared to 12Afor this PM design started on the VFD.

The use of a VFD also provides the opportunity to offersome additional features that across the line systems do not. The drive may beconfigured to apply a trickle current to the motor windings to act as a brakeduring down time. This prevents the fan from free wheeling due to nominal windsor adjacent cooling tower turbulence. However, a mechanical locking mechanismshould be used during any maintenance procedures. This trickle current alsoacts as an internal space heater by raising the winding temperature, preventingcondensation when the motor is not running.

Inside the fan stack is an extremely humid environment.Therefore, the insulation system on the stator windings must be robust andhighly moisture resistant. To this end, an insulation system derived from asystem originally developed for use by the U.S. Navy was employed. This systemutilizes an epoxy compound applied via a vacuum pressure impregnation system.

MechanicalConsiderations

Dueto the harsh environment inherent with a cooling tower application, the motor'sdrive end is protected by a metallic, non-contacting, non-wearing, permanentcompound labyrinth shaft seal that incorporates a vapor blocking ring toprevent an ingress of moisture. This seal has been proven to exclude all typesof bearing contamination and meets the requirements of the IEEE-841 motorspecification for severe duty applications.

Another consideration on this project was overall systemmaintenance. For motor/gearbox combination drives, the lubrication interval isdetermined by the high-speed gear set. The recommended lubrication interval forthis type of gear is typically 2,500 hours or six months, whichever comesfirst. In addition, gear manufacturers recommend a daily visual inspection foroil leaks, unusual noises or vibrations. With the elimination of the high speedinput due to application of the slow speed PM motor design, the lubricationcycle can now be extended up to two years. The PM motor need not be inspecteddaily for oil leaks, as the motor contains no oil.

Withthe elimination of the high speed input to the gearbox, the system dynamicsfrom a vibration standpoint have been simplified. There are no longer anyresonance issues with the driveshaft. The maximum rotational excitation is nowlimited to the rotational speed of the fan. The number of bearings in the drivesystem has been reduced from six to two for a single reduction gearbox and fromeight to two for a double reduction gearbox. This reduces the number of forcingfrequencies present in the system.

Manycooling towers are in locations where airborne noise can be an issue, such ashospitals and universities. To this end, a third-party testing company wasengaged to conduct comparative sound tests between the two cells. Data wastaken at both high speed and low speed for both cells. At high speed, the PMmotor cell was 4.6 dBA lower than the induction motor cell. For low-speedoperation, the PM motor cell was 5.4 dBA lower. The removal of the high-speedinduction motor from the outside of the fan stack appears to have the biggestinfluence on the noise level of the tower itself.

References

[1]Steve Evon, Robbie McElveen and Michael J. Melfi, "Permanent Magnet Motors forPower Density and Energy Savings in Industrial Applications," PPIC 2008
[2]NEMA MG 1-2006, Motors and Generators

RobbieMcElveen, Bill Martin and Ryan Smith are senior development engineers at Baldor. The authorsextend their thanks to Clemson University and Tower Engineering Inc. for theircontributions and participation in the project.

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