Posted on 30 July 2015

How Proper Applications of Thermally Conductive Materials will Improve Motor Power Density

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Motor designers have battled heat in motor designs for years. Thermal losses in electric machines rob motion systems of power and degrade efficiency. Excess heat can reduce reliability of motors and shorten their lifetimes. Meanwhile, electrification in all transportation sectors is driving requirements for motors with ever-higher power densities.

By Anita LaFond, Senior Editorial Manager, Constructive Communication, Inc.

Good thermal management in electric machines and their power electronic drives can minimize losses, particularly copper (I2R) losses, and yield improved performance, reliability and efficiency. Research shows a potting or encapsulation process using high thermal conductivity material from LORD Corporation can dramatically decrease the operating temperature of an electric machine at a given load. Proof-of-concept research explored the benefits of potting motor end windings and power electronics with thermally conductive materials. Results showed significant decreases in operating temperature that correlate to higher motor efficiency and double-digit increases in output power.

The ability of high thermal conductivity materials to improve performance in electric machines was demonstrated during research conducted by Shafigh Nategh while a graduate student at KTH School of Electrical Engineering, Stockholm, Sweden. Nategh’s research, “Thermal Analysis and Management of High-Performance Electrical Machines,” ( http://kth.diva-portal.org/smash/get/diva2:623376/ FULLTEXT01 ) dealt with thermal management aspects of electric machinery used in high-performance applications with particular focus on electric motors designed for hybrid electric vehicle applications.

LORD Thermoset SC-320 Thermally Conductive Potting Material, a relatively soft, high thermal conductivity material (3.2 W/m∙K) with sufficiently low viscosity to be used in vacuum potting, was evaluated in Nategh’s research. The electrically-insulating silicone encapsulant, designed for electrical/electronic applications, offers excellent thermal conductivity properties while retaining the desirable properties associated with silicons. Thermoset SC-320 is a two-component system that exhibits low shrinkage and stress on components as it cures, and maintains a low viscosity for ease of component encapsulation. Environmentally resistant and UL-rated to UL94V0 and 180C RTI, Thermoset SC-320 is composed of an addition-curing polydimethyl siloxane polymer that will not depolymerize when heated in confined spaces.

In the first part of Nategh’s thesis, new thermal models of liquid cooled (water and oil) electric machines (i.e., motors) were proposed based on a combination of lumped parameter (LP) and numerical methods. A permanent-magnet assisted synchronous reluctance machine (PMaSRM) equipped with a housing water jacket, as well as an oil-cooled induction motor where the oil was in direct contact with the stator laminations, were evaluated. In the second part of the thesis, the thermal impact of using different winding impregnation and steel lamination materials was evaluated.

Conventional varnish and epoxy, as well as a silicone-based thermally conductive impregnation material (LORD SC-320), were investigated and the resulting temperature distributions in three small permanent magnet motors were compared. The thermal impact of using different steel lamination materials was reviewed by simulations using the developed thermal model of the water-cooled PMaSRM. The differences in alloy contents and steel lamination thickness were studied and a comparison between the produced iron losses and the resulting hot-spot temperatures was outlined.

In comparing the effects of various impregnation materials, the hot spot temperatures of the windings were evaluated under various coolant flow rates and current levels for each of the potted motors. The hot spot temperatures of the motor impregnated with LORD SC- 320 were generally 40-degrees-C to 45-degrees-C cooler than the varnish-only motor, and about 12-degrees-C to 15-degrees-C cooler than the epoxy-potted motor.

Motor output power chart

No difficulties in vacuum filling were noted with LORD SC-320 despite its viscosity being somewhat higher than those of the epoxy and varnish. In addition, the potting compound was effective at decreasing the hot spot temperatures even when the potting was not 100-percent dense. For example, the hot-spot temperature of motors potted with LORD SC-320 increased by only 3-degrees-C as the potting density was decreased from 80-percent to 50-percent, whereas the hot spot temperatures of epoxy- and varnish-potted motors increased by 19-degrees-C and 25-degrees-C, respectively, as their potting densities were lowered from 80- percent to 50-percent.

Nategh’s research showed that hot spot temperatures can be reduced by 35-percent to 50- percent using LORD SC-320 material as compared to an un-potted motor, compared to improvements of only 20-percent to 30-percent using typical epoxy potting materials. The potting compound may provide significant improvements in power density of electric motors.

A decrease in the hot-spot temperature of motors, depending on current, may provide an increase in achievable power/torque for a given motor size, decrease in motor size for a required power/torque, and longer motor operation before reaching temperature limits.

Analyzing the Test Data

Additional analysis of the data found in Nategh’s dissertation was performed at LORD Corporation. Hot-spot temperatures from the dissertation were tabulated based on the potting materials thermal conductivity (varnish only, 0.25 W/m·K; Epoxylite, 0.9 W/m·K; and LORD SC-320, 3.2 W/m·K), coolant flow rate, and applied current. Data were analyzed as a three-variable general factorial experimental design using common statistical analysis software. The results showed that end winding temperatures depended strongly on the current and thermal conductivity of the impregnation material, but there was no statistically significant dependence on coolant flow rate.

Average end winding temperature chart

Statistical analysis further showed that with the LORD SC-320 potting compound, the average end-winding temperature was 25-degrees-C to 45-degress-C lower than the varnish-only motor; and 10-degrees-C to 15-degrees-C lower than the epoxy-impregnated motor. Compared to varnish alone, the estimated achievable current required to reach the maximum temperature of 150-degrees-C is 14-percent higher with Epoxylite and 26-percent higher with LORD SC-320.

Since the current is directly related to the torque, improved thermal performance of the potted motors offers several benefits:

  • Higher Power Density – more torque/horsepower from a similarlysized motor; the same torque/horsepower from a smaller motor; and the ability to remove copper or steel from the motor
  • Better Reliability – Low-temperature operation prevents insulation from degrading quickly, increasing the lifetime of motor insulation
  • Lower Copper Losses – Lower operation temperatures translates to lower copper losses; less resistance to current flow = lower I2R losses

Independent Potting Study

To independently verify the efficacy of using high conductivity thermal materials to improve motor performance, LORD Corporation worked with Advanced MotorTech, an engineering consulting and services company. Advanced MotorTech is involved in the research, development, fabrication, testing, and re-design of industrial and high-performance machines.

Advanced MotorTech tested the potting compound on a different type of motor from the ones used in the LORD Corporation research. They purchased Class F, 3 hp, 182T frame, totally enclosed, fan-cooled (TEFC) motors with a rated rpm of 1760; voltage: 208-230/460, threephase, 60 Hz; full-load current: 8.4-7.8/3.0. For the test procedures, they knew that using a resin in a standard motor could improve a motor’s performance by making the motor run cooler or making it run harder without exceeding its temperature rating. Advance MotorTech went into the testing with no preconceived notions of the outcome.

Average coil temperature drive chart

Average coil temperature fan chart

For testing purposes, the motors were coupled to a DC generator. A dynamometer accurately controlled the power output of the generator, and a digital meter provided readout of the torque transducer. The testing was done in a before-and-after manner; thermocouples were added to the motor before the epoxy was added to insure that the testing was done on an equal-match basis.

Six thermocouples were installed on each motor end (drive-end and fan-end), two per winding base, for a total of 12 thermocouples measuring winding temperature. Thermocouples were also added to the motor skin to monitor the outside temperature. Four “as-received” motors were tested using a “quick, full-load test.” From that testing, two motors with similar current, power and temperature rise were selected for subsequent tests.

A controlled test for equal comparison was done on the unpotted motors. To determine an accurate comparison between the motors, the testing was accomplished in the same room, with the same load, generator and meters; and the same ambient temperature (24.8-degrees- C). During the testing, the motors were run until they were thermally stable. Temperatures rose in the motors over time. The fan-end ran cooler and the shaft-end ran warmer, and both motors performed according to the manufacturer’s instructions.

Of the two motors chosen for the testing, one was left unpotted and one was encapsulated with LORD SC-320 silicone. For the potted motor testing, Thermoset SC-320 resin was mixed with Thermoset SC-320 hardener. The silicone can be applied with handheld cartridges or automatic meter/mix/dispense equipment. A vacuum was used to minimize air bubbles. The encapsulant was allowed to cure for 24-hours at room temperature. It can also be cured faster at 125-degrees-C for 60-minutes. After curing, the insert was removed and the air gap cleaned. During the encapsulation process, workers at Advanced MotorTech found that Thermoset SC-320 exhibited none of the “strong” odor or “eye-irritating” fumes commonly noticed in other epoxy compounds.

The same load conditions were used for the potted vs. unpotted motors – a full-load test (100-percent torque). Testing results showed that the average overall temperature was 7-degrees-C cooler with the potted motor (54-degrees-C compared to 61-degrees-C for the unpotted control motor). The temperature difference between the fan and drive sides was only 3-degrees-C compared to 5-degrees-C to 6-degrees-C for the unpotted motor. Therefore, the potted motor, encapsulated with Thermoset SC-320, ran cooler than the unpotted motor.

Both motors were also tested at overload conditions (164-percent and 174-percent rated torque) to determine the thermal effects under extreme conditions. The potted motor averaged 30-degrees-C cooler than the unpotted motor at the highest torque. This is a significant difference, and could lead to a projected eight-time longer dielectric life. The temperature difference between the drive and fan sides was less in the potted motor (ΔT = 9-degrees-C compared to 16-degrees-C for potted compared to unpotted motors, respectively).

The potted compared to the unpotted motors also exhibited a dramatic difference in skin temperatures. Testing showed that, while the average skin temperatures were about the same, the potted motor had only an 8-degree-C difference between the hottest and coldest spots, whereas the unpotted motor had a 20-degrees-C difference.

Thus, there was a more even distribution of skin temperature in the potted motor. This can be attributed to the air-gap-filling properties of the silicone resin. Thermoset SC-320 evenly and smoothly fills in all air gaps around the end windings. This results in even differential temperatures around the motor’s frame, and helps to mitigate any stress and performance issues that might be created as the motor operates.

Conclusion

The results of testing both at LORD Corporation and at KTH clearly show the benefits of potting with thermally conductive materials. Motors potted with Thermoset SC-320 offer more torque and horsepower at a given temperature and should provide higher reliability and lower copper losses at the same torque. The tests demonstrated a 30-degrees-C difference at the highest applied torque and load, and an improvement in copper losses of about 10-percent. Alternatively, at the same highest temperature, there was a 16-percent improvement in torque with the potted motor.

Developed to provide exceptional thermal conductivity for electrical/ electronic encapsulating, Thermoset SC-320 also retains the desirable properties associated with silicones. The low-viscosity encapsulant provides better flow-ability while reducing the risk of air entrapment. It is designed with lower durometer to reduce mechanical stress on electronic components and lower the probability of failure. Its low coefficient of thermal expansion results in lower stresses due to temperature change.

Possible applications for SC-320 include those in which higher power at lighter weight is needed, such as motors for electric vehicles; aerospace actuators and motors; and portable power generation equipment. Better thermal management in electric motors can reduce the current, and therefore, the energy required to provide the necessary power; and can also extend a motor’s lifetime. Cost savings may also be possible due to the reduced amount of copper wire required for the motor windings. Thus, savings in weight, energy, and/or cost may all be possible depending on the application.

LORD Corporation is continuing to collaborate with development partners to verify the benefits of using high thermal conductivity materials in motor potting applications. These benefits include: higher power densities - reduced copper and steel weight via thermal potting material; improved reliability – substantially lower operating temperatures and elimination of hot spots; and improved efficiency.

 

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