Think thermal to increase motor efficiency

Aug. 1, 2009
Segmented lamination stator technology delivers higher torque and power density, better thermal efficiency, cooler operation, and lower energy costs compared to traditional brushless dc motor design.

Basic thermodynamic laws tell us that, no matter what its design, in every electric motor some of the input electrical power is undoubtedly lost (via heat) as it is converted to output moving power. The more power lost, the higher the operational and environmental costs, which is why a motor's thermal efficiency is key.

With high efficiency, not only does an electric motor waste less power, it also requires less power for a given application. One way that engineers can find a proper balance is to consider using segmented lamination stator technology in place of traditional brushless dc motor design. This technology can pack 40% higher torque and power density into the same-sized motor. It also allows the motor to run cooler, potentially extending its operational life.

How stator design improves motor efficiency

Image 1

A brushless dc motor is a synchronous electric motor powered by direct current electricity that has an electronically controlled commutation system, in which the permanent magnets, fixed to the rotor, rotate within a static stator. Brushless dc motors provide an efficient solution for converting the input electricity into output mechanical power, specifically in applications requiring velocity, position, and torque control.

High slot fill, reduced end turns improve performance

When segmented lamination stator technology is incorporated into the motor design, it creates a unit with higher power density along with better heat transfer within a smaller overall package. This design also produces lower winding resistance, which reduces the heat generation inside an electric motor to maximize motor efficiency. Segmented lamination stator technology also incorporates higher slot fill with reduced end turn waste and uses thermally conductive epoxy potting to produce higher power efficiency.

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Epoxy encapsulation, minimized wire strain

Segmented lamination stators allow for 20 to 30% higher slot fill with larger diameter copper wire, compared to more traditional brushless dc motor stators of equal size. The corresponding reduction in winding resistance boosts the motor's current rating, horsepower, and torque output. The stator technology also reduces end turn waste. The end turns of a traditionally wound brushless motor do not provide additional power or torque, but rather make a motor less efficient and generate unnecessary heat. The end turns are also the most susceptible area to heat and voltage damage because they are surrounded by air — without a good thermal path for heat to escape. This damage can create a short in the windings, rendering the motor inoperable. Forming or compressing the end turns reduces their size, but also puts significant strain on the wire that can potentially lead to insulation breakdown.

Motors using the segmented lamination stator technology generally have at least 10% less end turn waste compared to a traditional brushless dc motor. Along with improved heat transfer, this reduces the end turn's susceptibility to heat or voltage damage. In addition, reduced end turns reduces overall stator length, making the motor more compact and easier to integrate into tight areas without wire strain.

A segmented lamination stator is completely encapsulated in a thermally conductive epoxy to provide reliable operation despite high temperatures. Combined with plastic lamination caps and slots insulated with flame-resistant material, this stator design provides a Class 180(H), 460 Vrms (650 Vdc bus) rating. It also provides for a motor temperature rating of 180° C, compared to the 155° C temperature rating commonly associated with traditional brushless dc motor stators.

The stator's end turns are also completely encapsulated in the thermally conductive epoxy, which further increases motor thermal efficiency and protects the end turns from overheating. Greater thermal efficiency also means that the motor runs cooler at a given power output, which may result in longer motor life.

Another difference: Traditional brushless dc motor wires from one phase come into contact with wires from the other two phases within the same area. This leads to measurable voltage differences between adjacent phase end turns, thereby leading to potential motor failure, as well as significantly stressed insulation. In contrast, in a segmented stator, only one wire passes from phase to phase, resulting in negligible voltage differences between these wires. The wire used to wind segmented stators is heavy build, inverter-grade wire, suitable for even IGBT-based servo amplifiers.

Designed to conserve labor

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To create the least amount of stress to the individual phase wires in a segmented stator, the wire is externally wound on a straight stack of laminations. Because complete phases are wound using a single contiguous wire, this design removes any current-carrying (but potentially unreliable) solder joints from the stator winding. This straight winding design, paired with minimized end turns — with no compressing or forming — provides exceptionally effective phase insulation. In comparison, the phase windings in traditional brushless dc motors are wound externally of the stator and inserted after each coil is wound, creating high stress on the wires.

Traditional brushless dc motors often require that the lamination stack be skewed the width of one slot to reduce cogging torque, further stressing the wires. The straight coil design of a segmented stator eliminates this need to skew, resulting in less strain on the wire during assembly. Cogging torque is still minimized while usable torque is maximized through the magnet and lamination geometry.

Besides saving energy costs through improved thermal efficiency and operational life, segmented lamination stator technology can also reduce labor and replacement costs. The design eliminates many of the difficulties associated with insulating inserted coil-type brushless dc motors, which can lead to voltage-related failures. In addition, the “concentrated” winding allows for maximum flexibility by allowing custom windings to be designed and manufactured in short time periods, and without the need for special tooling, as the winding is done on a simple bobbin winder.

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