High-current motors demand novel sensing technology

The increasing size of the currents in AC motors is making it difficult to use devices such as Hall sensors inside motors. One company affected by this trend is SKF which, for several years, has been producing combined sensor-bearings that use Hall devices to detect motor positions and speeds without needing vulnerable encoders.

When SKF first developed its sensor-bearings in the mid-1990s, typical motor currents were in the range 200-300A, allowing Hall sensors to be used safely inside the motors. But since then, currents have been creeping up, with many of today`s motors consuming 400-600A, and future machines expected to draw 1kA or more. The problem is particularly prevalent in the compact, high-power-density motors being developed for electric vehicles.

The fields created by these currents are powerful enough to interfere with, and distort, the outputs of the Hall sensors. SKF therefore started to look for a new sensing technology which would not be affected by the massive currents.

"We needed a real technological breakthrough," recalls Jean-Jaques Duquenoy of SKF`s sensors business unit. After considerable research and development work, the company has come up with a technology which, it believes, will be immune from the effects of large motor currents.

The technique is based on tiny high-frequency sensor coils which are excited inductively. The excitation patterns are modified by the presence or absence of metallic teeth on an "impulse ring" (shown in the cutaway above), based on the eddy current principle.

This creates a phase shift in the excitation patterns which is processed by a dedicated chip to generate digital outputs. This technique ensures that the signal detection and processing are not affected by the powerful fields in the motor.

According to Duqueroy, the new, maintenance-free sensing technique will open up new possibilities for motor designers, such as placing sensors inside electromagnetic brakes. It could even lead to new motor designs, he predicts.

SKF is targeting motors used in demanding applications such as forklift trucks and conveyors, as well as electric vehicles. At present the technology is being tested by some of SKF`s partners before being launched commercially early next year.