Dyson vacuums 104,000rpm brushless DC technology

Dyson has developed gone to 104,000rpm brushless DC technology to combine efficiency and manufacturability in its latest handheld vacuum cleaner.

"Because it is two pole, it is very simple, and when you make it run at high speed, you can make it incredibly small," Dyson's Andy Clothier told Electronics Weekly. "It is 84% efficient, which is high at this small size. At this voltage and power level, to get a brushed motor this efficient is very difficult. Our old motor was 40% efficient."

Overall the motor, dubbed DDM (Dyson digital motor) V2, is 55.8mm in diameter and weighs 139g.

Notoriously tricky to start predictably, the motor uses asymmetric poles. "You have to have enough saliency on the poles to make it start in the right direction," said Clothier.

With brushless motors, there is a choice: sensored or sensorless – incorporate a magnetic sensor have tell the electronics when to switch coil polarity, or use a more powerful processor and sense rotor position from back-EMF.

"The way we designed it was to integrate the electronics into the motor. It is the least expensive way of doing it," said Clothier. "The PCB is in exactly the right place to carry a Hall sensor."

Control comes from a simple 8-bit Microchip microcontroller, not one that has special motor control peripherals, said Clothier: "We used our own motor control technology. To get the absolute best, we make sure the motor produces constant power regardless of speed and the battery voltage."

The power control is largely open loop – determined from detailed knowledge of the motor and impeller dynamics, combined with motor speed derived from the Hall sensor. Up to 3,300 adjustment per second are made.

The battery is either six or four lithium ion cells depending on the vacuum cleaner model: DC31 (pictured) or DC30 respectively.

Up to 10A at 20V, and up to 13A when the battery voltage drops, is switched into the motor by an H-bridge of mosfets.

To get current to change direction fast enough with such a low supply voltage requires low-inductance windings – in this case twin coils wound in parallel.

The whole motor, and its mechanical and air environment, was modelled extensively.

"That is where most of the work went in: we developed our own simulation tools to model the whole motor including its electronics," said Clothier. "We also used some commercial finite element software for spot checks and detailed work, but 90% was designed by our own software."

Modelling, for example, showed the sintered neodymium permanent magnet rotor was small enough not need a carbon fibre sleeve to stop it flying apart at full speed.

"This is the kind of thing that looks simple and needs a lot of work," Mathew Childe told Electronics Weekly. "We modelled the motor dynamics and made sure it was stable against vibration right up through its acceleration range, checked the acoustic noise and checked the resonances."

The team also built prototypes that were tested using accelerometers and laser displacement instruments, then fed-back the results. "All the way through, you learn to improve and adapt the modelling process," said Childe.

High rotational speed put means the impeller can be small, but means it is subjected to high forces. "Most people would use aluminium," said Childe. "Through simulation we designed out as much stress as possible and so we can make the impeller out of carbon fibre-reinforced polymer."

A plastic impeller and steel shaft means welding is out of the question. "Everything in the vacuum cleaner is dependent on bonding," said Childe. "We have had an engineer working for two years on adhesives for the product."

The motor has been dubbed DDM (Dyson digital motor) V2.

What was effectively DDM V1 was actually dubbed X020 and is the switched reluctance designed used in the company's Airblade hand dryer.