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.