General > General Technical Chat
How Does the Dyson Motor Work?
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rob77:

--- Quote from: Hero999 on October 23, 2016, 09:07:46 pm ---Technically it's not a DC but an AC motor.

--- End quote ---

yes correct, brush-less DC motors are technically AC motors.. everyone knows that, yet everyone calls them brush-less DC motors ;) and technically the brush-less DC motors are the closes relatives to stepper motors. and technically stepper motors are also AC motors... aren't they ?

the reason for that might be that a brush-less DC motor requires a controller to be operated (which does the switching and also might do sensing of rotor position), therefore the motor and the controller can be considered as a single unit - and that unit is fed by DC.
stj:
good, now that that is settled.  ;)
how do they keep the bearings from being destroyed at that speed?
Zero999:

--- Quote from: rob77 on October 22, 2016, 09:53:32 pm ---
--- Quote from: mikeselectricstuff on October 22, 2016, 09:43:02 pm ---I'd imagine that at high speeds, a strong permanent magnet  might cause eddy current losses.
With only 2 connections I'm curious as to how they make sure it starts in the right direction.

--- End quote ---

personaly i would give it a nice pulse to make it turn and measure the back-emf till zero cross - if it goes from positive to zero or negative to zero would tell the direction.. counting 2 zero crosses would give the speed - and with a precisely timed pulse you would change it's direction if needed...and then start to comutate to spin it up (pulse, measure back emf till zero cross, comutate , measeure back-emf...etc...)

--- End quote ---
I don't see how you could sense the direction of a symmetrical single phase motor, without another winding slightly out of phase, a Hall effect or optical sensor. When the motor is unpowered, the pole of the rotor will be aligned with the stator. If a field is applied to the stator to flip the pole, it could go in either direction and the generated back EMF will be identical, thus there is no way to control the direction.

If the motor is made to be asymmetrical, then it could be done but then it's easier to just design the motor so it always starts in the same direction, without any complicated software control algorithm.
rob77:

--- Quote from: Hero999 on October 24, 2016, 08:07:00 am ---
--- Quote from: rob77 on October 22, 2016, 09:53:32 pm ---
--- Quote from: mikeselectricstuff on October 22, 2016, 09:43:02 pm ---I'd imagine that at high speeds, a strong permanent magnet  might cause eddy current losses.
With only 2 connections I'm curious as to how they make sure it starts in the right direction.

--- End quote ---

personaly i would give it a nice pulse to make it turn and measure the back-emf till zero cross - if it goes from positive to zero or negative to zero would tell the direction.. counting 2 zero crosses would give the speed - and with a precisely timed pulse you would change it's direction if needed...and then start to comutate to spin it up (pulse, measure back emf till zero cross, comutate , measeure back-emf...etc...)

--- End quote ---
I don't see how you could sense the direction of a symmetrical single phase motor, without another winding slightly out of phase, a Hall effect or optical sensor. When the motor is unpowered, the pole of the rotor will be aligned with the stator. If a field is applied to the stator to flip the pole, it could go in either direction and the generated back EMF will be identical, thus there is no way to control the direction.

If the motor is made to be asymmetrical, then it could be done but then it's easier to just design the motor so it always starts in the same direction, without any complicated software control algorithm.

--- End quote ---

yes you're right about the back-emf, so it's not doable that way.
and i think SeanB provided the most plausible solution for the start in the same direction.
johansen:
so for what its worth, i've never run across a shaded pole motor with a variable air gap. meaning, the air gap across the section after the shorted turn is the same as the rest of it.

and i've never run across a single phase 400hz aircraft blower motor....

nor have i ever heard nor read about causing the saturation of half of the pole piece as a way of generating a rotating field.


the math behind a shaded pole motor is rather nutty, the same sort of mechanism is going on in a regular induction motor that have a start coil, (high resistance) but they have no start capacitor (btw, adding a capacitor to these motors should increase the starting torque)
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