# The perfect BLDC sensorless controller 3 quadrants



## Tesseract (Sep 27, 2008)

wingover said:


> ...As regenerating is as important as the propulsion in a sailing boat, motor will run 24h/24h when sailing. 1000 hours of use is reached only in a few months, so i think PM brushless motors are almost compulsory for my application.
> Looking at actual applications of this kind of motor and sensorless controllers, it seems they are used a lot in aerospace and military applications. This let me think it is a good technical choice for bulletproof applications.


Sensorless motors _are_ tougher than ones with Hall Effect sensors in them, but not in a way that is relevant to a sailing vessel. Hall Effect sensors do not tolerate vibration very well (it causes piezoelectric effects) and they also have a fairly limited operating temperature range (to be military/aerospace qualified a part must function properly from -55C to +125C). A sailboat is about as low a vibration environment as I can imagine and the people on the boat will probably have a more restricted temperature range than any Hall Effect sensor so I would not restrict myself to only sensorless motors/controllers for this application. 

That said, there's no reason to exclude sensorless motors/controllers, though, because while the information in the quoted post by _abudabit_ is correct, a sailboat rarely reverses direction and changes speed very gradually (compared to an automobile, for example) so those concerns aren't really relevant in such an application.




wingover said:


> Also, for a direct drive low revering (compulsory for efficency) of the propeller shaft i will use a 24 to 12 poles motor giving full speed at 900 to 1200 tr/mm. What will be the effect on the controller ? I guess this helps ?


Yes - more poles means more BEMF for a given shaft RPM.




wingover said:


> My application runs under 50 volts and motor will be 4 to 5Kw, but for the moment i dont not know if it will be AC synchronous, asynchronous, PMAC...as it is a little confused in my head about advantages of each as far as efficency, weight and regeneration efficency. I found a 24 poles motor from Perm in Germany, but for the moment this motor is only qualified as a wind generator and they ask for time to do more test to use it as a motor. So if you have ideas about other motors...let me know !


You're not the only one confused by all the different types of "rotating machines". Broadly speaking, brushless DC motors are the same as synchronous AC motors but with permanent magnets in the rotor instead of a wound field and slip rings. Asynchronous AC motors (e.g. - what are commonly referred to simply as "induction" motors) are very easy to drive in all 4 quadrants - simply command a lower speed than the shaft is turning at and an induction motor automatically turns into a generator. Another plus is that induction motors work equally well as a generator or a motor because they do not have a fixed field position (the field is "induced" in the rotor by the currents flowing through the stator windings). 

Anyway, one very well respected manufacturer of brushless motors here in the US is Mars Electric. See, for example, this page:

http://www.marselectricllc.com/burnishermotor.html


As for the rest of your post, I have not enough experience with brushless motors (except industrially, in servodrives) to really comment on which controllers work better, etc...


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## wingover (Feb 26, 2009)

Tesseract your information are already a great help as it answers several questions i was asking myself !

Yes Mars brushless are great motors but for inboard boat application, a 2:1 to 3:1 mechanical reduction is compulsory, and i do not want this.

- First because i do not know where i can buy this reduction
- Second because i think i will lose 10 to 15 % in it.
- Third it means more parts, so more problems.
- Fourth Noise will be higher as a reduction will make noise

But to be honest reduction on a ship has a 2 advantages:
- motor choice is much wider, so this means lower cost
- Motor is in an upper position in the boat, so less exposed to water always present in a ship when sailing very heavy weather

So unless i would find a good quality, low loss, resonnable cost belt reduction i won't use reduction.

Belt drive reduction looks like this :
http://www.youtube.com/watch?v=GdEDZuvk7wE&feature=related
http://www.youtube.com/watch?v=Bd4EIU4QC5k&feature=related


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## Tesseract (Sep 27, 2008)

wingover said:


> Tesseract your information are already a great help as it answers several questions i was asking myself !


You are most welcome, monsieur.

I like toothed-belt drives. They can be very quiet and very efficient (95% efficiency is relatively easy to achieve) and, of course, they are a lot easier to fabricate yourself than a gear reduction and much more efficient than a v-belt or flat belt (a little noisier though).

This link has about the best explanation on how to design a belt drive I have come across:

http://www.sdp-si.com/D260/D260cat.htm


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## etischer (Jun 16, 2008)

DCBL Motor is a DC motor which uses an encoder or some sort of feedback device for commutation. Typically an absolute encoder is used, the encoder must be precision aligned with the stator (by the factory). 

AC Induction motor does not require commutation. 

You have to put power into the AC induction motor to regen, there needs to be some magnetizing current to create a field. So if your battery is dead to the point you can't reach minimum voltage on the controller, you might just be SOL. Sensorless vector mode running on an AC induction motor is fine as long as you don't intend to spend alot of time at low speed (less than 5% base speed, or around 100 rpm)


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## wingover (Feb 26, 2009)

Battery can't be dead unless accident as i also have solar panels, windmill and a "low voltage cut everything" security, but it was worth to point out this limit.

What happens exactly when i slow down too much the AC motor going close or under 5% or rated speed ? Do i loose efficiency or will i loose also control of the motor by loosing BEMF ?

"you might just be SOL" what means "SOL" ;-) 

Another question coming to my mind, what will be the usual maximum frequency that will accept a sensorless controller, with positive and negative torque, that is to say when motoring and when regenerating ?
Ideally, 200 Hz would be great for positive, 250/300 for negative...Do i dream awake ?

Thank's !

For school time, here is an interesting paper about different motor technologies and drive solutions
http://www.lg-motion.co.uk/product ...n_Sensorless_Control_of_BLDC_Motors_04-05.pdf
And this one about regeneration 
http://www.roboteq.com/how-to/understanding-regeneration.html


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## etischer (Jun 16, 2008)

The low speed range of an AC motor running in sensorless vector may have a problem with stablity. This is only in sensorless vector or Volts/hz mode. Closed loop vector mode (using encoder feedback) should not have this problem. Your application should not have the problem in sensorless vector either, because you will be running in "Fan mode" not "Linear mode". Baiscally, you do not need high torque at low speed, your torque increases with speed. 

For an electric car let's put it this way:

60 hz and 230 volts = 2000 RPM
30 hz and 115 volts = 1000 RPM

If you want to go 10 RPM you would need

0.3 hz and 1.15 volts

you cannot get much torque with 1.15 volts, so the AC controller will throw some dc offset in. this setting is called boost. 

On flat ground to get 10 rpm, you may use 0.3 hz and 30 volts. 
On a steep hill you may need to use 0.3hz and 50 volts. 
For a boat, the boost setting should always be the same cause you cannot go up hill and the torque should always be the same (unless the prop is grounded or something). 


Max speed on most AC controllers would be around 400 hz, or about 12,000 rpm. 


SOL means sorry, out of luck. Also means S*** outta luck. It's a TLA 






wingover said:


> Battery can't be dead unless accident as i also have solar panels, windmill and a "low voltage cut everything" security, but it was worth to point out this limit.
> 
> What happens exactly when i slow down too much the AC motor going close or under 5% or rated speed ? Do i loose efficiency or will i loose also control of the motor by loosing BEMF ?
> 
> ...


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## wingover (Feb 26, 2009)

About synchr motor speed versus frequency i thought that it was :

Rpm synchronous = (120 x Frequency)/ Number of poles

Is it correct, and what is the formula for torque ?

So in my case, just by choosing a 18 to 24 poles motor will give me the rpm range i want (and more in fact) 



etischer said:


> For a boat, the boost setting should always be the same cause you cannot go up hill and the torque should always be the same (unless the prop is grounded or something).


In fact yes you are right for most of time , but no when you have very big waves, very long period, coming for thousand miles away. Then you go up and down hill, boat decelerating a lot and accelerating slightly less , and then... i usually puke.


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## etischer (Jun 16, 2008)

I don't think you will ever find a 18 or 24 pole motor. Most are 4, some are 6, you might find one that is 8. 

there is some slip involved, so RPM synchronus is never achieved. I believe slip is directly related to torque. Too much slip and the motor runs jerky and cogs. This is where boost comes in. Not enough boost, you slip and loose your synch. Too much boost and the motor is overshooting its position, and feels like it is cogging. When in closed loop mode using an encoder, the drive knows exactly how fast the motor is turning and can apply the right boost, volts and frequency. 









wingover said:


> About synchr motor speed versus frequency i thought that it was :
> 
> Rpm synchronous = (120 x Frequency)/ Number of poles
> 
> ...


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## major (Apr 4, 2008)

etischer said:


> there is some slip involved, so RPM synchronus is never achieved.


Hi etischer,

He's talking about BLDC, so there is no slip. Synchronous RPM is a must. There is a torque angle, if I am not mistaken, like you have with a synchronous motor.

Regards,

major


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## wingover (Feb 26, 2009)

in fact my perfect motor would be BLDC, with samarium cobalt (SmCo5, Sm2Co17) magnets, that are far superior.
Controller would be sensorless to avoid a sensor that is another part to go wrong.

If i understand well, confusion is coming from the fact that BLDC and PMAC are quite the same. Controller does the difference.

On wiki 
http://en.wikipedia.org/wiki/Brushless_DC_electric_motor
i found this

"Although BLDC motors are practically identical to permanent magnet AC motors, the controller implementation is what makes them DC. While AC motors feed sinusoidal current simultaneously to each of the legs (with an equal phase distribution), DC controllers only approximate this by feeding full positive and negative current to two of the legs at a time. The major advantage of this is that both the logic controllers and battery power sources operate on DC, such as in computers and electric cars. In addition, the approximated sine wave leaves one leg undriven at all times, allowing for back-EMF-based sensorless feedback.
Vector drives are DC controllers that take the extra step of converting back to AC for the motor. The DC-to-AC conversion circuitry is usually expensive and less efficient, but they have the advantage of being able to run smoothly at very low speeds or completely stop in a position not directly aligned with a pole. Motors used with a vector drive are typically called AC motors."


And about more than 8 poles, i still have faith and wait to know more about this perm 24 poles generator Type PGSR 170
http://www.perm-motor.de/index_en.php?linkid=h


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## Tesseract (Sep 27, 2008)

wingover said:


> ...And about more than 8 poles, i still have faith and wait to know more about this perm 24 poles generator Type PGSR 170
> http://www.perm-motor.de/index_en.php?linkid=h


My understanding is that motors with more than 8 poles become increasingly impractical because each pole must then be wound with smaller diameter wire and routing all of the interconnections starts to use up more space. Also, motors with more poles tend to be less efficient, which sort of defeats the purpose if one is trying to avoid the efficiency hit from a reduction drive.

The slowest speed induction motors I have ever seen used in an industrial application were 900rpm (8 pole, 60Hz). I do know they make them with more poles, it is just that I have never personally seen such used industrially.

Go with the toothed-belt drive and a proven-reliable motor... or pray for wind


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## wingover (Feb 26, 2009)

Tesseract said:


> Go with the toothed-belt drive and a proven-reliable motor... or pray for wind


I always pray for more wind, and very often for much less !

I have done a careful reading of your site on toothed-belt drive , and of course keep this as a possible good solution. Look, i'm ready !
http://www.thunderstruck-ev.com/sailboatkit.htm

But i do not give up so easily...

By the way this SEVCOM millipak SBPM controller is using the mars motor as a BLDC sensorless or not ?


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## Tesseract (Sep 27, 2008)

wingover said:


> ...
> By the way this SEVCOM millipak SBPM controller is using the mars motor as a BLDC sensorless or not ?


The wiring diagram shows the encoder being used, so I would say it is NOT being used as a "sensorless" motor.

That German site you referred to a couple posts back has some motors with planetary gear reductions built-in (proof right there that a high number of poles is self-defeating!). I would also consider them if the price is reasonable. Keeping in mind that the Thunderstruck-EV kit costs less than $1500 (which is, around 1150 Euros I think) and those German motors look very expensive to me


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## wingover (Feb 26, 2009)

Tesseract said:


> The wiring diagram shows the encoder being used, so I would say it is NOT being used as a "sensorless" motor.
> 
> That German site you referred to a couple posts back has some motors with planetary gear reductions built-in (proof right there that a high number of poles is self-defeating!). I would also consider them if the price is reasonable. Keeping in mind that the Thunderstruck-EV kit costs less than $1500 (which is, around 1150 Euros I think) and those German motors look very expensive to me



Do you know an equivalent controller to this SEVCON, doing sensorless ?

As far as price is concerned, it is clear that euro products are much more expensive. Gernam products being among the most expensive with Austria, so mars+sevcon is an unbeatable couple !

Eric Tabarly said that sailing is the slowiest, unconfortable and expensive way to go from a place where you are fine to somewhere else where you have nothing to do.
This certainly explains why i search though i have found someting acceptable ! Sorry !

Anyway thank's for your usefull advices


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## Tesseract (Sep 27, 2008)

wingover said:


> Do you know an equivalent controller to this SEVCON, doing sensorless ?...


Hmmm... I don't think you are going to find a sensorless controller at this power level - they are typically used for fans and small pumps under 500W (loads that require very little starting torque and that have little inertia). While a propeller does require very little starting torque, it will likely have too much inertia for a sensorless controller to get it moving.


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## wingover (Feb 26, 2009)

Tesseract said:


> Hmmm... I don't think you are going to find a sensorless controller at this power level ... ...., it will likely have too much inertia for a sensorless controller to get it moving.


Yes, furthermore, sea grass and other animal growing on the propeller shaft and bearing under water, stick lightly everything in less than 2 weeks in hot water...

I will investigate this fact about starting sensorless, with manufacturer but i'am afraid you are right.


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## etischer (Jun 16, 2008)

You can always do encoder feedback, and in the event the encoder fails, you can always resort to using sensorless mode, it should be an easy to configure change in the controller. 



wingover said:


> Yes, furthermore, sea grass and other animal growing on the propeller shaft and bearing under water, stick lightly everything in less than 2 weeks in hot water...
> 
> I will investigate this fact about starting sensorless, with manufacturer but i'am afraid you are right.


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