# DIY brushless dc motor



## major (Apr 4, 2008)

r4ndom said:


> Hi!
> 
> Just a quick question to those who have made a motor by themself or anyone else who can answer...
> 
> ...


Hi r4,

Do you understand the concept of generated voltage or back EMF (BEMF) as some call it?

I am not an expert on BLDC, but I think what you have written above is incorrect. Show me a mathematical expression for coil reactance which has a factor for speed.*

Regards,

major

*oops, thinking of inductance. Reactance is proportional to frequency (which is speed). But I think that BEMF would be the major contributor to the impedance of current.


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## r4ndom (Apr 13, 2010)

Hi,

Ok, i try to explain:

1. Inductance (or self-inductance) is a property of an electric circuit. A coil which has multiple turns tends to have quite amount of inductance. You can calculate multilayer coils inductance f.eg. here.
For example coil inductance which has 500 turns, diameter 2cm, coil length 1cm and thickness of wiring 1cm: L = 3150uH


2. Inductive reactance tries to opposite changes in direction of current, creating "resistance". Here is the formula: X(ohms) = 2*pi*f*L, where f =
frequency.


3. Speed controller changes the polarity of stator coils. Direction of current changes quite rapidly depending on setup and RPM. For example 2000RPM motor that has 24 permanent magnets it changes 800 times / sec. f=800hz


In this case X = 2*pi*800hz*3150uH = 15.8ohms

So at 2000RPM there is dc resistance and inductive reactance present and power to the motor has decreased significantly.



Please feel free to correct me.


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## major (Apr 4, 2008)

r4ndom said:


> 2. Inductive reactance tries to opposite changes in direction of current, creating "resistance". Here is the formula: X(ohms) = 2*pi*f*L, where f =
> frequency.


Hi r4,

Yes. Reactance increases. But the impedance to the square root of the sum of the squares of the resistance and the reactance. Minor point. But I still think that the BEMF is the big factor.

major


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## AmpEater (Mar 10, 2008)

Posted to the wrong thread, sorry guys...continue


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## jlgh (Mar 6, 2010)

major said:


> Hi r4,
> 
> Do you understand the concept of generated voltage or back EMF (BEMF) as some call it?
> 
> ...


I'm not a BLDC expert either, but I would assume that Back EMF is indeed the issue


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## peggus (Feb 18, 2008)

r4ndom said:


> Hi!
> 
> 
> At low RPM motors impedance (resistance and inductive reactance) is almost same as resistance, right?


Reactance is the product of speed, Iq and winding inductance. At zero speed it is zero and the impedance will be purely resistive. 



r4ndom said:


> So if we are running a 48V motor at very low RPM and amps are 125A then we know a motor resistance is about 0.384ohm and power is 6kW.


The impedance will be 0.384ohm, not the resistance.

The impedance is the hypotenuse of a triangle were one leg is the back EMF + resistance and the other is the reactance described above. The resistance tends to be drowned out by the back EMF as it is disapearingly small and can be ignored.

If you're running at low RPM and high current the reactance tends to be large compared to the back emf and you end up processing a lot of reactive power in your inverter. This is what is known as poor power factor. 



r4ndom said:


> But when motor accelerates inductive reactance of coils increases.
> Depending on the coil setup the inductive reactance will soon be the same as the resistance of 0.384ohm (giving total 0.768ohm) or much more and power will dramatically decrease, right?



If current (torque) is kept the same while you speed up the reactance will go up proportionally to speed but so will the back EMF, thus the power factor remains the same. 

If power is kept constant while speeding up the back EMF will still increase proportionally to speed but the reactance will be constant due to the current being the reciprocal of speed. Thus the power factor will improve and at high enough speed the impedance will be mostly resistive in the form of back EMF. 



r4ndom said:


> So, what are the options to prevent a power loss at full RPM?
> Speed controller can handle 150A so coils must have some resistance to operate at low RPM also.
> Changing motor wiring from series to parallel at some point? Any other methods?


Fewer turns of thicker wire leads to less back emf as well as less inductance. Too few turns and you run out of current and can't produce the flux needed to get your desired take off torque. Too many turns and the back EMF and reactance will limit your speed. 

The resistance doesn't factor into it, the less the better. What you need for your controller to be able to regulate current at low speed is inductance.


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## Olaf-Lampe (Feb 24, 2010)

I can only copy what peggus wrote.
What would help is starting in _wye_ and later switch to _delta_ for higher RPM.
Or use a winding scheme like Dahlander.

-Olaf


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