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How to calculate the BLDC motor no load speed ?

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alek

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Hi

I have BLDC motor that has no load speed at 15V equal to 8800 rpm. If i supplied this motor with 12 V, then how i can calculate the new no load speed from the motor spec sheet?

thank.
 

how do i calculate kv : rpm/v?

alek said:
I have BLDC motor that has no load speed at 15V equal to 8800 rpm. If i supplied this motor with 12 V, then how i can calculate the new no load speed from the motor spec sheet?
Click to expand...
At no load, motor current is usually very small compared to rated current of the motor. Therefore,

Eemf ≈ Vs

where Eemf is the back emf and Vs is the voltage supply across the motor winding terminals.

But Eemf = kw

where k is motor constant and w is the speed of the motor.

Therefore, for 12V voltage supply, the motor speed will be:

Eemf(15V) = k * 8800
Eemf(12V) = k * w(12V)

so, w(12V) = 12/15 * 8800 = 7040 RPM

Hope it helps.
 

bldc motor friction coefficient

Thanks Nicleo;

If I include the motor current (Im) into the eq.

Vs= Im*R + kw

Now what is the value of Im ?

Is it correct if I define Im = Io - Ispd ?

Io is the no load current (from motor spec sheet) and Ispd is the reduced current from the supplied voltage change (15V to 12V).
 

no-load loss 12v kilowatts

The no-load current is, of course, caused by friction losses. If you want a better approximation for the no-load speed at 12V, then you can first calculate k from the equation:

k=(15 - R*Io)/w15.

Now you can consider that the friction losses will be constant (you neglect viscuous friction and variation of friction coefficient with speed) and assume the no-load current will be the same. Then, the new no-load speed is:

w12=(12 - R*Io)/k.

However, I think this is an unnecessary complication. You can check that yourself, by using the actual numbers from the datasheet.

Nicleo's solution should provide a reasonable approximation.
 

bldc kw

Hi guys, new to the board and this particular thread is what I'm currently researching. I am working on a tool to calculate a brushless motor design given user inputs. Currently I am able to calculate the Kv (RPM/V) and R (winding resistance) but I'm not exactly sure how or if I can calculate the no load current Io. If I can, then I can plug these number into another set of functions I have to calculate motor performance.

Any guidance on this would be great.

Thanks,
Jay
 

no load current bldc

No load current is due to friction losses. If you can estimate those, they represent power loss. The no load current is then simply Io=Po/V.
 

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