How to calculate the BLDC motor no load speed ?

[alek]

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.

 

[nicleo]

how do i calculate kv : rpm/v?

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?

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.

 

[alek]

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).

 

[VVV]

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.

 

[jay C]

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

 

[VVV]

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.