Calculating the voltage constant Kv

[2moto]

Is it possible to calculate the voltage constant from a measured phase voltage when rotating the motor by hand? In other words, the known values are:
- peak-to-peak voltage
- frequency of voltage
- number of motor poles
- number of slots

Is that enough? BTW, it's a standard brushless direct drive hub motor.

Cheers.

 

[2moto]

Hmmmm:

(Frequency x 120)/(number of poles x number of phases)

If correct that makes mine approximately 1.22 RPM/V which is definitely not correct as the data sheet says about 9.3 RPM/V which is probably about how it actually spins, i.e. 450RPM at 48V.

Where is the rub?

 

[nieles]

not 100% sure on the calculations, but try these:

electrical frequency x 60 / pole pairs = mechanical rpm (frequency)
(so if you have 14 magnets, that would be 7 pole pairs)


mechanical rpm / peak-to-peak voltage = kv (the voltage constant in rpm / V) (

now the torque constant (kt)

kt = 9.55 / kv (with kt in N-m/A)

Edit: additions in red

 

[Jeremy Harris]

The voltage constant is Kv. Kt is the torque constant.

Kv can be measured by spinning the motor at a known RPM and measuring the RMS voltage between any two windings.

Kt can be calculated from Kv, as it's 1/Kv (using consistent units).

 

[2moto]

Yes, sorry, meant Kv (edited now).

 

[nieles]

The voltage constant is Kv. Kt is the torque constant.

Kv can be measured by spinning the motor at a known RPM and measuring the RMS voltage between any two windings.

Kt can be calculated from Kv, as it's 1/Kv (using consistent units).

hi Jeremy,

why should the RMS voltage be used opposed to the Peak-to-Peak voltage?

 

[Miles]

If Kt is the torque constant in Nm/A, then it seems inconsistent to call Kv (rpm/V) the voltage constant...... I wish everybody would call it the velocity constant and use Ke for the voltage constant. The abbreviation for volts is V (upper case)

[Excuse the interruption....]