BLDC motor constants

[hyboxis79]

I have catalog specs for a brushless DC motor saying

[I_0] no load current @ 8.4V = 3.84[amp]
[R_a] = 0.0063
Continuous Current = 45 Amp
No. Winding = 6
Voltage= 14.8 - 18.5
max power = 700Watt (15 Sec)
Kv=5000RPM/Volt

1. Is it possible to find the motor torque constant with the help of the non load current someway?
2. Is it possible to predict how much current the motor will draw in case nominal power is not achieved?

 

[Clyde38]

1. Yes, you need the no load speed at the applied voltage.
Then (UN-Io*R)/N[sub]O[/sub]
Where; UN is terminal voltage (Volts), Io is no load current (Amps), R is terminal resistance (ohms) and N[sub]O[/sub] is no load speed (v/r). This will give you Ke or voltage constant (v/r/sec) wich is equal to the Kt or torque constant (nM/A)(SI) units. If you use rpm for no load speed you will have the voltage constant (Ke) in v/rpm. Solve for the torque constant Kt depending on the units:

Multiply
1.0000 V/Krpm or mV/rpm
Times
1.35229499= ozin/Amp
0.08452= lbin/Amp
0.00704= lbft/Amp
0.00955= NM/Amp
9.549296586= mNM/Amp
0.00955= V/R/S
97.4275928= g-cm/Amp
0.097427593= Kg-cm/Amp

However, you already have the "Kv" rpm/v and the reciprocal (1/Kv) is the voltage constant. Multiply by the constants above for the units you need (torque per amp).

2. You will need more information to determine "how much current the motor will draw in case nominal power is not achieved".

Download the book the MotorExpert reccomended at

http://www.specamotor.com

referenced in you other post.

 

[hyboxis79]

thanks Clyde38. I've downloaded the book and still havn't looked at it.