 # Friction Torque, T_f

Hi!
I have problems concerning the friction torque in a motor(here used as a generator), and I hardly know the magnitude of it. In the specification sheet, it says that the friction torque T_f is equal to 0.08 [mNm], but I find this to be inaccurate with my results of a miniature wind turbine model. Shouldn’t the friction vary with the rotational speed of the generator, and hence not be constant over the whole operating range of the motor? The motor type is called Athlonix 12g88.
Since I am working on the performance of a wind turbine, I need to accurately find the friction losses: P_mech=P_converted + P_friction= (I*K_m + Q_f) *omega. I am finding the electromagnetic torque in the system by measuring the current flowing in the circuit, and multiplying this with the torque constant, K_m [mNm/A]. Until now, I have only estimated the friction in the system based on the friction torque. The turbine operates at a range between 0-4000 RPM, and the values are therefore far smaller than the given values for no-load, and max conditions, which are included in the same specification sheet.

My question is therefore: Are there far lower friction values for lower rotational velocities, or should T_f =0.08, be pretty accurate for the operating range of my turbine, too?
Hope you understood my question, and I would really appreciate any help.

I’d expect friction torque in a motor with rolling element (e.g. ball) bearings to be fairly well decoupled from rotational speed. It’d be a different story were there a built-in fan in the unit, but since it appears there’s none, using the datasheet value as a constant would seem a reasonable starting point for design guesstimations.

Note however, that this figure is a characterization and not a specification. The effective friction torque value will be subject to variation, due to factors such as bearing wear and temperature.