How does field weakening affect torque output of a DC motor?

[KLoux]

Hello,

I have a question regarding calculating the torque output of a DC motor. I can record current and field from a motor drive. I can also record commanded torque. I have time histories for a series of motor commands, and I'm comparing the torque command to a torque that I calculate based on the field and the current. All of my units are % of rated, so I'm using the following relationship to calculate the torque output:

Torque = Current * Field

So below base speed, the torque is proportional to current, but above base speed, we take into account the reduction in the field.

My problem is that my calculated torque only matches my commanded torque curve below the motor's base speed. Is there something else going on dynamically that invalidates this equation? Curiously, the curves match very well if I divide the field reduction by two:

Torque = Current * (100% - (100% - Field) / 2)

Is this purely chance? Should I check my data again? Am I missing something obvious?

Thanks for your help!

-Kerry

 

[anorlunda]

You don't mention the load. Power=torque*speed is another relation. So we need to know how power varies with speed also.