#### valdorf

##### Newbie level 4

First of all, sorry for my english, which is not my first language. I am currently making a design of AC universal motor (series motor) as a nutrunner (torque wrencher) for bolts wrenching at adjusted known torque. I would like to calculate a load torque using electric parameters such as current, speed. I know that there are an equation of torque T=P/w=IU/w. But there are also another equation T=k*I^2 (k - torque constant). So I am a bit confused which equation I really have to use. My controller will adjust desired torque (i.e. 500Nm) and controller will calculate torque from electrical parameters and run the motor until it reaches desired torque.

I have made a test, running motor for 5mins at no load and I have seen that motor temperature increases and a current decreases, and this means that my torque will drop depending on temperature. Also I observed that motor speed during this test was increased. My design at the moment has current and motor speed sensors, so I can track these two parameters. But I do not know if I really need to sense a voltage.

So my question would be: Could I create a relationship between motor current and motor speed to estimate desired load torque, or do I need to measure voltage in addition?