# Universal AC motor calculations

#### castle

Joined Oct 5, 2016
4
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?

Joined Jul 18, 2013
28,026
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.
With a series field motor, unlike a shunt field version, the torque increases with load, due to the field current increasing proportionately.
Motor temp should decrease with current decrease as the rpm's increase and the motor receives more cooling at lower current.
With no load at all, the motor operates in a run-away condition.
Max.

#### shortbus

Joined Sep 30, 2009
10,038
I am currently making a design of AC universal motor (series motor) as a nutrunner (torque wrencher)
I don't or wouldn't think that's how it's done, too many variables when measuring motor current. It is probably done with what is called a torque sensor/transducer. Many links to what I mean - https://www.google.com/search?client=firefox-b-1-d&q=torque+sensor

Or if you only need one value on the tool a torque limiter would be used. Some of the assembly lines where i worked had them on the conveyor, to keep from breaking things if something went wrong. Again many options https://www.google.com/search?client=firefox-b-1-d&q=torque+limiter

#### castle

Joined Oct 5, 2016
4
With a series field motor, unlike a shunt field version, the torque increases with load, due to the field current increasing proportionately.
Motor temp should decrease with current decrease as the rpm's increase and the motor receives more cooling at lower current.
With no load at all, the motor operates in a run-away condition.
Max.
So please tell me, which torque equation would you advice me. Can I use a relationship between speed and current to make sure the load nut/bolt ir wrenched with the same torque at various temperatures?