Estimating torque of AC motor - Electrical Engineering Stack Exchange most recent 30 from electronics.stackexchange.com 2019-06-19T23:15:53Z https://electronics.stackexchange.com/feeds/question/407969 http://www.creativecommons.org/licenses/by-sa/3.0/rdf https://electronics.stackexchange.com/q/407969 0 Estimating torque of AC motor Jason C https://electronics.stackexchange.com/users/30761 2018-11-21T05:11:18Z 2018-11-21T22:47:09Z <p>Given voltage, measured current draw, and RPMs of an AC motor, and an estimated efficiency rating, is it possible to calculate the actual instantaneous output torque? Or is other information required?</p> <p>I can also measure its physical dimensions; shaft diameter, etc. With some effort I should be able to measure the time it takes to spin up to full speed as well. </p> <p>Here is the motor I have and the only label on it. I don't know where it came from:</p> <p><a href="https://i.stack.imgur.com/So1Q3.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/So1Q3m.jpg" alt="enter image description here"></a></p> <p><a href="https://i.stack.imgur.com/ANtsO.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ANtsOm.jpg" alt="enter image description here"></a></p> https://electronics.stackexchange.com/questions/407969/-/407971#407971 1 Answer by M KS for Estimating torque of AC motor M KS https://electronics.stackexchange.com/users/192288 2018-11-21T05:31:00Z 2018-11-21T05:31:00Z <p><span class="math-container">\$cos(\phi)\$</span> is also required, The phase difference between voltage and current. The efficiency in that load is available in the catalog. <span class="math-container">\$V * I * cos(\phi)*efficiency= rotational speed(rad/s)*Torque(NM)\$</span></p> https://electronics.stackexchange.com/questions/407969/-/407985#407985 1 Answer by mkeith for Estimating torque of AC motor mkeith https://electronics.stackexchange.com/users/57037 2018-11-21T07:16:37Z 2018-11-21T07:26:11Z <p>I think that is an induction motor from a washing machine. 180W is the rated output power (mechanical power). 1600 rpm is the rated speed. So that means that when you power it from 120V at 60Hz, it will deliver 180W when it is spinning at 1600 RPM. This also implies that the synchronous speed of the motor is 1800 rpm. Conceptually, synchronous speed is the speed at which the torque is zero. As you load the motor, the motor slows down (this is called slip) and the torque increases linearly with slip.</p> <p>So 1800-1600 = 200 rpm. That 200 rpm is the slip required for the motor to put out its rated power and torque. Let's calculate torque.</p> <p>Power = speed * torque</p> <p>If we use rad/sec and N-m, power will be in Watts. So let's convert. 1600 rpm = 26.7 rev/sec. 26.7 * 2pi = 168 rad/sec.</p> <p>If Power = speed * torque, then torque = power/speed. So 180W / 168 rad/sec = 1.07 N-m at rated power. So now we know that when slip = 200 rpm, torque will be 1.07 Newton meters. Likewise, if slip = 100 rpm, then torque must be 0.5035 Newton meters.</p> <p>So in general, torque is as follows:</p> <p>T = 1.07 * (slip / 200)</p> <p>It will be in Newton meters. Slip is in rpm. You calculate slip as follows: 1800 - rpm, where "rpm" is the actual rpm.</p> <p>So, breaking it all down, measure the actual no-load rpm. Take 1800 rpm, and subtract the actual no-load rpm. This is the no-load slip.</p> <p>Divide by 200, and multiply by 1.07. Now you have the no-load torque. Approximately. You can also calculate the no load output power, since all you need is torque and speed (just multiply rad/sec * Newton-meters to get power in Watts).</p>