# How is speed torque curve of an induction motor plotted experimentally?

I'm wondering how is obtained a 3-phase induction motor's speed-torque curve obtained. I couldnt find any procedure.

Are they first applying fixed voltages with fixed freq. to stators. and then gradually increasing the torque or they speed up the motor with a variable torque such as wind fan?

• When I was at university, you connected the motor to a leather belt driving a stationary (fixed) wheel, with the wheel mounted on a spring. All energy "generated" by the motor was dissipated by belt slipping on the wheel, and the torque was read off the spring. Yes, you could do the same driving a fan. – david May 11 '15 at 2:00
• But for diffetent torques different loads needed right? Otherwise we cannot plot anything. How do they adjust the load? – user16307 May 11 '15 at 6:42
• By tightening the belt. (See screw wheels at top of picture posted by @raj) The belt is slipping, and the tighter it gets, the more energy is used by friction – david May 12 '15 at 7:04
• I see, so the readings will show the torque? why there are two readings? – user16307 May 12 '15 at 7:31

Usually in college laboratory they connect brake drum and spring balance meters with motor's pulley to measure the force. (you may refer this image below i attached)

While running test you need to measure the speed using tachometer and spring balance force.

And need to find the radius of brake drum .

Torque= force*distance

Torque= (difference between spring balance )* (radius of brake drum)

In industires they may use test benches they can easily do this

• +1 Neat approach. Could probably use cheap digital luggage scales. – Spehro Pefhany May 11 '15 at 2:16
• Thanks for your comment. Yes. We can use its depends on accuracy, How long u are going to use. @ spehro – Photon001 May 11 '15 at 2:23
• Incidentally, the term for these type of devices in general is a dynamometer. There are several different designs, this is one of the simplest I've seen. – helloworld922 May 11 '15 at 2:37
• But how does the rotor rotate if it is connected to break drums? – user16307 May 11 '15 at 6:38
• The brake is a progressive brake - it slows the motor but doesn't stop it. (The brake will get hot!) You increase its pressure slowly with the handwheels at top. As the motor is pulling the brake one way, the torque is the difference between gauge readings * the radius. – user_1818839 May 11 '15 at 9:02