Induction Motors: Don’t write them off just yet!

March 26, 2014 // By Chris Clearman, Texas Instruments
I’ve been doing some light reading recently on the European Council for an Energy Efficient Economy website, specifically their recommendations for electric motors and controls from their recent “Ecodesign Seminars.” There is some excellent information included about the efficiency and lifetime costs of various motors, including the workhorse of industry – the induction motor.

There was also information on the more natural-resource-consuming, permanent-magnet synchronous machine, as well as the new kid on the block – the synchronous-reluctance machine. Some very intelligent decisions were drawn regarding where best to focus efforts in target applications with specific technology. But (you knew there had to be a downside), I feel they neglected a very important consideration.

Check out this image (Figure 1) from the overview presentation that communicates one of their key points. I took the liberty of summarising their conclusions:

- Top graph: Induction motors are wildly inefficient when they don’t need to produce torque (the load needed to drive is not near the maximum), even when they are superiorly designed for excellent efficiency at load as the motors described have been.

- Middle graph: Variable speed drives (VSDs), used typically in a digitally controlled three-phase inverter, are reasonably efficient components on their own.

- Comparing the bottom graph to the top: VSDs do very little to help efficiency of induction

Figure 1 is drawn from Page 22 of “Consideration of system approaches for motors and motor-driven units: rationale and implementation challenges” presented by Anibal de Almeida, Coimbra University. (online PDF)

Induction motors are not much different than they were when invented about 130 years ago. Their efficiency is poor, primarily due to the namesake: Induction. Because there is not a permanent magnet to use for the rotor, you must induce a magnetic field on the rotor through the magnetic field you are producing in the stator. In a synchronous machine, you simply create a rotating magnetic field on the stator, and the existing magnetic field of the magnetic rotor will be attracted and follow. The simple way to think about it is that for an induction motor, we are always using a near constant amount of current (called the magnetizing current), even if it’s not required, to induce this rotor field to a specific level to produce a specific shaft torque. But this is also done if, at times, no torque is required! Power dissipated with no work accomplished is the definition of poor efficiency.