Although efficiency is key these days there are more niches for the AC induction motor to fit in as you may think. Most of the induction machines are tied to the grid to run on a more or less precise and constant speed but by adding intelligent electronics you discover some interesting features of these machines. Additionally a FOC controlled induction machine can also reach very high efficiency.
These machines can have 3 main advantages:
- Intelligent control of this machine makes it possible to start from standstill with full torque even in sensorless control, as long as the machine parameters are known. This startup torque is one of the major challenges with synchronous motors in a sensorless configuration.
- You have also full control of the motor flux. By enhancing the field for more torque or reduce the field for more speed. At synchronous machines you need to waste a lot of energy if you go with field weakening into high speed ranges, at the induction machine this is not the case.
- The machine construction is very simple so they can be purchased for a cheap price and do not follow the varying magnet prices. This makes them also very reliable and they require only little maintenance.
TI’s InstaSpin offers a very convenient and powerful solution for induction machines. The encoder algorithm relies on simple 4 parameter motor models and offers an automated identification process which helps to find the right stator/rotor resistance and inductance. Be aware that there are a number of Induction machines which show a distinct current displacement effect which alters the rotor resistance and make them hard to handle.
The control theory or the machine is one topic. It is definitely trickier to grasp what is going on in the machine if you run in field oriented control so it might already scare off developers to dig into it.
The majority of induction machines were designed to run on the grid voltage or on a fixed voltage vector for which motor manufacturers have developed sophisticated design methods to achieve certain behaviors of it. If you have a digitally controller attached to it you do not need those specialties because you have anyway full control of your machine. Moreover it also creates big issues at the drive controller.
A common problem is the following:
In theory an efficient induction machine for the grid looks like shown below. You have a low slip at the rated speed, a low rotor resistance and also a low startup torque.
To increase the startup torque the motor would need a higher rotor resistance. Then the behavior looks like the following figure. High slip at rated speed, high rotor resistance and also high startup torque but low efficiency.
The solution to create an ideal machine which runs efficient and with high startup torque is to use the skin effect of the rotor squirrel cage. If the slip is high then the rotor skin effect is also high and generates a high rotor resistance – this is useful for the startup where the slip is the electric frequency since the rotor is at standstill.
When the motor has accelerated and is close to the operating point then the rotor resistance is lower because there is no more skin effect due to the lower slip – this helps to get a very high efficiency at the operating point.
Like said above, having a FOC controlled motor you don’t need this behavior since you anyway control your frequency/slip to have full torque at startup and high efficiency at the operating point. But if your chosen motor uses this effect then the machine varies its rotor resistance and this can be very hard to handle for an algorithm that relies on a precise motor model.
I may also add here some technique to find out this magnitude of the current displacement in the rotor.
Let me know what you think! Do you already have experience with different induction machines, please leave a comment! Cheers!