The Motor Controller

Before we looked why electric vehicles are important, the architecture of an electric vehicle and started looking at the components of the electric powertrain. Last time we saw the battery management controller and its functions. Today we will glance over the motor controller.

Similar to the BMS (Battery management system) which manages the interaction of the battery with various other components of the powertrain, the motor controller manages the interaction of the motor with the battery and rest of the vehicle. In most of the electric vehicles the motor used is 3ph AC induction motor. You should be studying this in your ES2 (Electric sciences 2) course in second year. AC motor is used because it has many advantages, the most important being high efficiency and low maintenance. Now this being a “3ph-AC” motor, it requires 3ph AC current whereas the battery outputs DC current. Hence there is a need of an “Inverter” in between the battery and the motor which converts DC current into AC current and the role of the motor controller is to control the functionality of this inverter.

The control action

The most important function of the motor controller is the 3ph AC current generation. Without going into detail mathematics, I would say that this is achieved by transforming the  current vector by a series of transformations, controlling their magnitudes by tuned controller gains and feeding them to the IGBTs of the inverter to convert them in 3ph-AC. 

So to be more specific the motor controller does the following things

1. It measures the fed back current and the speed (RPM) of the motor via appropriate sensors. 
2. It takes in these values and applies a few transformations transform these quantities into quantities that are easy to control. The controller uses Clarke and Park transformations.
3. The PI (proportional-Integral) controller compares these values with the ideal values which are generated from the pedal position. (More the pedal is pressed, more velocity/ power is requested and hence more magnitude of AC current is needed). 
4. This controller outputs corresponding voltages that would be required to generate appropriate PWMs for the 3 phases. 
5. These voltages are then passed in a SVPWM (Space vector PWM) algorithm coded inside this controller which gives out the 3 ph PWM signals, which are in turn fed to the IGBTs with the DC current from the battery to generate the correct 3ph AC currents. This current is then fed to the motor windings to generate torque.

Hence the motor controller is a closed loop controller which ensures the correct 3ph currents are generated to achieve the requested torque and power levels.

If you guys want to go deeper into the design of motor controller you should be aware of the following things

1. Clarke/ Park transformations
2. Feedback controller design (PI, LQR)
3. Space Vector PWM techniques
4. Inverter (IGBT) Electronics
5. Field Weakening algorithms etc.

I will try to give you brief information on these to give you a head start for the intereseted. Tons of information is available on motor control techniques online. This is a very well developed area and it should not be difficult to find research papers on this. I think Texas A&M university in the US has a good research lab on motor controls.