Continuing our exploration into the Secrets of the Electric Drivetrain Universe, and moving on from our motors post, let's look at motor controllers.  Here's where things get interesting.

The clearest and simplest explanation of your basic motors and how you drive them is found on the Carotron website - an industrial motor controller company.  Here it is, in a nutshell:

1.)  DC Drives – Torque Control:

To control DC motors torque, a DC drive will regulate armature current.


The armature voltage is unregulated allowing the motor to operate at whatever speed is necessary to achieve the set current /torque level.

2.)  AC Drives – Torque Control:

An AC drive uses complex processing of motor voltage, current, frequency and rotational position to give it torque regulation capability.  TORQUE mode operation usually requires encoder feedback.  ... Complete torque control may be dependent on the use of an external torque reference circuit or control that has flexibility and adjustability to compensate for any drive/motor shortcomings.

3.)  DC Drives – Velocity (Speed) Control:

To regulate DC motor speed, the drive will normally control the armature voltage.  How well it does this depends on what feedback signal is used to represent the motor speed.


Common selections for some DC drives are as follows:

  • AFB – Armature feedback
  • TFB – Tachometer feedback
  • EFB – Encoder feedback

4.)  AC Drives – Velocity (Speed) Control:

AC Inverter drives can have several selectable control methods.  Some examples are:

  • V/F Control (V/F, voltage/frequency – also called Volts-per-Hertz control)
  • V/F Control with PG or Tachometer Feedback
  • Open Loop Vector
  • Closed Loop or Flux Vector
The V/F, voltage/frequency, Control method – also called Volts-per-Hertz control is the most common inverter control method.  Requiring no feedback device, it is suitable for general purpose and multiple motor applications.

V/F Control with PG Feedback gives the better speed regulation of a closed loop system.

Open Loop Vector, sometimes called sensorless vector, utilizes a more complex control algorithm to give precision speed control, quick response and higher torque at low speed.

Flux Vector or closed loop vector requires encoder feedback and gives precise speed and full rated torque control over a wide speed range – sometimes even at zero RPM.

Inverters and their motors can also be operated in a “Constant Horsepower” profile where motor speed can be extended beyond the base speed rating with torque capacity de-rating.

If you're paying attention, you've noticed there are two basic ways to control the motor - one is using "Speed Control", one is using "Torque Control", and in fact, the most common is a combination of the two.  That's not a surprise.  Keep in mind you have two basic things going on with electricity - volts and amps.  As far as an electric motor is concerned, your voltage determines your RPM.  Your amps determine your torque - or "twisting force" as it's commonly explained.  (Power, whether Watts or Horsepower, is the combination of the two: torque and RPMs.)  So we can control either the voltage, or speed, or we can control the amps, or torque.

What are the advantages to one over another?  For a more detailed discussion, stop over at The Electric Chronicles, but simply?  True speed control isn’t a great thing, whereas true torque control is “tolerable”.  From the AEVA forum post:

True speed control would be very unpleasant. A small change in throttle position would result in a sudden jerk as you accelerated at maximum torque, then another jerk once the set speed is reached and the torque drops down.

Torque control is very smooth, but relies on the driver to regulate the vehicle speed by varying the throttle position.

A DC drive using open loop voltage control is in between the two. It’s basically a softer, less jerky version speed control, but isn’t as smooth as torque control. Some people prefer soft speed control, others prefer torque control.

For AC drives, torque is what’s intrinsically controlled, so you’d have to build a feedback system around that to emulate speed control if that’s what’s desired. 

Now things start to get a little clearer when you begin to understand the basic motor types along with how you can drive them, if you're wondering why AC motors seem to be the predominant solution that's emerging with everything from scooters to Teslas.  Not only are they lighter and more efficient than the old-style PMDC motors, and you can cool them, plus they need literally no maintenance, but, you have a whole lot more ability to control their every function.  Think in terms of gas motors again - what's the big difference between your daddy's big block V8 with a 4-barrel carb and an electronically fuel injected turbo?  Control.  Pure and simple.