EV NerdGasm (Part 1): Basic EV Motor Types


Motor bling from EV West - a very pretty 3-phase AC Induction motor from HPEV

Motor bling from EV West – a very pretty custom 3-phase AC Induction motor from HPEVS

If you’re like us, you’ve spent most of your life infatuated with machines and speed, and up until the last decade or so, the only outlet for that was through tampering with infernal internal combustion propulsion.  Now that we can get some serious speed and power from electric drivetrains, we find ourselves trying to repackage our understanding of what makes things go fast into a new model – batteries, controllers and motors – and it’s not always a graceful fit.  Much of what we understand in a dinosaur-burning powerplant simply doesn’t apply to a one-moving-part magnetic flux drive – or, as a friend puts it, the “elegant simplicity of electro-motive force”*.

This leaves us garage gear-heads trying to understand some fundamental things that many in the industrial power industries take for granted, and the two starting points are basic motor types, and basic motor speed controllers and how they work.  Over at The Electric Chronicles there are a few posts going into some great detail on these topics, but let’s break it down simply, here.  Let’s start with motors.

There are a host of different motor types, but a handful that we run into in the EV world.  Here’s a brief outline of the types, and their characteristics:

PMDC: Permanent Magnet DC (brushes)

Motenergy ME1003 PMDC  motor

Motenergy ME1003 PMDC motor

These are the oldest and among the simplest designed motors.  They have magnets and an armature which is essentially an electromagnet.  You feed the armature a pulse of current, it literally pushes off the magnetic field of the magnets.  That pulse is controlled by brushes making contact, physically, with the poles of the electromagnet’s coils.

On the plus side, they’re cheap and simple to control.  They have pretty good starting torque characteristics. On the minus side, they need maintenance because of the brushes, and they’re not quite as efficient as the next type – BLDC motors.

BLDC: Brushless DC (no brushes)

BLDC motors are similar to PMDC motors in that they have magnets, but they’ve done away with the brushes.  Thus, they’re what is called “externally commutated” or commutated via the motor speed controller.  (Commutation is how you time the pulse of the current to the motor – whether electronically through position sensors, or mechanically, using brushes.)

Here are the advantages:

  • High efficiency
  • More reliable, no arcing on commutation  and no brushes to maintain
  • Higher speed and power to size ratio
  • Heat is generated in stator – easy to remove
  • Lower inertia – no commutator
  • Higher acceleration rate

AC motors – Induction and Synchronous, PMAC (Permanent Magnet AC) (no brushes)

AC motors are brushless, and can either use a magnetic field generated by coils, or by permanent magnets.  Here’s the most clear and simple explanation of that, from the Wikipedia:

There are two main types of AC motors, depending on the type of rotor used. The first type is the induction motor or asynchronous motor; this type relies on a small difference in speed between the rotating magnetic field and the rotor to induce rotor current.

The second type is the synchronous motor, which does not rely on induction. The magnetic field on the rotor is either generated by current delivered through slip rings or by a permanent magnet (PMAC).

Here’s where it gets a bit confusing.

BLDC motors vs PMAC (no brushes on either)

A BLDC motor is driven by DC, where a PMAC is driven by AC?  It’s really simpler than that.  The two types are virtually identical in design and structure.  They’re simply driven by a different type of signal from the controller.  BLDC is driven by a trapezoidal waveform, and PMAC is sinusoidal.  What that means in practical terms is a little beyond the scope here, but suffice to say there’s little difference between the two in design, structure or performance. 

Just to confuse us, PMAC motors are also what you call those plug-in-the-wall, brushed motors that you find everywhere throughout your house and have no speed controls.  That type of PMAC motor is not something that is much of a concern to EV enthusiasts, yet still will muddy the waters when you’re trying to understand the differences.

For more details and links on motor types, be sure to visit The Electric Chronicles: Basic Motor Types: PMDC, BLDC, AC Induction, Synchronous and Series DC.

On to Part 2: Basic Motor Speed Controllers.

*Yes.  We’re aware of the technical definition of electro-motive force, and how this is not quite accurate.  Call it poetic license, and leave us to our errant ways, if you please.

Category: General


17 responses to "EV NerdGasm (Part 1): Basic EV Motor Types"
  1. Josephus says:

    Are we to assume that EVs all use BLDC then?

    1. Ted Dillard says:

      By no stretch. Tesla, for example, uses a “three phase, four pole AC induction motor”. Many smaller EVs use PMDC motors. The motor shown at the top of the page is a favorite for conversions, and that’s also AC induction.

    2. Djoni says:

      There also AC asynchronous and synchronous.
      Tesla has AC asynchronous an Nissan use the later.
      BLDC tend to be in smalleur engine SmartED is one of them if I’an not mistaken.

      1. Unplugged says:

        The following cars all use the AC permanent-magnet synchronous motors:
        Spark EV, Fiat 500e, Focus Electric, Honda Fit EV, Nissan Leaf, Smart ED.

  2. Mark H says:

    What type of motor was used in the early 1900 EVs?

    1. Ted Dillard says:

      Not entirely sure, but the Baker was the gold standard of electric cars at that time, and they claim a “series wound” DC motor in this ad:

      More here: http://en.wikipedia.org/wiki/Baker_Motor_Vehicle

      Here’s a detailed explanation of series motors: http://www.lmphotonics.com/DCSpeed/series_dc.htm

  3. Anderlan says:

    Infernal combustion is pretty close to the truth. In a very poetic sense, fossil energy is the fire of hell. It’s, like, from deep in the earth. We thought we needed it, but really, no we didn’t, in fact we passed the allowable rate of burning it *decades* ago. So we can see it for what it is now. Hellfire, and with it comes damnation.

  4. Anderlan says:

    Brush DC: https://www.youtube.com/watch?v=RAc1RYilugI
    Brushless: https://www.youtube.com/watch?v=F9CPX3EJoN4

    This guys whole channel and website looks pretty awesome.

    1. Ted Dillard says:

      THANKS! Awesome videos!

  5. Rob says:

    Can we simplify it a bit more?
    Which one is the cheapest for a diy builder?

    1. Ted Dillard says:

      For cheap and simple, you can’t beat the (brushed) PMDC.

    2. Bill Howland says:

      Most of the cheaper kits use the Series – Woulnd (no permanent magnet, it has an electromagnetic field) motor.

      As was mentioned, the Baker Electric used this since it has by far the highest starting torque per starting ampere. If you want to know why, ask.

      Regeneration with simple controls is out of the question with this motor type, but as I say, some of the conversion kits are only $5000.

  6. Thank you for this article – it answers the questions that many of us have.

    I have a question about AC induction motors (with no permanent magnets) vs all the types of motors with permanent magnets, and that is AC induction are the only type that can coast (in an EV) and not *always* generate power, right?

    In other words, all electric motors can be used for regenerative braking, but only AC induction motors can be spun during coasting and *not* create regenerative braking if coasting is preferred. Obviously, AC induction motors can be used to do regen braking, if the controller is designed for it.

    My question leads to what I think is true; that AC induction motors are able to be more efficient in the “real world”, even if they do not match the peak efficiency of the best brushless permanent motors.

    And the follow up question is, can permanent magnet motors truly coast? Can they be spun without having magnetic fields generated, or does this require some way to negate this in the controller?

    1. Bill Howland says:

      Oh man, Not to be a killjoy here but these explanations —- well, i’ll shut here.

      All I’ll say is that if something does seem clear, it … well,I’m not saying any more because I can’t critique things here.

      On to your questions:

      1). “…AC induction are the only type that can coast (in an EV) and not *always* generate power, right? …”

      All motors can coast… Just disconnect the wiring, and the thing will only slow down due to windage (the amount of ‘wind’ the motor self-generates), and bearing losses.

      2).”…that AC induction motors are able to be more efficient in the “real world”, even if they do not match the peak efficiency of the best brushless permanent motors…”.

      Induction machines as they are used in a car (either motor or generator at different times) are overall less efficient than the synchronous (permanent magnet) kinds, but its not a super big deal. I don’t hear anyone complaining about the efficiency in the basically 130 year old induction motors in the Roadster or Model S. Of course, there the main loss of efficiency is the ludicrous speed those things spin at, plus the overloaded gearbox in the S.

      So yeah the simplest induction motor is a tad bit less efficient but its kinda like who cares. Tesla didn’t bother maximizing motor efficiency by changing the motor type, and, in this case, I’m glad they didn’t since the induction motor they chose is the cheapest and easiest to get to work. Its the screaming speed I have a slight issue with, but HEY, it works.

      Tesla isn’t having great problems with motor
      types, they’re having trouble with gears.

      3). “…can permanent magnet motors truly coast? Can they be spun without having magnetic fields generated…?”

      If the wiring is disconnected from the armature, conductors will ‘cut’ magnetic lines from the magnet, but since no current will flow there will be no restraining torque.

      So, this is the same answer as 1). that is, coasting is easy.

      The rule to rememeber is you need a CHANGING magnetic field to transfer power in non-ferrous objects.

  7. Leaf Errickson says:

    Don’t mean to split hairs here, but motors are designer to pull the armature pole into the stator pole, not push. Pulling concentrates the lines of flux, increasing force, while pushing disperses lines of flux, decreasing force. Pistons are pushed, electric motors are pulled.

    1. Ted Dillard says:

      Not at all! That’s just the kind of nerdgastic hairsplitting we loves – Thanks!