- 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
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 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.
*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.