Basic Motor Types: PMDC, BLDC, AC Induction, Synchronous and Series DC

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Going along with my controller post, I want to set down, once and for all, a good outline of basic motor types and how they’re different.  I’m going to restrict this to motors that we can buy for motorcycle conversions, because if I don’t it’s going to be a bottomless bit of motor design information – ’cause there are a whole lotta motors.  Also have a look at my post on motor stall torque, which is at least a starting point.  OK, that was kind of a bad pun.  If you don’t get it, read the post, here.  I’m going to indicate right in the title whether there are brushes or not – or, as some would say “self commutated” (brushes) or “externally commutated” (without brushes via a 3-phase controller).

Motenergy ME1003 PMDC motor

PMDC: Permanent Magnet DC (brushes)

These are the oldest and among the simplest designed motors.  They are what you made in Science class in school – if you had a cool teacher – with two magnets, some coils, and the way they work is to use the coils as electromagnets which quite literally bounce off of the fields of the permanent magnets.

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: Brushless DC (no brushes)

Here’s the confusing part about BLDC motors.  Though they’re called DC, they actually are running from an AC power source that comes from a DC power source. What?  Here’s the Wikipedia explain: “Brushless DC electric motor (BLDC motors, BL motors) also known as electronically commutated motors (ECMs, EC motors) are synchronous motors that are powered by a DC electric source via an integrated inverter/switching power supply, which produces an AC electric signal to drive the motor.

Here are the advantages:

  • High efficiency
  • More reliable and no arcing on commutation – 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 and as a result can rotate exactly at the supply frequency or a sub-multiple of the supply frequency. The magnetic field on the rotor is either generated by current delivered through slip rings or by a permanent magnet (PMAC).

Take note: this is where you get the PMAC designation of motor, like the PMAC liquid-cooled motor we talked about back here: NEW PMAC 38 kW Liquid-Cooled 120V Motor (from Electric Motorsport) 

The photo shown here is a common choice for EV applications – though this one is a lot prettier than most – but it’s an AC20/30 type AC induction motor.

BLDC motors vs PMAC (no brushes on either)

This is probably the most confusing part for anybody but an electrical engineer.  So 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 waveform from the controller.  BLDC is trapezoidal.  PMAC is sinusoidal.  What that means, I guess, will have to be the topic of another post (when I actually have it figured out, maybe).  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, but that’s just to confuse us.  That type of PMAC motor is not something that is a concern to EV builders.

Series-Wound DC motors (brushes)

There’s a good description of a series-wound DC motor here, on the IMPhotonics page: Parts and Principles of operation of a Series DC Motor:

In series motors stator windings and field windings are connected in series with each other. As a result the field current and armature current are equal. Heavy currents flow directly from the supply to the field windings.

In a series motor electric power is supplied between one end of the series field windings and one end of the armature. When voltage is applied, current flows from power supply terminals through the series winding and armature winding. The large conductors present in the armature and field windings provide the only resistance to the flow of this current. Since these conductors are so large, their resistance is very low. This causes the motor to draw a large amount of current from the power supply. When the large current begins to flow through the field and armature windings, the coils reach saturation that results in the production of strongest magnetic field possible.

The strength of these magnetic fields provides the armature shafts with the greatest amount of torque possible. The large torque causes the armature to begin to spin with the maximum amount of power and the armature starts to rotate.

Series motors are a common choice for competition drag applications, presumably because of their relative light weight compared to induction AC motors, and their huge starting torque.  They’re are called “Universal” motors commonly, and are most typically used in power tools, though you can find them in stuff as big as locomotives.

How’m I doin’?

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3 responses to “Basic Motor Types: PMDC, BLDC, AC Induction, Synchronous and Series DC

  1. I stumbled on this while trying to decide what set up to go for with my intended scratch-built NEV.

    Your article was AWESOME.

    Funny, plain-speaking and informative, and I loved your blog’s “catchphrase” or mantra-thingy at the top – Just walk away.

    From the links I followed, it seems you’re pretty consistent with your humorous but practical advice too.

    Keep up the good work. Now I am going off to read more of your blog…

  2. Congratz on the good post. I would like to hear more about the BLDC vs PMAC topic. I read that the differnce arrises from the back-emf in the windings, but i dont understand how this difference arrises since their construction is so simmilar.

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