• Jul 16th 2009 at 7:47PM
  • 22
Tesla Roadster with AC drive unit - Click above for high-res image gallery

AC/DC? We're not referring to the classic rock band; we're taking a look at electric motors. In today's Greenlings post, we'll delve into the ins and outs of the powerplants that we hope will soon be transporting millions of people around the world to and fro. That said, there are lots of choices when it comes to selecting the proper motor for any given application, and the merits of each has been debated since before any of us were born.

Way back in the late 1880s, Nikola Tesla dared to challenge powerhouse American inventor Thomas Edison by suggesting Alternating Current (AC) was a more desirable method to transfer electricity from one place to another than Direct Current (DC). So, which genius inventor was right? Suffice it to say that just about everything with a plug these days relies on AC current for power. But that was a long time ago, what's changed since then? Which is better for today's – and tomorrow's – electric cars?

Interestingly, while a lot has changed as far as AC and DC technology goes, the debate still rages on over which is better for specific applications, including automobiles. One of the main advantages that AC power holds over DC is its ability to easily change voltages with a transformer. This is an important distinction and one that holds true to this day. Why does it matter?

Here's the simple version: The total amount of power from an electrical system can be calculated by multiplying the voltage by the current. Therefore, the higher the voltage, the less current is required for a given amount of power. Without getting into the science of it all too much, high-voltage AC systems are generally more efficient than lower-voltage DC systems and the gap gets bigger as the transmission distance increases.

So, that's it, right? Since AC is the clear winner in terms of efficiency, it's the way to go? Not so fast. Even though the power coming from the wall through an electric car's plug is Alternating Current, the car's batteries (or ultracapacitors or solar cells or hydrogen fuel cells) give off Direct Current. This means that more expensive electronic doohickery is needed to convert the DC power to AC power, and, since there are not a few hundred miles of wiring to travel, the efficiency benefits of the high-voltage low-current nature of AC is somewhat mitigated.

Today, there are multiple kinds of DC motor technologies on the market, including series wound motors, Permanent Magnet motors and Brushless motors. Each type of DC motor has its advantages. Naturally, the more efficient and powerful the motor, the more it probably costs. Even relatively inexpensive series wound DC motors, though, are well-known for producing a ton of torque right off the bat, and they are also readily available on the open market to anyone looking to create their own electric car conversion.

Permanent Magnet DC motors make lots of power and are often used in low-cost electric car conversions, but they also use components (brushes and commutators) that naturally wear out over time and will eventually need to be replaced. Brushless DC motors do away with many of these wear items and generally run very cool, which means they can last for a very long time. Downsides to Brushless DC motors include higher initial costs and the extra electronics required to make them function.

So, what's currently (no pun intended) being used in mainstream eco-friendly cars and trucks? Modern hybrids generally rely on Brushless DC motors for motive power while the now-discontinued EV1 from GM, the current Tesla Roadster and the upcoming Chevy Volt all use AC drives.

Can you tell where all of this is going? After all these years, there's still no simple answer to the AC versus DC argument for electric cars. Which technology is best really depends on the application after considering variables like price, desired power and desired range. No matter what kind of electric motor is used, one thing remains constant: a noticeable lack of emissions exiting the vehicle's tail pipe.

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    • 1 Second Ago
      • 6 Years Ago
      Uh... wait... can someone tell me what the difference is between an AC and DC brushless (like you are really going to use a DC motor with brushes these days) motor when it comes to the motor itself?

      For those of you who know your electric motors, you most likely said NOTHING, and you are correct. AC motors and DC motors are identical as far as how they look on the inside. What varies is the power controls going into each technology. Some here have been giving opinions on 'which is more complicated' or which is 'more interesting'... which is kind of a joke.

      AC and DC brushless both have stators and a rotor, a shaft, windings, etc... At the motor level, a DC motor requires that the power controls invert or change the polarity of the stators as the rotor spins... just like an AC motor. A DC motor usually means that you supply DC power to the controller, and then the controller breaks up that power into some waveform which gets fed into any number of poles in the motor... square wave 4-pole, sawtooth 6-pole, sinusoidal 12 pole, etc... AC motors usually mean you have a single phase sinusoidal wave that you can either feed right into the motor (so a 2-pole or 4-pole), but many motors break this up into more poles and can break up the phases into multiple smaller phases as well as manipulate the waveform. The major advantage of DC motors is that you can manipulate the frequency and therefore speed of the incoming waveforms at the controller. The thing is... using PWM or VFC drives on an AC motor gives you the exact same thing. One thing to keep in mind in this 'seeming debate' between AC and DC motors is that at the motor level itself, they are all AC, and that if you are talking automotive applications, you are most likely talking some sort of power storage which will be DC, and through a series of control electronics, gets converted to AC for the motor (you can not have a motor that runs on straight DC really... you need some way to vary the phases). So for the most part, every motor in a car would be considered a DC motor because its run off a DC source.
        • 8 Months Ago
        There is a difference between the electric machines used in AC induction and Brushless DC motors. AC induction motors are asynchronous AC machines, while brushless DC are synchronous. The former has a squirrel-cage conductor (or windings other type of conductor) in the rotor, while the latter has permanent magnets in the rotor, allowing for synchronous rotation of the rotor and stator fields. In asynchronous machines, there is a difference in rotational between the rotor and stator fields. This slip is what gives rise to torque in these motors, since the rotor field comes from the current induced in the rotor conductors.

        DC Brushless tend to be more efficient, but are more expensive due to the permanent magnets.
      • 6 Years Ago
      "Whether induction motors or PM motors is the more relevant question."

      This hits the spot. There are significant advantages and disadvantages to AC induction and Brushless PM "DC" motors, and the terms AC and DC don't necessarily mean what they did before complex drive electronics came into play. AC Induction can give you simpler control electronics and good efficiency, but stall torque is poor, and the electromagnetic circuit is pretty much a point design that's a bear to optimize. Simulating / forecasting performance is a challenge, and they can be very sensitive to materials processing. And you need an AC power source. At least that's what I would hear from the AC engineers :)

      Brushless PM "DC" motors typically run cool (the copper's all where the heat can conduct out of the motor), have very high power densities, and also run efficiently. But their control electronics are expensive, relatively unreliable (lots of ICs and they run hot), and they usually require some type of external commutation circuit. They're also usually much more expensive, due in large part to the exotic magnet material and the electronics costs. They may have temperature range limitations as well, due to magnet material choices. The "DC" is usually a PWM input to the motor, and you'll typically have either a Sine or Trap waveform depending on the winding and what your goals are with the output (higher power density or lower output torque disturbance, respectively). They're referred to as DC motors these days because the equations that govern their performance (Kv, Kt, Km, etc.) are the same as those used for brushed DC motors. They scale easily, and it's much easier to design a BLDC E-M circuit since the interdependencies are not nearly as tricky as in an Induction motor.

      I wouldn't say that Brush DC motors are a misnomer; the commutated windings are seeing only a DC power input, but they are switched on and off based on the shaft rotation and the commutator bars contacting the brushes. They have the lowest reliability and life due to high mechanical complexity and designed wear surfaces. They also run very hot as their copper is typically on the rotor, where it's quite difficult to remove heat from a closed design. PM brush motors have high power density and low cost, and the control system is essentially a rheostat. They still have their uses.

      Jick, the laid off motor engineer...
        • 8 Months Ago
        I can't believe the author didn't write the biggest advantage of AC over DC: regenerative braking is only supported thru AC because it acts as a generator while DC, to my understanding, is not.
      • 6 Years Ago
      That wasn't really about AC motors at all. Not the greatest title, especially since AC motors are much more interesting than DC motors.
      • 6 Years Ago
      The article almost says that AC will be more efficient for power transfers over longer distances, which is not generally the case. High voltage DC current can actually be more efficient than high voltage AC current.

      For more information, just read this: http://en.wikipedia.org/wiki/HVDC
        • 8 Months Ago
        right, but it's only recently that we've been able to make DC-DC converters capable of handling that kind of power. For much of history, it was AC or nothing.
      • 6 Years Ago
      Wow. This one is messy. The original article and all the comments.

      Batteries are DC. That's about all that is DC in a car.

      The Series Wound DC Motor, vs. the AC induction motor is the debate guys. And it really has not much to do with AC vs DC.

      BOTH motors get a pulse width modulated AC waveform, typically in the 16-20 kHz range, quite different from 60Hz AC power you are alluding to. By varying this waveform, we can very efficiently vary the power to the motor with very low losses.

      But in both cases, we have a DC power source that is converted to an AC waveform by the "controller" and applied to the motor.

      Now the "debate" is between two types of motors, and their controllers are quite different as well.

      The series wound motor is typically referred to as a DC series wound motor causing part of the confusion. In a static setup, it could indeed be run on a set DC source. We don't do this in cars, but it could be to run a fan or something. The power is applied to both the case windings and the rotor windings in series. This causes a magnetic reaction between them and so rotory motion. In MOST series wound motors, there is an armature and brushes to sequentially reverse the current direction in the rotor.

      Because full power goes through both the case windings (stator) and the armature (rotor) it produces very high torque on startup. Because of the brushes and armature, you are somewhat limited in the peak voltage you can apply, (typically less than 200v) and the rpms (typically less than 5000). It is also somewhat difficult to reverse the connections for a series wound motor to deploy regenerative braking.

      The AC induction motor, notably has no brushes and no armature. Rather, 3 separate and distinct AC waveforms are applied to the case windings and effectively "rotate" or cycle around the case windings. The rotor has no brushes, nor indeed any electrical connection at all. The conductors in the rotor are magnetized by current "induced" in them across the air gap by the magnetic field in the case windings. This causes the rotor to "react" to the rotation of the fields in the case. Basically the rotor then follows the rotation of the 3 phases applied to the field windings in the case.

      Because there is no power applied directly to the rotor, it is a bit torque challenged when compared to a series wound motor of the same size. But since there are no brushes and commutator, much higher voltages can be used - typically 300-600 volts and as a result the conductors to the motor can be smaller. For the same reason, the rpm limitation becomes the limitation of the bearings primarily and there are AC induction motors that can spin up to 12000 rpm.

      Broadly, the AC induction motor has a slight edge in that it just requires no maintenance, no brushes, and indeed, there are examples of AC induction motors in continuous use with no maintenance for nearly 100 years.

      But while the series wound motor controller has a single PWM waveform, the AC controller, also referred to as an "inverter" for purely historical reasons, provides THREE PWM waveform outputs and of course they have to be very closely coordinated with each other. The AC controller must also be very closely matched to the specific motor design. You can't just take any AC controller and run any AC induction motor with it.

      So the AC controller becomes a pricey item.

      Bottom line is that excellent DC series wound motors and controllers can be paired up for $7000 or so. Good AC induction motor/controller combinations are somewhat rare, a little bit tricky in operation, and are often in the $10,000-$12,000 range. Systems from AC Propulsion can be $25,000 and UQM as high as $35,000.

      Regenerative braking is EASIER to incorporate into the AC controller.

      The advantage of higher voltages and correspondingly lower currents, zero maintenance, and the incorporation of BMS, DC-DC conversion, and charging functions INTO the AC controller, rather point to AC induction motors as the future for electric cars.

      Jack Rickard
      • 6 Years Ago
      "the powerplants that we hope will soon be transporting millions of people"

      They already do:

      • 6 Years Ago
      This one needs a full re-write. See the above comments but do not get too far into the weeds, include a better balance of AC to DC motor tech and include some helpful motor diagrams not a photo of a Toyota logo.
      • 6 Years Ago
      AC has lost nearly all its advantages since Edison versus Tesla (which Tesla rightly won). In high power applications, DC is still tough to convert, but in applications up to and including these cars, DC is the clear winner. You can use it easier and you can convert it (and regulate it) as easily or easier.
        • 8 Months Ago
        If DC is a clear winner, then why did the T-zero, Tesla roadster, and Chevy Volt use AC motors? You know something they don't?
      • 6 Years Ago
      I think that in the futur we will see many different electric motors transmissions combo. This game just started. The choice are endless and an engineer with some driving skills and taste can manage a car with good drivability and efficiency, battery or fuelcell or battery+ generator. Me i will like electric 4 wheels drive with hub-motor with or without a demultiplication gear ratio, that way the stress on mechanical components is reduced to the max contrary to a corvette where the propulsion system overload and upset the chassis with huge weight and huge inertia and loose in the entire drivetrain and a lot of mechanical losses.
        • 8 Months Ago
        Since the dawn of the automobile, the differential and its various incarnations have allowed us to ignore the fact that when a vehicle turning, wheel speeds are never matched nor should they be if we want optimum traction. The torque/hp characteristics of IC motors coupled with this deliberately differentiated delivery of power means we don't get in situations (or minimize them) where a single drivewheel produces way too much forward motion resulting in torque steer or a spin. Hub motors could certainly be made to guess what to do in all situations, but don't discount the fact that the differential is a such a simple solution to an early problem, that we take it for granted. I predict if we go hub motor, that a least one manufacturer along the way will end up with an Explorer/Firestone design nightmare where people get hurt due to smart computers/sensors failing to work precisely at speed.

        Of course the weight in the wheels is freaky bad too, but hopefully materials will keep that in check. Not saying hub wheels can't work but 4 are probably overkill in complexity/cost/weight, except for snow/sand.
      • 6 Years Ago
      hi frankbank, I'm preparing to convert a car to electric. What motor will you suggest then to cheaply convert one without sacrificing efficiency and power. thanks a lot.
        • 8 Months Ago
        DC motors are far less expensive than AC when it comes to electric cars because the control circuitry is simpler. All it has to do is basically control voltage with an enormous transistor(usually made up of smaller mosfets), while an A/C motor requires that the power fed to it is is a sine wave of consistent frequency. Additionally a DC motor could even be controlled by a rheostat or even using 2 resistors and some switches to get 3 different power outputs. Although it would be much less efficient and precise than PWM. While AC motors are most efficient when using 3-phase power. If you are considering AC I would suggest AC propulsions kit, I think its around $10-15000 though.
      • 6 Years Ago
      The advantage of brushless DC and AC motors is all in the lack of brushes, which does a couple of things. First, by eliminating the brushes, you significantly decrease the service required on the motor. Secondly, because of the lack of brushes, the rotor of the motor is only supported by bearings at either end of the casing, which allow it to reach very high speeds. A good brush DC motor is limited to roughly 4000-5000 RPM, while an AC induction motor can easily top out at 12000-14000 RPM. That's where the high power density numbers of AC motors is found, because they spin like 600 cc sport bike motors
      • 6 Years Ago

      You really do need to get into the science of a little bit. If you really don;t understand motors, no crime there, but lets not just make up stuff and pretend.

      Edison vs Tesla on AC s DC was about power lines, not about motors.

      Seriously, all motors are AC in sense that they are commutated and there is an AC waveform. The AC could be sinusoidal or trapezoidal. The AC could be made with brushes or with an inverter. When brushes are used the wavefrom is trapezoidal (sometimes " called squarewave"). These motors have been referred to as "DC" since there is no inverter and the commutating brushes are in the motor frame. DC connects directly to the motor. But there is an AC waveform running the motor. When a PM motor uses brushes, they are sometimes called "PM brushed motors". WHen PM motros are trapezoidally commutated, they are sometimes call "Brushless PM motors". A total misnomer to call these DC, but there you have it. Toyotas, GM;s Hondas, Ford's and pretty much everyone's motors are inverter commutated PM motors that can make square waves or make sinusoidal waves, depending on driving conditions.

      AC induction are also sinusoidally commutated. These are the motors in the tesla and the were in EV1s. They are AC induction motors, but they require inverters and controls with the same or more sophistication than PM motors. I am sure that the Volt motors will be inverter commutated and not brushed, Whether induction motors or PM motors is the more relevant question.
        • 8 Months Ago

        my thoughts exactly. except I couldn't have explained it that well. thanks!

        • 8 Months Ago
        Now, I know it's tough to say DC is "square" when you have all that inductance (and some capacitance) in there playing hell, aren't motors all run on switched DC now? I mean, I know it will alternate when fed (as you switch it different ways), but you start from DC and you switch it as necessary to get the speed and torque you want at the efficiency you want.

        None of these motors work as a traditional synchronous AC motor where you feed in power with an unchanging frequency and the motor just runs at a single speed. In that way, I always think of them as DC, even though if you put a scope on the inputs it'll show inversions and nulls (perhaps more accurately gaps) over the place.

        I mean, a key part of running these motors is you want to apply power exactly when you need to, and if you fed in AC, the nulls on the supply power would play hell with this idea, so you turn it into DC first, then chop it up as needed.

        Am I wrong?
        • 8 Months Ago
        hi frankbank, I'm preparing to convert a car to electric. What motor will you suggest then to cheaply convert one without sacrificing efficiency and power. thanks a lot.
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