What Are Electric Motors?
In their simplest form, electric motors are devices which convert electrical energy into mechanical energy. They can run off DC or AC electricity, something that we will compare later in the article.
Electrical current is used to drive a rotor. This rotor can be connected to a multitude of things, including shafts and even refrigerators through a mechanism.
More recently, electric motors have been used for a variety of purposes. For example, a small vibration motor in some smartphones creates feedback based on the users input. As a result, the phone may vibrate when ringing. In order to create a vibration, the motor has an off centre weight attached to its output shaft, creating an asymmetric motion which goes on to cause a vibration.
On a larger scale, electric motors are used inside electric cars, trains, as well as prototypes of electric planes.
A Short History of Electric Motors:
The first electric motors came about in the 1740s, with the help of American experimenter Benjamin Franklin. These could only be described as simple electrostatic devices, but were the first ember in a technology that would go on to change the way we generate and consume electricity for the next few centuries.
The first DC (direct current) motor capable of turning machinery was created in 1832 by William Sturgeon, a British scientist. Over the years to follow, there were may attempts to create useful and successful DC electric motors.
The newly founded technology soon found its niche in running printing presses and other industrial machinery, however it was riddled with drawbacks. Although the motors were perfectly capable of running machines, the cost of batteries to run them was simply too high at the time and the grid was not sufficiently developed.
By the time the first practical DC motor arrived in 1886, the grid had advanced sufficiently to allow an industrial scale application of it. Frank Julian Sprague invented this new, more useful motor, which was put to use running the first electric trolley system in Richmond, Virginia.
A little later on, a motor similar to this was behind the functioning of the first electric subway with independently controlled cars. Instead of a system like a winch driven drum running the cars from afar, electric motors could be attached to the cars individually, running them independently of one another.
Given that many of the worlds systems ran on AC, including the power grid, the search for an AC motor was on.
Although multiple attempts were made to produce electric motors, Nikola Tesla conceptualised the first industrial development of them. Yes, that’s where Tesla the electric car manufacturer got its name from.
Despite the anticipation, the AC induction motor was first found to be unsuitable for street cars. Consequently, it was used in the mining industry instead.
Shortly after that, developments of the AC induction motor allowed for hydroelectricity to be produced at the Lauffen waterfall on the river Neckar in Germany. This was one of the first sources of clean, sustainable energy in 1891 when it tested.
Following on from the success of the AC motor so far, the General Electric company decided to start developing the technology further. Ever since then, the AC motor has become incrementally better and better.
AC vs DC Motors
It goes without saying that the main difference between the two motors is the power source they use. AC motors use alternating current, which changes direction based on a set frequency (usually around 50 or 60 times per second in the national grid). This creates a sine wave on an oscilloscope. Conversely, DC motors use direct current, which constantly flows in one direction. This creates a straight, horizontal line on an oscilloscope.
How Does a DC Motor Work?
In its simplest form, a DC motor consists of a coil of wire placed between the poles of a permanent magnet. The coil is free to rotate within the magnets casing, and can be attached to an output shaft to rotate something.
In order to make the coil rotate and drive the output shaft, a current is passed through the wire. To understand what is happening here, it’s important to understand a simple concept; the motor effect.
When opposite poles of a magnet are placed close to each other, they produce strong magnetic fields. Moreover, a coil of wire (solenoid) with a current flowing through it also produces a magnetic field.
When these two fields are combined and placed perpendicular to one another, a force is produced at right angles to the two magnetic fields. The direction of this force is determined by the directions that the magnetic and electric fields act, but within a motor, the force causes the coil to rotate.
However, there’s one problem. Once the coil passes its halfway point (i.e. is vertical on the diagram above), the force then starts acting to push the coil in the opposite direction. As I’m sure you can imagine, a motor that reverses itself every half turn is pretty useless in most applications, so engineers worked to change that.
If the polarity (determines which way the current is flowing) was simply just reversed, then the rotation of the coil would continue in the same direction, just what we want. To achieve this, a commutator was installed where the coil connects to the power source. This does just that, flipping the polarity every half turn and keeping the motor turning the same direction.
Despite the dramatic improvement, the rotation of the coil still slows down and speeds up in a sinusoidal manner as it approaches and passes the flip in polarity caused by the commutator.
The solution was as simple as adding more evenly spaced coils around the rotor, smoothing out the transition. As a general rule, the more coils, the more consistent the angular rotation of the motor.
How Does an AC Motor Work?
Similarly to the its DC counterpart, the AC induction motor consists of a stator (stationary part) and rotor (rotating part). The stator is on the outside of the motor and the rotor is on the inside.
Usually, the stator is filled with coils of wire, as well as tightly packed steel laminations to enhance induction. When three-phase AC is passed through the coils, it induces an EMF in the magnetic rotor, causing it to spin. This spin can then be used to drive a shaft
I’ve explained how the motor produces a rotation in another article on electric car acceleration, so I’ve pasted that here:
Take a look at the picture above, you can see blue and red coils on the motor, they are wired in series, but in opposite directions. Here’s how it works:
As the electromagnet coils are supplied with AC, they produce magnetic fields. Since AC current comes in the form of a sine wave, the movement of the rotor is smooth. Additionally, when the red coils are at peak activity, the blue coils are at zero activity, since the AC current is opposite for each.
Furthermore, the magnetic fields produced by the coils create an induced potential current on the rotor in the centre. Since the current in the rotor produces it’s own magnetic field, it opposes the magnetic field that the coils produce, exerting a force on the rotor. Finally, since the current keeps changing between the coils, the rotor continually spins in one direction.
The rotor then goes on to drive something. In the case of an electric vehicle, this goes on to rotate the wheels.
Why Are AC Motors Favoured in Electric Cars?
In many EVs, controllers are used to convert the DC from the battery to three-phase AC for the motors. It does this using very large transistors which rapidly turn the batteries voltage on and off in the form of a sine wave.
There are a few key advantages of using AC motors over DC motors in electric vehicles, despite the need to convert the electricity to use it.
Supplying the motor with electricity causes a rotational output. However, AC motors have the special property that a rotational input can be converted into electrical output.
For example, when travelling downhill, you would usually put the brakes on to slow down. This works purely off friction, wasting all the kinetic energy that is being accumulated by going downhill.
Conversely, electric cars can turn their motors into generators. This is called regenerative braking. In many cases, EV owners never really need to use their brakes as the regenerative braking is strong enough to slow them down if given some warning.
In fact, regenerative braking is one of the largest contributing reasons as to why Tesla’s are able to travel as far as they can on a single charge.
The AC induction motor is extremely easy to control. It’s as simple as changing the frequency of the input electricity. A higher frequency causes the motor to spin faster, directly translating to the wheels spinning faster and, as a result, the car going faster.
Furthermore, the simplicity of controlling them means that there are fewer complex parts needed in the vehicle. For the consumer, this means a cheaper car (as manufacturing costs are lower). For the manufacturer, it makes the car easier to repair.
DC motors aren’t quite as easy to control as AC, meaning they are less favourable.
We’ve already gone over controllability, however this controllability also acts over a vast range of RPMs, especially compared to the DC motor. They produce vast amounts of torque for a large range of RPMs, meaning the car just keeps accelerating until around 100mph or so when the acceleration slows.
Due to this, most electric cars only use a single speed transmission, meaning there’s no gears. With no gears and regenerative braking, it’s not too difficult to drive with just one pedal, something that Nissan are working on with their Leaf.
Furthermore, a perk of a single speed transmission is that, hypothetically, an EV can go almost as fast in reverse as it can going forwards. Fortunately, Tesla made the sensible decision to limit this to 15mph through software.
Conclusion: The Future of the Electric Vehicle Drivetrain
In the near future, it’s extremely likely that AC motors will remain a prominent feature of electric cars. Currently, motors are by no means the bottleneck of electric cars. It’s more battery energy density that’s the issue. Until they become a bottleneck, they probably won’t be replaced.
Over the next few years, you may see experimental technology being trialled as a propulsion method, for example the SpaceX options package on the Tesla Roadster 2nd generation. Although this isn’t particular new technology, cold gas thrusters have mostly been used on motorbikes for cornering up until now, rather than cars.
In the distant future, AC motors will probably be replaced. Some sort of technology will supersede them as it will be better in almost every way.
Despite this, I think we’re going to be seeing the humble AC induction motor hanging around for a great number of years to come.