One Finite Planet

EV Literacy: AC, DC and Electric Motors for dummies.

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Table of Contents

For almost 100 years, people have grown up in an age of the internal combustion engine. For many people this has meant an understanding of engine capacity, cylinders, spark plugs, engine compression, crankshafts, valves, turbochargers, exhausts etc.

The bad news is that EVs mean so much that previous literacy is about to be consigned to history and replaced by induction and permanent magnet motors and other new terms.

The good news is, it is easy to build and EV literacy on the those ICEV foundations, so there is no need to feel illiterate in this new EV world.

For almost 100 years, people have grown up in an age of the internal combustion engine. For many people, this has meant an understanding of engine capacity, cylinders, spark plugs, engine compression, crankshafts, valves, turbochargers, exhausts etc.

The bad news is that EVs mean so much that previous literacy is about to be consigned to history and replaced by permanent magnet and induction motors, regen braking, and other new terms.

The good news is, it is easy to build and EV literacy on the those ICEV foundations, so there is no need to feel illiterate in this new EV world.

What is worth understanding?

At least a basic understanding the two main motor types used in EVs is useful. So many articles available are confusing, often because the author is outside their field, that confidence is needed not be mislead.

While I feel this background is useful, the section on what is changed by having electric motor is more relevant to everyday driving EVs, as opposed to comparing technologies.

It is also worth looking at how the distinction between AC and DC can become blurred, and while induction is a more advanced topic, some understanding may also be interesting, and
I will provide links more and content for more and more in depth information in future.

Of course, knowledge on batteries, charging and range all also help understanding EVs.

While I believe it is easier to understand EVs than internal combustion engine vehicles (ICEVs), most people went through various steps over many years in gaining their understanding of internal combustion engines vehicles, and there has been less time to build equivalent knowledge. The sections here are my suggested steps, and my suggestions is to pause at any step that feels new to allow it to sink in.

Magnets, DC and AC electricity.

Electricity and magnetism.

Electrons always move, and when they do, the create a magnetic field. In any material, even at absolute zero, electrons have motion, so there are magnetic fields. Normally, as these fields are randomly arranged, magnetic fields mostly cancel each other out, so when looking beyond individual atoms, there are only extremely weak magnetic fields. However, weak magnetic fields are everywhere, and MRI machines (magnetic resonance imaging) work by imaging the magnetic fields inside the human body.

However, instead of the magnetic fields mostly cancelling out resulting in weak magnetic fields, getting electrons mostly moving in the same manner generates a far more useful magnetic field. Then the magnetic fields can reinforce each other instead of cancelling out. Generally there are two ways to get electrons moving in the same manner:

  • A crystal structure that resulting in electron movement being aligned: a ‘permanent’ magnet.
  • Make a flow of electrons as ‘electric current’, creating an ‘electro-magnet’.

Every wire with electric current flowing through it has a magnetic field around that wire. Every wire. This gives some idea of just how prevalent magnetic fields are.

This means magnetic fields are everywhere, but not all magnets fields result in what we recognise as magnet. A magnet requires the magnetic field to result in magnetic poles.

see also: How Electricity Actually Works

In reality, it is not the electrons moving though the wire that carries the energy from one point to another, but rather, it is the electrical and magnetic fields that transfer the energy from point to point. The role of the movement of electrons, it to generate the magnetic field that carries the energy, as explained in the Veritasium video.

But to focus the magnetic field to create a magnet, with north and south poles, requires making a coil with the wire generating the magnetic field.

Magnetic fields from electromagnets have the advantage that they can be switched off and on, and even reversed by reversing direction of the flow of electrons. In practice, there are enhancements to just making a coil such as adding a core to concentrate the magnetic field, but principle of using a coil to generate a magnetic field is used over and over within electric vehicles.

DC and AC.

Most of us have some idea of DC and AC as ‘direct current’ and ‘alternating current’. The constant voltage from a battery or DC plug pack is clearly DC, and the power from a main socket is clearly AC.

The DC most of us are familiar with is low voltage, and low power. The first hint that those old rules do not always apply is that batteries in EVs can produce even 900v of DC, and for EV charging, the AC is lower power than the DC charging.

The graph here shows a DC voltage that is slowly decreasing in the manner of a battery losing charge, and AC waveform and ‘Square wave’ which takes the form of a series of DC voltage pulses. Note that a real ‘square’ wave is not quite square, as it always takes time for the voltage to rise or fall, so the sides of the pulses are never quite vertical.

But is the ‘square’ wave AC or DC? The square wave ‘alternates’ in amplitude, but relative to the zero volts line, it is always a positive voltage, which could suggest this is still a DC signal. However, relative to the grey line, the ‘square’ wave is no longer always positive, so is the square wave DC relative to zero volts, and AC relative to the grey line? If you ask a large group of electrical engineers, you will get some answers that a square wave can be DC if it never passes zero volts, and other answers that say in practice, zero volts is always relative to something else, and there is nothing unique about any voltage, a square wave can never be considered DC.

Following this logic, an AC voltage is any voltage that alternates over the time relative to any fixed voltage, and can be seen to have a frequency over the time being considered.

This means over the time of this graph, the ‘square’ wave, and the red voltage are both AC, and the green voltage is DC.

But time matters. Within the time of the first pulse of the square wave, the square wave is DC, and so is the red voltage, as while rises during that time, it does not fall. Over a period of a week, our green voltage would also become AC if that battery is charged once every day.

It becomes clear that while the difference between AC and DC can in some cases be very clear, there are also times when the difference is no so clear. It turns out that when when considering motors, this ‘it depends on the perspective’ difference between AC and DC can result in motors which are the same, can sometimes be described as AC, and other times be described as DC.

Electric Motors: and you could say they are all AC!

Principles of all electric motors.

If you get a rotor and attach some magnets to it, it becomes obvious how another magnet could be used to cause the rotor to rotate to a new position. But to make the rotor keep moving and spin, it becomes clear that the magnet used to first move the rotor would need to either be moved, or reversed to move the rotor further. Keep moving the magnet around the rotor and you then you can make the rotor spin, but that is suing something already spinning, to make something spin.

The alternative is to switch to polarity of either the magnets on the rotor, or on the housing of the rotor at the right time so that the rotation of the rotor never ‘catches up’ and becomes aligned with magnets on the housing. As you can’t switch the polarity of permanent magnet, either the rotor magnets, or the casing magnets, or both, will need to be electromagnets. To summarise:

  • A set of magnets on the rotor, and another on the casing, where at least one set is electromagnets.
  • A way of reversing the polarity of electromagnets at the right time to keep the rotor spinning.

Reversing the polarity of electromagnets periodically requires reversing the current periodically, which requires AC power for the electromagnets. This means all electric motors require AC power for their electromagnets, and the frequency of the AC power will determine the speed of the motor.

With two types of motors, and two places to put each type, that makes 4 ways to build a motor:

  1. Permanent magnet rotor, permanent magnet case: Won’t keep spinning, doesn’t work.
  2. Electromagnet rotor, permanent magnet case: Brushed DC Motors.
  3. Permanent magnet rotor, electromagnet case: Permanent magnet / Synchronous motor.
  4. Electromagnet rotor, electromagnet case: Induction / Asynchronous motor.

With only 3 of them really possible, and only the last 2 used in EVs.

Brushed DC Motors.

How can a motor use DC, if the the electromagnets will require AC?

The solutions is the make the rotor magnets the electromagnets, and connect these magnets to the DC power though ‘brushes’ which make contact with connectors which rotate with the rotor. The result is the current through the electromagnets reverses, in this case with two brushes, twice every revolution. If the power to the motor increases, it spins faster, which means the electromagnets get reverse current at an increased rate.

The electromagnets get AC current, that automatically varies in frequency to match the speed of the motor. Brilliant!

The setup is:

  • fixed permanent magnets on the casing.
  • reversible electromagnets on the rotor.

However, the brushes themselves are a problem. The nature of electric circuits is that the flow of electricity though coils is ‘reluctant’ to change, which makes the brushes and contacts highly likely to spark, increasing the wear even beyond that already present from the friction.

The AC for the electromagnets is made in a clever way, but the life of the brushes can be a problem.

BLDC / Brushless DC / Permanent Magnet AC / Synchronous / PMSM.

General Principles.

Overall, any of these labels above can be applied to these motors that are basically the reverse of the brushed motor:

  • permanent magnets on the rotor.
  • fixed reversible electromagnets on the casing.

Adjusting the speed of the motor requires changing the frequency of the AC power to the electromagnets, which means a motor powered by mains frequency AC, would have a fixed speed.

Motor with 8 permanent magnets on the rotor surrounded by 12 electro magnets on the casing.

Since variable frequency AC requires starting from DC, this type of motor either is a fixed speed if starting from AC power, or requires DC power, which it will then be converted to either a square wave, or smoother waves like sine wave(s), at variable frequency to enable varying motor speed.

For variable speed motors, as used in an EV, it is necessary to start with DC and then created the variable frequency ‘AC’ that is needed for the motor.

The simplest way to get a variable form of ‘AC’ for this type of motor from DC power, is to switch the DC power off and on at the required speed so as to create a square wave, as shown in blue on the graph in ‘DC and AC‘ above. To a solenoid, such a square wave will act as very rough edged AC signal.

Changing the frequency of the switching off and on will, provided there is enough power, set the speed of the motor, and the rotor will rotate synchronously with the magnetic field created by ‘AC’ frequency.

The all ‘on’ and then all ‘off’ of a square wave is less efficient then using more complex circuits to generate waves more like sine waves, and most EVs will have such circuits which are sophisticated inverters with the ability to produce variable frequency, and often also variable voltage.

Confusion.

As you can see there are different names for the one overall motor type. Plus, there is confusion on DC vs AC, with both used within the same type of motor. Many articles written about motors where the author is trying to learn while writing, and getting confused on the way. Here is an example where BLDC and PMSM are listed as two motor types:

Similar to a BLDC, but the PMSM has – as you’d guess from the name – permanent magnets embedded in the rotor to create a constant magnetic field. 

Typical mistake: Everything you need to know about EV motors

Of course, every BLDC also fits that same description, which makes sense as BLDC and PMSM are the same type of motor. There are many, many articles with the same type of mistakes, as most are written by people trying to come to grips with the new topic.

One confusing area is this same type of motor can be used directly with 50 Hz or 60Hz AC mains power. When this motor type is used that way, the motor will have one fixed speed, and will be inefficient, but technically it is the same type of motor, and what has changed is how it is used, and how it is powered.

It can be seen that synchronous AC motors spin at the frequency of the AC, which can be taken to mean 50Hz or 60Hz AC, but in EVs, that is not what it means. When used in EVs these motors are driven by their own AC power, generated from DC and at a variable frequency in order to control motor speed, which is the type of use that gave rise to the label BLDC.

Sometimes, in some resources, the labels are considered reflect differences in the control circuits used to control the motor, but there is nothing universal. Some people consider small variations better fit using slightly different naming, while many others use the labels as interchangeable. This results in statements like “EVs use BLDC motors” on one site, and another site saying “EVs don’t use BLDC, motors” simply because one site is using the label ‘BLDC’ for the family of motors, and another is using BLDC only for motors within the the BLDC family that use a specific controller configuration, or do not smooth the square wave into a sine wave, even though the motors are all basically the same.

Variations: Axial, Radial, & Transverse.

There can be mentions of axial and radial flux, as in this discussion on the KoKoenigsegg Quark Motor, but these are all just different ways of arranging the windings withing permanent magnet motors.

Induction / Asynchronous Motors.

Brushed DC motors have electromagnets on the rotor, and Brushless permanent magnet motors have the electromagnets in fixed position on the casing, which leaves the only other option as electromagnets for both. OK, there would be fourth option of permanent magnets for both rotor and case, but it does not seem logical there would be a way to make that work.

Induction motors used the magnetic flux of electromagnets in the casing, to power the magnets in the rotor by induction, using the same induction principles as a wireless charger. This means no brushes, and no need for permanent magnets on the rotor.

The same general principle of needing to vary the frequency of the power supplied to the casing electromagnetic coils applies as with brushless DC/ permanent magnet motors.

However, they are also called asynchronous motors, as the rotor must rotate slightly slower magnetic field created by fixed electromagnets to induce current in the electromagnets in the rotor. The difference in speed provides the power for the rotor coils.

A key differentiating characteristic of induction motors is that they can spin relatively freely without power. Spinning either of the permanent magnet motor types will always generate electricity, as moving magnet relative to a coil will generate electricity in the coil. With the induction motor, when there is no power, there is no magnet, so the ‘generator’ is off. This enables a mode where no significant is power required to spin the motor, allowing a passive mode of operation. Induction motors when passive provide no regeneration power, but once activated with the right control logic can still provide regeneration.

What is changed by having electric motors?

No reverse of other gears ratios, no clutch, no starter.

Normally, the electric motor connected via a fixed gear ratio directly to the driveshafts to drive the vehicle. In some cases, the electric motors are in the wheels. Unlike ICEV where the motor always rotates the same way and a special gear is used for reverse, in an EV the motor simply spins backwards for reverse. The motor can be always connected, so no clutch is required.

Having a motor always connected to drive wheels means that unlike even an automatic ICEV, there can be no way to roll the vehicle without the motor rotating.

There are some EVs with a clutch, including the very rare EVs with two gearbox speeds, and some that such as the Hyundai Ioniq 5 AWD models which use a clutch disengage the second motor and save fuel when AWD is not needed and one motor is enough.

Of course there is no starter, nor alternator, nor associated belts, as electric motor(s) to drive the vehicle perform all these roles. With maximum torque often available at zero revs, the whole ‘launch’ problem is non-existent.

Regenerative Braking and One Pedal Driving.

Principle.

All EVs offer regenerative braking. In fact, as most EVs have permanent magnet motors, just trying to push the EV when it is off will result in some regenerative braking. The natural behaviour of an electric motor is to produce regenerative braking when the motor is forcibly rotated.

To put the physics in the simplest form, regenerative braking generates energy, and to obtain maximum regeneration, that energy must be consumed. Friction braking also generates energy, which is consumed as heat energy, but with regeneration, the energy is normally consumed by charging the battery. If the battery cannot accept charge, the regeneration effect will be minimal unless some other way can be found to consume the energy generated by the motor when it is rotated.

How much braking power? Why friction and regenerative brakes combine so well.

The more energy produced and consumed by regenerative braking, the more ‘stopping power’.

For friction brakes, the energy is consumed as heat, so the more heat that can be generated and dissipated by the friction brakes, the more stopping power. To generate the heat, the callipers need sufficient force, and the faster the vehicle is moving, the more force is required. Friction brakes normally increase in effectiveness as vehicle speed decreases. Friction brakes are very effective at bringing a slow moving vehicle to a complete stop.

For an regenerative braking, the more power from braking that can be applied to charge the EV battery, the more ‘stopping power’. To generate the electrical energy, the motor needs to be a powerful electric motor, and this time the slower the vehicle is moving, and thus the slower the motor is rotation, the more powerful the motor needs to be to generate the maximum power that can be applied to charging the battery from the rotation. Regenerative brakes normally decrease in effectiveness as vehicle speed decreases.

Regenerative brakes are almost useless, at bringing a slow moving vehicle to a complete stop.

ICEV brake pedal and accelerator pedal vs EV brake pedal and accelerator pedal.

On an ICE vehicle, the accelerator controls the engine, and the brake pedal controls the brakes. On newer vehicles, autonomous emergency stopping other systems dictate that no only the brake pedal applies the brakes, and ABS systems moderate how much braking effect is applied by the brake pedal, but there is most often a direct connection. On modern ICE cars, the accelerator usually no longer directly controls the fuel intake and now simply provides input to a computer, but it still controls only the engine.

On and EV the accelerator often also controls the brakes, and the brake pedal always also controls the engine. This is because braking always includes regenerative braking, and only sometimes include friction braking. So pressing the brake pedal will always activate regenerative braking by the engine.

For an EV to have competitive level of efficiency on an WLTP test cycle, given how it is calculated, (or EPA or NEDC test) , it is essential that regenerative braking is the predominant braking system during normal driving. Due to the much lower energy density of batteries compared to gasoline/petrol, prioritising energy recovery is essential to useful range, especially on urban cycles.

So the brake pedal must control both regenerative and friction brakes, or more accurately, must result in the vehicle’s control systems using regenerative and friction brakes as required. Generally friction braking will only be used when regenerative braking is insufficient, which will be either in emergencies, at very low speeds when regenerative braking becomes increasingly insufficient. The goal is usually to try and simulate the behaviour of friction brakes alone, as that is experience most familiar to drivers. With friction brakes, it is normal to ease off the brakes as the vehicles reaches very low speeds, to avoid aggressively bring the vehicle to a halt as the friction breaking become highly effective as speed falls. In contrast, if braking with regeneration alone, the driver would need to increase the amount of braking as speed dropped, as regenerative braking becomes less effective. The challenger for control systems is to smoothly manage the introduction of friction braking as speed drops towards zero.

When accelerating, input from the accelerator is used to control both amount of power from the battery to the motor, and the frequency of that power. These two factors cannot be directly controlled by by a pedal, as they do not retain the same ratios, the only solution is for the accelerator pedal to be an input to the vehicles computer, which can then do whatever software has been programmed to do, including anything controllable by software. What is also normally controlled by the accelerator pedal is regenerative braking, and sometimes even friction braking.

Regeneration Settings.

With any vehicle, ICE or electric, lifting off the accelerator produces deceleration. With an ICE vehicle, the driver can select different rates of deceleration by their choice of gear ratio, but with an EV, the deceleration from ‘lifting off’ is determine by how much power is directed to the battery. Most EVs allow the driver to select between different rates of deceleration in response to lifting off the accelerator. Making this deceleration in response to lift off smooth is not easy, as the regenerative braking is effect is initially quite strong, but then falls away as speed drops. To avoid drivers being caught out by the loss of declaration as speed drops, most vehicles will increase the level of attempted as speed falls, and the maximum power available from regeneration also falls.

One pedal driving.

With the strong initial deceleration available from lifting off the accelerator, it seems logical to continue that deceleration until the vehicle comes to a halt. This behaviour of continuing even strong deceleration from lifting off, until the vehicle comes to a halt, is known as ‘one pedal driving’, since it allows stop start motoring using just the accelerator pedal.

However, there are two myths around once pedal driving:

  • Some people think one pedal driving is a result of powerful regenerative braking.
    • Reality: Regeneration settings only change the amount of regeneration before using the brake pedal.
  • It can give the impression the vehicle can be stopped by regenerative braking.
    • Reality: Regenerative braking is almost non-existent at very low speeds, and far too weak to be used for fully stopping. Actual stopping either result from the motor being powered to stop the vehicle, or by applying friction braking.
  • There can be the thought that maximum regeneration will result in maximum efficiency.
    • Reality: Driving for maximum can be achieved equally well with any regeneration setting, and the settings just change how the pedals work, not how the vehicle works or its efficiency.

The most efficient driving is achieved by braking as little as possible. When braking is needed, it ideally would be regenerative braking as often as possible, but his can be achieved with either pedal.

AC vs DC motors in EVs.

There is a lot of confusing content about whether motors are AC or DC, even with EVs. As covered previously, inside all electric motors, the electromagnets are powered by AC.

However in all but fixed speed motors, where the speed is set by incoming AC mains power and are not suitable for EVs, the variable frequency AC for the motor(s) must being as DC, not AC.

So the motors use AC internally, even when described as BLDC (brushless DC motors), but the power for the motor always starts as DC.

So there is always both DC and AC in use within the motor, making AC or DC labels ambiguous and best avoided. Just stick the fact that EVs use only either:

The Uses of Different EV Motor Types: Permanent magnet vs induction.

Despite the use of different names for the one motor type, almost all EVs use a permanent magnet motor for their main power, and use an inverter to produce smooth 3 phase sine wave AC power for the electromagnets, as this is normally the most efficient arrangement.

Different sources list these same motors using different names, and sometimes really stumble over AC vs DC, thinking AC and DC are motor types.

EVs do also use induction motors. The main use of induction motors is nor for the main motor but, for additional motors, such as the second motor in a 2 motor AWD vehicle. The advantage for the second motor being induction is that when the second motor is not needed, simply cutting power to the induction motor, can allow that motor to spin freely. This is the simply solution for part time AWD.

Appendix.

Some simple terminology.

Power vs energy.

I plan to add to this section over time, but first the difference between power and energy is that energy is power multiplied by time. Power is instant, and a 100W light bulb will consume 100W at any instant when in operation, but will requite 100W hours of energy to operate for an hour.

Updates.

  • 2022 September 17: Added regeneration information.
  • 2022 Aug 29: First version.

Future Updates: Induction and other content to follow….