Tech Info: Motors


At the heart of every EV is an electric motor. This is what makes the car actually move, instead of an Internal Combustion Engine as used in conventional vehicles. By comparison electric motors are very simple machines offering extremely high reliability, with virtually zero maintenance, and efficiency up to 98% (vs around 25% for an ICE). Electric motors come in a huge variety of designs, but the three most common topologies used in EVs are are DC, BLDC, and AC induction.

DC Motors

There are four main types of DC motor, namely permanent magnet, series, shunt and seperately excited. The latter three all use field coils in the stator (the part which doesn't move) to generate a magnetic field for the rotor to spin in, and their name simply refers to the way the field coils are wired with respect to the rotor coils. All four types use a commutator to control which rotor coils are energised at any given time in order to maintain rotation, and it is enough just to apply a DC voltage across their terminals to get the motor to spin, so they are relatively easy to control.

Currently series DC are the most economical and commonly used type of motor in electric vehicle conversions. Being a mature technology, they do perform well, with efficiencies up to 90% and only needing servicing every 100,000kms or so. However using a commutator is restrictive and is a source of inefficiency. Also, with series DC motors regenerative braking is very difficult to do, so is not commonly seen. Regen can increase your range by 10-20%, so is quite valuable.

Though still popular for EV conversions, all modern mass-produced electric vehicles tend not to use DC motors, instead favouring BLDC or AC induction motors.

Some popular brands of DC motors for EVs are Advanced DC, NetGain and Kostov Motors.

Permanent Magnet DC Motor

DC Motor with wound stator


Brushless DC Motors

In a brushless DC motor (BLDC, also known as Permanent Magnet Synchronous Motor/PMSM or Permanent Magnet AC), the rotor has permanent magnets and the stator has an electronically-controlled rotating field, using sensors (rotor angle sensors or back-EMF) to detect rotor position. As such they have no commutator, and tend to be more efficient and more power-dense than commutated motors. They do require a more complicated motor controller, although as the technology matures and costs come down they are becoming increasingly popular, particularly for smaller motors.

The main disadvantage for EV use is the cost of the large permanent magnet(s) required for the rotor, and the added expense of the more complicated motor controller. At present BLDC drive systems are still considerable more expensive than their brushed DC counterparts.

Brushless DC Motor ("in-runner" type)


AC Motors

Although there are a variety of motors which will run on AC power (including BLDC above), for EV use it usually refers to AC induction.

The operation of induction motors is a somewhat difficult concept to grasp at first. Basically they use a rotating magnetic field in the stator to induce a magnetic field in the rotor and hence a current to flow in the rotor's coils. The rotor coils actually just loop around on themself - they are not explicitly powered. The induced field in the rotor tries to stay aligned with the rotating field of the stator, so it turns to chase the stator's field. Due to loads on the motor, the rotor's field is forced to rotate slightly slower than the stator's field (if it kept up exactly, there would be no difference in the fields and hence no torque).

Three phase induction motors are very common for industrial use because they are highly efficient and reliable. These same advantages apply for electric vehicle use, except for the added complication that a variable-speed inverter (sometimes called a Variable Frequency Drive or VFD) is required to control the AC motor from a DC power supply (the battery). These are a relatively expensive piece of hardware. Although they do include regenerative braking and are generally more efficient, primarily due to the expensive controller AC systems currently cost at least twice as much as series DC for equivalent power.

The most popular brands for AC induction motors suitable for EVs are Siemens and Azure Dynamics. It is also possible to use industrial AC equipment from manufacturers such as ABB and Danfoss.

AC Induction Motor


The Future

Currently, the most economical (and indeed common) option for electric vehicles is series DC technology. However, it is likely that all commutated motors will be phased out over the next decade or two, since a commutator's functionality can be replaced by clever electronics (which is getting cheaper every day), and with improved reliability and efficiency. At present, AC induction and permanent magnet brushless DC are the best technologies available, with efficiencies up to 98%, very quiet operation, and almost never requiring any servicing. They each have various advantages and disadvantages over one another so it will be interesting to see which one becomes the new standard in the years to come.