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 vehicles. Being a tried-and-tested technology, they are actually quite good – with efficiencies up to 90% and only needing servicing every 100,000kms or so. However using a commutator is restrictive and a source of inefficiency. Also, with series DC motors regenerative braking is very difficult to do (in fact, you basically have to operate the motor as a sepex DC motor).

Permanent Magnet DC Motor

DC Motor with wound stator

In a brushless DC motor (BLDC), the rotor has permanent magnets and the stator has an electronically-controlled rotating field, using sensors (rotary encoders or back-EMF) to detect rotor position. As such they have no commutator, and tend to be more efficient and more powerful 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 speed controller. Unfortunately, at present there are no economically viable BLDC options available for EV use. ZEVA is hoping to change this in the near future.

Brushless DC Motor ("in-runner" type)

Although there are a variety of motors which will run on AC power, 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 tried 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 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, AC systems currently cost about twice as much as series DC.

AC Induction Motor