Electric vehicle motors (9)

AC/DC

There are two main kinds of electricity: alternating current and direct current.

Alternating Current (AC) is where electric current constantly reverses its direction. It is that used in grid power supplies in Australia and many other countries  cycling at 50 times a second -  and in America and some other countries where it cycles at 60 times a second. 


Tesla Roadster AC motor. Pic: Tesla.

The AC induction motors used in a few electric vehicles have a stator (stationary coils of wire) that when AC electricity flows through them generates a strong rotating magnetic field. That rotating field causes a rotatable so-called armature to revolve in the direction of the rotating stator’s magnetic field. It does so in synchronism with the frequency of the alternating current oscillations.

That a basic induction motor revolves at a speed directly associated with the frequency of the applied ac voltage necessitates that frequency to be instantly variable to allow for changes in the vehicle’s speed. This is not an issue as the batteries DC must be converted to AC by a so called ‘inverter’. It readily feasible to have one that has its frequency variable over whatever range is required.

Tesla presumably chose an AC induction motor for its Roadster model because whilst less efficient it can produce more power.  Significantly, however, Tesla uses the brushless BLDC motor (described below) in its smaller and well selling Model 3.

By and large AC induction motors have been confined to hybrid vehicles –that use the electric drive for limited commuting – so efficiency and hence range not a major factor. There is however (as of April 2019) an increasing trend towards brushless direct current DC motors for electric vehicles generally.

Direct current (DC)

Direct current is a flow of electrons in one direction. Whilst often (wrongly) credited to Edison it was originally conceived in 1800 by Alessandro Volta. It is that which is stored in all batteries.

Most do-it-yourself builders use DC as it is simpler and also enables such builders to use DC electric motors designed initially for other purposes. Another advantage of DC motors is that they have so much low-speed torque that no gearbox is required. They are also electrically very efficient. There are, however, many variants of DC motors that combine the benefits of both AC and DC. (One, the Brushless DC motor is described later in this article).

A basic DC motor has fixed external magnets that surround a so-called revolving armature that is an electromagnet (and also the drive shaft). Direct current is fed to this electromagnet via a commutator.

Commutator & brushes

The commutator is the weak point of a basic DC motor. It is a small ‘drum’ (part of the drive shaft) made of an electrically-insulating material on which are a number of copper segments. Carbon brushes are sprung against these segments. 

A DC electric motor’s commutator – one carbon brush is attached to the yellow lead
– a second (out of sight) is on the left

Direct current, fed to the revolving armature via those brushes, creates a magnetic field in the armature. This causes the armature to spin through 180 degrees. To keep it spinning, a further mechanism causes the DC current fed to the brushes to change the DC’s polarity for the second 180 degrees, and so on . . .

These motors work well but the spring-loaded carbon brushes constantly pressed against fast rotating copper segments wear out and constantly spark – causing a potential fire hazard and electrical noise that must be suppressed.

Brushless DC motors

The brushless DC motor (BLDC), in effect turns the motor inside out. It has permanent magnets on the rotor and the ability to generate a rotatable magnetic field on its outside. Rotation is caused by an electronic sensor that monitors the angle of the rotor and, via high power transistors, applies current to generate that external electromagnetic field so as to create a torque (turning force) in the required direction.

Brushless DC motor – Pic: original source unknown

Maximum torque at zero speedA major advantage of brushless DC motors for electric vehicles is that they develop maximum torque at zero rpm (revolutions per minute). They are also more efficient electrically. They have no brushes that wear out and no need for internal cooling – enabling its internal bits and pieces to be fully enclosed and thus free of contamination.

Another and truly major benefit is that these motors not only produce far more torque than fossil-fuelled motors of the same size and/or weight, but can revolve at far greater speed. They are relatively light and compact – with their available power limited primarily by heat.

There are minor downsides: BLDC motors cost more to make than their brushed counterparts and at present the strength the permanent magnets’ field is not adjustable. Work is in progress, however, to make it so, thereby increasing maximum torque at low speeds when required.

Brushless DC motors cost more than most electric motors but are nevertheless proving commercially successful – particularly with Tesla’s Model 3. It currently  seems likely they will dominate the market.

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