2.4.- TYPES OF ELECTRIC MOTORS

There are different existing types of electric motors depending on the poisitioning of their electromagnets and permanent magnets, as well as the kind of electric current in which they are fed:

2.4.1- BRUSHED DC MOTOR

It is based on the inversion of the dynamo. The stator, provided with electromagnets, is powered by DC, while the rotor, which also allows electromagnets, gets polarised trough conductive brushes which drive the electricity from the stator. This current equalises the polarity of the electromagnets to the one of non-switching magnets, a fact that induces a par in the rotor due to the magnetic repulsion.

PROS: It is simple to apply since it runs directly from batteries (which supply DC power) with needlessness of complex electronics. It is cheaper to produce than any alternative, and it is the most mature electric motor.

CONS: It requires important maintenance because of the erosion caused by the friction of its brushes. This friction produces important energy and speed losses in form of heat, and makes necessary a refrigerating system.

EFFICIENCY: Its efficiency rate is of 75-80%.

2.4.2- BRUSHLESS DC MOTOR

In this case, the rotor contains permanent magnets and the stator allows switching electromagnets. The polarity swapping of those magnets produces the necessary torque to move the rotor. This fact makes brushes unnecessary, but makes convenient an electronic system in order to determinate when is needed to change the polarity.

PROS: It requires less maintenance costs, and has reduced noise and energy losses. It works directly from batteries and has a higher top speed.

CONS: It needs complex electronic speed controllers to work, which increase its price. The properties under high temperatures, so it is convenient to install a refrigeration system.

EFFICIENCTY: Around 85%.

2.4.3.- ASYNCHRONOUS/INDUCED AC MOTOR

The stator is fed by a three-phased AC current which creates a magnetic field that induces current in the rotor, which is formed by a squirrel-cage or windings, producing movement. Currently, it is the most used motor for all ranges of EVs.

PROS: The engine is simple and cheap to build, and has low maintenance requirements. It produces low noise, has a good efficiency and a high top speed.

CONS: This motor occupies much size. It needs complex speed control systems, as well as DC/AC converters, which increase the final price.

EFFICIENCY: Around 85%.

2.4.4 SYNCHRONOUS AC MOTOR

This type of motors is based on the inversion of the alternator. Both parts of the motor are composed by windings. The rotor turns synchronously with the stator magnetization, whose speed is controlled trough the frequency regulation. Due to its intrinsic characteristics, it can maintain high torques without losing speed.

PROS: This motor produces low noise and requires low maintenance. It offers a high maximum speed, a great efficiency, a great power output and has a low size.

CONS: The same ones as the previous motor, except the size problem. This motor is more expensive to build, the starting maneuver is complicated and in the case of a large charge alteration the motor can stall.

EFFICIENCY: Around 90%.

2.4.5 SWITCHED RELUCTANCE MOTOR

The stator, fed by AC current, is similar, but simpler, to the one placed in the induction motor. Its rotor, made in copper or aluminum, is based on the squirrel-cage structure and it gets magnetized when the stator is fed by three-phased current because of the magnetic reluctance phenomenon.

PROS: This engine virtually doesn’t require any maintenance, offers a high speed and a lower cost and size than induced motors.

CONS: It results particulary noisy. Nowadays, it is not still enough developed for use in EVs. It has the same electronic requirements that the other AC motors.

EFFICIENCY: Between 85-90%.