Motors in Electric Cars

Electric Cars- The prodigy of the automobiles and electricity. The concept of electric cars is in our midst for quite some time now. Though the commercial sale of these cars is not very popular all over the globe, in the future it is expected to proliferate. Electric cars need not completely run on electricity, it can even go partially. Few cars are plug-in hybrids that give us both a conventional component which is fuel and the new electric component.

The need for electric cars is massive. The pressure from the government is one of the benefactors for a steep rise in future of these electric cars. Britain and France have stated that a pure fossil fuel served car would not be available in the country post-2040. Cities like Athens, Mexico City, Paris and Madrid claim that they will not allow diesel cars by 2025. These tightening regulations will enhance the automakers and the public to use electric cars.

The electric car has three main components for its drive propulsion.

  1. Propulsion Electric Motor– It consists of a vehicle controller, an electronic power converter, electric motor, manual transmission and driving wheels.
  2. Power Supply– It consists of a power supply, a power management unit, and unit energy recharge.
  3. Auxiliary System– It consists of drive power steering, climate control room and auxiliary power unit.

(This article is light about other stuff and mainly focuses on Motors in cars)

Motors Used in Electric Cars

  1. Brushed DC Motor

Architecture   

Parts of a brushed DC Motor
Source: Fuelab

It contains two main parts

  • Static- The part that consists of field windings and receives the supply
  • Rotor- The part that bring about mechanical rotations

Other subsidiary parts consist of

  • Yoke of DC Motor
  • Poles of DC motor
  • Field winding of DC motor
  • Armature winding of DC motor
  • Commutator of DC motor
  • Brushes of DC motor

Advantages

  • High torque at low speeds can be achieved, which is apt for traction requirement
  • Can be rebuilt for longevity in the lifespan
  • Controller not needed for varying the speed. The speed can be altered by changing the voltage

Disadvantages

  • Bulky Construction which makes it heavy and expensive
  • Low efficiency
  • Low reliability
  • High maintenance due to the presence of mechanical commutators and brushes
  • Friction between the brushes and commutators restrict the maximum rotor speed
  1. Induction Motor

Architecture

Induction Motor
Source: EV West

It also contains two main parts

Stator- It consists of

  • Stator Frame
  • Stator Core
  • Stator Winding or Field Winding

Rotor

The Rotor is further classified into 2 types

  • Slip Ring Rotor or Wound Rotor or Phase Wound Rotor
  • Squirrel Cage Rotor

Advantages

  • Simple, reliable construction that can withstand tough conditions
  • Low maintenance and inexpensive
  • Increase in speed as the brush friction is absent, giving a high output
  • 3-phase induction motor has self-starting torque, unlike synchronous motor. Hence no starting method required.
  • Due to the absence of brushes, it cannot cause sparks

Disadvantages

  • Induction motor controllers are expensive
  • Breakdown torque restricts the extended constant power operation
  • If the speed is increased twice the synchronous speed breakdown torque occurs
  • The induction motor is less efficient than permanent magnet motors and switch reluctance motors due to the absence of rotor winding and rotor copper losses
  1. Permanent Magnet Brushless DC Motors (PM BLDC)

Architecture

It consists of two main parts

permanent magnet brushless dc motor
Source: Slideshare

Stator- It is made up of silicon steel stampings and slots. Slots are accommodated with armature windings. The winding has a specific number of poles and is connected to a DC supply.

Motor- It accommodates the permanent magnet. The rotor shaft carries the rotor position sensor. The sensor provides the information about the position of the shaft.

Advantages

  • Small in size
  • Regenerative braking possible
  • High efficiency and high power density
  • Optimal dissipation of heat happens in the surrounding

Disadvantages

  • Expensive motor
  • Due to the mechanical strength of the magnet, the build-up of large torque is difficult
  • This motor has limited field weakening too

Switch Reluctance Motor (SRM)

Architecture 

It consists of two main parts

Switched Reluctance Motor
Source: commons.wikimed

Stator- It has concentrated coils and salient poles. On diametrically opposite poles, stator windings are connected in series or parallel to form one phase of the motor.

Motor- It has a basic structure with no coil or magnets but has salient poles.

Advantages

  • Simple to construct and can withstand rough conditions
  • A high fault-tolerant operation, simple control and torque-speed characteristics
  • They can inherently operate with an extreme long constant power range
  • Fast acceleration
  • It has high speed operation with wide constant power region
  • Can withstand high temperature and easy to cool

Disadvantages

  • They have to suffer from torque ripple
  • High acoustic noises

Why is Induction Motor commonly used?

The induction motor has been used very widely in electric cars nowadays. Tesla’s take on using an induction motor over Brushed DC and PM BLDC is as follows

  1. Cost
  • Brushed DC- Regenerative Braking is too complex
  • PM BLDC- Magnet cost is high
  • IM- Relatively Cheap
  1. Torque
  • Brushed DC- Torque is inversely proportional to speed. Requires 3-4 gear ratios for complete automotive functioning
  • PM BLDC- Constant torque up to base speed and single gear ratio required for a complete automotive functioning
  • IM- Same as PM BLDC
  1. Heat Rejection
  • Brushed DC- Heat removal from rotor and commutator is challenging due to windings
  • PM BLDC- Heat removal from the stator is easy due to windings. Magnets on rotor have low-moderate eddy current-induced heating
  • IM- Windings make heat rejection easy. Induced currents in the rotor can require oil cooling in high power applications
  1. Wear and Tear
  • Brushed DC- Brushes require periodic replacement
  • PM BLDC- Long lasting bearings
  • IM- Long lasting bearings
  1. Efficiency
  • Brushed DC- Net efficiency is low
  • PM BLDC- The peak efficiency is higher than IM
  • IM- The peak efficiency is slightly low, but average efficiency is high
  1. The magnetic field is fixed in PM BLDC but IM does not have a magnet, so the field is adjustable.

Other motors apart from IM

  1. Tesla– Tesla Model 3 very unlikely to its name uses Permanent Magnet Motor. This is due to increased performance and efficiency.
  2. Land Rover- LandRover Defender uses a Switched Reluctance Motor. It delivers high torque in quick time which is suited for SUVs which offers an all-terrain traction.
  3. Nissan- Nissan Leaf uses a permanent magnet motor
  4. Mitsubishi- Mitsubishi i-MiEV uses a permanent magnet motor
  5. Volkswagen- The Volkswagen Passat GTE uses permanent magnet motor
  6. Ford- Ford Focus Electric uses a permanent magnet electric motor

Conclusion

Seeing the PM BLDC and IM motors differing minutely in their compatibility for the electric cars, the cost is a major factor why induction motor is chosen over PM BLDC. Due to lack of advancements in Brushed DC Motor, it is not taken used in electric vehicles much. Tesla Roadster one of the fastest cars in the world uses induction motor.

On the other hand, why is switched reluctance motors not used? Well, that is again because of the cost. The SRM produces a lot of noise which can be toned down with additional designs. The peak efficiency is low but the average range is high. Power is switched rapidly from one set of coils to the next, so 2 times the mean current flows through the electronic cars making it more expensive. So due to the cost-effectiveness and basic features, the induction motor is preferred over all the motors

 

 

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