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Motors : heart of an electric vehicle

Electric vehicles are gaining popularity due to their zero emissions and the ever-rising fuel prices.The core elements of electric vehicles are electric motor, a motor speed controller and a battery. Electric power is required to generate the torque and power output to propel the electric vehicle forward and enable motion in it. Traditionally, this is done by the internal combustion engine(ICE) which required fuel to have enough energy to rotate the wheels of the vehicle and overcome the resistive forces to the motion. So, the electric motor has similar function but completely different application. The motors used in EV should have characteristics like high starting torque, high power density, good efficiency, etc.

Different types of electric motors used in electric vehicles:

1) DC Series Motor : DC Series Motor is best to be used in electric traction. The values of maximum active torque are very high, when different load torques is applied. The DC Series Motor can easily drive the car with its loads. When rotating speed of DC Series Motor is reduced till zero during the car running, DC Series Motor can operate as a generator to regenerate energy, but this needs special additional electric circuits. This regeneration is fully used in the electric trains and diesel electric trains and can be used to charge the batteries or supplied to the electric network. This motor comes under the category of DC brushed motors. It uses commutators and brushes and therefore requires high maintenance. Also, DC series motors are more expensive, have more complicated construction and, more volume and weight as compared to induction motors.

2) Brushless DC Motor : Brushless DC Motor is similar to DC motors with Permanent Magnets. It does not have the commutator and brush arrangement (therefore know as brushless motors) as commutation is done electronically and are maintenance free. These motors have traction characteristics like high starting torque, high efficiency around 95-98%, etc. and are suitable for high power density design approach. In comparison to AC Induction Motors ( ACIMs), BLDCMs are expensive to manufacture but are very efficient, lighter in weight, compact in size & Simpler to control the speed and reverse function than the ACIMs. Motors like PMSMs and IPMMs are much better(in efficiency with no torque ripple & less noise) than BLDC but they are more expensive to manufacture. So due to more availability and good performance BLDC Motors are being used in almost every light vehicles.

BLDC motors further have two types:

  • i. Out-runner type BLDC Motor : In this type of motor, the rotor of the motor is present on the outer side and the stator is present inside. The wheel is directly connected to the exterior rotor there is also know as Hub motors. Out-runner type BLDC Motor has inbuilt planetary gears and therefore this type of motor does not require external gear system. This motor makes the overall vehicle less bulky and also eliminates the space required for mounting the motor. The in-runner configuration imposes a restriction on the motor dimensions which limits the power output. This motor is widely preferred by electric cycle manufacturers like Hullikal, Tronx, Spero, light speed bicycles, etc. It is also used by two-wheeler manufacturers like 22 Motors, NDS Eco Motors, etc.

  • ii. In-runner type BLDC Motor : In this type, the rotor of the motor is present on the inner side and the stator is outside like other conventional motors. The out-runner configuration is little bulky when compared to the in-runner configuration, because of which these motors require an external transmission system to transfer the power to the wheels. BLDC motors are used in many three- wheeler like Goenka Electric Motors, Speego Vehicles, Kinetic Green, Volta Automotive . Low and medium performance scooter manufacturers also use BLDC motors for propulsion.It is one of the widely preferred motor for electric vehicle application. The main drawback is the high cost due to permanent magnets. Overloading the motor beyond a certain limit reduces the life of permanent magnets due to thermal conditions.

3) Permanent Magnet Synchronous Motor (PMSM) : PMSM is also similar to BLDC motor which has permanent magnets on the rotor. Similar to BLDC motors these motors also have traction characteristics like high power density and high efficiency. The major difference between them is that PMSM has sinusoidal back EMF whereas BLDC has trapezoidal back EMF. Permanent Magnet Synchronous motors are available for higher power ratings, so used in high-performance applications such as sports cars, buses etc.. PMSM requires permanent magnets and their performance is also based on the quality of the magnet.The magnets with the desired features are rare and expensive. Despite the high cost, PMSM is providing stiff competition to induction motors due to increased efficiency than induction motor. PMSM motors find their application in hybrid and electric vehicles. For example, Toyota Prius, Chevrolet Bolt EV, Ford Focus Electric, zero motorcycles S/SR, Nissan Leaf, Honda Accord, BMW i3, etc use PMSM motor for propulsion.

4) Three Phase AC Induction Motors : Unlike the DC motors, the induction motors do not have a high starting toque under fixed voltage and fixed frequency operation. But this characteristic can be improved by using various control techniques like FOC or v/f methods. Application of these methods helps to maximise the starting torque of the motor which is suitable for traction application. Due to less maintenance, squirrel cage induction motors have a long life. Induction motors can be designed up to an efficiency of 92-95% and can withstand rugged environmental conditions.. The drawback of an induction motor is that it requires complex inverter circuit and it is difficult to control the motor. In permanent magnet motors, the magnets contribute to the flux density B. Therefore, adjusting the value of B in induction motors is easy because in Induction motors the value of B can be adjusted by varying the voltage and frequency (V/f) based on torque requirements. This helps in reducing the losses which in turn improves the efficiency.

Induction motors are the preferred choice for performance oriented electric vehicles due to its cheap cost. Tesla Model S uses this type of motor. Even Tata and TVS are planning to use induction motors in their electric vehicles. Indian Railways have also started using induction motors over DC motors.

5) Switched Reluctance Motors (SRM) : The switched reluctance motor (SRM) is an electric motor that runs by reluctance torque. These motors have simple construction and are robust. The rotor of the SRM is a piece of laminated steel with no windings or permanent magnets on it which makes decrease the inertia of the rotor which in turn helps in high acceleration. The robust nature of SRM makes it suitable for the high speed application. It also offers high power density which are some required characteristics of Electric Vehicles. It is easier to cool the motor as the heat generated is mostly confined to the stator. The switched reluctance motor's efficiency is very flat, dropping from 93.5% at full load to 93.0% at the reduced load. Unlike common brushed DC motor types, power is delivered to windings in the stator (case) rather than the rotor, this complicates the electrical design as switching circuit needs to be used to deliver power to the different winding. It also has some noise issues. Once SRM enters the commercial market, it can replace the PMSM and Induction motors in the future.

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