Week 3 Session 7

1.

The major types of motors used in electric vehicles are as follows,

i)DC motor

ii)Brushless DC motor

iii)Permanent magnet synchronous motor

iV)Induction motor.

DC Motor

A DC motor is an electrical machine that converts electrical energy into mechanical energy. The basic working principle of the DC motor is that whenever a current-carrying conductor places in the magnetic field, it experiences a mechanical force. A commutator brush arrangement is used to facilitate the production of unidirectional torque. The high starting torque capability of the DC motor makes it a suitable option for EV application.

Advantages

*Easy speed control.

*Can withstand a sudden increase in load.

*Robust construction and simple control.

Disadvantages

*High maintenance due to brushes and commutators.

*Higher size

*Low efficiency, and limited speed because of the friction between brushes and collectors.

*Cogging at low rpm.

Brushless DC motor

PM BLDC motors use permanent magnets instead of rotor windings and it does not have the commutator and brush arrangement which effectively makes the motor compact. The commutation in this type of motor is done electronically and thereby it doesn't require maintenance as a DC motor. Also, as they do not include rotor losses their efficiency is higher than other types of motors. BLDC motors have traction characteristics like high starting torque and high efficiency.

PM BLDC motors have a short constant power operation region because of their permanent magnet field weakened by a stator field. The permanent magnets also limit the motor torque to be too high. The magnets are significantly influenced by the high temperature which reduces the remnant flux density and thus the motor torque capacity. The mechanical forces and magnet prices are the biggest disadvantages of this type of motor. The increased centrifugal forces caused by higher motor rotation speed can cause safety issues due to the possible breaking of the magnets. The BLDC motors are more expensive and require complex control circuits for faithful operation.

Permanent magnet synchronous motor

This motor 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 permanent magnet synchronous motors are one of the types of AC synchronous motors, where the field is excited by permanent magnets that generate sinusoidal back EMF. The permanent magnet synchronous motor working principle is similar to the synchronous motor. It depends on the rotating magnetic field that generates electromotive force at synchronous speed. When the stator winding is energized by giving the 3-phase supply, a rotating magnetic field is created in between the air gaps. This produces the torque when the rotor field poles hold the rotating magnetic field at synchronous speed and the rotor rotates continuously.

Advantages

The advantages of permanent magnet synchronous motor include,

*Higher efficiency at high speeds available in small sizes

*Maintenance and installation are very easy

*This motor has a low torque ripple and can generate constant torque. Also, its able to maintain full torque at low speeds.

*High efficiency and reliability.

*Gives smooth torque and dynamic performance.

Disadvantages

The disadvantages of permanent magnet synchronous motors are,

*These types of motors are very expensive.

*They are not self-starting motors.

*Complex electronic circuit is required for speed control.

3-Phase Induction motor

An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. induction motors do not have a high starting torque like DC series motors under fixed voltage and fixed frequency operation. But this characteristic can be altered by using various control techniques like FOC or v/f methods. By using these control methods, the maximum torque can be made available at the starting of the motor and thus makes it suitable for traction application. The drawback of an induction motor is that it requires a complex inverter circuit and control of the motor is difficult.

Advantages

*The construction of an induction motor is robust and sturdy and it can operate in harsh environmental conditions.

*It is very cheap compared to the other motors.

*It is a highly efficient motor.

*It requires very less maintenance.

*3 Phase IM is self-starting.

*It is comparatively small in size.

Disadvantages

*The starting torque of the induction motor is low.

*The motor’s power factor is low during the light load condition.

*Speed control is difficult.

*High power losses in the rotor as heat.Effective cooling is required.