Fundamentals of Electrical Machines

Introduction

The topics covered in this chapter are as follows:

•  Electrical Machines in EVs and HEVs

•  Physical Concepts of Torque production

•  Why Should the Number of Poles on Stator Equal to the Number of Poles on Rotor

•  How Continuous Torque is Produced by a Motor

•  Rotating Magnetic Field

•  How to Create the Second Magnetic Field

•  Electrical and Mechanical Angle

Electrical Machines in EVs and HEVs

Vehicle propulsion has specific requirements that distinguish stationary and onboard motors. Every kilogram onboard the vehicle represents an increase in structural load. This increase structural load results in lower efficiency due to increase in the friction that the vehicle has to overcome. Higher efficiency is equivalent to a reduction in energy demand and hence, reduced battery weight.

The fundamental requirement for traction motors used in EVs is to generate propulsion torque over a wide speed range. These motors have intrinsically neither nominal speed nor nominal power. The power rating mentioned in the catalog and on the name plate of the motor corresponds to the maximum power that the drive can deliver. Two most commonly used motors in EV propulsion are Permanent Magnet (PM) Motors and Induction Motors (IM). These two motors will be investigated in detail in the coming lectures. However, before going into the details of these machines some basic fundamentals of electrical machines, such as torque production, are discussed in this chapter.

 

Physical Concepts of Torque Production

In Figure 1a a stator with 2 poles and a cylindrical rotor with a coil are shown. When only the stator coils are energized, stator magnetic flux is set up as shown in Figure 1a . The magnetic field for case when only the rotor coil is energized is shown in Figure 1b . In case when both the stator and rotor coils are energized, the magnetic resultant magnetic field is shown in Figure 1c . Since in this case the magnetic flux lines behave like stretched band, the rotor conductor experiences a torque in the direction shown in Figure 1c . From Figure 1c it can be seen that the stator S pole attracts the rotor N pole and repels the rotor S pole, resulting in clockwise torque. Similarly stator N pole attracts rotor S pole and repels rotor N pole, resulting again in clockwise torque.