Motor NotesHighly Efficient Motor Driving is the Key to the EV Revolution

The following four items regarding motors, which are important parts of EVs (electric vehicles), will be discussed in the articles that follow.

In 2022, with the launch of new subcompact EVs in Japan, we can say that EVs from companies around the world are now available. The following goals have been announced, with 2030 as a milestone year:

• 2030:

  Over 50% of the world’s vehicles will be EVs (Volkswagen)

• 2035:

  100% of new cars will be electric* (Japanese Government)
  * Including plug-in hybrids (PHVs), hybrids (HEVs), and fuel cell vehicles (FCVs)

As for cruising range, which has been a major bottleneck, 80-kWh class batteries have extended ranges to more than 500 km in WLTC mode. Charging infrastructure has also been promoted by the Japanese government, setting a target of 150,000 charging stations by 2030.

The selling price is no longer a disadvantage when fuel efficiency and maintenance costs are taken into account.

In fact, EVs have many advantages over gasoline vehicles. For example, the use of a motor instead of an internal combustion engine enables better electrical controllability and superior driving performance, especially when accelerating from a standstill. This is due to the motor’s unique torque performance.

Now we will discuss motors, which are indispensable to the evolution of EVs.

Basics of EV Motor Driving

Principle of motor rotation

A motor is rotated by using magnetic force. When outer magnets rotate around a permanent magnet having a rotation axis, the N and S poles attract and repel each other. This magnetic force causes the magnets to rotate.

Figure 1: Principle of motor rotation using permanent magnets

In an actual motor, however, instead of rotating outer magnets, an outer magnetic field is rotated.

A magnetic field exists in a region within which a magnetic force acts. There is a magnetic field around a permanent magnet (Fig. 2). The magnetic field can also be generated by supplying an electric current through a conductor (Fig. 3a). To visualize the direction of magnetic field, magnetic field lines are used.

The directions of current and magnetic field lines are based on the right-hand screw rule. When current flows in the direction of advance of a screw, a magnetic field will appear in the direction in which the screw turns.

The unit of magnetic field strength (magnetic flux density) is the Tesla (T), which is also the name of a well-known EV manufacturer.

Figure 2: Magnetic field

Figure 3a　Figure 3b

We have so far described the magnetic field formed around a conductor (Fig. 3a), but a magnetic field is also generated when a coil with conductors wound in a loop is used (Fig. 3b). When a current is applied to the coil, the respective magnetic field lines are combined to form a large bundle of magnetic field lines (magnetic flux), i.e., high magnetic flux density, which generates N and S poles.

To further increase the magnetic force, there are three methods: (1) increase the number of windings, (2) apply a larger current, or (3) insert an iron rod (iron core) in the center of the coil. The reason for (3) is that iron allows magnetic flux to pass through more easily than air. A material’s ability to allow magnetic flux to pass through is called its magnetic permeability. As Figure 3b also suggests, changing the direction of the current causes the N and S poles to switch.

Generating a rotating magnetic field to rotate a motor

Rotating magnetic fields are explained using a three-phase motor with a basic motor structure. A 3-phase motor has three coils (U-phase, V-phase, and W-phase). These coils are connected in a Y-connection, and currents flow in six directions (Fig. 4).

Figure 4: Y-connected 3-phase motor

First, current flows (1) from the U-phase to the W-phase, then (2) from the V-phase to the W-phase. By repeating this for the currents (1) through (6), a rotating magnetic field is generated and the permanent magnet is rotated. This phenomenon is illustrated in Figure 5.

Figure 5: Magnetic field and magnet position due to coil currents

In Figure 5, note that the angle between the magnetic field axis and the magnets axis is 90°. The S pole is not in front of the N pole as shown in Figure 1. This is the key to highly efficient motor driving.

In the next article, we will discuss the EV driver circuit configuration that produces the rotating magnetic field.