China permanent magnet manufacturer: www.rizinia.com

# Types and principles of common driving motors for new energy vehicles

Based on the basic performance requirements of new energy vehicle drive motor, the commonly used drive motor types mainly include three categories, namely AC asynchronous motor, permanent magnet synchronous motor and switched reluctance motor. At present, in the statistics of matching models of various vehicle enterprises, the type of drive motor selected for each model is also different.

Therefore, it is very important to understand the structure, working principle and performance advantages and disadvantages of the drive motor in order to select the type of motor for new energy vehicles.

## AC asynchronous motor

### Structure of AC asynchronous motor

AC asynchronous motor, also known as induction motor, mainly consists of stator, rotor, motor shaft, front and rear bearings, end cover, position sensor, temperature sensor, low voltage harness and high voltage power harness. The stator consists of stator core and three-phase winding; The cage rotor is commonly used in rotor, including rotor core and cage winding. Depending on the power of the motor, water cooling or air cooling mode will be selected.

### Working principle of AC asynchronous motor

#### Driving principle of AC asynchronous motor

The stator provides a rotating magnetic field
In order to drive and provide torque for AC asynchronous motor, three-phase AC needs to be connected into the stator coil to generate a rotating magnetic field (the magnetic field speed is NS). AC asynchronous motor requires that the stator three-phase windings must be symmetrical, and the stator core space is 120 degrees different from each other; The current into the three-phase symmetrical winding must also be symmetrical, with the same size and frequency and phase difference of 120 degrees. The rotational speed of the rotating magnetic field is shown in equation (1).

• ns=60f/p (1)

In the formula, NS is the rotational speed of rotating magnetic field (also known as synchronous speed), R / min; F is the three-phase AC frequency, Hz; P is the number of magnetic poles. For the drive motor that has been designed and produced, the logarithm of magnetic level has been determined, so the factor determining the rotation speed of magnetic field is three-phase AC frequency. Since the frequency of power grid in China is f = 50 Hz, there is a linear relationship between the speed of motor and the number of poles.

Cage rotor provides induced eddy current
Due to the rotating magnetic field provided by the stator, eddy current is induced on the cage rotor conductor, as shown in Figure 3. In the magnetic conduction region between the cage winding conductors C and B, there is an outward magnetic line of force, which is enhanced by the rotating magnetic field. Therefore, I1 eddy current will be induced on the conductors C and B; In the same way, I2 eddy current will be induced on the conductor by the weakened magnetic lines of force in the region of conductor a and conductor B. Under the action of stator rotating magnetic field, the current on conductor B will cause the conductor of cage winding B to receive electromagnetic force, so that the rotor will produce electromagnetic torque and rotate. The rotating rotor gradually catches up with the rotating magnetic field and rotates at a speed n slightly slower than the “synchronous speed ns” of the magnetic field. The phenomenon that the rotation speed n of the rotor is slightly slower than the speed ns of the stator magnetic field is called the rotor slip. This asynchronous slip causes the cage rotor conductor to continuously cut the magnetic line of force and produce the induced eddy current. Therefore, on the rotor, the electrical energy is converted into mechanical energy to ensure the continuous external output.

#### Generation principle of AC asynchronous motor

According to Faraday’s law of electromagnetic induction, when a part of the conductor of a closed circuit cuts the magnetic induction line in the magnetic field, the induced current will be generated in the conductor, and the generated electromotive force becomes the induced electromotive force. In an AC asynchronous motor, when the motor is used as a generator, the three-phase current in the stator is the excitation current to provide the magnetic field, and the upper winding of the rotor provides the conductor. When the rotor shaft is driven by external mechanical force, such as the automobile drive shaft, so as to drive the rotor movement, if the speed of the rotor is higher than the synchronous speed of the stator rotating magnetic field, At this time, the AC asynchronous motor is the generator, and the direction of the rotor cutting the rotating magnetic field is opposite to that of the rotor working as the driving motor, so the direction of the rotor induced electromotive force is also opposite. In the process of power generation, the rotor of the motor is driven by the opposite electromagnetic resistance moment, which makes the rotor speed decrease.

The advantage of AC asynchronous motor is that the output torque can be adjusted in a wide range, and it can be forced to increase the output torque in a short time when accelerating or climbing. The electric drive vehicle of permanent magnet synchronous motor usually increases the torque by increasing the gearbox mechanism to improve the speed. But the disadvantage of AC asynchronous motor is that it needs more current to produce unit torque because of unilateral excitation and larger starting current, and there is reactive excitation current in stator, so it consumes more energy and lags behind power factor than permanent magnet synchronous motor; Overload often occurs in heavy-duty drive; The structure is relatively complex, its control technology requirements are high, and the manufacturing cost is high; The power density is relatively low. At present, the electric drive vehicles developed in the United States mostly use AC asynchronous motor as the driving motor.

## Permanent magnet synchronous motor

### Structure of permanent magnet synchronous motor

The structure of permanent magnet synchronous motor includes stator, rotor, motor shaft, front and rear bearings, end cover, cooling channel, position sensor, temperature sensor, low voltage harness and power harness. The stator consists of stator core and three-phase winding; The rotor is composed of permanent magnet pole and iron core, which is made of silicon steel sheet. According to the arrangement of permanent magnets in the rotor, it mainly includes surface protruding permanent magnet rotor, surface embedded permanent magnet rotor and built-in permanent magnet rotor. At present, built-in permanent magnet rotor is commonly used in new energy motors.

### Working principle of permanent magnet synchronous motor

#### Driving principle of permanent magnet synchronous motor

The rotating magnetic field is provided by the stator, and the magnetic field is generated in the same way and speed as the AC asynchronous motor. The magnetic pole is provided by the rotor permanent magnet. In this way, the rotating magnetic field generated by the stator forms a loop with the permanent magnet pole and the rotor core. According to the principle of minimum magnetic resistance, that is, the magnetic flux is always closed along the path of minimum magnetic resistance, and the electromagnetic force of the rotating magnetic field is used to pull the rotor to rotate, so the permanent magnet rotor will rotate synchronously with the rotating magnetic field generated by the stator, thus driving the motor shaft to rotate.

#### Generation principle of permanent magnet synchronous motor

According to Faraday’s law of electromagnetic induction, a part of the conductors in the closed circuit are provided by the three-phase stator winding, and the magnetic field is provided by the permanent magnet on the rotor. When the external torque drives the rotor to rotate, a rotating magnetic field is generated, which cuts part of the conductors in the three-phase stator winding, and induces three-phase symmetrical current. At this time, the kinetic energy of the rotor is converted into electric energy, Permanent magnet synchronous motor works as a generator.

### Advantages and disadvantages and application scope of permanent magnet synchronous motor

The advantages of permanent magnet synchronous motor are small volume, light weight, high power density. Compared with asynchronous motor, the energy consumption is small, the temperature rise is low, and the efficiency is high. According to the requirements, the motor can be designed with high starting torque and high overload capacity. Permanent magnet synchronous motor (PMSM) has strict synchronization and good dynamic response performance, which is suitable for frequency conversion control. Adjusting the current and frequency can adjust the torque and speed of the motor in a wide range. However, NdFeB strong magnetic material is usually used in permanent magnet synchronous motor, which is brittle and hard, and may be broken by strong vibration; Moreover, the rotor is made of permanent magnet material, which will lead to magnetic recession and power decline when the motor is used and overheated. At present, permanent magnet synchronous motor (PMSM) is widely used in new energy vehicle motor. In Asia and Europe, PMSM is mainly used as new energy motor.

## Switched reluctance motor

### Structure of switched reluctance motor

Switched reluctance motor is a typical mechatronics motor, also known as “switched reluctance motor drive system”. This kind of motor mainly includes switched reluctance motor body, power converter, rotor position sensor and controller. The main structure of switched reluctance motor body includes stator, rotor, position sensor, front and rear bearings, front and rear end covers, motor shell, etc. the stator includes stator core and winding. The salient pole structure is adopted in the stator core and rotor. The salient pole core and rotor are made of silicon steel sheets. The windings are arranged on the salient pole of the stator, and the rotor has no windings and permanent magnets.
The three-phase 6/4-pole structure shows that the stator of the motor has six salient poles and the rotor has one salient pole. The concentrated windings on the two salient poles of the stator are connected in series to form a phase, and the number of phases is one half of the number of salient poles of the stator. The three-phase 12/8-pole structure shows that the stator of the motor has 12 salient poles and the rotor has 8 salient poles. The windings on the four symmetrical salient poles of the stator are connected in series to form a phase, and the number of phases is one fourth of the number of salient poles of the stator.
The more the phase number of SRM is, the smaller the step angle is, the more stable the operation is, and the more conducive to reducing the torque ripple is. However, the more complex the control is, the more the main switching devices and the higher the cost are.
For the calculation of step angle, see formula (2):

• α= 360° ×（ (2) for a three-phase 6/4-pole motor, its step angle a = 360 degree × 2（/6 × 4）=30°

### Working principle of switched reluctance motor

#### Driving principle of switched reluctance motor

According to the working principle diagram of three-phase 12/8-pole switched reluctance motor, when the main switches S1 and S2 of phase a winding current control are closed, a is electrically excited, and the magnetic field force generated in the motor is a radial magnetic field with OA as the axis. The magnetic field force line is bent at the air gap between the stator salient pole and the rotor salient pole, The magnetic resistance of the magnetic circuit is greater than that when the salient pole of the stator coincides with the salient pole of the rotor. Therefore, the salient pole of the rotor is pulled by the magnetic field, which makes the polar axis OA of the rotor coincide with the polar axis OA of the stator. Thus, the electromagnetic torque of the magnetoresistance property is generated, which makes the rotor rotate counterclockwise. Turn off the A-phase power supply and establish the B-phase power supply. At this time, the internal magnetic field of the motor rotates 30 degrees, and the rotor continuously rotates 15 degrees counter clockwise under the action of the electromagnetic pull. If the a-b-c-a phase winding is energized in sequence, the rotor will rotate continuously in a counter clockwise direction; When the stator windings in each phase are energized once in turn, the stator magnetic field turns 3 × 30 degrees, the rotor turns one, and the polar distance of the rotor is 3 × 15 degrees (i.e. 360 degrees / rotor salient pole number). If the a-c-b-a phase windings are energized in turn, the rotor will rotate clockwise. Switched reluctance motor has nothing to do with the direction of current, but depends on the sequence of energizing stator phase winding. In the actual operation of multiphase motor, two or more windings are often conducting at the same time.

Working principle diagram of three phase 12/8 pole switched reluctance motor

#### Working principle of switched reluctance motor

The phase inductance of SRG has three states: excitation state, freewheeling state and power generation state. The L waveform of phase inductance is shown in the figure.

In the picture, θ The angle is defined as the angle between the rotor pole axis and the stator slot axis. When the axis of rotor tooth pole coincides with the axis of corresponding stator tooth slot, the inductance of this phase is the minimum (defined as θ= 0°）； Until the leading edge of rotor salient pole meets the trailing edge of stator salient pole（ θ=θ 1) The winding phase inductance is always kept constant; When the rotor continues to rotate, the salient pole of the rotor begins to coincide with the salient pole of the stator until the trailing edge of the salient pole of the rotor completely coincides with the trailing edge of the salient pole of the stator θ=θ 2) The winding phase inductance increases linearly in this region until the maximum value Lmax; When the rotor continues to rotate until the leading edge of the rotor salient pole coincides with the leading edge of the stator salient pole, the θ=θ 4, the inductance of this phase lasts for Lmax.

According to the basic theory of electromagnetic field, the electromagnetic torque of motor rotor exists simultaneously with the existence of magnetic field, which can be expressed as equation (3).

If the windings of SRM are θ 3 and θ 4, then the motor operates as a generator. At this time, the current is formed in the inductor drop region, then dL/dθ＜0, When there is current passing through the phase winding, the braking torque (T) will be generated (T (θ, i) ＜0). If the external mechanical force keeps the motor rotating, the motor absorbs the mechanical energy and converts it into electrical energy for output. At this time, the switched reluctance motor is in the generator working mode.

Switched reluctance motor has the advantages of simple and reliable structure, good starting performance, high efficiency and low cost. It can adjust the speed by changing the turn-on and turn off angle and voltage, and has a wide speed range and ability. The disadvantage of switched reluctance motor is large torque ripple and noise. At present, it is used in some small electric drive vehicles, such as electric drive four-wheel scooter, patrol car, etc.

## Conclusion

According to the performance requirements of new energy vehicle drive motor, the drive motor used in the market is different. This paper describes the structure and working principle of AC asynchronous motor, permanent magnet synchronous motor and switched reluctance motor, which are commonly used in new energy drive motor. It will help to better understand the drive motor.
Moreover, due to the different structure and principle of each motor, the application range is also very different. According to the national industrial strategic planning, the research on the electric drive system of environment-friendly new energy vehicles will be more and more extensive, and the motor types and technical level will also be continuously improved.
Source: China Permanent Magnet Manufacturer – www.rizinia.com