Electric vehicle motors come in various forms depending on their operating environment and frequency. Different types of motors also have different characteristics. Electric vehicle motors generally use permanent magnet DC motors. A permanent magnet motor refers to a motor whose coils are excited by permanent magnets, rather than by coils. This saves the electrical energy consumed by the excitation coils, improving the motor's electromechanical conversion efficiency. For electric vehicles using limited onboard energy, this reduces the driving current and extends the driving range. Electric vehicle motors can be classified into two main categories based on their power supply method: brushed motors and brushless motors. Based on the mechanical structure of the motor assembly, they are generally divided into two main categories: "geared" (high-speed motor requiring gear reduction) and "gearless" (motor torque output without any reduction gear). An electric motor is a rotating electric machine that converts electrical energy into mechanical energy. It mainly consists of an electromagnet winding or distributed stator winding to generate a magnetic field and a rotating armature or rotor.
Composition and structure
1. Permanent magnet DC motor:
It consists of stator poles, rotor, brushes, and housing.
The stator poles use permanent magnets (permanent magnetic steel), including materials such as ferrite, AlNiCo, and NdFeB. Based on their structural form, they can be divided into several types, such as cylindrical and tile-shaped.
The rotor is generally made of stacked silicon steel sheets, with enameled wire wound between two slots of the rotor core (three slots means three windings), and each joint is welded to the metal sheet of the commutator.
A brush is a conductive component that connects the power supply to the rotor windings, possessing both conductivity and wear resistance. Permanent magnet motors use single-metal sheets, metal-graphite brushes, or electrographite brushes.
2. Brushless DC motor:
It consists of a permanent magnet rotor, a multi-pole winding stator, and a position sensor. The characteristic of a brushless DC motor is that it is brushless, using semiconductor switching devices (such as Hall elements) to achieve electronic commutation, that is, replacing the traditional contact commutator and brushes with electronic switching devices. It has advantages such as high reliability, no commutation sparks, and low mechanical noise.
The position sensor commutates the current in the stator winding according to the change in rotor position in a certain sequence (that is, it detects the position of the rotor magnetic pole relative to the stator winding, generates a position sensing signal at a determined position, and after being processed by the signal conversion circuit, it controls the power switching circuit to switch the winding current according to a certain logic relationship).
3. High-speed permanent magnet brushless motor:
It consists of a stator core, a magnetic rotor, a sun gear, a reduction clutch, and a hub housing.
A Hall sensor can be installed on the motor cover for speed measurement.
Position sensors come in three types: magnetic, photoelectric, and electromagnetic.
Brushless DC motors employing magnetic position sensors have their magnetic sensor components (such as Hall elements, magnetic diodes, magnetic transistors, magnetoresistive devices, or dedicated integrated circuits) mounted on the stator assembly to detect changes in the magnetic field generated by the rotation of the permanent magnet and rotor. Hall elements are commonly used in electric vehicles.
A brushless DC motor employing photoelectric position sensors has photoelectric sensor components positioned at specific locations on the stator assembly, and a light-shielding plate mounted on the rotor. The light source is a light-emitting diode or a small bulb. As the rotor rotates, due to the effect of the light-shielding plate, the photosensitive components on the stator will intermittently generate pulse signals at a certain frequency.
A brushless DC motor that uses an electromagnetic position sensor has an electromagnetic sensor component (such as a coupling transformer, proximity switch, LC resonant circuit, etc.) installed on the stator assembly. When the position of the permanent magnet rotor changes, the electromagnetic effect will cause the electromagnetic sensor to generate a high-frequency modulation signal (the amplitude of which varies with the rotor position).
The operating voltage of the stator winding is provided by an electronic switching circuit controlled by the position sensor output.
Equipment Classification
Drive motors used in electric vehicles differ from conventional industrial motors. Electric vehicle drive motors typically require frequent starts/stops, acceleration/deceleration, high torque at low speeds or when climbing hills, low torque at high speeds, and a wide speed range. Industrial motors, on the other hand, are usually optimized for their rated operating point. Therefore, electric vehicle drive motors are unique and should be classified separately.
For brushless motors, depending on whether they have a position sensor, they are divided into brushless motors with position sensors and brushless motors without position sensors. For brushless motors without position sensors, the vehicle must first be pedaled to reach a certain rotational speed before the controller can recognize the phase of the brushless motor and then supply power to it. Because brushless motors without position sensors cannot achieve zero-speed start, they are less commonly used in electric vehicles manufactured after 2000. The brushless motors used in the electric vehicle industry are generally brushless motors with position sensors. Rotating 180°, with the coil stationary, the Hall element senses the S-pole magnetic field. At this time, P1 and R2 are cut off, while P2 and R1 are connected. Current i' can be seen flowing from the positive terminal of the battery through R1, the coil, and P2 to the negative terminal. The direction of the current i' at point A in the energized coil is towards the terminal (the vector direction is opposite to the vector direction of i'). The magnet experiences a reaction force from the coil, generating a counterclockwise rotational torque. Brushless motors used in electric vehicles have a relatively large number of magnets and typically three sets of coils. Each set of coils has a corresponding Hall element (three Hall elements for a three-phase coil). This makes the motor rotate more smoothly and is more efficient. When the magnets rotate, the Hall elements sense the change in the direction of the magnetic field and give a corresponding control signal. The brushless controller uses this signal to control the conduction and cutoff of the upper three and lower three power transistors.
Comparison of brushed motors and brushless motors
The difference in the power-on principle between brushed and brushless motors: Brushed motors use carbon brushes and a commutator for mechanical commutation, while brushless motors rely on Hall effect sensors to sense signals and have the controller complete electronic commutation. Because their power-on principles are different, their internal structures also differ. For hub motors, the way the motor torque is output (whether or not it is reduced by a gear reduction mechanism) is different, and their mechanical structures are also different.
1. Internal mechanical structure of common high-speed brushed motors. This type of hub motor consists of a built-in high-speed brushed motor core, a reduction gear set, an overrunning clutch, a hub end cover, and other components. High-speed brushed geared hub motors belong to the category of internal rotor motors.
2. Internal mechanical structure of common low-speed brushed motors. This type of hub motor consists of carbon brushes, a commutator, a rotor, a stator, a shaft, end covers, bearings, and other components. Low-speed brushed gearless hub motors are external rotor motors.
3. Internal Mechanical Structure of Common High-Speed Brushless Motors. This type of hub motor consists of a built-in high-speed brushless motor core, planetary friction rollers, an overload clutch, an output flange, end covers, and a hub housing. High-speed brushless geared hub motors are internal rotor motors.
4. Internal mechanical structure of common low-speed brushless motors. This type of hub motor consists of a motor rotor, motor stator, motor shaft, motor end cover, bearings, and other components. Low-speed brushless gearless hub motors belong to the category of external rotor motors.
Key features
Brushless DC motors are widely used in electric vehicles because they have the following two advantages compared to traditional brushed DC motors.
(1) Long lifespan, maintenance-free operation, and high reliability. In brushed DC motors, due to the high motor speed, the brushes and commutator wear out quickly, generally requiring brush replacement after about 1000 hours of operation. Furthermore, the reduction gearbox presents significant technical challenges, particularly the lubrication of the transmission gears, which remains a major difficulty in current brushed solutions. Therefore, brushed motors suffer from problems such as high noise, low efficiency, and susceptibility to failure. Thus, the advantages of brushless DC motors are obvious.
(2) High efficiency and energy saving. Generally speaking, since brushless DC motors do not have the friction loss of mechanical commutation, the consumption of gearbox, and the loss of speed control circuit, the efficiency can usually be higher than 85%. However, considering the best cost performance in actual design, in order to reduce material consumption, it is generally designed to be 76%. The efficiency of brushed DC motors is usually around 70% due to the consumption of gearbox and overrunning clutch.
Working principle
An electric motor is a device that converts electrical energy into mechanical energy. It utilizes a rotating magnetic field generated by a current-carrying coil (stator winding) to act on the rotor, a squirrel-cage closed aluminum frame, creating a magnetoelectric torque. Electric motors are classified into DC motors and AC motors based on the power source used. Most motors in power systems are AC motors, which can be synchronous or asynchronous (the stator magnetic field speed and rotor rotation speed are not synchronized). An electric motor mainly consists of a stator and a rotor. The direction of the force on a current-carrying conductor in a magnetic field depends on the direction of the current and the direction of the magnetic field lines. The working principle of an electric motor is that the magnetic field exerts a force on the current, causing the motor to rotate.
Basic Introduction
An electric motor is a rotating electrical machine that converts electrical energy into mechanical energy. It mainly consists of an electromagnet winding or distributed stator winding to generate a magnetic field and a rotating armature or rotor. Under the influence of the rotating magnetic field of the stator winding, current flows through the armature's squirrel-cage aluminum frame, causing it to rotate. Some types of these machines can be used as electric motors or generators. It is a machine that converts electrical energy into mechanical energy. Typically, the working part of an electric motor rotates; this type of motor is called a rotor motor. There are also motors that move linearly; these are called linear motors.
Basic structure
I. The structure of a three-phase asynchronous motor consists of a stator, a rotor, and other accessories.
(I) Stator (stationary part)
1. Stator core
Function: Part of the motor's magnetic circuit, on which the stator windings are placed.
Structure: The stator core is generally made of silicon steel sheets with an insulating layer on the surface, which are stamped and stacked. The inner circle of the core has evenly distributed slots for embedding the stator winding.
The stator core slot types include the following:
Semi-closed slots: These motors have higher efficiency and power factor, but winding and insulation are more difficult. They are generally used in small, low-voltage motors.
Semi-open slots: These slots can accommodate pre-formed windings and are generally used in large and medium-sized low-voltage motors. Pre-formed windings refer to windings that have been pre-insulated before being placed into the slot.
Open slots: used to embed shaped windings, providing convenient insulation methods, and are mainly used in high-voltage motors.
2. Stator winding
Function: It is the circuit part of the electric motor. When three-phase alternating current is applied, it generates a rotating magnetic field.
Construction: It consists of three identical windings arranged in a symmetrical manner, spaced 120° apart in space. The coils of these windings are respectively embedded in the slots of the stator according to a certain pattern.
The main insulation items of the stator winding are as follows: (ensuring reliable insulation between each conductive part of the winding and the core, and reliable insulation between the windings themselves).
(1) Ground insulation: insulation between the stator winding as a whole and the stator core.
(2) Phase-to-phase insulation: Insulation between stator windings of each phase.
(3) Inter-turn insulation: Insulation between turns of each phase stator winding.
Wiring inside the motor junction box:
Every motor junction box contains a terminal block. The six wire ends of the three-phase winding are arranged in two rows, top and bottom. The top three terminals are numbered from left to right as 1 (U1), 2 (V1), and 3 (W1), and the bottom three terminals are numbered from left to right as 6 (W2), 4 (U2), and 5 (V2). The three-phase windings are connected in either a star or delta configuration. This numbering should be followed during manufacturing and maintenance.
3. Base
Function: To fix the stator core and the front and rear end covers to support the rotor, and to provide protection and heat dissipation.
Construction: The frame is usually made of cast iron. The frame of large asynchronous motors is generally welded from steel plates, while the frame of micro motors is made of cast aluminum. The frame of enclosed motors has heat dissipation fins on the outside to increase the heat dissipation area, and the frame of protective motors has ventilation holes at both ends of the end cover to allow direct air convection between the inside and outside of the motor, which is conducive to heat dissipation.
(II) Rotor (Rotating Part)
1. Rotor core of a three-phase asynchronous motor:
Function: As part of the motor's magnetic circuit and to house the rotor windings within the iron core slots.
Construction: The materials used are the same as the stator, made of 0.5 mm thick silicon steel sheets, punched and stacked. The outer circumference of the silicon steel sheets has evenly distributed holes punched to house the rotor windings. The rotor core is usually made by punching the inner circumference of the silicon steel sheets after the stator core has been punched. Generally, the rotor core of small asynchronous motors is directly pressed onto the shaft, while the rotor core of large and medium-sized asynchronous motors (rotor diameter of 300-400 mm or more) is pressed onto the shaft with the help of a rotor support.
2. Rotor windings of a three-phase asynchronous motor
Function: It cuts the rotating magnetic field of the stator to generate induced electromotive force and current, and forms electromagnetic torque to make the motor rotate.
Structure: It is divided into squirrel cage rotor and wound rotor.
(1) Squirrel-cage rotor: The rotor winding consists of multiple conductor bars inserted into the rotor slots and two annular end rings. If the rotor core is removed, the shape of the entire winding resembles a squirrel cage, hence the name squirrel-cage winding. Small squirrel-cage motors use cast aluminum rotor windings, while motors above 100KW use copper bars and copper end rings welded together.
(2) Wound rotor: The winding of the wound rotor is similar to that of the stator winding. It is also a symmetrical three-phase winding, which is generally connected in a star configuration. The three leads are connected to the three current collectors on the shaft and then connected to the external circuit through brushes.
Features: Due to its more complex structure, wound-rotor motors are not as widely used as squirrel-cage motors. However, by using slip rings and brushes to connect additional resistors and other components in series in the rotor winding circuit, the starting, braking, and speed regulation performance of asynchronous motors can be improved. Therefore, they are used in equipment requiring smooth speed regulation within a certain range, such as cranes, elevators, and air compressors.
(III) Other accessories for phase asynchronous motors
1. End cap: provides support.
2. Bearing: Connects the rotating part to the stationary part.
3. Bearing end cap: Protects the bearing.
4. Fan: Cools the electric motor.
II. The DC motor adopts an octagonal fully laminated structure, which not only has high space utilization but also can withstand pulsating current and rapid load current changes when powered by a static rectifier. DC motors generally do not have a series winding, making them suitable for automatic control technologies requiring forward and reverse rotation. Series windings can also be manufactured according to user needs. Motors with a center height of 100–280 mm have no compensating winding, but motors with a center height of 250 mm and 280 mm can be manufactured with compensating windings depending on specific circumstances and needs. Motors with a center height of 315–450 mm have compensating windings. The external dimensions and technical requirements of motors with a center height of 500–710 mm conform to IEC international standards, and the mechanical dimensional tolerances of the motors conform to ISO international standards.
Working principle of DC motor:
In the diagram, the coil is connected to the commutator segments, which are fixed to the shaft and rotate with the motor shaft. The commutator segments are insulated from each other and from the shaft. The entire assembly they form is called the commutator. Brushes A and B are fixed in space.
When a DC voltage is applied to the two brush terminals of a motor, the brushes and commutator draw electrical energy into the armature coils, ensuring that the current in the coil side of the same pole always flows in the same direction. This, in turn, ensures that the direction of the electromagnetic force on the coil side of that pole remains constant, allowing the motor to rotate continuously and convert electrical energy into mechanical energy to drive production machinery. This is the working principle of a DC motor. Note: The direction of the current in each coil side is alternating.
2. Working principle of a DC generator:
As shown in the figure, when the armature is driven to rotate counterclockwise by a prime mover, the coil sides will cut the magnetic lines of force and induce an electromotive force (EMF). The direction of the EMF can be determined by the right-hand rule. Due to the continuous rotation of the armature, the coil sides ab and cd will alternately cut the magnetic lines of force under the N and S poles. The direction of the induced EMF in each coil side and the entire coil is alternating. The induced EMF in the coil is an alternating EMF, but due to the action of the brushes and commutator, the current flowing through the load is a unidirectional direct current (DC). This DC current is generally pulsating.
Operating method
In the electric bicycle industry, "motor" generally refers to the motor assembly, including the motor core, reduction gear, etc. The electric bicycles discussed below refer to the motor assembly.
I. Disassembly of the motor
Before disassembling the motor, first disconnect the motor and controller leads. Be sure to record the one-to-one correspondence between the motor lead colors and the controller lead colors. Before opening the motor end cover, clean the work area to prevent debris from being attracted to the magnets inside the motor. Mark the relative positions of the end cover and the hub. Note: Loosen the screws diagonally to avoid deforming the motor housing. The radial clearance between the motor rotor and stator is called the air gap, which is generally between 0.25-0.8mm. After disassembling the motor and troubleshooting, reassemble it according to the original end cover markings to prevent rotor rubbing during reassembly.
II. Lubrication of the internal gears of the motor
If the noise from a brushless geared hub motor or a brushless geared hub motor starts to increase, or if the gears inside the motor have been replaced, all the tooth surfaces of the gears should be coated with grease, generally No. 3 grease or the lubricating oil specified by the manufacturer.
III. Motor Assembly
Before assembling the brushed motor, check the elasticity of the spring inside the brush holder, check if the carbon brush and brush holder are rubbing against each other, check if the carbon brush can reach its maximum stroke in the brush holder, and pay attention to the correct positioning of the carbon brush and commutator to avoid damaging the carbon brush or brush holder.
When installing the motor, first clean any impurities from the surface of the motor components to avoid affecting its normal operation. Ensure the hub is securely fixed to prevent damage from collisions caused by the strong attraction of the magnets during installation. The 36V output is normal, the controller outputs 5V and 12V are normal, and the motor resistance is normal. Connecting the motor directly to the 36V battery results in normal operation.
IV. Wiring Method
Due to their different commutation methods, brushed motors and brushless motors not only have different internal structures, but also very different wiring methods.
1. Wiring method for brushed motors. Brushed motors generally have two leads, positive and negative. The red wire is usually the positive terminal, and the black wire is the negative terminal. Swapping the positive and negative wires will only cause the motor to reverse; it generally will not damage the motor.
2. Determining the Phase Angle of a Brushless Motor. The phase angle of a brushless motor is short for the algebraic phase angle, which refers to the angle by which the direction of the current inside the coil changes during one energizing cycle. Common algebraic phase angles for brushless motors used in electric vehicles are 120° and 60°.
The phase angle of a brushless motor can be determined by observing the spatial position of the Hall element installation. The spatial position of the Hall element installation is different for motors with phase angles of 120° and 60°.
Determining the phase angle of a brushless motor by measuring the Hall effect true value signal.
First, it's necessary to explain what the magnetic pull angle of a brushless motor is. Brushless motors typically have 12, 16, or 18 magnets, corresponding to 36, 48, or 54 stator slots. When the motor is stationary, the magnetic lines of force on the rotor magnets have the characteristic of traveling along the direction of least magnetic reluctance. Therefore, the position where the rotor magnet stops is precisely the position of the stator slot salient pole. The magnet will not stop at the center of the stator slot, thus limiting the relative positions of the rotor and stator to a limited number of 36, 48, or 54 positions. Therefore, the minimum magnetic pull angle of a brushless motor is 360/36°, 360/48°, or 360/54°.
The Hall effect sensor of a brushless motor has five leads: the common positive terminal, the common negative terminal, and the Hall effect outputs for phases A, B, and C. We can utilize the five Hall effect leads of the brushless controller (60° or 120°) to connect the positive and negative power supplies of the brushless motor's Hall effect sensor leads. Then, connect the leads of the remaining three phase sensors (A, B, and C) arbitrarily to the Hall effect signal leads of the controller. By turning on the controller power and supplying power to the Hall effect sensor, the phase angle of the brushless motor can be detected. The method is as follows: Use a multimeter set to +20V DC voltage range. Connect the black probe to ground and the red probe to measure the voltage of each of the three leads, recording the high and low voltages. Slightly rotate the motor, allowing it to pass through a minimum magnetic pull angle, and measure and record the high and low voltages of the three leads again. Repeat this measurement and recording process six times. We use 1 to represent high potential and 0 to represent low potential, then—
If it is a 60° brushless motor, and it rotates continuously for 6 minimum magnetic pull angles, the measured Hall effect true signal should be: 100, 110, 111, 011, 001, 000. Adjust the pin order of the three Hall effect element leads so that the true signal changes strictly according to the above true value order. In this way, the A, B, and C phases of the 60° brushless motor can be determined.
If it is a 120° brushless motor, after rotating continuously for 6 minimum magnetic pull angles, the measured Hall true signal should change according to the pattern of 100, 110, 010, 011, 001, 101. In this way, the energizing phase sequence of the Hall element leads can be determined.
To quickly determine whether a brushless motor is 60° or 120°, use a multimeter set to the +20V DC voltage range. Connect the black probe to the ground wire and the red probe to measure the voltage of each of the three leads. If all three leads show voltage or no voltage, it is a 60° motor; otherwise, it is a 120° motor.
3. Wiring method for brushless motors. Brushless motors have 3 coil leads and 5 Hall effect leads. These 8 leads must correspond one-to-one with the corresponding leads on the controller; otherwise, the motor will not rotate properly.
Generally speaking, brushless motors with phase angles of 60° and 120° require corresponding brushless motor controllers with phase angles of 60° and 120°, respectively. The controllers for the two phase angles cannot be directly interchanged. There are two correct wiring methods for the eight wires connecting a 60° phase angle brushless motor to a 60° phase angle controller: one for forward rotation and one for reverse rotation.
For a brushless motor with a 120° phase angle, by adjusting the phase sequence of the coil leads and the Hall leads, there are 6 correct wiring methods for the 8 wires connecting the motor and the controller. Among them, 3 wiring methods allow the motor to rotate forward, and the other 3 wiring methods allow the motor to rotate in reverse.
If the brushless motor reverses direction, it indicates that the phase angle of the brushless controller and the brushless motor is matched. We can adjust the motor's direction of rotation as follows: swap the A and C wires of the Hall effect leads of the brushless motor and the brushless controller; simultaneously swap the A and B wires of the main phase leads of the brushless motor and the brushless controller. Electric bicycles can be broadly classified into three types: 1. DC hub motors, i.e., brushed motors, with two leads, connected to an external PWM controller. 2. AC hub motors, some with Hall effect sensors and some without, with three or more leads, connected to an external frequency converter. 3. DC brushless hub motors, containing an electronic commutator, with two leads, connected to an external PWM controller. It is crucial to distinguish between these types to avoid confusion.
Technical Requirements
They have different specific requirements in terms of load requirements, technical performance, and working environment:
1. Electric vehicle drive motors need 4-5 times overload to meet the requirements of short-term acceleration or hill climbing; while industrial motors only need 2 times overload.
2. Electric vehicles require a maximum speed of 4-5 times their basic speed when cruising on the road, while industrial motors only need to achieve a constant power of 2 times their basic speed.
3. Electric vehicle drive motors need to be designed according to the vehicle model and the driver's driving habits, while industrial motors only need to be designed according to typical working modes.
4. Electric vehicle drive motors require high power density (generally required to be within 1kg/kw) and good efficiency profile (high efficiency over a wide speed and torque range) to reduce vehicle weight and extend driving range; while industrial motors usually take into account power density, efficiency and cost, and optimize efficiency near the rated operating point.
5. Electric vehicle drive motors require high controllability, high steady-state accuracy, and good dynamic performance; while industrial motors only have one specific performance requirement.
6. Electric vehicle drive motors are mounted on the vehicle itself, in a confined space, and operate in harsh environments such as high temperatures, bad weather, and frequent vibrations. Industrial motors, on the other hand, typically operate in a fixed location.
As consumer demands evolve, the development of electric vehicle motors must also keep pace with the times. Consumers not only require electric vehicles to have great power, but also to accelerate quickly, and even make higher demands. When climbing hills, if the motors can automatically shift gears or maintain strong torque, riding on mountainous and hilly terrain will be as smooth as riding on flat ground.
