1 Foreword The traction motor of a railway locomotive has limited space for mounting on a bogie, so its volume should be small. The high speed of the train requires it to be lightweight and have a large output power. Moreover, the torque characteristics of the motor require a large torque at start-up, and can operate over a wide range of speeds, as well as facilitating torque control.
DC motors can meet these requirements, so traction motors have been using DC motors for many years. However, with the advancement of power electronics technology, asynchronous motors controlled by VVVF inverters can meet these requirements. Compared with DC motors, asynchronous motors do not have commutators, and maintenance is reduced. At the same time, they can be small and lightweight. Therefore, the traction motors of the new electric transmission locomotives are basically all asynchronous motors.
Now, permanent magnet synchronous motors have attracted attention. Not only does it have the same characteristics as a traction motor required for an asynchronous motor, but it can also be more efficient and less bulky and weight than an asynchronous motor. Firstly, the structure and characteristics of permanent magnet synchronous motor are introduced. Then, according to the concept that permanent magnet synchronous motor is suitable for locomotives of railway locomotives, the results obtained by using permanent magnet synchronous motors as direct-drive traction motors and fully-closed traction motors are reanalyzed. Elucidated the possibility of permanent magnet synchronous traction motor applications.
2 Structure and characteristics of permanent magnet synchronous motor 2.1 Structure of permanent magnet synchronous motor Synchronous motor for generating magnetic field. The method of mounting the magnet on the rotor can be classified into two types: surface magnet type and buried magnet type. The permanent magnet synchronous motor stator is basically the same as the induction motor, and is composed of a laminated stator core composed of laminated Gui steel sheets and a stator coil embedded in the stator core slots. The connection of the coils causes the normal three-phase AC power supply to generate a rotating magnetic field.
The torque of the permanent magnet synchronous motor is generated by the interaction of the magnetic field of the permanent magnet and the magnetic field established by the stator coil current. The rotor and the stator rotating magnetic field powered by the three-phase AC power supply operate in synchronization and generate torque. Such torque is called Magnet torque. In addition, reluctance torque can also be expected by changing the shape of the rotor core. The reluctance torque is generated by the magnetic salient pole structure of the rotor due to the direction of the magnetic pole of the magnet (the axis of the direction is the d axis) and the direction of the phase shift by 90. (the electrical angle) (in the coordinate The torque generated by the difference in the passage of the magnetic flux on the q-axis).
In simple terms, in the rotating magnetic field generated by the stator coil, the force generated by the permanent magnet of the rotor due to attraction and repulsion is the magnet torque; the torque generated by the magnet on the rotor in the rotating magnetic field is the reluctance torque.
It is a sectional view perpendicular to the rotation shaft of various typical permanent magnet synchronous motor rotors. (a) and (b) are called surface magnet types. As the name implies, permanent magnets are fixed on the rotor surface. Under normal circumstances, the surface magnet type permanent magnet synchronous motor is covered with a layer of non-magnetic structural material on the outside of the rotor to press the permanent magnet to prevent the surface magnet from flying out when the motor runs at a high speed. (a) The core shape of the structure does not have a saliency. Basically, no reluctance torque is generated, and only magnet torque is generated. (b) The core of the structure has a salient pole structure, and therefore, a reluctance torque can also be generated.
The structure of (c) is a built-in magnet type structure. As its name implies, its magnet is buried in the middle of the iron core.
The core of the built-in magnet structure usually has a magnetic saliency shape and can generate a reluctance torque. Moreover, the built-in magnet structure is simple, and the motor constants and torque characteristics of the permanent magnet synchronous motor with a corner frequency (rad/s) are 3 under the following conditions: /m is the maximum current RMS value of x10,000.
This situation means that the permanent magnets produce more magnetic flux than the stator. Therefore, the magnetic flux generated by the magnet in the high speed area is strong, and the magnetic flux cannot be sufficiently weakened and the voltage is too large, so there is an output limit speed. For this reason, the output power of high-speed areas will drop sharply. In this kind of motor constant relationship, the output limit speed must be much greater than the maximum speed. On the other hand, because of the small armature reaction flux and high power factor, a larger maximum output power can be used when the inverter capacity is the same. This is an advantage.
In this case, the output limiting speed is theoretically infinite. Therefore, the output power in the high speed zone does not drop, and the constant power characteristic can be maintained to the maximum speed.
This situation means that the magnet produces less magnetic flux than the stator. The magnetic flux of the magnet can be cancelled by the armature flux. Therefore, there is no output limit speed, and the power drop in the high speed zone is very small. On the other hand, the power factor is relatively low, and the maximum power that can be output is small when the inverter capacity is the same.
The traction motor requires a wide range of speed control, and the output power at high speed should be reduced. Moreover, the price and weight of the inverter increase as the inverter capacity increases. Therefore, the required inverter capacity for the same output power is smaller. As will be described later, in the design of a permanent magnet synchronous motor used as a traction motor, the value is limited to a certain value or less, and the motor constant is often (c). However, in the high-speed area, in order to maximize the output power at a certain inverter capacity, the design of the motor constant should be as close as possible to the characteristic of (b).
The value of -L, is kept constant so that the value of the starting torque remains constant. When the values ​​of 1 and ~ are changed, the characteristics of the speed and torque in the above three cases (a) to (c) are obtained. Traction motor design according to (b) has the following advantages. First of all, due to the increase in acceleration at high speeds, it is expected to shorten the operating time. In addition, the rate of regenerative braking at high speeds can also be increased. An increase in regenerative braking rate means more energy savings, and a high regenerative braking rate at high speeds can reduce the burden on mechanical brakes and reduce maintenance.
Therefore, compared with the asynchronous traction motor, the permanent magnet synchronous traction motor can be easily fabricated in the cross-sectional shape of the rotor of the permanent magnet synchronous motor. The brittle magnet is not on the surface and the structure is very strong.
The traction motor for locomotives of railway locomotives is expected to have a strong structure and it is desired to make effective use of the reluctance torque, so that the linkage flux generated by the permanent magnets can be made sufficiently small (see section 4.1). Therefore, it can be said that a permanent magnet synchronous motor with a built-in magnet rotor structure is suitable for use as a traction motor for a locomotive of a locomotive.
2.2 Features of permanent magnet synchronous motor 2.2.1 High efficiency, small size Permanent magnet synchronous motor is characterized by its high efficiency. Efficiency can be expressed as (input power)-(loss)/(input power). The magnetic field of the permanent magnet synchronous motor does not require current, and in principle the rotor does not generate losses. Therefore, the maximum loss of the motor, that is, copper consumption (joule heat generated by the current) is only about half that of the asynchronous motor, and the efficiency is much higher than that of the asynchronous motor. With high efficiency and low power consumption, the railway is more energy-efficient than ever, and it is also expected to reduce electricity bills.
In addition, as described later, the smaller the loss, the smaller the volume of the motor. In this way, using a permanent magnet synchronous motor can achieve small size and high power. Therefore, when the volume is the same, the permanent magnet synchronous motor can have a larger power than the asynchronous motor; when the power is the same, the permanent magnet synchronous motor can be smaller than the asynchronous motor.
2.2.2 Speed ​​Traction Characteristics The speed of the electric locomotive is controlled by the speed and torque characteristics of the traction motor. The torque of the asynchronous motor and the DC series motor is inversely proportional to the speed in the high speed zone and the speed of 50%. Therefore, the speed traction characteristics of the electric vehicle are generally constant torque in the low speed zone, and the torque and speed in the medium speed zone become As the inverse ratio decreases, the torque in the high speed area decreases inversely proportional to the square of the speed.
On the other hand, the basic characteristics of a permanent magnet synchronous motor can be expressed by the following equations.
The maximum value of the interlinkage flux produced by permanent magnets; Ld d-axis inductance; Lq q-axis inductance; r - torque; 匕 - terminal voltage; w - angular frequency; - shaft current; - q-axis current . In addition, the resistance and iron loss of the stator coil are not considered.
As shown, since the permanent magnet synchronous motor is a synchronous machine and must be powered by an AC power supply that is synchronized with the rotation of the motor, an independent control method is provided in which one inverter supplies power to one traction motor.
In addition, the permanent magnet synchronous motor generates magnetic flux even when there is no external power supply, so that a voltage is also generated at the traction motor terminal during coasting.
Therefore, when a fault such as a phase-to-phase fault occurs in the inverter, the inverter must be disconnected from the traction motor and a contactor (called a load contactor) must be provided between the inverter and the traction motor. The traction motor can be disconnected.
The cost analysis is also very important in the practical application of permanent magnet synchronous motors.
First consider the initial cost. The rotor of the permanent magnet synchronous motor is simpler than the rotor of the asynchronous motor, and the price is low when mass production. On the other hand, the permanent magnet synchronous motor must be controlled independently, and the inverter has a higher price than when the asynchronous motor adopts centralized control. Moreover, when a permanent magnet synchronous motor is used, there must be a load contactor between the inverter and the motor, and this part is also expensive.
Followed by operating costs. The permanent magnet synchronous motor has high efficiency and can increase the regenerative braking rate, so the power consumption is less and the power cost can be reduced. In the existing size and weight conditions, fully enclosed traction motors or direct drive traction motors can be realized, which can reduce various maintenance and save manpower.
In summary, the initial cost of the permanent magnet synchronous motor is higher than that of the centrally controlled asynchronous motor, but the use of the permanent magnet synchronous motor is expected to reduce various costs in view of the operating cost. Therefore, in terms of cost, the asynchronous motor and permanent magnet synchronous motor which can not be generalized.
3 Application of Permanent Magnet Synchronous Motors on Railway Vehicles of Railway Locomotives 3.1 Small and lightweight traction motors for traction motors are more stringent in terms of weight and size, and are more compact and lightweight than general motors. Under normal circumstances, the output power and size of the motor have the following relationship.
Air gap diameter (m); - iron core length (m); n - rotation speed (r/min). ;) Improve the performance of magnetic materials;) Increase the speed;) Increase the number of poles.
The rotor of the permanent magnet synchronous motor does not flow current, so the rotor basically does not generate heat, and the excitation current is small, the copper consumption is also small, and the efficiency is high. Therefore, by satisfying the above Article (3), it is possible to reduce the size and weight.
For example, in the development of a traction motor integrated with a wheel, the design of the asynchronous motor and the permanent magnet motor was performed under the same design conditions. The result shows that the weight of the permanent magnet synchronous motor is approximately 2/3 of the asynchronous motor and can be large. Decrease in magnitude.
3.2 Direct Drive Type Permanent Magnet Synchronous Traction Motor As described in the previous section, the traction motor can be operated at a high speed through a gear transmission device, enabling the traction motor to be compact and lightweight. Therefore, a conventional traction motor transmits power to the axle through a gear transmission to drive the vehicle. However, the use of gear transmissions also causes problems such as transmission loss, noise, and maintenance.
With the direct drive method, gear transmissions are not required, and these problems can be solved. At this time, the traction motor will increase in size, resulting in increased unsprung weight, increased impact on the track, and increased impact on the traction motor. Therefore, the weight and size are rigorous. It is difficult to use direct drive under the restricted floor of the vehicle body.
However, the permanent magnet synchronous motor can be greatly reduced in size and weight compared with the conventional direct current motor and asynchronous motor, so that direct drive can be achieved under the existing size and weight conditions.
Therefore, we have been developing direct-drive permanent magnet synchronous traction motors. The characteristics of direct-drive traction motors are shown in Table 1.
Table 1 Characteristics of Directly Operated Motors Advantages Gears that do not need to be repaired Do not install gears Space for gears No power transmission losses (high efficiency) Small noises Small shocks to traction motors Overall Speed ​​is low, weight Larger bogie undersprung weight increase Torque pulsation is directly transmitted to the wheels. The loading test of the prototype to be used in the existing commuter electric commuter train is carried out, and the noise near the underfloor traction motor of the vehicle is measured at a speed of 64km. When /h is reduced by 14dB, noise is greatly reduced. In addition, with the simple structure of direct-drive traction motors, besides being able to be used in existing commuter electric trains, as well as variable-gauge variable-speed electric vehicles (EMUs) and low-floor light rail vehicles, it is hoped that research will be conducted in this area in the future. .
3.3 Fully-enclosed permanent magnet synchronous traction motor Railway locomotive traction motor requires small size and high power, and ventilation cooling is usually adopted. However, the dust contained in the cooling air contaminates the inside of the traction motor. Therefore, the traction motor needs to be periodically disassembled and cleaned. Moreover, most of the traction motors of the existing line vehicles have a structure in which the rotor and the fan are directly connected (self-ventilation structure), and the noise of the fan during high-speed operation is very high.
If a fully enclosed structure is used, dust cannot enter the traction motor and there is no need to disassemble the motor for cleaning. At the same time, the noise inside the motor is isolated and the realization of a low-noise traction motor is possible. To this end, a fully enclosed traction motor was developed. However, totally enclosed motors have poorer cooling performance than ventilated and cooled motors.
Therefore, to achieve the same size and performance of the fully-enclosed motor as in the conventional motor, it is necessary to use a motor with less heat, and to study a new cooling structure so that the temperature rise of each part is controlled within the specified limit.
Using a permanent magnet synchronous motor with high efficiency and low heat can reduce the temperature rise. However, in the fully enclosed traction motor, the overall temperature of the motor is increased, and the temperature rise limit of the bearing portion is relatively low, so it is necessary to prevent the temperature rise of this portion from being too high. For this purpose, we studied the cooling structure around the bearing and prototyped a fully-enclosed permanent magnet synchronous motor with a new bearing cooling structure. We verified the effect of the cooling structure of the bearing and the noise reduction effect.
The results of the longitudinal profile of the fully enclosed traction motor prototype show that, with the same volume as the conventional self-ventilated asynchronous traction motor, a fully-enclosed traction motor with the same power can be realized, and the noise ratio of the fully-enclosed motor is reduced by about 10 dB during high-speed operation. Moreover, compared with the conventional electric motor, the fully-enclosed traction motor is simultaneously satisfied with light weight and high efficiency.
4 Problems Related to Permanent Magnet Synchronous Motors 4.1 No-load induced voltage Permanent magnets of permanent-magnet synchronous motors can generate interlinkage magnetic flux even when external power is not supplied to the stator coils, and they can also generate voltages at the terminals of the traction motor during inertia. This voltage is called the no-load induced voltage. Application of Permanent Magnet Synchronous Electric Permanent Magnet Synchronous Motors in Railway Locomotives by Common Voltage Source Inverters When converter technology and electric traction 1/2003 machines, no-load induced voltage may bring the following problems. In the event of a fault such as a short-circuit between phases, the motor supplies power to the fault and generates a short-circuit current, which may increase the effect of the fault.) If the peak value of the no-load induced voltage exceeds the withstand voltage of the inverter component, the component will be damaged;) If the load voltage is no load If the peak value is higher than the DC-link voltage of the inverter, the diode acting in anti-parallel with the inverter switching element acts as a rectifying circuit during coasting, so that regenerative braking occurs.
For point (1), a load contactor is provided between the traction motor and the inverter as described in 2.2.3. When the fault occurs, the traction motor and the inverter can be disconnected.
For point (2), components with relatively high withstand voltage can be used, but this will increase the price of the inverter. It is also possible to adopt a method of passing a weakened field current in a traction motor so that the voltage generated during coasting is not too high, or to isolate the traction motor with a load contactor during coasting. To ensure reliability, the load induced voltage must be sufficiently lower than the withstand voltage of the component. Therefore, the permanent magnet synchronous motor must be designed so that the interlinkage flux generated by the magnet is as small as possible, and thus the reduced magnet torque is compensated by the reluctance torque, which is the most realistic solution. The point is also the same as point (2). To make the interlinkage flux generated by the permanent magnet as small as possible, the insufficient torque is compensated by the reluctance torque, and the problem can be solved. In addition, it is also possible to control the traction motor so that it does not generate a braking torque when traveling.
4.2 Inter-layer short circuit Inter-layer short circuit is one of the faults of the motor. This is because the insulation layer in the specified sub-coil is damaged due to heat generation and the like, resulting in a short circuit between the copper wires in the coil. Inter-layer short circuit occurs in the asynchronous motor, but the permanent magnet's magnetic flux can also make the coil generate electromotive force when the fault motor is sent back and run after the interlayer short circuit occurs in the permanent magnet synchronous motor. As a result, a short-circuit current may be generated in the coil short-circuited between the layers.
The effect of the short-circuit current in this case on the traction motor must first be ascertained.
The author deliberately short-circuited and tested the permanent magnet synchronous motor layers, and investigated the phenomena that occurred during the short circuit between layers, and reached the following conclusions. ) The motor torque change caused by the short circuit between the layers is very small, about 5% of the rated torque.
(2) In the loopback operation under interlayer short-circuit conditions, the damage caused by the interlayer short-circuit will not develop at the constant speed (equivalent to 70km/h), and no smoke will be emitted. But above a certain speed, it will blow in the short circuit time.
Therefore, in order to make the motor not smoke when returning, the train speed must be suppressed below a certain speed. Or, as with the bearing sticking, it will be returned by the van. This inter-layer short-circuit fault is actually a very rare phenomenon, but in practical applications, it is necessary to understand the above method in order to deal with when an interlayer short circuit occurs.
4.3 Adsorption of Iron Powder The adsorption of iron powder by permanent magnets is a well-known phenomenon. The magnetic flux through the permanent magnet synchronous motor is basically the same as the asynchronous motor, but it can always produce magnetic flux which is different from the asynchronous motor.
Therefore, after the iron powder enters the permanent magnet synchronous motor, it may be absorbed in the traction motor.
In order to analyze the effect of iron powder adsorption on the performance of the motor, I used a forced-air-cooled external rotor permanent-magnet synchronous motor to deliberately cast iron powder into the traction motor. We investigated the performance of the traction motor while investigating the iron powder adsorption condition. Analysis.
The results show that iron powder is mainly adsorbed on the rotor core end (as shown). The adsorbed iron powder can reduce the effective interlinkage flux of the permanent magnet, but in the motor performance test, there is no significant change in the motor performance before and after the iron powder adsorption, and it can be confirmed that although the iron powder is attached, the motor performance is confirmed. The impact is not great.
The iron powder attachment point in the rotor of the traction motor 5 Conclusion The traction motor of the railway locomotive has been pursuing compactness and light weight. Permanent magnet synchronous motor is essentially a kind of high-efficiency motor, and it can also be made compact and lightweight. Therefore, it is undoubtedly suitable for use as a traction motor for railway rolling stock. Not only that, intrinsically efficient permanent-magnet synchronous motors, which are widely concerned about energy and environmental issues, are also motors that meet the requirements of the times.
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