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1 Introduction The traction motor of a railway rolling stock is limited in space on the bogie, so the volume is small; the high speed of the train requires it to be light in weight and large in output power. Moreover, the torque characteristics of the motor require a large torque to be output at start-up and can operate over a wide range of speeds as well as for 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, VVVF inverter-controlled asynchronous motors can also meet these requirements. Compared with the DC motor, the asynchronous motor has no commutator, the maintenance is reduced, and the compact and lightweight can be achieved at the same time. Therefore, the traction motor of the new electric transmission locomotive is basically an asynchronous motor.
Nowadays, permanent magnet synchronous motors have attracted people's attention. It not only has the same characteristics as the traction motor of the asynchronous motor, but also has higher efficiency and smaller size and weight than the asynchronous motor. This paper first introduces the structure and characteristics of permanent magnet synchronous motor, and then re-analyzes the results obtained by using permanent magnet synchronous motor as direct drive traction motor and fully enclosed traction motor according to the viewpoint that permanent magnet synchronous motor is suitable for railway locomotive. The possibility of application of a permanent magnet synchronous traction motor is clarified.
2 Permanent magnet synchronous motor structure and characteristics 2.1 Permanent magnet synchronous motor structure of the magnetic field of the synchronous motor. The method of mounting the magnet on the rotor can be divided into two types: a surface magnet type and a buried magnet type. The stator of the permanent magnet synchronous motor is basically the same as the asynchronous motor, and consists of a stator core composed of laminated kiln steel sheets and a stator coil embedded in a stator core slot, and the coil is connected to generate a rotating magnetic field by a usual three-phase AC power source.
The torque of the permanent magnet synchronous motor is generated by the interaction between the magnetic field of the permanent magnet and the magnetic field of the stator coil current. The rotor operates synchronously with the rotating magnetic field of the stator powered by the three-phase AC power supply and generates torque. This torque is called For the magnet torque. In addition, by changing the shape of the rotor core, reluctance torque can also be expected. The reluctance torque is generated by the salient magnet 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. (electrical angle) (in the coordinates) For the q-axis, the torque generated by the difference in the difficulty of passing the magnetic lines of force.
Briefly, 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 cross-sectional view perpendicular to the rotating shaft of various typical permanent magnet synchronous motor rotors. (a) and (b) are called surface magnet types, and as the name implies, permanent magnets are fixed to the surface of the rotor. Usually, the surface magnet type permanent magnet synchronous motor is covered with a non-magnetic structural material on the outer side of the rotor, and the permanent magnet is pressed to prevent the surface magnet from flying out when the motor is running at a high speed. (a) The shape of the core of the structure has no convex polarity, and substantially no reluctance torque is generated, and only the magnet torque can be generated. (b) The core of the structure has a salient pole structure, so that reluctance torque can also be generated.
The structure of (c) is a buried magnet type structure. As the name implies, its magnet is buried in the middle of the iron core.
The core of the embedded magnet type structure usually has a magnetic convex shape and can generate reluctance torque. Moreover, the embedded magnet type has a simple structure, three kinds of cases of the motor constant and the torque characteristic of the angular frequency (rad/s) permanent magnet synchronous motor, and /m is the maximum current effective value x 10,000.
This situation means that the magnetic flux generated by the permanent magnet is larger than the magnetic flux generated by the stator. Therefore, the magnetic flux generated by the magnet in the high-speed zone is strong, and the magnetic flux cannot be sufficiently weakened, and the voltage is excessively large, so there is an output limit velocity. For this reason, the output power of the high speed zone will drop sharply. In this motor constant relationship, the output limit speed must be made much faster than the maximum speed. On the other hand, because the armature reaction has a small magnetic flux and a high power factor, a larger maximum output power can be used in the case of the same inverter capacity. This is an advantage.
In this case, the output bounding speed is theoretically infinite. Therefore, the output power of the high-speed area does not decrease, and the constant power characteristic can be maintained to the highest speed.
This situation means that the magnetic flux generated by the magnet is smaller than the magnetic flux generated by the stator. The magnetic flux of the magnet can be offset by the armature flux, so there is no output limit speed and the power drop in the high speed zone is 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 speed range of the traction motor is very wide, and the output power drop at high speed should be small. Moreover, since the price and weight of the inverter are increased as the inverter capacity is increased. Therefore, the inverter capacity required for the same output power is small. As will be described later, when designing 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 zone, in order to maximize the output power under a certain inverter capacity, the design of the motor constant should be as close as possible to the characteristics of (b).
The value of -L, is kept constant so that the value of the starting torque is kept constant, and when the values ​​of 1 and ~ are changed, the rotational speed torque characteristics in the above three cases (a) to (c) are obtained. The design of the traction motor according to the characteristics of (b) has the following advantages. First, it is expected to shorten the operation time due to an increase in the acceleration force at a high speed. In addition, the ratio of regenerative braking at high speed can be increased, and the increase in regenerative braking rate means more energy saving, and the high regenerative braking rate at high speed can reduce the burden of mechanical braking and reduce maintenance.
Therefore, compared with the asynchronous traction motor, the permanent magnet synchronous traction motor can also make the rotor cross-sectional shape of the permanent magnet synchronous motor easy. The brittle magnet is not on the surface and the structure is very strong.
The traction motor of a railway locomotive is expected to have a firm structure and it is desirable to utilize the reluctance torque effectively so that the interlinkage flux generated by the permanent magnet is sufficiently small (see 4.1). Therefore, it can be said that the permanent magnet synchronous motor in which the magnet type rotor structure is embedded is suitable for use as a traction motor for a railway locomotive.
2.2 Characteristics of permanent magnet synchronous motor 2.2.1 High efficiency, small size The biggest characteristic of permanent magnet synchronous motor is high efficiency. Efficiency can be expressed in terms of (input power) - (loss) / (input power). The magnetic field of a permanent magnet synchronous motor does not require current, and in principle the rotor does not produce losses. Therefore, the maximum loss of the motor, that is, the copper consumption (the Joule heat generated by the current) is about half that of the asynchronous motor, and the efficiency is much higher than that of the asynchronous motor. High efficiency and low power consumption make the railway more energy efficient than before, and it is also expected to reduce electricity bills.
Further, as will be described later, the smaller the loss, the smaller the volume of the motor. Thus, the use of a permanent magnet synchronous motor can achieve a small size and a large power. Therefore, when the volume is the same, the permanent magnet synchronous motor can be larger 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 drive locomotive is limited by the torque characteristics of the traction motor. The torque of the asynchronous motor and the DC series-excited motor is proportional to the speed in the high-speed range; 5 is inversely proportional. Therefore, the speed traction characteristic of the electric vehicle is usually constant torque in the low-speed region, and the torque and speed in the medium-speed region are The inverse ratio is reduced, and the torque in the high speed zone is inversely proportional to the square of the speed.
On the other hand, the basic characteristics of the permanent magnet synchronous motor can be expressed by the following equation.
The maximum value of the interlinkage flux generated by the permanent magnet; 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 in the figure, since the permanent magnet synchronous motor is a synchronous machine and must be powered by an alternating current power source synchronized with the rotation of the motor, an independent control method of supplying power to one traction motor by one inverter is adopted.
In addition, the permanent magnet synchronous motor generates a magnetic flux even if there is no external power supply, so a voltage is generated at the traction motor terminal during coasting.
Therefore, when the inverter has a phase-to-phase short circuit and other faults, the inverter must be disconnected from the traction motor. Therefore, a contactor (called a load contactor) is provided between the inverter and the traction motor. , the traction motor can be disconnected.
Cost analysis is also important when practical applications of permanent magnet synchronous motors.
First consider the initial cost. The rotor of the permanent magnet synchronous motor has a simpler rotor structure than the asynchronous motor, and the price is lower in mass production. On the other hand, the permanent magnet synchronous motor must be independently controlled, and the inverter is more expensive than the asynchronous motor when it is centralized. 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 electric energy consumption is relatively small, and the electric power cost can be reduced. In the existing size and weight conditions, a fully enclosed traction motor or a direct drive traction motor can be realized, which can reduce various maintenance and save manpower.
In summary, the initial cost of a permanent magnet synchronous motor is higher than that of a centrally controlled asynchronous motor, but from the perspective of operating cost, the use of a permanent magnet synchronous motor is expected to reduce various costs. Therefore, in terms of cost, which of the asynchronous motor and the permanent magnet synchronous motor is better cannot be generalized.
3 The application of permanent magnet synchronous motor on railway locomotives 3.1 The small and lightweight traction motor of traction motor has strict restrictions on weight and size, and the requirements for small size and light weight are quite high compared with general motors. Normally, there is a relationship between the motor output power and the size.
Air gap diameter (m); "core length (m); n - speed (r / min). ;) Improve the performance of magnetic materials;) Increase the speed;) Increase the number of poles.
The permanent magnet synchronous motor rotor does not flow current, so the rotor basically does not generate heat, and the excitation current is small, the copper consumption is small, and the efficiency is high. Therefore, it is possible to achieve small size and light weight by satisfying the above item (3).
For example, when developing a traction motor integrated with a wheel, the design analysis of the asynchronous motor and the permanent magnet motor under the same design conditions shows that the weight of the permanent magnet synchronous motor is about 2/3 of that of the asynchronous motor, which can be large. The amplitude is reduced.
3.2 Direct Drive Permanent Magnet Synchronous Traction Motor As described in the above section, the traction motor can be operated at high speed by the gear transmission, which makes the traction motor small and lightweight. Therefore, the conventional traction motor drives the vehicle by transmitting power to the axle through a gear transmission. However, when using a gear transmission, problems such as transmission loss, noise, and maintenance are also caused.
With the direct drive method, no gear transmission is required, these problems can be solved, but the volume of the traction motor will increase, resulting in an increase in the unsprung weight, an increase in the impact on the track, and an increase in the impact on the traction motor. Therefore, the weight and size are strictly controlled. It is very difficult to use a direct drive method under the limited body floor.
However, permanent magnet synchronous motors can be significantly reduced in size and weight compared to conventional DC motors and asynchronous motors, enabling direct drive in existing sizes and weights.
Therefore, we have been developing direct drive permanent magnet synchronous traction motors. The characteristics of the direct drive traction motor are shown in Table 1.
Table 1 Features of direct-acting motor Advantages No gear-driven device to be repaired No space for gear transmission is required. No power transmission loss (high efficiency). Low noise. The impact of the traction motor is small. Larger bogie unsprung weight increased torque ripple directly transmitted to the wheel. The prototype of the proposed commuter electric vehicle group was loaded and tested. The noise near the traction motor under the floor was measured at a speed of 64km. The /h can be reduced by 14dB, which greatly reduces the noise. In addition, with the simple structure of the direct drive traction motor, it can be used for both on-line commuter electric vehicles and on track-variable electric vehicles () and low-floor light rail vehicles. It is hoped that research will be carried out in this area in the future. .
3.3 Fully enclosed permanent magnet synchronous traction motor The railway locomotive traction motor requires small volume and high power, and usually adopts ventilation cooling. However, the dust contained in the cooling air pollutes the inside of the traction motor, so the traction motor needs to be dismantled and cleaned regularly. Moreover, most of the traction motor of the wired vehicle is a structure in which the rotor and the fan are directly connected (self-ventilating structure), and the noise of the fan is high at high speed operation.
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 has been developed. However, the hermetic motor has poor cooling performance compared to the ventilation and cooling motor.
Therefore, the size and performance of the fully enclosed motor must be the same as that of the conventional motor. It is necessary to use a motor with less heat and study the new cooling structure so that the temperature rise of each part is within the specified limits.
A permanent magnet synchronous motor with high efficiency and low heat generation can reduce the temperature rise. However, the fully enclosed traction motor is such that the overall temperature of the motor rises and the temperature rise limit of the bearing portion is low, so it is necessary to prevent the temperature rise of this portion from being too high. To this end, we have studied the cooling structure around the bearing, and trial-produced a fully enclosed permanent magnet synchronous motor () with a new bearing cooling structure, which verified the effect of the bearing cooling structure and the noise reduction effect.
The longitudinal section view of the fully enclosed traction motor prototype shows that the fully enclosed traction motor of the same power can be realized under the same volume as the conventional self-ventilating asynchronous traction motor. The noise of the fully enclosed motor is reduced by about 10 dB at high speed. Moreover, such a hermetic traction motor is at the same time more lightweight and efficient than the previous motor.
4 Problems related to permanent magnet synchronous motor 4.1 No-load induced voltage Even if the external power supply is not supplied, the permanent magnet of the permanent magnet synchronous motor can generate the interlinkage magnetic flux of the stator coil, and the voltage can be generated at the traction motor terminal when inert. This voltage is called the no-load induced voltage. When a permanent magnet synchronous electric permanent magnet synchronous motor is driven by a conventional voltage type inverter to apply the current conversion technology on the railway locomotive and the electric traction 1/2003 machine, the no-load induced voltage may bring the following problems to the inverter. When a phase-to-phase short circuit or other fault occurs, the motor supplies power to the fault point, which generates a short-circuit current, which may increase the impact of the fault;) if the peak value of the no-load induced voltage exceeds the withstand voltage of the inverter component, the component may be damaged;) if the no-load induced voltage When the peak value is higher than the DC side voltage of the inverter, regenerative braking occurs when the diode that is anti-parallel to the inverter switching element acts as a rectifying circuit during coasting.
For point (1), a load contactor is placed between the traction motor and the inverter as described in 2.2.3, and the traction motor and the inverter can be disconnected in the event of a fault.
For the point (2), a component with a 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 reducing the field current in the traction motor so that the voltage generated during the coasting is not too high, or the traction motor is isolated by the load contactor during coasting. To ensure that it is very reliable, the load induced voltage must be sufficiently low compared to the withstand voltage of the component. Therefore, the permanent magnet synchronous motor must be designed such 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 the point (2). To make the interlinkage flux generated by the permanent magnet as small as possible, and the insufficient torque is compensated by the reluctance torque, the problem can be solved. In addition, the traction motor can be controlled to prevent the braking torque from being generated during the trip.
4.2 Inter-layer short circuit The short circuit between layers is one of the faults of the motor. It is a phenomenon in which the insulating layer in the specified sub-coil is damaged due to heat generation or the like, resulting in a short circuit between the copper wires in the coil. The inter-layer short circuit is also generated in the asynchronous motor, but after the inter-layer short-circuit occurs in the permanent-magnet synchronous motor, the magnetic flux of the permanent magnet can also cause the coil to generate an electromotive force when the faulty motor is turned back. This will cause a short circuit current on the coil that is shorted between the layers.
The effect of the short-circuit current in this case on the traction motor must be clarified.
The author deliberately short-circuits and tests the permanent magnet synchronous motor between layers, and investigates the phenomenon that occurs when the interlayer is short-circuited, and draws the following conclusions. The change in motor torque due to the interlayer short circuit is small, about 5% of the rated torque.
(2) When returning to the operation in the inter-layer short-circuit state, at a certain speed (corresponding to 70 km/h), the damage caused by the interlayer short-circuit does not develop and smoke does not occur. However, above a certain speed, the short circuit time will be blown.
Therefore, in order to prevent the motor from smoking when the return is made, the train speed must be suppressed below a certain speed. Or, when it is stuck to the bearing, it will be returned by the truck. This type of interlayer short-circuit fault is actually a very rare phenomenon, but in practice it is necessary to understand the above method in order to deal with the occurrence of interlayer short circuit.
4.3 Adsorption of iron powder The adsorption of iron powder by permanent magnets is a well-known phenomenon. The magnetic flux passage inside the permanent magnet synchronous motor is basically the same as that of the asynchronous motor, but the magnetic flux can always be generated differently from the asynchronous motor.
Therefore, after the iron powder enters the permanent magnet synchronous motor, it may be adsorbed inside the traction motor.
In order to analyze the influence of iron powder adsorption on the performance of the motor, the author uses forced-air-cooled external rotor permanent magnet synchronous motor to intentionally throw iron powder into the traction motor, and investigate the change of traction motor performance while investigating the adsorption condition of iron powder. Analysis.
The results show that the iron powder is mainly adsorbed at the end of the rotor core (as shown). The adsorbed iron powder can reduce the effective interlinkage flux of the permanent magnet. However, in the performance test of the motor, there is no significant change in the performance of the motor before and after the adsorption of the iron powder. It can be confirmed that although the iron powder adheres, the performance of the motor is improved. The impact is small.
The iron powder attachment point in the traction motor rotor 5 Conclusion The traction motor of the railway locomotive has always pursued small and lightweight. The permanent magnet synchronous motor is essentially a high-efficiency motor and can be compact and lightweight, so it is undoubtedly suitable as a traction motor for railway rolling stock. Not only that, the intrinsically high-efficiency permanent magnet synchronous motor, which is widely concerned with energy and environmental issues, is also a motor that meets the requirements of the times.
