Cableway drive background
The special operating environment of passenger cableways determines some special requirements for their electric drive systems. First of all, many cableways are built in high mountains and hills with high altitudes or in remote scenic areas far away from cities with inconvenient transportation. The natural environment is harsh, equipment accessories are inconvenient to purchase, and technical support channels are not smooth. This requires that cableway equipment, especially core components such as motor drivers, should have simple structures, strong environmental adaptability, not easy to damage, and convenient maintenance. Secondly, according to the size of passenger flow, the speed of the cableway will often change during operation. Therefore, it is also required that its speed control is convenient and easy to adjust. In addition, regardless of the height difference of the cableway and how the load changes, it should always be able to maintain smooth operation without speed creep, which puts high demands on the speed regulation performance of the motor. Finally, as an aerial passenger transport tool, the cableway drive system must also have reliable equipment and mature technology to ensure stable operation and less failure.
In the early motor drive, due to the complex speed regulation system of AC motors and the speed regulation performance cannot be comparable to that of DC motors, DC motor drives are mainly used in drive applications that require speed regulation, while AC motors are mostly used in situations where speed regulation is not required. The same is true for passenger ropeway drives. Small ropeways, especially fixed-grip ropeways, basically do not require speed regulation due to their low operating speed. Even if speed regulation is required, it is not demanding due to the small number of passengers and small loads, so AC fixed-speed drives or AC series resistor speed regulation drives are mainly used. Large-capacity ropeways, especially circular detachable ropeways, require frequent speed changes due to their high operating speeds. At the same time, due to complex terrain, large height differences, large passenger capacity, and load size that changes at any time, higher requirements are placed on ropeway speed control and speed regulation performance, so most of them use DC motor drive systems.
In recent years, with the development of modern control theory, power electronics technology and intelligent control technology, as well as the continuous improvement of the performance of rare earth permanent magnet materials, coupled with the advantages of AC motors such as simple structure, small size, light weight, and basically no maintenance, AC motor speed control systems have been increasingly used. In particular, the application of permanent magnet synchronous motors, combined with variable frequency speed control, not only makes the motor structure simpler, but also the speed regulation performance is completely comparable to that of first-flow motors. Therefore, the electric drive of larger domestic cableways has also experienced an evolutionary process from the dominance of DC motor rectifier drive to the gradual introduction of AC asynchronous motor variable frequency drive, until the current water-magnetic synchronous motor variable frequency direct drive has emerged.
The structure of a permanent magnet synchronous motor is similar to that of an ordinary AC motor, mainly including a base, stator core, stator winding, rotor core, permanent magnet, rotor shaft, bearing and end cover. In addition, there are generally ventilation holes or cooling water channels, junction boxes, etc. The special feature of a permanent magnet motor is that in addition to the core, shaft, bearing, etc., its rotor also has permanent magnets. In the cross section of the rotor core, there is space for the installation of permanent magnets and rotor shafts. Since the permanent magnets generate a magnetic field, it is no longer necessary to generate an electromagnetic field by power supply. After the rotor is started by an appropriate method to rotate, a rotating magnetic pole is formed. Under the magnetic pull of the stator rotating magnetic field, the rotating magnetic pole is dragged to rotate synchronously and output stable mechanical energy.
Permanent magnet synchronous motors have simple manufacturing processes, low costs, high energy efficiency and power factor, small size and low noise for the same capacity, and at the same time take into account the simple structure of AC motors and good speed regulation performance of DC motors. Therefore, they are widely used. In particular, in recent years, this type of AC permanent magnet synchronous motor has been widely used in AC direct drive (DD) cableways.
Starting of permanent magnet synchronous motor
Through the analysis of the mathematical model of permanent magnet synchronous motor, we can know that there is the following relationship between the rotor speed n (r/min) and the stator rotating magnetic field speed n (r/min), as well as the stator power supply frequency f (Hz) and the number of magnetic pole pairs P of the motor: n=n,=(60f)/P
Through further analysis, we can also know that the electromagnetic torque of PMSM is stable only when it is synchronously running. At asynchronous speed, the average electromagnetic torque is zero. During the starting process of the motor, the rotor speed increases from zero, which is the asynchronous running state. Since the electromagnetic torque is zero, the rotor cannot be accelerated, so the synchronous motor cannot start by itself. According to the above speed formula n=n,=(60f)/P, it can be seen that the speed is related to the stator power supply frequency and the number of motor pole pairs. When the number of motor pole pairs has been determined, the starting and speed regulation can be achieved by changing the stator power supply frequency. The device that can provide this variable frequency power supply is called a variable frequency drive (VFD), also known as an inverter (VVVF).
Direct drive of permanent magnet synchronous motor variable frequency speed regulation
The formula of synchronous motor speed n=（60f）/P can be used to calculate that when the motor pole pair number P=45 and the frequency f=16.5Hz, the speed n=22r/min. For the drive wheel with a diameter D of 5.2m, the linear speed L is:
L=(πD*n)=(3.14*5.2*22)/60=6m/s
It just meets the need of the maximum cableway running speed of 6m/s. When the control frequency of the inverter is between 0~16.5Hz, the cableway can run at a speed between 0~6m/s without the need to use a reducer for speed adjustment. The variable frequency speed regulation of the permanent magnet synchronous motor can also fully meet the low speed and high torque requirements at startup. This also provides the possibility for the variable frequency direct drive of the permanent magnet AC synchronous motor.
Conclusion
Through the above analysis, we can see that the permanent magnet synchronous motor has the same simple structure as the ordinary AC motor, and the synchronous motor variable frequency control has the same superior performance as the DC motor rectification speed regulation. In particular, with the continuous improvement of the performance of rare earth permanent magnet materials, the magnetic properties and thermal stability of permanent magnets can fully meet the requirements of motor use. In addition, the synchronous motor has a stronger tolerance to torque disturbances than the asynchronous motor and can respond relatively quickly. As long as the power angle of the motor is adjusted in time, its speed will always maintain synchronous speed operation, and the change of load torque will not affect the stability of the speed, which is very suitable for constant torque loads such as ropeways. At the same time, the existence of the permanent magnetic field of the synchronous motor rotor enables the motor to operate stably at extremely low frequencies, which makes it possible for the motor to start directly at low frequency and drive directly without a reducer. Therefore, it can be foreseen that the permanent magnet synchronous motor variable frequency direct drive will be more and more used in large-load passenger ropeways.
