1 Introduction
In recent years, with the widespread application of machine-room-less and small-machine-room elevators, gearless transmission using permanent magnet synchronous motors has become a major technological trend in the elevator industry. Gearless elevators connect the motor's output shaft directly to the traction sheave of the main unit, eliminating the need for gear reduction or other speed reduction mechanisms and directly driving the elevator. This transmission method features high efficiency, low noise, and simple mechanical structure, making it the optimal solution among various traction transmission methods for elevators. Currently, most elevator products use variable frequency drives (VFDs). This article will discuss the differences and requirements in VFD control between traditional geared elevators and gearless elevators using permanent magnet synchronous motors.
2. Characteristics of Permanent Magnet Synchronous Motors
(1) The electromagnetic torque of a permanent magnet synchronous motor is greater than that of a conventional AC or DC motor of the same capacity.
Ignoring the additional torque, the electromagnetic torque expression of a permanent magnet synchronous motor is:
As can be seen from equations (1) and (2), t and φ are directly proportional to b. The rotor magnetic poles of the permanent magnet synchronous motor are made of high-performance, low-cost neodymium iron boron (ndfeb) rare earth permanent magnet material. This material has a remanent magnetic induction intensity of up to 1.47t at room temperature and a maximum magnetic energy product of 397.9kj/m2, which is much larger than b of ordinary motors. Therefore, its output torque is much larger.
(2) High efficiency and power factor
The motor does not require reactive excitation current, which can significantly improve the power factor (up to 1) and reduce stator current and stator resistance losses. When the motor is working stably, the rotor and stator magnetic fields operate synchronously, and there is no induced current in the rotor, so there is no rotor resistance loss. Both of these factors improve the efficiency of the motor by 2% to 8%. Moreover, the motor can maintain high efficiency and power factor in the load rate (p2/pn) range of 25% to 120%, making the energy-saving effect more significant when operating under light load, as shown in Figures 1 and 2. [3]
Figure 1. Comparison of efficiency curves of permanent magnet synchronous motor and asynchronous motor.
Figure 2 Comparison of power factor curves of permanent magnet synchronous motor and asynchronous motor
As can be seen from Figures 1 and 2, because the rotor winding of the asynchronous motor needs to absorb part of the electrical energy from the grid for excitation, the efficiency and power factor of the motor are relatively low, especially when the motor load rate is less than 50%, both of which drop significantly.
(3) No current flows through the rotor of the permanent magnet synchronous motor, and there is little or almost no reactive current in the stator winding, which results in a low temperature rise of the motor and extends the service life of the motor.
In summary, although the use of high-performance permanent magnet materials increases the cost of the motor, eliminating the reduction mechanism lowers the mechanical manufacturing cost. Furthermore, the increased efficiency of the motor and transmission significantly reduces the capacity of the elevator's variable frequency drive (VFD). For example, an elevator with geared transmission and a speed of 2 m/s and a load capacity of 1000 kg requires a 22 kW VFD, while the same elevator using a permanent magnet synchronous motor with gearless drive requires only a 15 kW VFD.
3 Design Requirements for Elevator Variable Frequency Drive
(1) Since it is a gearless direct traction system, the motor's response changes will act directly on the elevator car through the wire rope. Therefore, to consider the elevator's vibration, comfort, and other indicators, a high-performance variable frequency speed controller with high control accuracy and fast response speed needs to be designed. In particular, the detection accuracy of the current loop and the speed of calculating the response are crucial.
(2) Rotary encoder
In the control system of a permanent magnet synchronous motor, the encoder, in addition to providing feedback on the motor's speed, also needs to detect the position of the motor's magnetic poles. Therefore, the encoder needs to be able to provide feedback on the magnetic pole position signal. Furthermore, in gearless traction systems, the motor speed is relatively low, thus requiring a higher encoder resolution, generally above 4096 c/t, to ensure good system control performance. Therefore, this type of elevator system typically uses high-performance absolute or sine/cosine rotary encoders. However, current standard configurations for frequency converters generally use incremental rotary encoders with phases A and B. This necessitates considering the interface and application design of the new rotary encoder in the frequency converter design.
(3) Compared with AC asynchronous motors, permanent magnet synchronous motors cannot start automatically at the rated frequency and require starting under the drive of a frequency converter. This is because the rotor of a permanent magnet synchronous motor is a permanent magnet, and its rotor magnetic field is constant. Therefore, the generation of the stator magnetic field needs to be coordinated with the position of the rotor magnetic field poles to generate the torque required for the motor to run. Therefore, in elevator systems using permanent magnet synchronous motors, the frequency converter needs to be designed with the function of detecting the magnetic pole position signal through encoder feedback and generating a stator rotating magnetic field based on the magnetic pole position, motor speed, required torque, etc.
(4) In the current system, the magnetic pole detection of the permanent magnet synchronous motor requires the inverter parameters to be initialized in the inverter design. In many cases, the inverter needs to run the motor under no-load conditions to set the position. However, for elevator products, after an elevator is installed, the motor is already loaded with loads such as the car and counterweight. It is not practical to run it under no-load conditions to initialize the magnetic pole position. Therefore, in such elevator systems, the inverter needs to have the function of initializing the magnetic pole position without running it under no-load conditions.
(5) To ensure good starting and braking characteristics of the elevator under various load conditions, it is essential to install a load detection device in the gearless traction drive control system. In the system, as mentioned earlier, a high-performance rotary encoder with absolute value or sine/cosine properties is used. The inverter design can utilize the performance of the rotary encoder to calculate the torque that needs compensation at the moment of elevator start-up and compensate for the input and output torque, thereby achieving smooth start-up. This requires the inverter to have improved signal detection accuracy, anti-interference capability, and calculation response speed to a certain extent.
(6) Considering that gearless traction requires low speed and high torque, the motor must be designed with multiple poles, generally more than 20 poles, and even 40 or higher. Therefore, the frequency converter used in elevators needs to be able to adapt to the requirements of higher motor pole numbers than before.
(7) Noise
Traditional elevator systems, which use geared reduction gearboxes, primarily generate noise in the elevator machine room from the gearbox. However, gearless elevators using permanent magnet synchronous motors eliminate the gearbox, making the motor's electromagnetic noise the main component of the machine room noise. Therefore, these systems place higher demands on the frequency converter's motor noise suppression capabilities.
(8) Protection
In typical variable frequency speed control elevators, the inverter design incorporates significant considerations for both its own protection and the protection of the system and overall safety. However, for gearless elevators with permanent magnet synchronous motors, in addition to the traditional system design, new safety features are required based on the characteristics of gearless transmission and permanent magnet synchronous motors. For example, protection against motor stall due to incorrect magnetic pole position detection necessitates a specialized inverter design with higher detection and calculation speeds.
(9) Energy Feedback
As a vertical transportation tool, elevators inevitably operate in two states: electric and generator. Traditional elevator inverters typically dissipate the energy fed back during generator operation through resistors. This design is simple and cost-effective; however, traditional elevators use geared transmissions, which are inefficient, resulting in less energy feedback. In gearless elevators using permanent magnet synchronous motors, the lack of gears increases efficiency, leading to a significant increase in feedback energy. Using the traditional method of resistor dissipation would drastically increase the power, size, and cost of the resistors, and in today's energy-scarce environment, it contradicts the trend towards energy conservation and environmental protection. Therefore, in gearless elevator systems using permanent magnet synchronous motors, the inverter design needs to incorporate energy feedback functionality, feeding this feedback energy back to the power grid.
4. Some practical applications of the system
4.1 Noise during elevator operation
Tests were conducted at five points—top, front, back, left, and right—1 meter away from the motor. The average value was taken and corrected based on the difference between this value and the ambient noise. The result was 58 dB, which is less than the enterprise standard of 75 dB and much lower than the noise of the current elevator system.
4.2 Elevator Operation Comfort
It refers to the vibration acceleration during elevator operation, including acceleration, deceleration, and constant speed processes. As shown in Figures 3 and 4, the vibration acceleration during constant speed operation is less than 15 gal, which is lower than the design value of 20 gal. The vibration acceleration during acceleration and deceleration processes exceeds 20 gal but is less than 25 gal, which meets the design requirements.
Figure 3 Vibration acceleration curve of the elevator during upward movement.
Figure 4 Vibration acceleration curve of the elevator during upward movement.
The system has been successfully applied to elevator products after being designed for use. This year, it has been installed and is in operation for many customers across the country. Customers generally report that the product has the characteristics of low operating noise, low power consumption, and stable operation.
