Application of Frequency Converters in Automation Systems

Application of Permanent Magnet Synchronous Motor (PMSM) Variable Frequency Speed Control in the Chemical Fiber and Glass Industries​

In the 19th century, permanent magnets were used to create electric motors. Today, with the rapid advancement of electronic technology, Permanent Magnet Synchronous Motors (PMSMs) and frequency converters together form open-loop, high-speed, high-precision variable frequency speed control systems. These systems have been widely applied across various industrial sectors, replacing traditional DC speed control systems and electromagnetic slip speed control systems, demonstrating strong vitality.

It is well known that the rotational speed of a PMSM is strictly proportional to the supply frequency. Provided the precision of the power supply frequency is ensured, the rotational speed precision of the motor is also guaranteed, resulting in linear mechanical characteristics. For instance, in one enterprise, two independently operating synchronous systems ran continuously for several months, and the cumulative speed error was nearly zero.

Since the output frequency precision of frequency converters can reach 1.0‰ - 0.1‰, or even higher, the speed precision of the control system is also enhanced. Furthermore, the system has fewer control components, making its circuitry simpler than any other type of speed control system. Additionally, PMSMs possess advantages such as high power factor, high efficiency, energy savings, compact size, being brushless, and offering high safety and reliability. Consequently, this system is now extensively and commonly used in various industrial departments. Examples include winding, stretching, metering, and godet roller applications in the chemical fiber industry; and applications in annealing furnaces for flat glass, glass furnace stirring, edge rollers (or "pullers"), and bottle-forming machines within the glass industry.

​Application of PMSM Variable Frequency Speed Control in the Chemical Fiber Industry​

PMSM variable frequency speed control systems have been successfully implemented in melt-spinning machines for chemical fibers, as shown in the system diagram (Figure 12-1). The metering pump drive motor in the spinning machine employs a PMSM, requiring precise speed output to control the quantitative supply of the chemical fiber solution, thereby meeting the demands of the spinning process. When changing fiber product varieties, simply adjusting the speed of the metering pump drive motor fulfills the process requirements.

The power of the main metering pump typically ranges from 0.37 kW to 11 kW, with motors featuring 4 or 6 poles. The frequency variation range is 25 Hz to 150 Hz. Usually, one frequency converter is selected to drive multiple motors, though dedicated systems (one converter per motor) are also used, each approach having its own advantages and disadvantages.

Other essential processes in spinning, such as winding, stretching, and godet rollers, require either constant rotational speeds or specific speed ratios between paired rollers. The variable frequency speed control system is the ideal primary choice, a fact confirmed by long-term practical operation. After adopting variable frequency speed control, spinning line speeds can reach 3,000 to 7,000 m/min. Stretch rollers incorporating internal heating elements necessitate constant speed operation; the accompanying PMSM power ranges from 0.2 kW to 7.5 kW, with high-speed two-pole motors selected, featuring a frequency adjustment range of 50 Hz to 250 Hz. Utilizing variable frequency control provides high starting torque, rapid acceleration, and meets the requirements for demanding start-up conditions (hard starting).

​Application of PMSM Variable Frequency Speed Control in the Glass Industry​

Variable frequency speed control systems for the main drives of float glass annealing furnaces have been applied on dozens of production lines in China, replacing the original DC drives and achieving satisfactory economic benefits.

A float glass production line involves high-temperature glass liquid flowing from the melting furnace, gradually cooling down along the line. After solidifying, the glass undergoes heat treatment in the annealing furnace before proceeding to the cold end for cutting, inspection, packing, and other downstream processes. The annealing furnace process imposes stringent requirements; over its approximately 200-meter length, every roller must operate continuously and uniformly. Stoppages are absolutely unacceptable, as they would cause significant economic losses.

For this application, a TYB100-8 three-phase rare-earth PMSM paired with a Fuji G5 frequency converter was selected. This system has operated continuously and safely for tens of thousands of hours and received acclaim from all sides. Its main advantages are:

1. ​High Speed Precision (up to 0.4%)​: Ensures product thickness tolerances, saves raw materials, and yields obvious economic benefits.
2. ​High Reliability: Reduces maintenance workload.
3. ​Energy Saving: Due to fewer system components and the motor's inherent high efficiency.
4. ​Compact and Lightweight Design: The equipment is smaller and lighter.

Stirrers within glass melting furnaces previously used DC drives. However, considering the high-temperature environment and maintenance challenges, variable frequency speed control systems have been adopted since 1995. Specifically, two TYB400-8 motors are employed for this purpose. The operational requirements are as follows:

The two motors drive their respective stirrers, agitating the glass solution within the high-temperature furnace. To ensure mixing uniformity, "dead zones" inside the furnace bath are not permitted. Therefore, the working areas of the two stirrers must overlap slightly, yet be arranged so that the rotating paddles do not collide. A schematic of the working area is shown in Figure 12-2.

If the rotational speeds n1 and n2 differ, their cumulative effect could eventually lead to collisions between the stirrer paddles. Long-term practical application of this system has demonstrated no paddle collisions, confirming that the speed precision meets the requirements.

The application of variable frequency speed control on edge rollers (or "pullers") has also yielded excellent results. On the glass production line, as the molten glass gradually transforms from a liquid state to a plastic semi-solid state, it needs to be stretched and flattened into a plane. Edge rollers perform this crucial function. Their drive motors must offer continuously variable, stepless speed control. The speed of each puller motor should be pre-set based on factors such as the working point temperature and glass type. This demands excellent control system performance: wide speed range, high speed precision, and good dynamic response. Therefore, the existing Z₂-12, 0.6 kW, 1500 r/min DC motor speed control system was replaced with TYB500-6, 125 V, 50 Hz three-phase PMSMs. The frequency converter operates over a range of 10 Hz to 150 Hz, providing a motor speed range of 200 r/min to 3000 r/min. Replacing DC motors with synchronous motors offers advantages including higher production automation, superior quality, lighter weight, and easier centralized control.