Application of VFD (Variable-frequency Drive) in extruders

Most screw extruders are generally driven by DC motors, but in recent years, AC motor speed regulation has been increasingly used due to its technical and cost advantages.

DC drive for extruder

The structure of the screw extruder is shown in Figure 1. The plastic pellets are heated and plasticized after being fed from the hopper, and pushed to the exit of the extruder through the rotation of the screw. The final product shape is determined by the extrusion die. The typical screw speed of a common single-screw extruder is about 100 r/min, while the typical speed of a twin-screw extruder for mixing is 200~500 r/min, and the speed of a low-speed twin-screw extruder used for profile extrusion Usually 10~40r/min.

The DC motor used in traditional extruders requires regular replacement of brushes and has poor tolerance to harsh environments. Sometimes it also requires independent clean cooling air. In daily use, typical faults of DC motors include brush sparking, armature discoloration, insufficient heat dissipation, severe noise and vibration, etc., making daily maintenance costs higher than AC motors, and their cost is also higher.

Figure 1 Screw extruder structure

AC drive for extruder

A few years ago, AC drives could not provide the same excellent performance as DC drives and were expensive. However, with the rapid development of power electronic devices and microprocessor technology in recent years, the proposal of AC control theory and the improvement of AC control software, AC drive technology is more cost-effective than DC drive in many application fields. For example, Danfoss FC302 series VFD (Variable-frequency Drive) is used in extruders and has better speed control performance and torque response speed than DC drives. Among them, the open-loop speed control accuracy is better than 0.5%, and the closed-loop speed control accuracy is better than 0.5%. Better than 0.1%.

Since the starting torque of the extruder may reach 200%~250% of its operating torque, which is higher than the maximum 180% starting torque value of Danfoss FC300 series VFD (Variable-frequency Drive), one solution is to choose For a larger power drive, another way is to make the rated speed of the equipment work in the constant power range of the AC motor (because the load torque of the extruder at full speed does not require the rated torque of the motor).

Figure 2 Schematic diagram of working in constant power area

When using the method of making the rated speed of the equipment work in the constant power area of the AC motor (as shown in Figure 2), the general operating speed of the motor can be made to 70Hz by appropriately selecting the reduction ratio. At this time, the load torque is T load, According to theoretical calculation, the rated torque of the motor is:

Then the maximum torque of the motor is:

In this way, the starting torque requirements of the extruder can be met. Otherwise, if you follow the traditional selection method and select a motor with a general operating speed of 50Hz, in order to meet the maximum starting torque requirement of 250%, you must select a motor and driver with 1.4 times the power of the original.

Advantages of using AC drives

1. Improve efficiency

Using Danfoss FC300 AC drive