The plastic film blowing machine, which can process the plastic particles into plastic film through a series of processes, uses the frequency converter to control each processing link in the whole operation process, and the motor speed to control the thickness of the film and other process parameters. In the frequency conversion control of the plastic film blowing machine, other links are relatively easy to control, The most difficult to control is the frequency conversion control of the traction and winding parts, which is directly related to the ease of use and automation of the equipment, and the product quality of plastic film. Speed control, position control and torque control can be used for traction and winding of plastic film.

1、 Process introduction of film blowing machine:

1. Film blowing machine is a mechanical equipment that melts plastic particles and then blows them into film.

(1) Production process and principle:

The dried polyethylene particles are added into the hopper and enter the screw by the gravity of the particles themselves. When the particles contact with the screw, the plastic particles are pushed forward due to the vertical thrust generated by the rotating inclined surface and the friction between the plastic and the screw rod, the plastic and the barrel, and the collision friction between the particles, At the same time, the barrel is gradually melted by external heating. The molten plastic is filtered by the die head to remove impurities, and comes out from the die mouth of the die head. It is cooled by the wind ring, inflated, and rolled into a cylinder by herringbone plate, traction roller and coiling.

(2) Main structure of film blowing machine:

The main structure of film blowing machine is mainly composed of extruder, die head, cooling device, foam stabilizer, herringbone plate, traction roller, coiling device, etc.

2、 Site conditions:

There are four motors in the field traction winding part, and the motor parameters are as follows: two traction motors with power of 2.2kW and two winding motors with power of 2.2kW

Load requirements: the inverter runs stably, the master-slave synchronization is good, and the speed of each motor can be adjusted according to the actual requirements of the product to get different tension.

3、 Frequency conversion solution

1. Control scheme

2. Scheme description

The starting and stopping mode of frequency converter is terminal starting and stopping, that is, the starting and stopping command of frequency converter is given by external button through digital input terminal to realize remote starting and stopping control.

The frequency of the main engine is given by the analog quantity Ai1. At the same time, because of the need to ensure the synchronous speed of four motors, it is necessary for the slave to follow the speed of the main engine. Considering that the analog has zero drift, the high-speed pulse output is used for frequency setting, and the slave receives the main frequency setting through the high-speed pulse input. Considering the need to adjust the tension change through the free frequency, the auxiliary frequency source analog Ai1 is added to set the auxiliary frequency to adjust the tension manually.

Because the system requires high synchronization, the signal of the former inverter is sent to the slave to start the command, and the starting frequency is added to the host to improve the response accuracy of the slave. The fault signal of frequency converter is output through the programmable relay terminal. When the frequency converter is in fault, the system receives the fault signal to avoid the mechanical damage caused by misoperation. After the fault is cleared, the fault lock can be released through the reset terminal. Because the system uses two channels of frequency source superposition, it may bring inconvenience in the process of trial operation, so a single action switch is added to realize the frequency source switching through the single action button in the debugging mode.

The speed regulation mode of the system is the superposition of two frequency combinations. The main setting is given by the keyboard, the slave setting is given by the analog quantity Ai1, and the slave setting is given by a small amount of regulation. A small upper and lower limit can be set.

4、 Technical description of ski600 series inverter

Ski600 series inverter, environmental protection and energy saving, is widely used in fans, water pumps, air compressors, injection molding machines, winding machines, central air conditioning, textile machines, chemical industry, metallurgy, mining, pharmaceutical, ceramics, paper making, oil fields, plastics, printing, thermoelectricity, tobacco, food and other mechanical equipment.

Technical features:

(1) Accurate motor parameters self-learning: accurate rotating or static motor parameters self-learning, convenient debugging, simple operation, providing higher control accuracy and response speed

(2) Vectorized V / F control: automatic stator voltage drop compensation and slip compensation, ensuring excellent low-frequency high torque and torque dynamic response under VF control mode

(3) Software current and voltage limiting function: good voltage and current limiting, effective limiting of key control parameters to reduce the risk of inverter failure

(4) Multiple braking modes: provide multiple braking modes to ensure stable and fast system shutdown

(5) Strong environmental adaptability: high over temperature point of the whole machine, independent air duct design, thickened three proofing paint treatment, more suitable for harsh occasions such as high temperature and high humidity in plastic machinery industry

(6) Speed tracking restart function: realize the smooth starting of the rotating motor without impact

(7) Automatic voltage adjustment function: when the grid voltage changes, it can automatically keep the output voltage constant

(8) Comprehensive fault protection: over current, over voltage, under voltage, over temperature, phase loss, overload and other protection functions

5、 Concluding remarks

The control difficulty of plastic film blowing machine lies in the high synchronization of rotation speed, so a series of measures are needed to ensure the simultaneous stop. At the same time, due to the flexibility requirement of the actual tension, it is necessary to have a small range of self adjustment under the premise of ensuring a high degree of synchronization. The whole control system requires the inverter to have high sensitivity high-speed pulse input / output port, fast slope following, frequency superposition and switching performance.