Investigation of electromagnetic compatibility of high frequency polymer-welding machines

Lyuben Iliev, Ivaylo Stoyanov, Teodor Iliev, Elena Ivanova

University of Ruse, Department of Electrical and Electronic Engineering, Ruse, Bulgaria

Corresponding author, E-mail address: lailiev@uni-ruse.bg

Abstract The article presents the results of a measurement of the electromagnetic disturbances of a High Frequency (HF) plastic welding machine and their analysis. These types of machines are used in food processing, medical field, plastic or other dielectric material welding or heating. Because they operate with high power high frequency electromagnetic fields, they emit electromagnetic disturbances with high amplitudes up to several GHz. For these reasons, these machines are strictly regulated by standards for electromagnetic compatibility.

Key words: Electromagnetic Compatibility, Simulation.


1. Introduction

High frequency (HF) welding is the joining of material by supplying HF energy in the form of an electromagnetic field (27.12 MHz) and pressure to the material surfaces to be joined. A high-power high frequency generator produces the energy which is guided through a transmission line to the electrodes of the machine. The machine basically acts as a capacitor with high dielectric losses in the dielectric material used (Fig. 1). The electrical energy causes the molecules within the material to start moving, which generates heat that causes the material to soften and thereby fuse together. No external heat is applied, it is instead generated within the material. After cooling the welded surface under maintained pressure, the material is fused and a weld has been created. The weld seam can be at least as strong as the surrounding material or even stronger.

Fig. 1. Diagram of the welding machine

Four important factors that affect the final weld result are pressure, welding effect, welding time and cooling time. These parameters can be adjusted and combined in different ways to achieve the optimal welding result for a specific material.

The material most commonly used with HF welding is PVC (polyvinyl chloride), sometimes called vinyl and PU (Polyurethane). The material can be thick or thin, reinforced or coated. It can also be plain, colored or patterned [1]. A photo of the tested welding machines can be seen on (Fig. 2).

Figure 2. Tested Welding machine series E

A. EMC issues with HF welding machines

The power of the HF welding machines can vary from less than 1 kW up to 20 kW or more. The frequency of the magnetic field is around 27.12 MHz. This means that high-power high frequency electromagnetic disturbances are produced during the working cycle of the machine. The harmonics of the generated frequency can reach up to several GHz with very high amplitudes. These disturbances might create many problems with electrical equipment operating near the machines, disturbances in the communication devices and can even have harmful effects on humans. For these reasons, such machines are subject to regulation according to a standard for electromagnetic compatibility - EN 55011:2010 [2]. The machines are intended to operate in industrial sites, that is why the standard has higher limits for the disturbances than in the domestic or urban areas. Electromagnetic compatibility ensures that the electrical devices can operate in a given surroundings without disturbing other devices or being disturbed.

The HF dielectric welding machines are usually heavy (several hundred kilograms or up to several tons). For this reasons they are not usually tested in laboratory conditions, but on the operation sites in normal conditions. The machine contains electronic components, it is a finished appliance intended for end user. According to the Guide for the EMC Directive 2004/108/EC it is classified as an apparatus [3]. It is intended for incorporation into given fixed industrial installation. The directive provides set of requirements for these machines. According to the document the state of the art engineering practices should be applied for the machine manufacturing and also they should be documented and presented to the user and the authorities when required. Also if the machine creates disturbances which lead to improper operation or failure of other devices, then the manufacturer must take actions in order to fix the problems.

B. Standard EN 55011:2010

The title of the standard EN 55011:2010 is "Industrial, scientific and medical equipment - Radio-frequency disturbance characteristics - Limits and methods of measurement." In the Annex A of the standard it can be seen that the HF welding machine is classified as Group 2 "ISM RF equipment in which radio-frequency energy in the frequency range 9 kHz to 400 GHz." In the detailed description, plastic welding machines can be found. The standard also defines the requirements about the test site an how the measurement must be held. Fig. 3 shows the distance requirements.

Figure 3. Minimum size of the test area

The distance between the antenna and the test unit is defined to be 10m. There must be no other objects between the antenna and the measured machine. Also all other objects that might reflect the electromagnetic field must be removed from the room where the measurement takes place.

Table I shows the limits for the measurement results which are defined for 30 m distance from the test device and 10 m.

TABLE I. LIMIT LINES FOR RADIATION MEASUREMENTS

A quasi-peak detector should be used for the measurement in order to identify not only the peaks, but also the duration of the disturbances.

II. MEASUREMENT

A. Antenna

The antenna which is used for the measurement is log-periodic type with the following specifications:

  • Freq.Range: 30-1000MHz
  • Gain: -10~6.5dBi Typ..
  • VSWR: 2.5:1 typ. (10.0:1 typ. @ 30MHz)
  • Power Handling: 300W
  • Connector: N-Female
  • Material: Al
  • Size: 1720mm(L) x 1450mm(H)

Figure 4. Log-periodic antenna 30-1000 MHz

B. Spectrum analyzer

The spectrum analyzer used is a Hameg HM5014-2 with the following specifications (Fig 5):

  • Frequency Range : 0.15 MHz1.050GHz
  • Frequency Resolution: 1kHz (61?2 digit in readout)
  • Resolution Bandwidths (RBW) @ 6dB: 1 MHz, 120 kHz and 9 kHz
  • Video Bandwidth (VBW): 4kHz
  • Sweep Time (automatic selection): 40 ms, 320 ms,1 s*)
  • Measurement Range: -100dBm+10dBm []

Figure 5. Hameg HM5014-2 spectrum analyzer

C. Software

The software used to record the measurement results is AS100E and is provided by Hameg. All the measurement components with their specifications can be set up in the software and all the values are calculated automatically and then the measurement is displayed graphically.

Figure 7. Measurement results for welding machine E200

Figure 8. Measurement results for welding machine S200