Radiocommunications Agency  EMC Awareness

Industrial equipment

Description

Radio Frequency Heating (RFH) machines belong to a class of apparatus known as Industrial, Scientific and Medical (ISM) equipment which use high power radio waves in a wide range of material processing applications. A major use of RF equipment operating at HF is for the purpose of dielectric heating. The types of processes that utilise dielectric heating are plastic welding, woodworking, food processing, fibre and textile drying, plastic and rubber pre-heating and welding, paper and board drying.

ISM equipment uses RF and microwave energy to heat dielectric materials, however the leaked electromagnetic energy often radiates into the environment. The International Telecommunications Union (ITU) have authorised certain frequency bands for the use of ISM equipment, including the designated fundamental frequencies 13.560 MHz and 27.120 MHz. Their harmonics may cause RFI to various radio services operating within the frequency range 13 MHz to 244 MHz (9th harmonic for ISM equipment operating with a fundamental frequency of 27 MHz).

The individual incidents logged by the RA's Investigation Service have included:

• Interference to Greater Manchester Fire Service low band communication receiver

• Interference to the CAA VHF air band from a wood glue drying machine

• Interference to TV and Band 2 radio from an RF welder in Birmingham

• Interference to CAA band at 136.2 MHz from ISM equipment

• Interference to low band PMR (private mobile radio) from an RF wood glue drying machine

• Interference to the CAA VHF band from RF

• Interference to PMR low band from ISM equipment used by a textile company

• Interference to television reception from clothing factory

• Interference to telephone/CAA from RF heaters in Manchester

• Interference to TV and CAA channels from RF heating equipment at a plastics company

Commentary

Interference caused to air-traffic communications systems is by far the most common problem. Although various harmonics can exceed the specified templates, it seems that the 5th harmonic is particularly likely to create problems. The levels measured exceeded the specified limits by amounts varying from 3 to 60 dB.

The fundamental frequency should not drift outside the bands internationally allocated for ISM. However, the load can change during the heating process, thereby changing the resonant conditions and hence frequency of the circuit.

In order to reduce radiated emissions from an ISM equipment, the simplest way is to introduce metallic shielding around the machine. Most users of ISM equipment have used this technique, but suppression of harmonics is a major problem. Many dielectric heaters are large machines having electrodes which can be up to five metres or more length. The centre-fed push-pull systems can generate harmonics depending upon the separation distance and electrode lengths.

Drying ovens have a conveyer, onto which the product is placed and is passed through the dielectric heater, which acts as a good radiating antenna at certain frequencies. Placing a screened chamber over the dielectric heater alone would not prevent leakage of electromagnetic energy since the input and output port will simply be apertures. However, long vestibules covering the lengths of the conveyor system will create waveguides coming out of the dielectric heater source. The width of the vestibules are chosen such that they perform like waveguides operated below cut-off.

References and links

“Interference from radio frequency heating (RFH) machines”, Radiocommunications Agency leaflet RA4340, available from: http://www.radio.gov.uk/publication/ra_info/ra340.htm

“Suppression and mitigation techniques for interference from 900MHz microwave ovens”, Radiocommunications Agency final report AY 3499, available from http://www.radio.gov.uk/topics/research/topics/other/ay3499/intro.htm. Alternatively, there is a link at: http://www.yorkemc.co.uk/Research/index.html.

“Industrial Microwave Oven (ISM) Emissions and Mitigation Techniques”, by Dr D Welsh, Proceedings of EMC York 2000, 10-11 July 2000, http:/www.yorkemc.co.uk.

“Design and test of high performance choke tunnels for industrial microwave ovens”, and… “Investigation to determine optimisation routes for new choke tunnel design for 900MHz ISM machines”, both available from http://www.radio.gov.uk/topics/research/topics.htm#emc

Development of an ISM harmonic RF attenuation model for combating interference generated by 13 MHz and 27 MHz industrial RF processing machines, P Bansal, ERA Report 97-1015; RA project AY2902, Task A

Practical implementation of radiated emission reduction and frequency stabilisation of ISM equipment operating in the 27 MHz band, P Bansal et al, ERA Report 99-0164.

Links to Mitigation Techniques

	Installation	Design & Development	Resources
Transmitter frequency control
Segregation
Spurious suppression
Shielding of areas and volumes
Shielding of cables