Supervising Motor Temperatures

ATEX-certified over-temperature protection integrated in Vacon AC drive plays an important role at KemFine's plant in Kokkola, FinlandA customer's need is always an effective stimulus for a product development project. Vacon did not turn a deaf ear to a customer's need but developed an ATEX-certified thermistor input for supervising the temperature of motors. This feature can simplify supervision tasks and installation, and saves overall costs at KemFine's Kokkola plant. Kjell Hellsten, Day Supervisor, Electricity, at the Kokkola plant and a specialized instructor in electrical safety issues in EX environments - one of the persons behind the idea of the integrated thermistor input - shares his experiences.

When the ATEX directive came into force in July 2003, the requirements concerning the control and supervision of motors placed in potentially explosive environments (Ex motors) became considerably stricter. The directive classifies the thermistor input for supervising the temperature of a motor as a safety device, and its components must meet ATEX requirements. ''We asked various AC drives suppliers whether they have a thermistor input that meets the requirements of the ATEX directive. This was also discussed when I visited Vacon's stand at the Electricity, Telecommunications, Light & Audiovisual exhibition in Jyväskylä a few years ago,'' Mr Hellsten recalls.

Vacon worked with the German BGIA classification body and VTT Technical Research Centre of Finland to develop an AC-drive-integrated solution for the need. Vacon's motor over-temperature protection was certified in accordance with the ATEX 94/9/EC directive in 2006. The drives as such, however, cannot be placed in a hazardous area. This is one of the few solutions of the kind available on the market.

Safety first
Vacon's over-temperature protection utilizes the same certified technology as the integrated safe disable function designed by Vacon. This guarantees that over-temperature protection is reliable and safe and that the motor is stopped when the thermistor installed in the motor detects a temperature that is too high.

Safety and meeting the requirements of the ATEX directive as precisely as possible take priority at KemFine's Kokkola plant, where most of the production facilities are classified as Ex environments (Ex class 1, device class II2G). The plant, which manufactures for example agrochemicals and fine chemicals for the pharmaceutical industry, uses about 200 different chemicals and other raw materials, some of which are in solid and some in liquid form.

Most of the 1,250 electric motors at the plant are Ex motors. The number of AC drives from various suppliers totals about 250, and 30 of these are Vacon NXP drives installed with thermistor-based motor over-temperature protection. Many of the Vacon AC drives control pumps and mixers.

Simplicity, less cabling and cost savings
''The motor over-temperature protection developed by Vacon brings many benefits. It has a simple circuit diagram, and it uses fewer components than a conventional system since it has no thermistor relay, pilot lamp or reset button. This also reduces the amount of cabling in the electrical room, which gives greater reliability and savings in space and costs. Another benefit is that the cause of the alarm can be checked on the AC drive panel or through the fieldbus,'' says Hellsten.

''All in all, the over-temperature protection available for the Vacon NXP drives represents the very latest in technology. We will also choose this option in future when we order AC drives from Vacon,'' Hellsten adds.

Instructor is experienced professional
Hellsten has been working in the chemical industry since 1973 and with Ex-issues since 1984. He has extensive experience in electrical safety issues and with AC drives from different manufacturers. He has for several years lectured on electrical safety in Ex environments on courses organized by Inspecta Ltd, and has trained personnel both at KemFine and occasionally at other companies. He usually shows the circuit diagram for Vacon's over-temperature protection during his lectures, and the topic has aroused much interest and discussion.

Development of motor over-temperature protection
Until 2003, the Ex regulations required and allowed an AC-drive-solution in Ex environments in which the windings were equipped with thermistor input. In practice, there was a separate thermistor relay in the electrical room that, in one way or another, switched off the AC drive running the motor if the temperature of the windings was too high. In the following phase, all major AC drive manufacturers designed a thermistor input integrated in the basic I/O card of their drives. The ATEX directive that came into force in July 2003 classified thermistor protection as a safety device, for which the components must be ATEX certified. At this point, the separate thermistor relay was retaken into use - with the additional requirement that now it needed to comply with ATEX requirements. This inspired KemFine to contact AC drives suppliers to ask for an ATEX certified thermistor board. ''The solution designed by Vacon as it were returned us to the time before the ATEX directive came into force,'' says Kjell Hellsten, Day Supervisor, Electricity, KemFine Kokkola plant.