Staying cool over ATEX
19 Apr 2004
Many users of electric motors may be concerned that changes in the ATEX Directive have led to a requirement to test EEx d motors and inverters together for compliance when variable speed drive (VSD) systems are used in potentially flammable atmospheres. But this is emphatically not the case.
For most small and medium industrial Zone 1 applications, a flameproof motor is considered to be the safest option. But there are additional complications when using the motor with an inverter or electronic VSD. Part of the Ex certification procedure involves testing exposed surfaces of the motor to ensure that they do not become dangerously hot. Alongside this, when an inverter is used to slow the speed of the motor, losses within the motor increase and thus it runs hotter. To add to this, of course, the slower speed reduces the effectiveness of the cooling fan.
Motor manufacturers have addressed these issues by designing special motors suitable for variable speed duties in explosive and flammable atmospheres, and until recently that was where their responsibility stopped. It was left to users to install and run motors correctly, in particular ensuring that they do not exceed the certified T class.
However, the growing popularity of inverters has meant that non-specialist plant engineers have had to address the complicated issue of installing and running inverter-fed motors in dangerous environments. So now the law has been changed and the emphasis is clearly on manufacturer, installer and end-user. The law does not require motor-inverter systems to be tested as discrete entities, but it does say that if you are partly responsibility for an installation you won't be able to pass the buck.
Specifically, manufacturers have to ensure that surface temperatures do not exceed the specified T-class or certified temperature for the particular application (and this goes for the makers of the drives as well as the motors); installers are responsible for selecting suitable equipment and its correct fitting; and end-users have to operate and maintain the equipment to a safe standard.
EEx d and EEx de, T3 or T4 temperature classification motors supplied by Brook Crompton for use with inverters have been rigorously tested and inspected and backed by certified conformity to the ATEX Directive (94/9/EC). The certificates make it clear that the motors can be used with any make of inverter, without need of time-consuming and costly testing before installation.
Installers can thus confidently use these motors, although they are legally responsible for ensuring that the appropriate thermistors are connected to the control circuits during installation. They should, of course, also ensure that the control circuits work properly - that is, that they effectively disconnect the motor from the inverter, and other power sources, in the event of overheating.
Temperature rises are almost always centred in the motor's winding and heat is conducted from there to the motor's outer surfaces, the critical area in most Ex installations. Thermistors embedded in the windings effectively nip problems in the bud by tripping out before external surfaces are anywhere near critical temperatures.
Finally, for the record, EN 60079-14 deals with the requirements of testing motors for installation in explosive gas atmospheres. It quite clearly states that for EEx d motors, when protective devices are employed, combined testing is not a requirement.
Martin Swindell is the technical director of motor manufacturer Brook Crompton.
