Electric motors, inverters and the ATEX Directive

Motor users have expressed concern that changes in the ATEX Directive have lead to a requirement to test EEx d motors and inverters together for compliance when variable speed drive systems are used in potentially flammable or explosive atmospheres. However, this is emphatically not the case, as Martin Swindell, Technical Director of motor manufacturer Brook Crompton, explains.

All electrical equipment has the potential to spark, so must be installed with great care if it is to be used in any sort of potentially flammable or explosive atmosphere. There are several ways of minimising the danger, such as the use of flameproof enclosures or pressurising a conventional enclosure with inert gas. For moderate- and low-risk areas, other possible solutions lie in incorporating design details into the equipment to reduce the risk of sparking and temperature rises to a minimum.

For most small and medium industrial Zone 1 applications, a flameproof motor is considered to be the safest option. However, there are additional complications when using the motor with an inverter or electronic variable speed drive (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 it should be noted that, when an inverter is used to slow the speed of a motor, losses within the motor increase and therefore make it run hotter. To add to this, the slower speed also, of course, reduces the effectiveness of the cooling fan.

Responsible 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, notably ensuring that they are not used beyond their ratings so that they do not exceed the certified T class.

Change in the law

But 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 so that the emphasis is clearly on the manufacturer, installer and end-user. It does not require that motor-inverter systems be tested as discrete entities, but it does say that if you are partly responsibly for an installation you are not going to 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 (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 users have to operate the equipment in a safe manner and maintain it 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 subjected to rigorous testing and inspection and are backed by certified conformity to the ATEX Directive (94/9/EC). The certificates make it clear that they can safely be used with any make of inverter, without the need for any further time-consuming and costly testing before installation.

Installers can therefore use Brook Crompton motors with confidence, although they remain legally responsible for ensuring that the appropriate thermistors are connected to the control circuits during installation. They should also, of course, ensure that the control circuits work properly - ie that they effectively disconnect the motor from the inverter (and other power sources) in the event of overheating.

Brook Crompton pioneered the development of Ex-certified motors over 80 years ago, and the use of thermistor protection in the 1960s. It was also a frontrunner in the further developments required when Ex-certified motors began to be used with inverters.

Temperature rises are almost always centred in the motors winding and heat is conducted from there to the outer surfaces of the motor, which is the critical area in most Ex-installations. So thermistors embedded into the windings effectively nip problems in the bud by tripping out before external surfaces get anywhere near critical temperatures.

And finally, for the record, the harmonised standard EN 60079-14 deals with the requirements for testing motors that are to be installed in explosive gas atmospheres. It clearly states that, for EEx d motors, when protective devices are employed, combined testing is not a requirement.