Category : Electric DC motor, linear motor, stepper motor, transmission, drives & control, cylinder, gearbox > DC motors

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Engineers do not have to test EEx motors and drives together to comply with the latest version of the ATEX Directive, contrary to the current perception of many users. So says Martin Swindell, Technical Director of motor maker Brook Crompton, a recognised authority on using variable speed drive systems in potentially flammable atmospheres.

“The growing popularity of inverters has meant that non-specialist plant engineers are having to address the complicated issue of installing and running inverter-fed motors in dangerous environments,” he says.

Until recently only installers and users could be held responsible for the safe operation of motors; notably they had to ensure that motors were not used beyond their ratings so that they overheated. Motor manufacturers could not be held responsible provided that their products complied with the stated certification.

But now the ATEX Directive has been changed to include the manufacturer alongside the installer and end-user. It does not require that motor-inverter systems are 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, the manufacturers of the motors (and the drives) have to ensure that surface temperatures do not exceed the specified T-class or certified temperature for the particular application; 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.”

It is to be noted that when an inverter is used to slow the output speed of a motor, losses within the motor increase and thus it runs hotter. And to add to this, the slower speed also of course reduces the effectiveness of the cooling fan.

EEx d and EEx de 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 further testing before installation.

Thus installers can confidently use Brook Crompton motors, although they are 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, i.e. that they effectively disconnect the motor for the inverter (and other power sources) in the event of overheating.

“For the record, with EEX d and EEX de motors it is only the external temperatures that matter: while for EEx e and EEX na classification both internal and external temperatures must be kept within T-class limits,” furthers Swindell. “But the best advice has to be that if you are involved in safety critical work seek expert assistance, because the consequences of getting it wrong could be horrendous.

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