Drive systems often operate in potentially explosive atmospheres. One example is the inflow area of a sewage treatment plant. If frequency converters are used for speed control of drives in such areas, the facility must fulfill special conditions. The basis for this is provided by EU Directive 94/9/EC, which is called the ATEX directive.
The ATEX directive describes the use and operation of equipment and protective devices in potentially explosive atmospheres. This directive harmonizes regulations and requirements throughout the EU for the operation of electrical and electronic devices in potentially explosive atmospheres, such as may be caused by dust or gases.
If frequency converters are used to control motors in potentially explosive atmospheres, these motors must be equipped with temperature monitoring using a PTC temperature sensor. Motors with ignition protection class “d” or “e” may be used. These ignition protection classes differ in terms of how the ignition of an explosive medium is prevented. In practice, frequency converters are rarely used with “e” class motors. This combination must be approved as a unit, which involves elaborate and expensive type testing.
However, the PTB in Braunschweig (Germany) has developed a new approval procedure that will make the use of speed controllers with class “e” motors considerably more attractive in the future. The new concept calls for the acceptance of only the motor itself, while additionally defining specific requirements for thermal monitoring in the EC type test certification process. For instance, speed-dependent current limiting is required in addition to the usual certified PTC thermistor monitoring, in order to deal with the reduced cooling of self-ventilated motors with variable speed control.
Although this does not require separate approval of class “d” motors, feeding cables into the “d” area is very complicated. Motors with protection class “de” are the most widely used. In this case, the motor itself has a “d” ignition protection class, while the connection space is implemented in compliance with the “e” ignition protection class. The restriction on the “e” connection space consists of the maximum voltage that may be fed into this space.
Due to pulse-width modulation of the output voltage, most frequency converter outputs have peak voltages that exceed the allowable limits of class “e” ignition protection. In practice, using a sine-wave filter at the frequency converter output has proven to be an effective way to attenuate the high peak voltages.
Never install a frequency converter directly in an area with a potentially explosive atmosphere. It must be installed in a cabinet outside this area. Using a sinewave filter at the output of the frequency converter is also recommended because it attenuates the voltage rate of rise du/dt and the peak voltage Upeak. The connected motor cable should be kept as short as possible due to the voltage drop in the cable.
For ease of installation, it is advised to use frequency converters equipped with a PTB-certified motor thermistor sensor monitoring capability for potentially explosive atmospheres.