The relationship between variable speed drives and Ex certification

The explosion protection (Ex) certification of any motor is said to
be no longer valid when the motor is supplied from a variable-speed
drive (VSD). Ex certification of motors for use in a hazardous
area, as defined by SABS 0108, is valid for normal S1 duty and
connection to a standard sinusoidal supply only. According to Zest
Electric Motors and Drives manager of drives and automation Johan
van Niekerk, there are several good reasons for this.

Firstly, the VSD output causes additional heating in the motor, he

Secondosborn double toggle stone crusherly, running at reduced speeds may cause additional heating in
the motor, depending on the driven load.

Thirdly, running at increased speed may increase the power
consumption and, hence, the heating in the motor. Once again, this
depends3 ft zenith cone crusher overall dimensions on the driven load.

Van Niekerk explains that the above conditions are not a specified
part of the certification process and are, therefore, not covered
by the certification. He says that this is generally not understood
corprocess vibrating table recovery of aluminium in drossrectly in industry, with the result that many Ex certified
motors are installed in VSD applications without the certification
being valid. He emphasises that an Ex-certified motor, when
supplied from a VSD, no longer has valid certification regardless
of the motor and VSD make, type or Ex certification.

Depending on the specific area classifi- cation there are different
solutions to this.

The motor and VSD combination can be type-tested for the specific
load and speed range, or the motor certification can be linked to a
special condition requiring the use of thermistor protection.

“The requirements will differ, depending on the specific
hazardous area classification and, in each of these instances,
specific certification of the motor and VSD combination is
required,” Van Niekerk says.

The user need not despair at the seeming complexity of these
requirements. Certifi- cation can be arranged easily at a
certification authority such as Explolabs or the South African
Bureau of Standards, through a competent motor and drive supplier
such as Zest Electric Motors and Drives.

The user does need to contribute by making available certain
information that is required for certification. Van Niekerk
explains that the more complete the load and application details
are, the easier, quicker and cheaper the certification will

Details that must be known are the type of load, the load absorbed
power or load torque, the minimum speed for constant running, and
the maximum speed for constant running.