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Using ATEX motors with drives - the effects of harmonics

01 October, 2006

 

Ian Evans of Harmonic Solutions Co UK responds to an article by Steve Ruddell of ABB that appeared in Drives & Controls magazine (June/July, 2006 issue), on the use of ATEX-approved motors with variable speed drives. He also reveals a little-known restriction on the use of these motors.

In the late 1980s, I was a leading exponent in promoting the safe and correct use of inverter-fed, explosion-proof motors. As a direct result of this campaign, BASEEFA changed its certifying and test criteria with regard to all explosion-proof protection concepts when connected to variable frequency supplies (i.e. inverter drives).

In addition, HSE introduced a new Section 6 to the 1974 Act regarding the provision of information — which was ignored completely. This made it a legal requirement, in the case of explosion-proof motors, to advise customers, past and present, about the dangers and issues of supplying an explosion-proof motor with an inverter and also connecting explosion-proof motors to inverters.

This piece of legislation was aimed at motor and inverter manufacturers, suppliers, contractors and others, and was retrospective. At that time there were — and perhaps still are — hundreds of explosion-proof motors connected to inverters without certification or testing. Not surprising that this Pandora`s box remained firmly closed!

I therefore welcome the current strategy adopted in the EU with regard to explosion-proof motors on inverter supplies. Why it took 15 years to come to fruition is beyond me, because my company was supplying similar combined certified explosion-proof motor and inverter packages, based on PTB requirements, in the late 1980s.

In my opinion, PTB is the leading authority in this area. A paper, Static converter-fed electric drives — Safety assessment for use in hazardous areas by PTB`s Franz Lienesch (published in Stahl Ex Magazine in 2003) is recommended reading. Strangely, however, this does not mention the position of the inverter field-weakening point (and its effect on rotor heating). This is a serious omission.

On a more serious matter, I would like to pass on some information I gleaned when writing a 240-page harmonics guide for a major US marine classification body about a year ago. This concerns the subject of fixed-speed, explosion-proof motors used on supplies "polluted" with harmonics.

As is widely appreciated, EExd motors rely on the "dustbin" principle — it does not matter what happens inside the motor, it cannot escape. That however, does not hold true in the presence of harmonics, whether from an inverter or present in the supply voltage. These harmonics cause additional heating of the rotor, especially on deep bar or double-cage types. In some cases, hot rotors can result in lubrication evaporating and bearings collapsing, with possible subsequent friction sparking in a hazardous area.

Harmonics also produce shaft currents which degrade bearing lubrication. But how many fixed explosion-proof motors on polluted supplies have insulated bearings at the NDE?

There are similar issues with other protection concepts.

In EExd motors, flameproof seals are installed at the shaft ends to contain an internal explosion, should one occur. High rotor temperatures due to harmonics can damage these seals and if an internal explosion occurs — as is more likely with higher internal temperatures — then the explosion transmits outside the motor carcass, with potentially disastrous consequences.

It may not be well known that, in light of the above, current certification is based on Section 6 of EN 60034-1, which stipulates a maximum HVF — harmonic voltage factor (similar to total harmonic voltage distortion, Vthd) — of 2% for EExd, EExe and EExp motors, and 3% for design N motors (EN 60034-12). Above these limits, the motors may be deemed to be "operating outside the conditions envisaged when they were certified" and technically uncertified. Certification can be carried out for higher HVFs, but the motors must be designed and tested for the higher voltage distortion — but that is outside the standard scope of EN 60034.

Limits of 2% and 3% could be viewed as too cautious when compared to the UK`s harmonic recommendation G5/4 which looks at 5% Vthd being the maximum at the PCC (point of common coupling). If the Vthd is 5% at the PCC, then the Vthd within the plant will be somewhat higher. So what does a user do? Clearly, there is lack of joined-up thinking here.

In North America, there is currently no recognition of the problem of using fixed-speed, explosion-proof motors on polluted systems. NEMA simply refers the matter to UL, and UL refers the matter back to NEMA. The US standard MG-1 only addresses explosion-proof motors on an inverter supply. At least in the EU we have a more sensible approach.

On offshore installations, the Vthd is commonly in the range 20-30%. In April this year, a large electric motor on a North Sea platform reportedly caused an explosion and fire which took four hours to extinguish. The platform was renowned for high levels of harmonic distortion due to its widespread use of variable speed drives, which can comprise up to 85% of the load on such platforms. Unfortunately, ATEX does not apply to offshore installations.

Purchasers of fixed-speed, explosion-proof motors should ideally check with their suppliers about the permissible voltage distortion (and request a copy of the certification). The applicable HVF value then has to me gauged against the actual voltage distortion measurements from site and a judgement made.

This is not to say that the motors are dangerous, but somebody other than the certifying authority has to assume responsibility for their safe operation. This is ultimately the user. End-users should only assume that responsibility after being supplied with all of the relevant information by the supplier.




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