The Use of Explosion Proofs Motors on Distortion Supplies 6

[ICEMAN]

Can we start a discussion about the use (and safety) of fixed speed explosion proof motors on voltages supplies polluted with harmonics ?

We could also extend it to the use of explosion proof motors fed from VFDs.

If somebody kicks it off I will pick up the baton !

ICEMAN

 

[jbartos]

Suggestion: If the motor is explosion proof and receives power supply contaminated with harmonics, the worst case what can happen is that the motor becomes overheated or burns. However, the motor is in an explosion proof enclosure. Therefore, the motor enclosure is supposed to have contained all motor irregularity caused by the motor fire.

 

[ICEMAN]

Hi jbartos....In theory you are correct but in practical terms.....

In the US the term 'explosion proof motor' is used to describe 'flameproof motors', those for use in Zone 1 and Zone 2. In Europe 'explosion proof motors' is a generic term to decribe a range of motors for use in hazardous areas where the risk of explosion is a possibility. EEx 'd' is flameproof, EEx 'e' is increased safety, EEx 'p' is pressurised; all these can be used in Zone 1 or Zone 2. The other is EEx 'n' non sparking (the less said about this the better !) and it is only for use in Zone 2.

The first area of concern is the legal aspect of operating these motors on distorted supplies. The design, test and certification of these motors will have been done (unless otherwise stipulated) on sinusoidal supplies (i.e. with no harmonic distortion). So, if a such a motor would be connected to distorted voltage supplies it then would be legally "operating outwith the conditions envisaged when it was certified". It does not mean to say that it is unsafe but simply that the third party certification re its safety is now invalid and that the operator must assume full responsibility for the safety. If there is a incident due to the motor (e.g. an explosion due to the motor igniting gas or vapour then the insurance may not pay out if cause establish was the use of an uncertified motor. This is a serious consideration for any operator.

Let's look at explosion proof motors of the flameproof protection concept (EExd in Europe) then as jbartos mentioned. Flameproof motors reply on what is called the dustbin (or garbage bin) theory in that it doesnt matter what happens inside the motor it the result cannot escape. Whilst that is largely true (but not wholly) it hold not hold for motors connected to supplies distorted with harmonics. The problem is not the motor stator which is usually protected by thermistors but with the rotor, which has no thermal protection.

Harmonic currents increase the losses in the rotor due to increase iron and coppper losses, skin effect et al. On motors with deep bar or double cage rotors (common on offshore compressors for high starting torque/relatively low starting current type application the heating effect is very much motor pronounced. Hot rotor lead to excessive shaft heating which has two serious effects i) the bearing grease starts to 'disappear' leading to loss of lubrication and in a worst case bearing collapse (this results in frictional sparking in the hazardous area) {you also have the additional problem of bearing currents due to harmonics - the large VFD fed motors should have insulated bearings fitted to prevent this}. The other problem is that the heating on the shaft damages the flameproof seal to the extend that if there were an internal explosion due to high internal temperatures (remember that ALL motors suck in air (and gas or vapour) when they cool down)then the internal explosion would transmit outside the motor carcass and...BOOM if there has been an escape of gas or vapour.

As for VFD fed flameproof motor...they have similar problems and any flameproof motor/VFD package should be supplied and certified as a system with the thermal protection operating independently of the VFD. The documentatino should also detail the duty of the motor and have stipulated the 'field weakening point' (i.e. the point with the voltage is maximum is relation to the frequency ({e.g. 480V/60V} as adjusting this will have a serious effect on the motor (and rotor) heating due to harmonics.

The whole area of explosion proof motors whether on fixed or variable speed applications is often misunderstood. There is still a lot of ignorance out there. I led very successful campaigns in the late 1980s in the UK on these matters. The situation is better now but it certainly is not 100%.

Come guys, lets get a discussion going here !

ICEMAN

 

[jbartos]

Comment on the previous posting: Very impressive. Unfortunately, many electrical engineers, designers, technicians, electricians and ultimately "electrical managers," say with MBA degree, have to follow industry standards, e.g. NEMA 250 or IEC word by word for safety reasons. Visit

http://www.nema.org/DocUploads/7B767F84-0827-45F9-9BB01319E5BC55FB/enclosure.html

for the NEMA 250 enclosure categorization and comparisons to IEC enclosures.

 

[aolalde]

ICEMAN:

All arguments exposed are true. My opinion is that in USA people responsible for selection of motors in potentially explosive atmospheres follows the NFPA (National Fire Prevention Association) , National Electric Code (NEC), Underwriters Laboratories (UL) and NEMA Recommendations and Standards.

Certainly I have not seen specific recommendations or limitations for use of VFD as motor controllers; however it is understood that the motors must be installed to operate under Nameplate or Normal Operation Parameters. This implies the use of alternating current with sinus wave voltage and limited distortion (reduced harmonic content).

Motor manufacturers have started the manufacture of VFD duty motors due to multiple bearing and motor failures of standard motors under certain types of variable frequency drivers. Probably that trend will follow in regard of motors for operation into potentially explosive areas intended to be connected to VFD supplies.

I hope that the technical boards responsible for revision of related standards will include in a short future some articles clarifying this subject. Probably the insurance business will push for regulations.

 

[ICEMAN]

jbartos....

My words regarding flameproof motor shaft seal problems due to harmonic problems come from Ron Sinclair, Principal Certifying Officer, BASEEFA (British Approvals Service for Electrical Equipment in Flammable Atmospheres). They are dated Feb 1988 and were made during a conference we organised (Howieson & Evans, Drive Consultants) in the UK called "Exposing the Myth - Converter Fed Motors in Hazardous Areas". I still have a tape of his speech {which you are free to have a copy of} . His comments did take a lot of motor and drive manufacturers by surprise, including the UK biggest (who had been illegally supplying supposed 'certified ex motor/VFD packages' for some three years before it was found out and deemed illegal.) The campaign went as far as changing the UK Health Safety law under the 'provision of information' and the way ex motors were tested and certified. The bottom line was that all the parties were following the standards (when it suited them) but the standards were deficient (BASEEFA advised me of this during discussions but said it was not their job to rewrite standards, only the implement them ! That really gave me a lot of confidence. Standards very often fall behind in technological advances - VFDs is real example of this. 1000's of ex motor around the globe have or have had VFD connected to them. I met with so called experts, consultants, etc. and you would not have believed the ignorance (indeed arrogance) on this subject at that time (1987-1990). Indeed the most educated engineers I have met on this subject were with SCOP in Baghdad. I was preaching to the converted there. Good job I got out before Saddam invaded Kuwait shortly thereafter. That reminds me - I never got that order !

jbartos..can you guide me please to the NEMA standards for explosion proof motors fed from VFDs AND any standards re explosion proof motors on distorted supplies ?

Standards are not always 'up to date' and can be defficient. Look at the UK in 1988 re the above and you can see what the problems were. In fact if you look at EExN motors ('non sparking' {that's a joke for a start !}) then BS5000 at that time did not specify thermal protection for this motors for use in Zone 2 areas but only recommended. Also, 'normal operating conditions' were deemed only to be at 'synchronous speed' - so the motor could cause an explosion during start up and failure and still be covered with the standard.

Similarly, 3 large ExN (not Euronorm type) box frame motors 'exploded' according to the papers in the Scottish North Sea (note the lack of UK or English) from 1985 to 1987. There were 2 x 6MW and 1 x 10MW. What happened was the during a 'gas out' (rig/platform evacuation) the motors were switched off. When the gas or vapour had cleared the motors were switched on. On each occasion the motor blew up due to gas inside the enclosure being sucked in normally during cooling. In each case the bonding potential across the split frame was around 90V. It acted like a spark plug. All those motors were desogned, certified and operated according to the standard BS5000 et al. 'D' notices (UK Government warning notices) were posted on all similar ExN machines. All had to be converted to 'purged machines', similar to EExp pressurised machines so they could operated safely. So, there is one example where standards are not infallible. I won't even talk about the world's biggest oilfield disaster, Piper Alpha where some 157 guys died. That had high levels of voltage harmonics, fixed double cage ExN compressor motors and an escape of gas or vapour in the area of the compressor room. Harmonics weren't even considered {openly}. Can't say no more as I sign the UK Official Secrets Act and that's for like. Food for thought though !

ICEMAN

 

[ICEMAN]

Hi aolalde - it's lunchtime so it's OK for me to reply to you re ex motors and VFDs.

The US is a long way behind Europe in the safe application of ex motors and VFDs. It was in the early 1980's that Germany companies suh as Loher and F & G pioneered the ex motor/drive package concept with PTB (German certifying authority). Basically, the motor had to be designed specifically for variable voltage/variable frequency supplies re rotor design (no deep bar or double cage etc), bearings (insulated above a certain frame size), winding insulation (due to high dv/dt et al), etc. Various cooling methods were adopted depending on the application (constant or variable torque, speed range (if constant torque), etc. TEFC, forced ventilation and water cooled motors were all available from Loher from some 5HP to over 4000HP - all safe and certified packages. The stator heating was monitored by PTC thermistors or PT100s. During specific testing special paint was applied to the rotor which, when strip down would indicate the maximum temperature of the rotor attained.

In order to comply with IEC Standards the motors have to have two rating plates fitted. One stating normal sinusoidal duty and the other stating variable speed duty in terms of voltage, current, frequency, rpm and torque and kW (based on a min to mac duties). If this is not attached to the motor then it is not legally certified for variable speed.

The certification has to include the normal Certificate of Conformity, the variation detailing the specified duty (as laid out on the additional rating.

The thermal protection device has to be of an approval type (confirmed by test initially) and has to be independent of the VFD. In other words PTC going into the VFD are not acceptable - a seperate device which disconnects the VFD if the motor starts to overheat was required.

The main problem was that of the position of the 'field weakening poiny' on the VFD (where the voltage is at maximum and thereafter only the frequency is increased to increase speed). As you can see from a copy of the initial article to the Electrical Review (Feb 1988) the position of the field weakening point is crucial to the temperature of the rotor. NOTE THAT EVEN SOME 16 YEARS ON THIS IS NOT STIPULATED ON THE MAJOR OF MOTOR MANUFACTURERS CERTIFICATE FOR VFD OPERATION THUS MAKING IT POSSIBLE FOR THE CUSTOMER OR OPERATOR TO ALTER THE MOTOR/DRIVE/APPLICATION PARAMENTS SUCH THAT THE ROTOR OVERHEATS AND POTENTIALLY CAUSES AN EXPLOSION SHOULD GAS OR VAPOUR BE PRESENT.

===========================================================
Transcript of the article “ARE WE PLAYING WITH FIRE” by
Ian C Evans* which appeared in the ELECTRICAL REVIEW, Vol.221. No3 dated February 1988.
___________________________________________________________

UK standards for inverter fed motors in hazardous areas need urgent revision because ignorance of the risks is leading to dangerous installations.

Despite the popularity of variable frequency drives, or ‘inverters’ in industry, the inverter has failed to penetrate hazardous areas.

The application of inverters to ‘explosion proof motors’ is a substantial market segment, presently underdeveloped in the UK. In the rest of Europe however, there have been successful applications of inverters in hazardous areas due to a closer liaison between inverter manufacturers, certification bodies and users.

Inverters have been installed in the UK, in some instances with scant regard for the rules and guidelines. These instances may be due to ignorance more than deliberate intent.

Certification of explosion proof motors is based on sinusoidal supplies. No account is taken of the additional heating effects of the harmonics in non-sinusoidal supplies which are passed on to the motor by an inverter drive. If an explosion proof motor were connected to such a supply then the certification would no longer be valid.

The harmonic content of an inverter output significantly increases the machine losses, in particular, the iron and copper losses, which raise the machine temperature and alter the motor’s speed/torque characteristic. BS5000, Part 16, Clause 10.2 relating to ExN (so called ‘non sparking) motors states “When a rotating machine is intended to operate from a non-sinusoidal supply, e.g. from a static frequency converter or generator supplying a thyristor load, the temperature of the machine shall be determined by test, or in exceptional circumstances where it is not practical to carry out the test, by calculation”. But BASEEFA (the UK’s main certification authority) is not convinced of the validity calculations offered by the manufacturers and subsequently does not except them.

During combined testing with EExe (increased safety) or ExN (non sparking motors) the temperature rise caused by the non-sinusoidal waveforms must be considered for both the stator and rotor. With EExd (flameproof) motors it is general assumed that only external temperatures on the motor carcass have to be considered. However, this depends on the motor construction and is not always the case. Additionally, high rotor temperatures can lead to high shaft temperatures, outside the flameproof enclosure.

The author, Ian C Evans, has been involved in a number of investigations into failures, some catastrophic, in industrial applications on behalf of consultants, manufacturers and users. In one instance the motor bearing collapsed within three days of operation due to increased rotor heating. In this application, the machine was fitted with PTC thermistors and the temperature was within acceptable limits. On other application, a high speed machine failed catastrophically, splitting the stator pack and severely damaging the specialist mixer. It was established that due to the configured voltage/frequency ratio of the inverter, compared to the 50hz value the iron losses alone were increased by 370%. The rotor of the motor was blue in colour, indicating a surface temperature of between 350 – 500 C deg. The mixer manufacturer also designed a similar mixer for use in the chemical industry. Suffice to say, after this experience none were offered for use in a hazardous area.

The leading UK motor manufacturer suggests “self certification” for inverter fed explosion proof motors. This procedure allow customers or supplier to conduct combined testing on their equipment in order to establish the motor temperature rise on condition they provide temperature test records. However, this method is not acceptable to BASEEFA and in any case only monitors surface temperatures on the stator.

Those involved in the specification, purchase, application of use of inverters with explosion proof motors of whatever protection concept should consider these three questions :-

? Is your motor certification still valid ?

??Are you exposed to potential prosecution by Health & Safety Inspectorate ?

??Will your industrial insurance be honoured in the event of an incident ?

Whatever, the type of motor and converter used there is an interaction between these items of equipment. It is essential that these effects are understood to ensure the safe and reliable operation of the plant. This understanding can only be assured by close collaboration between manufacturers, consultants, systems designers, suppliers, contractors and users.

Safety and reliability should be paramount and consideration of cost should not prejudice the safe operation of the plant and equipment.

The demand of inverter-explosion proof motor packages will increase. To ensure equitable and safe solutions to the safety aspects, national and international standards must be revised. We do not want another incident like Flixborough to demonstrate the disastrous misuse of such equipment.

* Ian C Evans is a consultant with Howieson & Evans, Drive Consultants.

===========================================================

ANYBODY EVER NOTICE THAT THE ABOVE WAS PUBLISHED JUST BEFORE PIPER ALPHA AND THAT OUR CONFERENCE WAS A FEW WEEK LATER ?

With regard to the rotor temperature monitoring of Loher motors....Loher in the early 1980s developed a system called 'Calovert' which was approval by PTB and therefore all EU certifying bodies which monitored all the aspects of the motor - VFD interaction in the area which could effect the rotor temperature (V/f ratios, earth (ground leakage), unbalanced currents, etc. and if any of the parameters were exceeded then the device shut the VFD down. Many hundreds of such systems were installed in 1980s and 1990 throughout the world. I believe these that a software version is included im Loher's drive systems for oil, gas and petrochemical industries as standard.

Many of the lessons learned in the 1980s have not been lost or discared. Many still work under the blanket '2-100hz' certification still made by the major UK motor manufacturer and others which totally ignores the 'field weakening point' issue. Since the majority of hazardous area applications tend to be square law load with 50hz or 60hz the desired maximum frequency they have got away with it.

Anyway, that's all for now - there is more but I have to get on with something else.

ICEMAN

P.S. Please excuse any errors (as usual) - I don't have time to correct them OK.

 

[jbartos]

Comment: The explosion proof enclosures of motors and other electrical equipment have been around for a long time. Notice the age of refineries and petrochemical industry. I have a electrical design book written by Bishop dating way back to early 50s showing details of explosion proof electrical installations.
When it comes to VFD, there may be a need for a whole explosion proof room, considering some larger VFD sizes.

 

[ICEMAN]

jabartos....you have lost the thread here !

The motor is in the hazardous area and the VFD in the safe area ! It is the effect of the sythesised voltage and current waveforms, which contain harmonics, on the explosion motor is the problem (i.e. the interaction of the VFD output waveform and the motor) we are discussing.

It is unusual to house the VFD in the hazardous area but I have been involved in projects where this was a requirement. The most cost effective way is to use pressurised panels (housing the VFD(s)). These are airtight and purged prior to start up with inert gas. Once it has been established that no gas or vapour is present then the system can VFD can start. There is a UK company called EXPO Safety Systems who supply this type of kit.

The cooling on these type of drives is usually water cooled or via heat-exchangers. I have been involved in project where we supplied 12 pulse input - 6 phase output current fed VFDs and 1800kW EExp motors in the Middle East. The 6 phase VFD output with to reduce the torque ripple by 50% (i.e. the motors have two windings, each displaced by 30 degs - a 12 pulse output). They are still working outside in up to 55 deg C.

No. In my previous replies I have never mentioned the VFD being in the harardous area. If you want to email me I can send you a copy (a hard copy only) of a booklet I wrote in 1989 called "Hazardous Areas - A User's Guide to AC Drive Systems". That give a good grounding into the problems and solutions for VFD fed explosion proof motors. Mail me on mailevans@tiscali.co.uk

ICEMAN

 

[Ahr35181]

ICEMAN,

I read your threads with great interest, there aren’t many people on these forums with the depth of EX knowledge you seem to have.

I’m an E/I Inspector at a chemical plant, for several years now all our Exd motors supplied by invertors MUST be inverter rated with thermistor protection, although we still have quite a number of existing standard Exd motors which have thermistor protection but are not inverter rated. I don’t have BS EN 60079-14:2003 to hand right now, but I believe it states this is acceptable (as in the previous BS 4343), although the foot note did / does give a caution about certain faults causing over heating of the rotor, as you also have mentioned in your threads.

Would you advise replacing all existing non-inverter rated Exd motors (most are in Zone 2 areas), if so how would you back up this recommendation?

Regards,

Alan

 

[jbartos]

Comment on ICEMAN (Electrical) May 25, 2004 marked ///\\\
jabartos....you have lost the thread here !
///Possibly, due to ambiguous presentations.\\The motor is in the hazardous area and the VFD in the safe area !
///Thank you for briefing. Better late than never.\\ It is the effect of the sythesised voltage and current waveforms, which contain harmonics,
///Synthesized voltage and current waveforms form power waveforms.\\ on the explosion motor is the problem (i.e. the interaction of the VFD output waveform and the motor) we are discussing.
///Yes, I noticed.\\It is unusual to house the VFD in the hazardous area but I have been involved in projects where this was a requirement. The most cost effective way is to use pressurised panels (housing the VFD(s)).
///It depends on the physical dimensions and rating of VFD. If the VFD is small, perhaps; however, larger VFDs tend to have build an environment around them since it is of negligible cost with respect to the cost of the VFD.\\ These are airtight and purged prior to start up with inert gas. Once it has been established that no gas or vapour is present then the system can VFD can start. There is a UK company called EXPO Safety Systems who supply this type of kit.
///What is the largest size of VFD the EXPO Safety Systems support?\\The cooling on these type of drives is usually water cooled or via heat-exchangers. I have been involved in project where we supplied 12 pulse input - 6 phase output current fed VFDs and 1800kW EExp motors in the Middle East. The 6 phase VFD output with to reduce the torque ripple by 50% (i.e. the motors have two windings, each displaced by 30 degs - a 12 pulse output).
They are still working outside in up to 55 deg C.
///Good to know.\\No. In my previous replies I have never mentioned the VFD being in the harardous area. If you want to email me I can send you a copy (a hard copy only) of a booklet I wrote in 1989 called "Hazardous Areas - A User's Guide to AC Drive Systems". That give a good grounding into the problems and solutions for VFD fed explosion proof motors. Mail me on mailevans@tiscali.co.uk
///Impressive.\\

 

[jbartos]

Suggestion: Visit

http://www.kcentrifuge.com/001.asp?sort=StrModel

for explosion proof motors and drives

 

[ICEMAN]

Alan

I take it you are based in the UK (hence BS EN standards reference).

My direct involvement in these matters ceased almost 10 years ago so I may be out of date in some areas. Although nothing much, as far as I am aware, has changed during this time.

Despite what the standards say I believe the legal position to be that is the EExd motor were not ‘inverter rated’ (by this do you mean the blanket ‘2-100hz on variable frequency supplies’ coverage ? If so, the position of the field weakening point is still very much of an issue here) is that if the motor (certified for use only on sinusoidal supplies were to be connected to a VFD ‘(inverter’ in Europe) then it would be operating outside the conditions envisaged when it was certified and as such the certification would be invalid. At the end of the day the operator would have to assume full responsibility as to whether the motor was safe. Knowing how the UK Health & Safety Executive work they would probably deem it OK when it was working but may prosecute you if it caused an incident.

My opening shot in the explosion proof motor in variable frequency supplies campaign in 1988 was the article “Are We Playing With Fire” which I posted yesterday. That caused a major panic by a lot of companies including UK and International oil and petro-chemical companies. I got many calls asking for advice on issues exactly like yours. …..We have install VFDs on our EExd or ExN motors….what do I do. My advice initially was to for them to go back to their VFD suppliers, contractors, consultants, etc. and take them to task for this they were involved (many were). They ALL have legal responsibilities here regarding the Section 6 of Health & Safety legislation under the Provision of Information. In this ALL parties have a duty to inform their customers or clients of any new information which should come to like which is likely to have serious impacts as to safety. This new provision was brought into force solely due to my efforts in highlighting the dangers of incorrectly applying VFDs to explosion proof motors. The big sting here as that is was and is retrospective which means for example, if a certain ex motor or VFD manufacturer, supplier, consultant, contractor or whatsoever was involved in the sale, application or recommendation of a explosion proof motor or VFD etc., they have a legal responsibility to advise their customers or clients of the danger and risks regarding the of using ex motors and VFDs. This was/is s real can of worms which would implicate the vast majority of those involved from motor manufacturers, VFD supplies and consultants. The worst offenders were the major UK motor manufacturer and a Danish based VFD manufacturer who jointly promoted “certified AC Drive Packages”. This of course was a complete lie and both were severely taken to task by BASEEFA for the practice. By the way this was the same motor company who were promoting “self certification” as mentioned in Are We Playing With Fire until BASEEFA jumped on this as well. In actual fact BASEEFA used evidence I have gathered in against them. No wonder the Technical Director took early retirement ! In light of my campaign they also had to modify the testing methods on EExd motors on VFD to take in three extra temperature monitoring points (NDE shaft, one bearing cover and the rotor).

OK getting back to the legal aspect. In theory if you do supply EExd motors from VFDs and there is an incident (i.e. explosion) I would not sleep easy at night in the faith that the HSE will ignore it, whatever any standard says. As I said before, standard are fallible.

However, from the practicable standpoint it may be OK. The maximum temperature would normally occur at rated load (although in some early PWM VFDs the waveform was really crappy (i.e. even more full of harmonics) lower down the frequency curve but the current was less so the heating effect may not necessarily be worse (this depends on the torque curve, loading and rotor design). Most VFDs in hazardous areas were installed on applications like fin fans, mixers, etc., which usually had square law characteristics so the only point to worry about was at rated speed. In motor applications like these the effective power of the load was usually no more than 85% of the motor installed kW/HP. Using Class F insulation which, at that time, needed 10% of the motor temperature capability to support the additional heating effect from the VFD that leaves a 5% cushion (based on no more than 40 deg C ambient, standard maximum for ‘normal ex motors’). Not many people buy VFDs to run motors at rated speed continuously (although it does often figure in the operating profile) and as the speed decreases the power reduces by the cube of speed so the motor temperature drops off rapidly ASSUMING IT IS NOT DOUBLE CAGE OR DEEP BAR ROTORS !

If you have constant torque loads where the power decreases in direct proportion to the speed it is more complicated and serious as regards motor heating and on occasion due to motor starting (I don’t want to go into the latter right now OK).

The bottom line…..what would I do to sleep at night.

i) I would check the motor currents with a hall effect ammeter which read harmonic currents to establish worse case loading. I would not read the VFD loadmeter, ammeter, etc. these are usually a measure of the DC bus current and are usually 10-20% out !

ii). If you have a scope or Fluke 41 type instrument I would look at/capture the VFD output waveform. Look more for the voltage waveform here. Check close up for high dv/dt and other ‘crap’. If deemed not to good (most aren’t) then I would consider the use of ‘sinus output filters’ (not output inductors which could introduce ‘standing waves’ phenomena leading to in-turn winding failures on the motors 0 I am not going to elaborate on this either !) With sinus filters fitted the waveforms the motor sees is much better with less harmonic content and less motor heating.

iii) I would check the motor data and contact the motor manufacturer to establish what kind of rotor design is in the machine. If double cage or deep bar I would seriously considering binning the motor(s) and go for appropriately designed and certified EExd motors. I would ask for copies of sample certification from the company (you’ll see exactly what you need from the info I will mail you under separate cover).

iv) I would look at the thermal protective device which protects the motors. If it is wired into the VFD it is not independent. For example, there are certain faults within the VFD which may result in voltage being applied to motor even when the VFD is powered down (i.e. inhibited but not disconnected from the mains). Many VFDs no longer use ‘airgap disconnection’ (i.e. contactors) as the constant charging/dis-charging of the DC bus reduces the life of the capacitors. I have experience this myself; it is a remote possibility but still there.

I would install an approved PTC thermistor relay for use with the ex motor and VFD although there are not many in the UK. Loher (

http://www.loher.de)

do the CALOMAT unit approved by PTB; not much more expensive than non approval units. If the VFD has no input contactor you would have to use one of input contacts in the VFD to stop it in the event of overheating. At least then you have a degree of independence although I would always tend to go for airgap disconnection.

I would also check the certification and documentation on the ‘inverter rated motors’ you also have with the information I will mail on certification you to see if they comply fully (or partially).

I could go on but I’ve probably bored most people already and anyway I have to start work.

ICEMAN

P.S. Sorry if I’ve rambled a bit but this was written as from 0600hrs and I do wake up until around 1000hrs.

 

[ICEMAN]

jbartos...

1. Re VFDs housed in the hazardous area.....That is why it is unusual. It's usually a cost issue but on some applications it is a requirement. The smaller the VFDs the easier it is. On some sites (Middle East) the kit is miles away from the main switchboard so locating smaller drives locally is desired. The largest I have done was 1800kW. These were expensive but so were all the other options. The water cooling system dissipated 95% of the heat to the atmosphere (this was never inside the airtight enclosure anyway) so although it was big panel the heat inside was a maximum of 32 deg C. Obviously, there was a sun canopy. I would always advise placing the VFDs in the safe area unless no other option. Easier for maintenance, easier for fault finding and commissioning (this was the difficult part) having to commission the drives with no production!

2. Re Expo and largest cooling systems for VFDs..... Expo does not manufacture the larger cooling systems. They manufacture the monitoring and control system. They would work with the cooling system supplier and customer to provide the best technical or cost effective solution. By the way they did design the Ex. purged motor system I mentioned the other day for the North Sea and beyond.

3. Re the ex motor/VFD guidebook.... Don’t expect too much here. The booklet was written in 5 days as I was constantly bombarded with calls asking for advice (free). I therefore was pushed into writing it. It was and still is I believe the only specific publication on the subject. But suffice to say a lot of people wanted a copy in including motor manufacturers, consultants, safety engineers, insurance companies and even governments. 500 went to Iraq one month before Saddam, invaded Q8.

ICEMAN

P.S. It's short and sweet for once - I have to leave the office in 20 mins.

 

[Ahr35181]

ICEMAN,

Thanks for your help, it's certainly an eye opener!!

Another cause for concern is non EMC compliant VSD installations (typically no segregation of supply, motor & control cables) causing harmonics in the supply which may cause overheating of non VSD Ex motors???

With regard thermistor protection relays, we generally use Sprecher+Schuh RT3-M. Surely if these are designed & manufactured to British / European standards they will offer adequate protection to Ex motors without being approved by an Ex certifying body? If they were approved then they should be marked in the same way as I.S safe area barriers (i.e. safe area associated apparatus).

What about other safe area associated protection equipment, contactors, fuses, overload relays (non-VSD applications) & similarly ESD systems?

Sorry to sway from the initial subject & once again, thanks for your replies,

Alan

 

[Ahr35181]

ICEMAN,

I forgot to ask if you could point me in the right direction in gaining examples of fires / explosions caused by electrical equipment in explosive atmospheres?

Alan

 

[ozmosis]

ICEMAN
The new European Directive 94/9/EC (ATEX)has brought a lot of the concerns of 'self-certification' issues into the open. Prior to the new Directive, factory inspectors were able to issue Factory Inspector Certificates. This is no longer permissible based on the EC-examination certificate. This means, that especially for EExe motors, the notified body, either DMT or PTB(in Germany), must certify even small changes and revisions.
Whilst there is a feeling of 'adding further bureaucracy', it should mean people who purchase/use/specifiy may sleep easier, assuming of course the motors are purchased/used/specified correctly.
Siemens (the provider of my salary)have a considerable amount of information on this subject, both fixed speed running and variable speed control. As you're in the UK, you may wish to contact Siemens A&D (the LV motor team) in Manchester.

 

[ICEMAN]

Hi Alan

With regard to your questions….

Re approved thermistor relays. Having equipment manufactured to European Standards has little directly to do with safety in hazardous areas. Afterall you can buy an EExd motor built to these standards (they all are in the EU), connect a VFD to it and we are back to start of this issue. We are back to the often used BASEEFA phrase “Operating outside the conditions envisaged when it was certified”. Your IS barriers are “operating within the conditions envisaged when they were certified” – that’s the real point. Your S & S may be OK but according to whose rules – PTB are different (and far more advanced by BASEEFA). The main thing is that they do protect the motor and the PTC sensors are immune from the effects of high levels of dv/dt (which stuck in the hotspots should be).
BASEEFA at the time I am considering (late 1980s-early 1990s) were not as switched on as PTB. PTB looked at a drive system for use in hazardous areas which had to comprise of three discrete parts, namely :-

i) The explosion proof motor of whatever protection concept {e.g. EExd, EExe, etc.}.

ii) The VFD, inverter or frequency converter {different terms for the same item}.

iii) The protective device which tied the two piece of equipment together, for example the Loher CALOMAT approved relay or the CALOVERT protection device (integrated into the VFD but still independent regarding the protection function).

The above ‘system’ was them subject to test with the outcome that the motor(s) were certified for use on VFD supplies subject to strict design criteria. The protective devices were also approved for use with VFD fed explosion proof motors.

Loher then had the task of calculating (and confirmed by test) the maximum kW output from the mainly ‘flameproof’ (EExd/EExde II T4 up to EExde IIC T6) and ExN (before Euronorm) ‘non sparking standard motors’ motors’ up to 1000kW. Above that to 4000kW tests were on a case by caser basis. The achievable kW outputs were based on what could be achieve without exceeding the temperature class and machine insulation and lubrication criteria, with a additional application thermal cushion built in. The speed ranges for 2, 4, 6 and 8 pole 50hz or 60hz machines were for square law loads (10:1 speed range), constant torque (3:1, 5:1 and 10:1). Obviously on TEFC motors the higher the speed range on constant torque, the less the maximum continuous output. However, to compensate for that forced cooling or water cooled machines were available. We also used to offer ‘special windings’ for constant torque, which for a long time many people thinking we were ‘wired to the moon’. For example, a leading UK motor manufactured (based in Huddersfield) would offer for an application requiring 75kW at a speed range of 1500-150 constant, a 150kW EExd motor and 110kW VFD (a larger VFD is required due to higher magnetising current of the motor). Remember that the larger motor was offered for thermal considerations. We would offer a 75kW motor which with the special winding would provide 90kW at constant torque over a 17:1 speed range, TEFC. Often we also had to use a small reduction ratio gearbox but the price was around 60% of that offered by the competition. How did we achieve that ? Not as difficult as you think. Usually, the 150kW ‘offer’ stool no chance, not on price but on physical size – was just too big. Footprint too big, shaft to big to match the coupling, etc.

One notice of caution re the other party’s technical offer….even at rated load (75kW) the 150kW motor running at around 50% load so the motors efficiency and power factor would be down but hidden due to the DC bus of the VFD. No great issue here BUT as the VFD was at best operating at around 50% loading the input current drawn from the mains supply would always be discontinuous and as such the total harmonic current distortion (Ithd) would be in the region of 55-65% compared to our solution for around ~ 36-38% (based on 5% source impedance) for the same application. Doesn’t sound an issue for one drive but if you have say 10 mixer mixtures the harmonic currents does add up AND remember that fixed speed explosion proof motors on the PCC are also seeing those harmonics and are therefore legally uncertified and may pose a danger of risk of explosion should as or vapour be present !!! Back to where we started from !

Anyway, getting back to your query re approved devices et al >>>>>BASEEFA tended to only look at the motor and the effect of the VFD waveform on in regarding carcass temperature. In many ways the approach was significantly lacking (hence my comments re ExN motors and BS5000 the other day !) They did not seem to consider the protection device. They even accepted in straight into the VFD until I pointed out the problem here. In any case you could drive a double decker bus through many BS standards surrounding this issue.

Your fuses, contactors, etc. are not part of the issue as they DO NOT DIRECTLY protect the motor. The approved devices are only those directly protecting the motor.

Regarding information on explosions caused directly by problems with explosion proof equipment….In the UK I suggest you contact the HSE. The problem is that many of these are not widely published so information is often not ‘freely’ available (in any case oil companies et al has considerable power and do not wish to 'advertise' such incidents. It is only when there are fatalities does thing get more open. In the US is it different. The Freedom of Information Act does provide access to these reports et al. Most problems with explosion equipment is due to it not being maintained correctly or being misused (back to “operating outwith the conditions envisaged when it was certified”). The issues such as VFD – explosion proof motor interaction is not common. However, it is still there and still needs to be resolved correctly and above all, safely. To say “nothing to date has happened” will not stand up after the event !

Have to go…..

ICEMAN

 

[ICEMAN]

Sed2developer....

Guten tag. I would not go to Siemens UK (Stalag 17) - I have never found them that switched on. Go to Erlangen - no problem, real expertise there. UK - nein !

I know EExe motors as well having supplied a number of VFD/motor packages. A lot more problematic that any other protection concept due to 'TE times', etc. Usually easier and cheaper (less testing requirement) to supply EExd motor packages.

Self certification did on in the UK in the mid to late 1980's, mainly with the UK's largest motor company. It was certification by 'mail order'. I uncovered this as part of a covert investigation into VFD-explosion motor intereaction. I found that everybody was doing it {the application of ex motors and VFDs} illegally and dangerously (consultants, motor manufacturers, inverter makes, contractors, suppliers, customer). I took my evidence to BASEEFA and they took action against them. That was the start of my "Ex Campaign" which ended up i changes in the Health & Safety Law, methods of testing and certification. It is better now but now 100%, that's for sure !

They (the motor company) also made statements in catalogues that all their EExd motors had blanket 2-100hz certification for variable frequency supplies : didn't have it (BASEEFA told me) so they had to stop saying that also. After the conference "Exposing the Myth - Converter Fed Motors In Hazardous Areas" run by our consulting company, Howieson & Evans, they had to test all their motor at various speeds and loads, and in light of my comments were made to add three motor monitoring points on the motor carcass (shaft, bearing and rotor).

I really wasn't liked by these guys. I was told seriously by a leading authority that "I was to 'them' (can't say their name but the town Hudderfield comes to mind)what Salman Rushtie was the Ayahtolla Khomieni". When you are doing it wrong you should not complain when it is exposed.

The Germans were and still are much better at this game the English (I'm a Scotsman so I am independent). Companies like Loher, F&G, Siemens all do a good job (in that order !). Do Siemens still have the 2-100hz blanket certification ? If so, is the position of the field weakening point specified on the orginal or supplementary certificate ?

Re the new EU sitution. It's about time this happened; too many people getting off with murder. It' about time it also happened with CE marking for EC. I know the situation is about to change but to date it is a joke - I have experience of this. Even once established the fraudulent CED marking penalty is like getting bitten by a head sheep !

ICEMAN

 

[ICEMAN]

Alan

Re EMC compliance for systems. It is a seperate issue but......What a mess ! People are getting away with murder. What is the point of EMC filters, CE marking et al only for the kit to be stuffed willy-nilly in an enclosure, crossed cabling ? The problem is that nothing is policed. You have exactly then same problem with the new harmonic standard in the UK, ER G5/4 (2001). You have the same problem with CE marking. Nothing is policed. With no policing - many cowboys survive. I do say 'nothing is policed' a lot !

As a consultant in Dubai a few years ago I managed to convince the Government to adopt IEEE 519 (1992) {US harmonic standard) and to require on site EMC testing of all panels housing VFDs or any other non-linear loads. I don't know whether they still do it though although the requirement for IEEE 519 (1992) does remain. In many cases a 5% retention was held back until harmonics and EMC testing is successfully completed and ant remedial measures taken {these guys did not have a clue as to how much it costs to have harmonics and EMC problems resolved after the equipment was in ! It costs a much less to address the issues from the start and engineer them out from the beginning).

Of course both are EMC as far as the EU Directives are concerned; the frequencies are different - harmonic (<2khz), EMC (150khz - 30Ghz). Until then people were getting away with murder saying that their bid does include compliance with both issues (they didn't and in most case didn't even know what the issues were. Once they had the project, the harmonics and EMC issues were oftened quietly dropped when jobs went over budget).

I could go on and on re this matter also but I have work to do.

ICEMAN

P.S. There is an excellent book on this subject called "EMC for Systems & Installations" by Tim Williams and Keith Armstrong (ISBN 0 7506 4167 3). Mind cost me £ 30 four years ago.