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My direct involvement in these matters ceased almost 10 years ago so I may be out of date in some areas.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 (
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.
