Zone 2 / Div 2, VFD Duty Motor with Insulated NDE Bearing

[macmckim]

I have an application for a 6P-250HP, Class 1, Zone 2 / Div. 2, VFD duty motor, PD Pump, CT application, V-Belt with an insulation NDE bearing on jack pump (oil) application.
My pump OEM has advise me that his motor supplier has declined to quote, because it's their policy not quote motors in Zone 2/ Div 2 areas with insulated NDE bearing. It was explained to me it's bacause of possible shaft voltage issues that might cause / create a spark.

Has anyone heard of this possible issue.

thanks
Macmckim

 

[jraef]

Shaft voltages are real, especially when VFDs are used. I have never heard of it being high enough to be incendive though. Even an Intrinsically Safe circuit is allowed to be up to 20V at 250mA I think, shaft voltages are never that high. Skogsgura is the guru of motor shaft voltages, hopefully he sees this and chimes in.


"Will work for salami"

 

[DRWeig]

This is a really old memory, so pardon me if I'm way off base. Are they referring to shaft voltages arising from static electricity of the spinning, electrically isolated rotor? If so, precautions should be taken even in a Div 2 area.

Best to you,

Goober Dave

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[macmckim]

Dave

Not static voltage, but shaft current/voltages from the rotor when using on a VFD. I have look high and low in books
and internet and yet to find anything on the subject in a Zone 2 area.

macmckim

 

[DRWeig]

Ah, thanks for the clarification.

The key is: does the electrical device (motor in your case) create arc, spark, or surface temperature near or above the autoignition temperature of the flammable gas in question. If not, an enclosure for ordinary locations is fine. If so, it needs to be listed and labeled for Div 1 locations. If unsure, I think the proper choice will be the safe option. If the motor manufacturer is scared of it, I would take that as a strong hint.

Best to you,

Goober Dave

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[Skogsgurra]

I have been working with PWM and induction motors for decades. The worst voltage I have seen between shaft and stator (across bearing, that is) is around 60 volts. You could draw sparks with a screwdriver. This was a large cooling pump and the grease was very cold.

More typical voltages are between 10 and 20 V. Sometimes more. And the possible current from shaft to ground (when short-circuiting shaft to ground with a braid) is sometimes up to 10 A RMS. Siemens and ABB usually want to limit the current to under 1 A, but that is often not possible.

The available energy is enough to light any flammable gas* and I have never been involved in PWM fed motors in Ex zones. I guess because they are very rare. For a reason.

*It can also light an incandescent bulb in some cases.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

Wow, learned something new today. I had thought that the current and voltage was a lot lower. But thats why i was hoping you would check in Gunnar, you are the guru of sparky shafts!


"Will work for salami"

 

[LionelHutz]

Basically, you'd have to ensure the possible energy which could be stored on the rotor is below the energy required to ignite the gas if/when it discharges to ground. So, you're looking to find how many Joules of energy would be stored on the rotor. I'm just not sure how you'd do that. However, I would think using shaft brushes to discharge the energy would require them to be housed inside purged enclosures.

I believe there's a requirement for the motor and VFD to be a tested and rated combination for use in Hazardous locations. This is based on the fact that a rated motor is rated assuming sinusoidal voltage is applied and that the VFD output waveform will cause more motor heating.

 

[Skogsgurra]

Jeff, I must say that my shaft isn't so sparky any more. You will notice that yourself when you get as old as I am

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[DRWeig]

Shame on you Gunnar!



Best to you,

Goober Dave

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