Cooling Tower Fan Motor - Water Ingress

[mciver_53]

Hello All,

Having trouble with a "Tropical Proofed" 3 Phase Fan Motor.
The motor was replaced 3 months ago, lasted 2 months or so before the motor bearings flogged out. Have replaced that new motor a month ago with another brand spanker to find out that this morning that I had a VSD Earth Fault... The Motor had alot of condensation in it and was measuring 0 meg ohm to earth with a megger, when I reset it, the first time, it did start up for a few seconds and then tripped out on the earth fault, thats when I disconnected it from the vsd, and took of the terminal box cover and found the moisture, and then tested it with the megger to confirm it.

The Motor is mounted out of the air stream from the tower, and the terminal box is sealed up(apart from the inner of the conduit) which will be sealed the next time.

Speaking to the company who supplied the motor indicated that the motor might be drawing in moisture, as the motor is hot and fully sealed up, no breathing holes, so it draws in the moisture.. they reccomended that the replacement motor they will fit some breathing ports....

I think in a few days of not running the motor might dry out enough to get it ticking over again, but would the moisture have damaged any of the windings... bearings.....?

anyone had any experiences with this in the past?

Thanks

James

mciver_53@hotmail.com

[install monkey]

yep had a 3ph 1100w motor get damp- dried it out with the megger, now all ok
best stick it it an oven on a low heat

[RANGER1]

Hello All,

Having trouble with a "Tropical Proofed" 3 Phase Fan Motor.
The motor was replaced 3 months ago, lasted 2 months or so before the motor bearings flogged out. Have replaced that new motor a month ago with another brand spanker to find out that this morning that I had a VSD Earth Fault... The Motor had alot of condensation in it and was measuring 0 meg ohm to earth with a megger, when I reset it, the first time, it did start up for a few seconds and then tripped out on the earth fault, thats when I disconnected it from the vsd, and took of the terminal box cover and found the moisture, and then tested it with the megger to confirm it.

The Motor is mounted out of the air stream from the tower, and the terminal box is sealed up(apart from the inner of the conduit) which will be sealed the next time.

Speaking to the company who supplied the motor indicated that the motor might be drawing in moisture, as the motor is hot and fully sealed up, no breathing holes, so it draws in the moisture.. they reccomended that the replacement motor they will fit some breathing ports....

I think in a few days of not running the motor might dry out enough to get it ticking over again, but would the moisture have damaged any of the windings... bearings.....?

anyone had any experiences with this in the past?

Thanks

James

mciver_53@hotmail.com

I would have thought the opposite.
TEFC motor assembled using silicone on end cap flanges as well as everywhere else junction box etc.
You want everything totally sealed, so no moisture in at all.
Grease nipples on bearings to keep fresh grease 9as long as it has somewhere to vent old grease.
With breather holes it just makes it worse, the problem you just encounted.

[Frikkie]

When you call the motor tropical proof do you mean it's been treated to withstand moisture internally? If the windngs have been treated you should be able to oven dry the motor at around 80 celsius and refit the motor using an electrical sealing gel in the terminations box. If the motor is operating in very wet area maybe go for an Ex rated replacement.

The bearing failure could be because of moisture ingress but may be cause by something else such as rotor currents induced by the waveform from the VFD. It might be worth installing a rotor grounding kit.

[Magoo]

Hi mciver,
yes had similar problems, modified the output shaft bearing outer plate and fitted an oil seal, fitted a breather duck bill thing to bottom of terminal box, plus get rid of conduit and fit a cable gland. Once terminated spray terminals with a silicon sealer CRC type. Most of ground faults occur at terminals. I fitted sealed bearings and plugged the grease nipples. The motors come out for service every second year, no real problems, re-bearing and reseal etc.,

[PaulZ]

Hi mciver
I reckon Frikkie has picked the bearing problem. If you have a VSD on the motor the motor has to have one ceramic bearing and must have a shaft grounding brush.
When you run a motor with a VSD you get induced shaft voltages which will cause arcing in the bearings if the shaft has no grounding brush. This brush creates a path for this voltage to travel to ground other then through the bearings.
Moisture could also contribute to the bearing failure, but if you use a good water resistant grease this should help to protect the bearings also make sure there is a water slinger on the shaft positioned as close to the end case as possible.
If you are getting moisture in the terminal box I would be looking at where it is coming from, condensation or water hitting the motor. Condensation will form in the motor when it turns off only when it cools down, I would have thought the fan would be cycling fairly regularly or does the tower shut down for long periods.
I tend to agree with Ranger about a totally enclosed motor being the best.
Regards
Paul

[RANGER1]

I would have to disagree about rotor earthing on such a small motor, but of course it won't hurt!
Tropic proof windings is also a good idea, but would think its more for a drip proof motor.
If motor is sealed like this one should be, again it can't hurt but not necassary.

[ot1]

I am curious about the VFD inducing current into the shaft, has anyone ever measured the voltage potential being drained off the shaft? How much current is there?

[Frikkie]

Sorry, I think my old mind and poor english is leading down the wrong path. When the motor operates normally the magnetic field produced by the stator windings induces alternating current flows within the cage of the rotor. This is normal and even though current flows internally in the rotor, the potential between the rotor and the chassis of the motor remains at 0v.

I should have said that VFD's can cause the rotor to develope a voltage with respect to ground or earth. This voltage discharges in pulses through the bearings causing a current to flow which burns a very small imperfection on the ball and on the race with each discharge pulse. After a while the surfaces which are usually polished become rough and cause the bearing to run hot. As the heat increases the lubricating grease starts to break down which cause more heat and catastrophic failure.

I remember this was a fairly common problem with the earlier generation VFD's but I imagine that the newer types would have reduced or stopped entirely this problem.

[ot1]

Sorry, I think my old mind and poor english is leading down the wrong path. When the motor operates normally the magnetic field produced by the stator windings induces alternating current flows within the cage of the rotor. This is normal and even though current flows internally in the rotor, the potential between the rotor and the chassis of the motor remains at 0v.

I should have said that VFD's can cause the rotor to develope a voltage with respect to ground or earth. This voltage discharges in pulses through the bearings causing a current to flow which burns a very small imperfection on the ball and on the race with each discharge pulse. After a while the surfaces which are usually polished become rough and cause the bearing to run hot. As the heat increases the lubricating grease starts to break down which cause more heat and catastrophic failure.

I remember this was a fairly common problem with the earlier generation VFD's but I imagine that the newer types would have reduced or stopped entirely this problem.

My thoughts on this with another possibility, the bearings which are electrically coupled to the motor shaft which is collared to the fan and blades pick up an electrostatic discharge from the friction of air, finding its way to ground through the bearings and pitting them. I say this because it takes a relatively high voltage to microscopically pit metal. I would think if the VFD induced voltage it would be much lower in potential, I don't know, that's why I asked if anyone ever measured the voltage produced at the shaft, you would need oscilloscope to see it, and that's not a common tool to be carrying around for this kind of work.

[RANGER1]

We do not do any earthing or rotors up to 150kw.
Instead of using special bearings you can also sleeve bearing housing with I think, ceramic bush.

[Frikkie]

My thoughts on this with another possibility, the bearings which are electrically coupled to the motor shaft which is collared to the fan and blades pick up an electrostatic discharge from the friction of air, finding its way to ground through the bearings and pitting them.

I've never heard of static charge being induced on a blade set in clean air. I've seen environments such as flour silos where the contaminants in the air can cause static charge but usually in these special locations the motor would be Ex rated and bonding would be applied to any and all components.

I have very little knowledge of cooling towers but I would imagine if the air is very humid or saturated with spray then potential differences or static charges would be even less likely as moisture would allow any differing potentials to equalise by leakage.

[Yuri B.]

The Motor had alot of condensation in it

It is water - not condensation, I think. Bore small holes in the bottom cover for the water gathering to drain.

[PaulZ]

Hi All
This is an article from ASHRAE regarding shaft grounding.
http://www.sendspace.com/file/5c8j2v
Regards
Paul

[cool_tech]

what brand cooling tower is it. all cooling towers are in very humid conditions. may have the wrong IP rating. Is it a danfoss drive you have driving them. Also look at the seals around the shaft at the front of motor. excessive tightening of belts(if belt driven) or un balanced fan blades can cause shaft to move chewing out shaft seals.

[ot1]

Hi All
This is an article from ASHRAE regarding shaft grounding.
http://www.sendspace.com/file/5c8j2v
Regards
Paul

Thanks, good read.