Exhaust Motors Keep going bad help!!

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swflelectric

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2 months ago we were called from an A/C Company to look into why several exhaust fan motors in a large pool supply warehouse kept randomly going bad. There are six of them on the roof(which is about 3 stories high) and they are being fed by 3 separate thermostats when 1 therm would go on it will kick on 2 units. The original contractor used a single phase rely powered by 24v and keep 1 leg hot to the motors all the time. We eliminated that gave each motor it's own 3 phase Motor Combination starter w/ 24v coil , and gave that panel it's own surge. Well since then 4 motors have gone bad on the roof and 2 of them were just installed. The motors are 3hp 480v (the new motors are invertor motors, the old are induction) the A/C company has maintained the motors, but I did noticed they grounded the metal enclosures on the roof(the motor is mounted to) but did not bring the ground to the motor itself. I opened the MDP and checked bonding and grounding, I looked at the buildings ground and it looks good.
What am I missing? We do live in South Florida during rainy season it is the tallest building in that area. What should be my next move to tell the manager Should I rent a lift and check the grounding in the pipes, should I upsize the grounding in the pipes, Should we call in specialist for lightning protection. HELP!!!
 
The owner or tenant should be able to tell you if the building has been struck by lightning. Other than that grounding most likely has little to do with the failure of the fans. Are these fans on VFDs?

I would be checking for over current, phase loss, current and voltage imbalance. Were the fans properly applied from the start?
 
swflelectric said:
2 months ago we were called from an A/C Company to look into why several exhaust fan motors in a large pool supply warehouse kept randomly going bad. There are six of them on the roof(which is about 3 stories high) and they are being fed by 3 separate thermostats when 1 therm would go on it will kick on 2 units. The original contractor used a single phase rely powered by 24v and keep 1 leg hot to the motors all the time. We eliminated that gave each motor it's own 3 phase Motor Combination starter w/ 24v coil , and gave that panel it's own surge. Well since then 4 motors have gone bad on the roof and 2 of them were just installed. The motors are 3hp 480v (the new motors are invertor motors, the old are induction) the A/C company has maintained the motors, but I did noticed they grounded the metal enclosures on the roof(the motor is mounted to) but did not bring the ground to the motor itself. I opened the MDP and checked bonding and grounding, I looked at the buildings ground and it looks good.
What am I missing? We do live in South Florida during rainy season it is the tallest building in that area. What should be my next move to tell the manager Should I rent a lift and check the grounding in the pipes, should I upsize the grounding in the pipes, Should we call in specialist for lightning protection. HELP!!!
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First, I don't think your problem is going to have anything to do with grounding, or more accurately proper bonding of the equipment to ground. Second, you don't describe (in as much detail as possible) what "gone bad" means. Do the motors freeze up, start drawing too much current. open, ground fault, burn up and stink? Not sure I can help you, but some of the brilliant minds here probably can, but not likely without some detailed info.
 
Match the load

Match the load

Have you taken any amp readings and compared them to the nameplate?

Is the motor the right size for the job?

Is it belt-driven or direct-drive.

If the amp reading is the same as the nameplate,then you are generally ok, But if the motor is overdrawn, it might be possible to change the pulley size.

double check the size and speed of the unit and compare to manufactures specs.
 
did someone think they needed more air flow and maybe changed pulley size to increase fan speed, and now the motors are overloaded?

Even tampering with the housing may allow more airflow than original design and this will load the motor more.

If they are on VFD's maybe you have too long of cable run between the drive and motor, this will increase high voltage spiking from the drive to the motor, it can be minimized by installing line reactors on the load side of the drive.
 
All of the above

All of the above

All points mentioned are good ones.
HVAC Ventilator drives are generally pretty simple.
The running current is critical and I always try to allow some head room to the nameplate on exhaust motors.
I would look at starting inrush current as well.
The original wiring scheme is definitely a problem. If you have dedicated starters for the units now I am assuming that all 3PH wiring between points is no longer suspect.
No bad connections in intervening junction boxes, nothing out of sorts...etc.
I would also look at anything which would cause abnormal short cycling.
 
Another possibility - motor is in high ambient temperature has insufficient cooling air flow or both. This becomes more likely for any motor that is not located in the airstream of the air being moved.
 
Too many "ifs" for us to be able to offer much more than SWAGs. You mention "inverter duty motors" which implies VFDs, but then earlier you say that you are using contactors with 24V coils. Which is it? Because if there are VFDs, that adds a whole basket of potential issues that would be irrelevant if there are no VFDs. And then if there are VFDs, AND you are using contactors down stream of the VFDs to turn the motors on and off with your 24V signals, then that adds ANOTHER layer of problems.

A non-VFD related SWAG I can add is related to your 24V controls from the t-stat. Is there some sort of dead band built into that, to where the controllers don't just bang on and off really fast as the air flow drops the temperature when they come on, then it rises again fast when they turn off? Motors can't be cycled that fast unless they are seriously over sized.
 
All the above are good.

One other I've run into that has been mentioned (as in check the current):

System is likely 208Y (just a guess). If the motors are 240V and the voltage is on the low side, and the ambient is high, and the motors are close to full load, they will be running too hot. The fix in my case was to purchase 200V motors. Eliminated the high failure rate.

ice
 
swflelectric said:
The motors are 3hp 480v (the new motors are invertor motors, the old are induction)
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iceworm said:
System is likely 208Y (just a guess). If the motors are 240V and the voltage is on the low side.
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I assume that they wouldn't purchase a 480V motor for a 208V supply but that does bring up another thought.

Make sure the motors are tapped correctly for the 480 Volts supply. Then check to see what the operating voltage (under load) really is. Then check the normal operating current.

If the voltage and current are normal then I would check the belt tension and make sure they don't have the drive belts to tight. I once ran into a problem where they were just tightening the heck out of the drive belts and burning out the bearings and were dumb enough to think they had an electrical problem. Before that they had to many motors on one circuit and had a real voltage drop problem. The whole system just wasn't installed correctly to start with.

I say check everything just as if you were going to install the vent motors. Don't depend on anything being correct.


90% of the problems out there are simple and it's good to check for those before going after the 10% that are complicated. Just my thoughts.
 
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Jraef said:
A non-VFD related SWAG I can add is related to your 24V controls from the t-stat. Is there some sort of dead band built into that, to where the controllers don't just bang on and off really fast as the air flow drops the temperature when they come on, then it rises again fast when they turn off? Motors can't be cycled that fast unless they are seriously over sized.
Click to expand...

You would think there would be contactor problems to go along with that scenario, mainly failed contact points, unless the contactor is also over sized.
 
swflelectric said:
... 2 months ago.......keep 1 leg hot to the motors all the time.......480v ..... motors are invertor motors...
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If arcing and shorting out, partial discharge, corona due to differential voltages with 1 leg hot; if bearings going, HF bearing currents

2 months is about the right amount of time for partial discharge to eat thru wire insulation, if this is the cause, expect 100% of motors to fail and keep failing...

.... sounds like the relay is on the inverter output side? Use the tstat to control inverter on/off function.
 
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