480V Motor possible operating slower

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Nardie

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North Augusta, SC USA
I have a 480 VAC motor that normally pulls 50 in wc for exhaust. The other day the in wc dropped to 38 in wc. There was an initial concern of the motor was single-phasing, which I didn't think. However, today we checked the fuses, heater contact, and supply voltage. The fuses were good, and the heaters weren't tripped. The supply voltage was 479 VAC, so this was adequate. We also perform an ohm check on the motor winding's. They all read the same 0.6 ohms. From the operators inspection the inlet and discharge dampers were fully opened, so the motor wasn't dead headed. We are going to check the positions of the dampers again for verification. I also plan to perform static and dynamic testing on the motor to check the internal components of the motor. So I am wondering is there anything else I should do, or what else could cause the sudden drop in exhaust pressure? Thanks
 
drcampbell

drcampbell

Senior Member
Location
The Motor City, Michigan USA
Occupation
Registered Professional Engineer
If it's an induction motor, it's very, very unlikely that it has slowed down significantly without blowing fuses or tripping an overload.
Sounds like an aerodynamic issue, not electrical.
 
ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
Opening the exhaust Dampers (broken pipe) would cause the pressure to go down on all three types of fans/pumps.
Current would go up on the centrifugal, down on the axial and PD. Closing the dampers (plugged pipe) would be the opposite affect.

Closing the inlet dampers would cause pressure drop on the exhaust as well. with current drop on the centrifugal and increase on the other two.

We use pressure switches in air systems to detect plugged/broken pipes. High and low pressures respectively for blowing product.

Failed pump/fan or open pipe.

Has someone actually checked to see if the gauge is not plugged or have a loose fitting?
 
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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I would look at a couple things. The first being whether the pressure is being measured correctly. These very low pressure sensors tend to get plugged and not work right.

If you can verify the sensor is working correctly I would be looking at someone having changed a damper position.
 
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zbang

Senior Member
Location
Roughly 5346 miles from Earls Court
drcampbell said:
It's an antique/indirect/cockamamie expression of pressure. It stands for "inches of water column" and originated with a water-filled manometer.
Click to expand...

It's also highly useful when talking about fractions of a PSI (1 psi = ~27.7" water). Most HVAC air supply systems operate in the single digits. So do church pipe organs. A 50" pressure is very high for a blower. You could use Pascals, but most people don't understand that (1 psi = ~6900 Pa).
 
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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
zbang said:
It's also highly useful when talking about fractions of a PSI (1 psi = ~27.7" water). Most HVAC air supply systems operate in the single digits. So do church pipe organs. A 50" pressure is very high for a blower. You could use Pascals, but most people don't understand that (1 psi = ~6900 Pa).
Click to expand...
my guess is it is really 0.50".
 
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Nardie

Member
Location
North Augusta, SC USA
So I replied on the report section to several responses, but just in case it didn't go through I will summarize the responses. Once the operators noticed the issue, the standby fan was placed on-line. The magnehelic gauge returned to 50 in wc, so no blockage or loose fitting. This is an exhaust fan for gloveboxes, not a HVAC supply fan. The normal in wc is 50, not 0.50. Also, we have checked the inlet damper, and it appears to be in the fully open position. We plan to start the troubled fan on Friday to check the outlet damper once it is energized to ensure it is fully opened. The outlet damper is a pneumatic damper that is operated by a solenoid. I agree if the motor speed is reduced to cause the loss of vacuum it should have tripped something, especially the heaters. However, we did not see any abnormalities. The fuses continuity was good, the heater trip contact was closed, the motor winding's phase to phase was 0.6 ohms, and the supply voltage was at least 477 VAC. So besides an issue with the outlet damper, could there be something in the motor internal components, which I believe is an induction motor at 1800 rpm, to cause the motor to slow to down to reduce exhaust. Thanks for the responses.
 
Dennis Alwon

Dennis Alwon

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Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Nardie, what you did was report other members posts. This is usually done if there are inappropriate post or spam, etc. Please don't use that feature unless their is a problem with someones post
 
ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
Nardie said:
So I replied on the report section to several responses, but just in case it didn't go through I will summarize the responses. Once the operators noticed the issue, the standby fan was placed on-line. The magnehelic gauge returned to 50 in wc, so no blockage or loose fitting. This is an exhaust fan for gloveboxes, not a HVAC supply fan. The normal in wc is 50, not 0.50. Also, we have checked the inlet damper, and it appears to be in the fully open position. We plan to start the troubled fan on Friday to check the outlet damper once it is energized to ensure it is fully opened. The outlet damper is a pneumatic damper that is operated by a solenoid. I agree if the motor speed is reduced to cause the loss of vacuum it should have tripped something, especially the heaters. However, we did not see any abnormalities. The fuses continuity was good, the heater trip contact was closed, the motor winding's phase to phase was 0.6 ohms, and the supply voltage was at least 477 VAC. So besides an issue with the outlet damper, could there be something in the motor internal components, which I believe is an induction motor at 1800 rpm, to cause the motor to slow to down to reduce exhaust. Thanks for the responses.
Click to expand...
Not in the motor. Is this a direct drive or a belted fan? Is it controlled by a VFD?
 
synchro

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
Is the relatively high exhaust pressure that's considered normal due to pressure loss across subsequent filters? If so, then perhaps a filter or connection has a leak and that's reducing the exhaust pressure.
When the standby blower was placed on line was the normal unit still running or not? If they were both running then check if the standby unit can maintain 50 wc by itself. If it can't and it's the same size as the normal blower then something other than the blowers is the problem.
 
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Nardie

Member
Location
North Augusta, SC USA
This is a direct drive motor with no VFD control, it operates at about 3600 rpms. I was mistaken when I stated the motor operates 1800 rpms. When the standby fan was placed online, the troubled fan was shut off. The standby fan has been operating for a week with no issues. What is called high exhaust pressure is normal for us due to the number of gloveboxes we need to exhaust. Both fans are connected to the same HEPA filter bank, so since the wc level returned to normal so there should be no filter or connection issue. Thanks though for the responses.
 
Barbqranch

Barbqranch

Senior Member
Location
Arcata, CA
Occupation
Plant maintenance electrician Semi-retired
Is there a damper that prevents some of the air from going backwards through the second fan?
 
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