Motor over current

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tgreen

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I was looking at an HRV fan unit and both fan motors are drawings above name plate current.

if i leave the fan doors open (reducing the load on the motor) the motor current increases slightly. running the motors with the belt disconnect also increases the motor current about 10% above name plate.

anyone know a good resource that would explaine why this happens. i would have expected that to run the motor with little or no load that the current would go down.

the motors are a 1/3hp and a 1/4hp AC motor 120v.
 
Leaving the doors open likely increases the load as the fan will be moving more air and doing more work. I don't have any idea how you could get the current to go up by removing the drive belt and reducing the amount of work being done.
 
it didnt make sense to me either, in my mind the motor amperage should have dropped. so that is why i am asking the question. hoping someone can point me to a resource that will help me understand why?
 
Could power factor radically change between a loaded and unloaded motor? More amps being drawn doesn't always mean it is using more power.
 
power factor radically change between a loaded and unloaded motor

right you are, seems a lot of folks often forget about power factor.

Throw an ideal 16 mH choke across 120 V 60 Hz and you draw 20A or so, but zero power; real life inductor you would draw a few watts for core loss and I^2*R loss. Gets more complicated for a capactitor run motor.
 
HVACR Fans

HVACR Fans

If your motors are drawing nameplate with the cabinet closed, then typically you need to reduce the motor HP by dialing down[opening] the motor sheave. Just flying by rules of thumb in the field, its best to leave headroom to the RLA. IE, if you have an RLA of 12A at say 208V 1PH then start at 10A if the system allows it by adjustment. If the system is a constant volume configuration then your adjustments will hold. Loading of filters and or coils should be nominal with respect etc.
Assuring that the motor is within spec is usually the responsibility of Test and Balance or whoever is running start up on the system, typically HVAC Techs. Opening the blower door will increase the load on forward curved centrifugal fans, while blanking off the downstream side of the fan will decrease running load such as in old style VAV systems.
 
tgreen said:
it didnt make sense to me either, in my mind the motor amperage should have dropped. so that is why i am asking the question. hoping someone can point me to a resource that will help me understand why?
Click to expand...

By removing the door you eliminate the static pressure on the return side and allows the fan wheel to load, and the amperage to increase.
 
tgreen said:
I was looking at an HRV fan unit and both fan motors are drawings above name plate current.

if i leave the fan doors open (reducing the load on the motor) the motor current increases slightly. running the motors with the belt disconnect also increases the motor current about 10% above name plate.

anyone know a good resource that would explaine why this happens. i would have expected that to run the motor with little or no load that the current would go down.

the motors are a 1/3hp and a 1/4hp AC motor 120v.
Click to expand...

How are you reading the current? Amp clamp, flex CT? No way a fan motor with the belt removed will draw more current unless it's being fed by some exotic electronic controller. Pretty small motors to be reading current with an amp clamp. Just my humble opinion.
 
F.A.U ??? unit circuit breaker trips..

F.A.U ??? unit circuit breaker trips..

I did a trouble shoot in a house the f.a.u. breaker trip. I wanted to know why it trip so I started doing my search ,came up to be that f.a.u draws 16.5 when it kicks on . now my question is can I size this to a 20 amp breaker and keep the 14 awg wire ? is only 16.5 when it first kicks on then is at 9.6 amps ..following article 440.22 ac. sizing it to 175%
 
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