A quick question for some of you 'motor guru's'.

[ricm]

I was a member of HVAC Talk for several years. About 10 years ago. I used to post on here regularly. I'm a Fridgie, that SHOULD have retired by now, but still manage to bust knuckles, and frostbite fingers.

I was doing an Air Balance on Senior's lodge, Rather small, might I add. And during the testing of air flows, and such, I discovered that I was shy on Exhaust air. So the S&P fan houses a simple 1/2 hp motor that has a 7.9 amp rating FLA on the name plate. In order to 'tweek' a few extra CFM to bring the system into balance, I got the CFM I wanted simply by closing up the drive pulley one turn. However, I'm not drawing 8.4 amps... and the service factor on the name plate is 1.25.

What I've learned over the years, that an electric motor 1 phase, has a 'buffer' built into it (SF), that allows for the odd power spike, an unexpected load, or other situations that cause it to draw a little more than rated amperage. It's not really meant (at all) for constant service well into the SF of the motor's normal rating.

I'm only FAIR at math, and .5amps above my rated 7.9 amps... does this mean I'm searching for a higher HP motor to maintain the now perfect airflow I've achieved?

If I were to leave this motor chugging along at a 1/2 amp higher than rated, but still with in SF... Am I likely to see myself administering a service call in the near future?

Ric

[mark beiser]

Well, if is cooled by conditioned air from inside the building flowing around it, you may be ok......but maybe not.

[ricm]

Well, if is cooled by conditioned air from inside the building flowing around it, you may be ok......but maybe not.

Actually, it's a kitchen exhaust inter-locked with a direct fired MUA. So 'nope' on the A/C air over the motor, although it's roof top mounted, with the exhaust outlet NOT blowing over the motor. In the Canadian winters, I'm sure the temps should be fine... but our summers might be a different story.


Thanks for the quick response!

Ric

[mark beiser]

Yeah, I'm used to Texas summers. Around here, even running close to RLA, but not over it, is usually to hot, and going slightly over RLA is right out.

[hands]

I have seen several motors running .5 amp over RLA or more with no problems. I know of three fan motors that have been doing that for at least three years now with no problem.

[Gary R]

The other way to get the air flow is to increase the static pressure inside. We're you short on supply? How much outside air was being introduced? Are there other exhaust fans pulling more air than design?

The fan motor is working harder than design, which is why it is pulling more amps. The other possibility is that the voltage is not right.

Running above nameplate amps can be a problem on some motors. I have seen it kill motors within a few months. I also have seen motors run at amps over the service factor that last years.

If this exhaust fan has been in place for years, will senior blame you if it crashes in a few months?

[passintyme]

Motor 7.9 amps * s.f. 1.25 equals 9.8 amps....you are now pulling 8.4 which is still below max amps... Is say let it fly... Worst case scenario you are putting in a 3/4 h.p. motor which you would end up doing anyway..

[Mr.HVAC]

Motor 7.9 amps * s.f. 1.25 equals 9.8 amps....you are now pulling 8.4 which is still below max amps... Is say let it fly... Worst case scenario you are putting in a 3/4 h.p. motor which you would end up doing anyway..

I completely agree - I have left fan motors a little into the SF in extreme situations. Usually the issue is heat internally in the motor, and it shuts down on thermal overload before it actually burns out - that is if there is a problem and the motor doesn't like operating in the SF - which is usually not a problem.

[drunkenkoala]

What type of rest is the mua and motor on? I may have a suggestion that may help.

[Tommy knocker]

Let it eat. Do it all the time. Worst case it will trip and you can up it then. Never had to up one myself. Good luck.