OT: Motor Service Factor

[aiki202]

This is off topic, but I know there are excellent resources in this group that can provide an answer.

Pertaining to the service factor rating on an AC 3PH motor: The service factor rating value is a multiplier that is used to calculate the HP that the motor can provide above the rated (nameplate) HP. Before anyone goes into why this rating should not be used when designing applications, keep in mind I have no intention of using the rating to get extra out of our motors. This is just for my own understanding.

We recently had an issue where an existing motor driving a fan had the fan shaft mount worn out. The fan was replaced but the new one had a more aggressive blade pitch and thus put more of a load on the motor. The motor is 460VAC, 3PH, 2HP, 60Hz, across the line contactor for control, and heater overloads for over-current protection. The heater overloads continually trip out because of the increased load. One of our associates said why don't we just put in bigger heater overloads? This, of course, was not the correct solution, but it got me thinking.

Here is my question: The service factor is a multiplier of the HP rating. Is there a way to relate this multiplier to the motor current? If our 2 HP motor has a SF of 1.15, then it can produce 2.3 HP. What is the motor current at this rating? Correct me if I am wrong, but multiplying the FLA rating by the SF does not give a correct value. Or, is this one of those situations where there are too many variables for a generic relationship to be established, and each specific system would need to be analyzed individually? Or is there something I am missing totally?

Thanks for any feedback in advance.

 

[DickDV]

NEMA service factor is a multiplier on the FLA, not the hp. You need to check the NEC rules concerning the selection of heater values since the code rule does allow you to set the heaters for more than the FLA under some conditions.

Since we are talking about overload protection here anyway, let me suggest that the best bargain in motor protection today is the electronic overload block. You not only get adjustable current settings but you get switch-selectable Class 10-20-30 settings and--------the biggest benefit of all-----single phase protection. In my view, saving one motor from burning up with single-phasing will pay for all the electronic overload blocks you will buy. And, in many cases, the are physically interchangeable with the old heater types.

 

[aiki202]

Thanks for the reply DICKDV. I agree with you on the electronic overload advice, but this is an old system that we have no intention of upgrading in the near future.

I have searched many sites and articles on service factor and all define the service factor value as a multiplier of HP. Perhaps I am missing something or these resources are giving incorrect info.

 

[TConnolly]

Since the amount of power required is proportional to the mass of the air moved, putting a simple baffle in front of the fan will stop it from tripping the overloads without resorting to electrically risky solutions.

 

[milldrone]

I'm sure Alaric was thinking that you would use an ammeter to determine how much blockage to install, but forgot to mention it.

 

[TConnolly]

No, that would mean be being scientific.


Actually, I was thinking of trying one small baffle to see if it worked, and if it didn't to try a slightly larger one, until one worked.

 

[leitmotif]

What are the motor nameplate FLA. What is it drawing? What is the nameplate SF?

How much have you increased air flow??

ASSUMING blower load is constant

IF motor drawing < nameplate amps then increase heater size to no more than 100%.

As Alaric says you can throttle the blower
OR you can install a larger driven sheave (smaller drive sheave) to slow blower down and decrease power demand
ASSUMING in either solution you can live with decreased airflow
AND you do stay within the fans operation curves.

Dan Bentler