Correct sizing of motor overloads

[mull982]

There has been an ongoing argument here at our plant in regards to selecting proper motor overload sizes.

I have always followed NEC 430.32(A)(1) which states that for a motor with an 115% service factor the overloads can be sized at 125% FLA. Others here are agruing that the overloads should be sized exactly to the FLA rating or in the case I mentioned above to 115% FLA with a 15% service factor to match the service factor.

In the case without a service factor the NEC says that 115% of FLA can be selected while others here are arguing that the overolads have to be exactly at or less then the FLA or the motor will burn up.

Which is the correct answer?

Using a motor without a service factor as an example, why is there an extra 15% margin allowable for the selection of overloads over FLA. Can this 15% margin be harmful to motors as people here are claiming?

 

[jim dungar]

Each manufacturer has their own rules. Sizing depends on ambient temperatures and motor service factor.

One manufacturer says for motor and overload in same ambient:
motors with 1.15 to 1.25 service factor use 100% of FLC.
motors with 1.0 service factor use 90% of FLC.

 

[mull982]

Each manufacturer has their own rules. Sizing depends on ambient temperatures and motor service factor.

One manufacturer says for motor and overload in same ambient:
motors with 1.15 to 1.25 service factor use 100% of FLC.
motors with 1.0 service factor use 90% of FLC.

I guess motor overloads in a starter bucket inside an air conditioned electrical room would have a different ambient temperature than a motor outside in a plant? Maybe these location differences are reasons for rating margins given by the NEC.

 

[jim dungar]

I guess motor overloads in a starter bucket inside an air conditioned electrical room would have a different ambient temperature than a motor outside in a plant? Maybe these location differences are reasons for rating margins given by the NEC.

The NEC is about the maximum level of protection. The manufacturers selection procedures take these limits into account.

For the above manufacturer, if the motor is in an ambient 18?F warmer than the overload relay:
for a service factor 1.15 to 1.25, use the motor FLC x .9
for a service factor 1.0, use motor FLC x .8
if the relay is ambient compensated, no adjustment is needed.

 

[Jraef]

The point is, the NEC is all about the absolute maximum allowable limits, not the best protection for the motor. So to answer your internal debate, I would have to side with your colleagues. But as Jim Dungar says, the only real source is the selection procedure specific to the OL manufacturer in question, because different people have different specifications.

That said, because of specific IEC regulations, all IEC adjustable overload relays are supposed to be set specifically at the motor FLA (unless the OL relay is in an environment over 60degC (140F)). And because IEC motors never have anything like "Service Factor", the OL mfrs often don't have a procedure for compensating for that.

However, we can start a separate debate (bound to be long) on what "Service factor" really means. IMHO, you should NOT select or adjust OLs for Service Factor ratings unless you want to sacrifice the life of your motor. SF is intended to be a "short term" overload capability; essentially a reserve power for only when you need it, such as when voltage drops. If you set your OL protection into the SF all the time, you are allowing the motor to run hotter than it was designed for and it WILL break down the insulation faster.

 

[Cold Fusion]

Important Note: This discussion is NEC and industrial - whether NEMA or IEC doesn't matter.

(cut) as Jim Dungar says, the only real source is the selection procedure specific to the OL manufacturer in question, because different people (mfg) have different specifications. cut)

This is absolutely true. It's important to read the application notes. The mfg will tell you how they deal with the SF and the ambient. Some times deciphering the mfg literature is a bear. It's almost like they don't want you to know the actual trip point. I've had to go find the curves.

The point is, the NEC is all about the absolute maximum allowable limits, not the best protection for the motor. So to answer your internal debate, I would have to side with your colleagues. (cut).

I have a somewhat different method of looking at this. The NEC sets ovld limits to protect the motor from burning up - as in a fire. As Jr noted, the NEC is not concerned with the best protection for the motor. So far we agree.

The way I see it the overloads don't have much at all to do with motor longevity - set them up, set them down doesn't really matter (That's a bold statement bunko. How you going to back that up?)

The single thing that affects motor longevity is temperature. Assuming the ambient and supply voltage is not clear out of line, the single thing that affects temperature is the load - and that is determined by design. If the design has the motor loaded up too far, setting the overloads down won't fix anything ? in fact you may well burn up the motor sooner due to more frequent starts if the ovlds are tripping.

(cut). If you set your OL protection into the SF all the time, you are allowing the motor to run hotter than it was designed for and it WILL break down the insulation faster.

If the design sets the motor into the service factor then it will run hot and break down the insulation faster.

The mechanicals come and ask me, "I want to run this motor up in to the service factor, say 1.05. Is that okay? Otherwise we have to go to the next size motor, and you have to go to the next size up starter." My response is generally. "Sure. Just promise me that you absolutely know there are no errors in your hydraulic calculations, we will never have to put in a slightly larger impeller. Cause if you can do that, I don't mind a bit." Amazingly, the mechanicals have never pressed it.

We electricals don't pick the driven machinery. We don't pick the loads. The mechanicals do those things. We don't get to pick the motors - we may get to advise picking the motors.

After the mechanicals tell us what the motors are, and the civils tell us where they are, we get to pick the MCC, starters, conductors, ovlds. If they picked the right motor, it will run cool. You fooling with the overloads won?t fix an undersize motor ? setting the overloads down won?t fix an undersize motor.

(cut) you should NOT select or adjust OLs for Service Factor ratings unless you want to sacrifice the life of your motor. SF is intended to be a "short term" overload capability; essentially a reserve power for only when you need it, such as when voltage drops. (cut)

So, where do I set them? First I decipher the mfg literature. Next I set the ovlds at the max the mfg instructions and the NEC allows. For a 1.15sf I'm generally right next to 140%FLA.

(cut) If you set your OL protection into the SF all the time, you are allowing the motor to run hotter than it was designed for and it WILL break down the insulation faster.

I would say it quite differently: If the design of the driven machinery is such that the motor runs in the service factor ? then the motor is going to run hot and as Jr said, the insulation life drops.

I have never seen a motor burn up because the overloads were set to the max allowed. I have seen motors burn up because the machinery allowed the operators to overload the motor until it tripped and after they had had tripped and re-started enough times, it burned up. For that case, I?d say the equipment worked exactly as per plan.

cf

 

[Jraef]

I think your distinctions are valid.