75 hP Motor Application with 150 hP motor installed

[mayanees]

We have a client that runs a cement operation where they have a 460V, 3-phase blower motor that pulls 97 amps. The motor is 150 hP with overloads at 180 amps and a motor nameplate fla of 169 amps.
They recently had a fire at the motor and they'd like the starter to have dropped out.
I've never heard of sizing the overloads under the motor rating, but would that have tripped for say 125 amps, and would that be a recommendation? The logical recommendation is to size the motor appropriately, and I know the motor will be in a low power factor situation, but would undersized overloads signal a load change?
Thanks for any replies.

 

[BillyMac59]

Interesting question. Code states the maximum size of overloads. Your plan is to choke the motor's current draw by having the OLs tripping the control circuit BEFORE the smoke comes out?

 

[CaptainKu]

Is the motor on a VFD or DOL? Was the fire actually in the motor itself? If the blower is for a kiln the excess heat can be transferred into the motor and with the reduced speed the cooling of the motor can be reduced or uneven heating in the windings. A more elegant solution would be to install a motor RTD and wire it to a switch that will open the starter in the event of an over temp condition. Although this would be more expensive than just dropping the OL.

 

[mayanees]

The motor is Across-The-Line started. The RTDs for the motor were all disconnected and shorted out, so that's the first place they'll start. But yes, I was wondering if the OLs could be used to stop the fire in an oversized motor application. I grew up with Scott Paper, DuPont, GF, P&G and I've never seen an application where the motor was so grossly oversized and I've never seen overloads that were undersized because of the motor.

 

[jim dungar]

Leave the overloads where they are supposed to be per code and manufacturer. Then add a separate undercurrent relay. Because this would not be an NEC required relay you could set it at any current value and time delay you want. This type of alternate protection is quite common.

 

[winnie]

If you think about the way motors are rated, then 'underating' a motor is perfectly reasonable. Instead of a 150 Hp motor, you have a 90 Hp motor with a 1.7 SF

I think @jim dungar has the correct idea. Overloads are not very accurate. While you can have undersized overloads, you run the risk of nuisance tripping on normal loading or not detecting the abnormal loading.

I'm curious what sort of situation you are trying to detect that would cause a fire. Insulation failure? Bearing failure? Mechanical rubbing?

-Jonathan

 

[Jraef]

Leave the overloads where they are supposed to be per code and manufacturer.

This is the salient point, and is one of my pet peeves.

The motor is 150 hP with overloads at 180 amps and a motor nameplate fla of 169 amps.

People often misinterpret the part of 430.32.A allowing for 115% or 125% of the motor FLA. What the NEC doesn't mention, because it VARIES from manufacturer to manufacturer, is that some (most actually) of the OL manufacturers ALREADY build in the 115% or 125% in the trip curves of their OL relay, then instruct you as to how to select and adjust according to your motor and application. Many of them tell you to set/se;lect a value as close as possible to the actual motor nameplate FLA. So if, for example they already have 125% built-in, and YOU add another amount equivalent to 125%, the OL relay is not even going to BEGIN protecting that motor until it gets to 152% (125% of 125%) of the motor FLA! The ONLY valid method of selecting OL settings or heaters, is to follow the manufacturer's instructions, as required in 110.3.B.

So as that relates here: Motor FLA = 169A, if the OL is selected / set at 180A, that is already 106.35%, and if the relay mfr had 125% already built in to their trip curve, now it is 131.5% before the OL relay BEGINS to think about tripping. Without knowing the details on the OL relay mfr., we don't know what it SHOULD have been selected at. But to Don's point, that's where the process has to START.

 

[topgone]

This is the salient point, and is one of my pet peeves.


People often misinterpret the part of 430.32.A allowing for 115% or 125% of the motor FLA. What the NEC doesn't mention, because it VARIES from manufacturer to manufacturer, is that some (most actually) of the OL manufacturers ALREADY build in the 115% or 125% in the trip curves of their OL relay, then instruct you as to how to select and adjust according to your motor and application. Many of them tell you to set/se;lect a value as close as possible to the actual motor nameplate FLA. So if, for example they already have 125% built-in, and YOU add another amount equivalent to 125%, the OL relay is not even going to BEGIN protecting that motor until it gets to 152% (125% of 125%) of the motor FLA! The ONLY valid method of selecting OL settings or heaters, is to follow the manufacturer's instructions, as required in 110.3.B.

So as that relates here: Motor FLA = 169A, if the OL is selected / set at 180A, that is already 106.35%, and if the relay mfr had 125% already built in to their trip curve, now it is 131.5% before the OL relay BEGINS to think about tripping. Without knowing the details on the OL relay mfr., we don't know what it SHOULD have been selected at. But to Don's point, that's where the process has to START.

In my other life, we don't send TOL's right away by relying on the 115% built-in setting (just setting the trip lever to 100% of the motor FLA). Company procedures require techs to inject 600% current on those TOL that were set at 100% of FLA and note the trip time (depending on the class (10s, 20s, or 30 seconds). If it takes a longer time than specified, the knobs will have to be tweaked.

 

[ptonsparky]

I put current transducers on motors often enough. Let them control via PLC, or whatever, your choice.

I left the starter overloads as they were.

 

[ptonsparky]

A little preventive maintenance may go a long way.

 

[mayanees]

thanks for all the responses