Setting instantaneous trip based on motor kVA code, premium efficiency motors

[adamscb]

Forum,

Have a question about setting the instantaneous trip setting on an MCP circuit breaker. In the past in our plant, common practice has been to dial up the trip setting as far as it'll go, but I'm convinced that this could really screw up the system coordination. The way I'm trying to teach our plant personnel is to set the instantaneous trip setting based on the kVA code for a locked rotor on the motor nameplate, using calculation. So far using this method has caused me no problems, except for now. We recently replaced a standard efficiency motor with a NEMA Premium Efficiency motor, and we could not get it to start. We ended up replacing the old breaker with a new one that has a higher inrush capability, and it started right up without a problem. What's throwing me for a loop is the locked rotor kVA code on the nameplate for both motors is the same. Can you not set instantaneous trip settings for a premium efficiency motor based on the kVA code? Let me know your thoughts. I'm starting to think I'm not getting the full picture here, and maybe the old breaker just went bad at the same time we replaced the motor. Let me know your thoughts.

Some background info:

• Both motors are three-phase, 460v, 100hp motors, with FLA's of 113.
• Both kVA codes are G
• The old breaker was a Siemens 125A, with an inrush capability of 1250A (tripped right after pushing start)
• The new breaker is a Square D 200A, with an inrush cabability of 2000A (started the motor up just fine)

 

[Ingenieur]

2014

430.52 (C) (3)

Table 430.52: energy eff motors max setting is increased from 800 to 1100%
exception 1 allows 1700% if trouble starting
after engineering evaluation

why not turn it up to max allowable, start motor a few times measuring inrush. and use that to set it?
much depends on the load, not just the starting kva/hp rating

 

[Jraef]

Forum,

Have a question about setting the instantaneous trip setting on an MCP circuit breaker. In the past in our plant, common practice has been to dial up the trip setting as far as it'll go, but I'm convinced that this could really screw up the system coordination. The way I'm trying to teach our plant personnel is to set the instantaneous trip setting based on the kVA code for a locked rotor on the motor nameplate, using calculation. So far using this method has caused me no problems, except for now. We recently replaced a standard efficiency motor with a NEMA Premium Efficiency motor, and we could not get it to start. We ended up replacing the old breaker with a new one that has a higher inrush capability, and it started right up without a problem. What's throwing me for a loop is the locked rotor kVA code on the nameplate for both motors is the same. Can you not set instantaneous trip settings for a premium efficiency motor based on the kVA code? Let me know your thoughts. I'm starting to think I'm not getting the full picture here, and maybe the old breaker just went bad at the same time we replaced the motor. Let me know your thoughts.

Some background info:

• Both motors are three-phase, 460v, 100hp motors, with FLA's of 113.
• Both kVA codes are G
• The old breaker was a Siemens 125A, with an inrush capability of 1250A (tripped right after pushing start)
• The new breaker is a Square D 200A, with an inrush cabability of 2000A (started the motor up just fine)

It's a known consequence of newer energy efficient motor designs that the magnetizing inrush current is now higher than it was in the past. There was a really good paper on this put out by the "Motor Matters" program from the Dept. of Energy years ago, I think it's still out there, along with some good followup from EPRI (Electric Power Research Institute, an offshoot of that original DOE program). I'll see if I can locate one or the other.

The problem with the kVA code number is that this is for LOCKED ROTOR current, which is different than that initial magnetizing current. Although of much shorter duration than locked rotor current, the higher magnetic (flux establishing) inrush affects the magnetic trips in the circuit breakers nonetheless. That is what was behind the change, I think in 2002, of the exception to 430.52 allowing an increase to 1700% if tripping occurs (it used to stop at 1300%). As you have discovered, mag trips on CB frames are always going to max out at 10X the current rating, which is where the difficulty lies, especially in MCC buckets where a change in the MCP frame size may not always fit in a particular bucket size. It's a challenge sometimes.

Ingineur's suggestion of setting it to max and measuring the inrush, then backing down the setting accordingly is pretty much what everyone does, but be warned that it requires a very fast sampling rate on your ammeter. The overall concept is however backed up by the way the exception is worded;

Trip settings above 800 percent for other than Design B energy effıcient
motors and above 1100 percent for Design B energy effıcient
motors shall be permitted where the need has been
demonstrated by engineering evaluation. In such cases, it
shall not be necessary to first apply an instantaneous-trip circuit
breaker at 800 percent or 1100 percent.

(emphasis added)

The term "engineering evaluation" is not one that is defined in Article 100, so what that entails is kind of up to you, as long as you are ready to defend your decision. As you mentioned, it's effect on coordination is usually the primary concern.

PS: Found it; here is a link to an updated paper on the subject.
https://energy.gov/sites/prod/files/2014/04/f15/avoid_nuisance_motorsys_ts6.pdf

Here's another one with a graphic that shows the issue of the peak inrush being higher than the LRC.
http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/2100-wp005_-en-p.pdf

 

[Tshering]

Protective device trip settings have to be performed considering both coordination and arc flash hazard. m not sure what is the model of your breakers, but m ASSUMING 125Amp siemens MCP and 200Amp Schneider Electric's thermal mag breaker. I don't have the library for siemen's 125A MCP, but I used a similar rating. Upon plotting the TCC per your description, although the 125Amp MCP has setting upto 1250Amps, its continuous ampacity is only 125A whereas the motor full load amp is 113Amp. This stress made the MCP "Marginal" in equipment evaluation meaning the load flow current is higher than 90% of the continuous current ampacity. In such scenario, the breaker may trip or may not depending upon the load. I haven't seen a breaker rating of this low for 100hp motor. We normally use 200Amp for 100hp motors.
For the 200Amp Schneider electric's breaker, I modeled using 200Amp trip KA type breaker. Its instantaneous setting has 2000Amp maximum, matching your description. This breaker will run the 100hp pump even when the setting is at "5" corresponding to 1700Amp instantaneous current. I attached the TCC for your reference and hope this information helps.
Disclaimer: This TCC is approximate and is not the exact model of the equipment. The breakers in this TCC is not coordinated with the upstream devices of the system. Complete arc flash and coordination study has to be performed by qualified engineer.

 

[templdl]

2014

430.52 (C) (3)

Table 430.52: energy eff motors max setting is increased from 800 to 1100%
exception 1 allows 1700% if trouble starting
after engineering evaluation

why not turn it up to max allowable, start motor a few times measuring inrush. and use that to set it?
much depends on the load, not just the starting kva/hp rating

That is an important part of the NEC to know when setting the instantaneous trip of an MCP. But I always recommend that the instantaneous trip setting be set just above where it would nuisance trip. Set as such if it does trip in the future it sends up a red flag to take a look to see what is going on if the motor is in the process of developing a progresive line to ground winding fault. You goal is to limit damage to the motor and to address it on your own terms if posible..

 

[adamscb]

Thank you all for your input. As far as the sample rate of my ammeter is concerned, the below link shows that I use at work.

http://www.idealindustries.ca/products/test_measurement/clamp_meters/760_series_clamp_meter.php

Would something like this be able to accurately measure inrush current?

​Upon plotting the TCC per your description, although the 125Amp MCP has setting upto 1250Amps, its continuous ampacity is only 125A whereas the motor full load amp is 113Amp. This stress made the MCP "Marginal" in equipment evaluation meaning the load flow current is higher than 90% of the continuous current ampacity. In such scenario, the breaker may trip or may not depending upon the load. I haven't seen a breaker rating of this low for 100hp motor. We normally use 200Amp for 100hp motors.

I am slightly confused by that statement. Because it's an MCP, it only protects against inrush current, am I correct? So for example because it's a 125A breaker, if it draws 126A the breaker won't trip, because it doesn't protect against overloads, it only protects against the inrush. So in my mind I'm wondering why that would come into play.

I was also under the assumption that equipment evaluations in SKM had to with short circuit ratings.

 

[GoldDigger]

That is an important part of the NEC to know when setting the instantaneous trip of an MCP. But I always recommend that the instantaneous trip setting be set just above where it would nuisance trip. Set as such if it does trip in the future it sends up a red flag to take a look to see what is going on if the motor is in the process of developing a progresive line to ground winding fault. You goal is to limit damage to the motor and to address it on your own terms if posible..

Just recognize that to avoid nuisance tripping in the near future you are statistically required to conduct a good number of tests, not just one or two.
If you have a way to guarantee make at zero volts, then once is enough.

Sent from my XT1585 using Tapatalk

 

[Ingenieur]

as Jraef pointed out (and in the paper) there are 2 components to inrush magnitude
the transient and steady state
transient is determined by the inductance (primarily)
steady state by R (primarily)

crude concept
480 vac
fla 100 A
lrc 800
R ~ 480/800 = 0.6 Ohm
L ~ 0.0556 H
tc = 0.0333 sec
t = time

i = V/R (1 + e^-(t/tc)) = 800 (1 + e^(-30t))
.....t......cycles.....i
0.0167.....1.....1285
0.0333.....2.....1095
0.0500.....3......980
0.0667.....4......910
0.1000.....6......840
0.1667....10.....805

Just a rough example of magnitude
load basically stretches it out
this doesn't mean if set at 1000 it will trip
there is also a time component, it has to be >1000 for some duration

note if L improves tc gets bigger
say tc = 0.04
at 1 cycle now 1330

 

[ActionDave]

It's a known consequence of newer energy efficient motor designs that the magnetizing inrush current is now higher than it was in the past. There was a really good paper on this put out by the "Motor Matters" program from the Dept. of Energy years ago, I think it's still out there, along with some good followup from EPRI (Electric Power Research Institute, an offshoot of that original DOE program). I'll see if I can locate one or the other.

PS: Found it; here is a link to an updated paper on the subject.
https://energy.gov/sites/prod/files/2014/04/f15/avoid_nuisance_motorsys_ts6.pdf

Here's another one with a graphic that shows the issue of the peak inrush being higher than the LRC.
http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/2100-wp005_-en-p.pdf

Eeegad, you tricked me into reading an instruction manual.

What are the long term implications of this higher inrush? From my wiring pulling grunt's perspective it seems motor life might be shortened. Sure light bills may go down but equipment costs go up.

 

[templdl]

Just recognize that to avoid nuisance tripping in the near future you are statistically required to conduct a good number of tests, not just one or two.
If you have a way to guarantee make at zero volts, then once is enough.

Sent from my XT1585 using Tapatalk

I agree as you would like to give closer protection than the max allowed but call backs for neusance tripping don't make sense either as it will get the customer POed big time. That's where experience comes in, the school of hard knocks. You would like to be a responsible electrician and do what is the right thing to do instead of going with what is allowed to avoid call backs.

 

[Ingenieur]

2 or 3 test starts should do it, as was said, have an accurate and fast meter
at the max recommended starts per hour under normal load/operating conditions
the primary motor protection are the OL's

a quick short ckt calc may be beneficial to keep the setting above the measured inrush and below the avail fault current

 

[Jraef]

Eeegad, you tricked me into reading an instruction manual.

I'm sorry, but yes, that means your man-card is temporarily revoked... you must now drive somewhere new with your wife in the car without asking for directions to reinstate it.

What are the long term implications of this higher inrush? From my wiring pulling grunt's perspective it seems motor life might be shortened. Sure light bills may go down but equipment costs go up.

No long term ramifications have been reported. The high peak current is strictly during the establishment of flux in the stator core, so is not related to torque yet. Besides, EE motors are typically designed with better bearings and tolerances in order to further reduce losses.

 

[adamscb]

Thank you all for your input. As far as the sample rate of my ammeter is concerned, the below link shows that I use at work.

http://www.idealindustries.ca/products/test_measurement/clamp_meters/760_series_clamp_meter.php

Would something like this be able to accurately measure inrush current?



I am slightly confused by that statement. Because it's an MCP, it only protects against inrush current, am I correct? So for example because it's a 125A breaker, if it draws 126A the breaker won't trip, because it doesn't protect against overloads, it only protects against the inrush. So in my mind I'm wondering why that would come into play.

I was also under the assumption that equipment evaluations in SKM had to with short circuit ratings.

Bump

 

[GoldDigger]

Thank you all for your input. As far as the sample rate of my ammeter is concerned, the below link shows that I use at work.

http://www.idealindustries.ca/products/test_measurement/clamp_meters/760_series_clamp_meter.php

Would something like this be able to accurately measure inrush current?



I am slightly confused by that statement. Because it's an MCP, it only protects against inrush current, am I correct? So for example because it's a 125A breaker, if it draws 126A the breaker won't trip, because it doesn't protect against overloads, it only protects against the inrush. So in my mind I'm wondering why that would come into play.

I was also under the assumption that equipment evaluations in SKM had to with short circuit ratings.

First, if you have a true MCP (see http://forums.mikeholt.com/showthread.php?t=173252&p=1692113#post1692113 for some info from jraef)
then it is part of a listed assembly and you should at least look at the setting instructions for that listed assembly.
Second, you meter appears to have an AC frequency response up to 400Hz, which suggests that the peak hold should be at least useful for motor inrush.
You might have to convert that peak value back to RMS, depending on just what the meter is really telling you for AC. If it is catching peak RMS, then it may well try to integrate over one full cycle of the waveform, in which case it may not be as useful to you for peak current in the first fractional cycle.
Definitely test to see if it is giving you peak (amplitude) or "peak" RMS by using the peak function on a constant AC voltage or current.

 

[Tshering]

I am slightly confused by that statement. Because it's an MCP, it only protects against inrush current, am I correct? So for example because it's a 125A breaker, if it draws 126A the breaker won't trip, because it doesn't protect against overloads, it only protects against the inrush. So in my mind I'm wondering why that would come into play. I was also under the assumption that equipment evaluations in SKM had to with short circuit ratings.

Happy New Year!
Coming back to our discussion, the MCP protects against short circuit & ground fault and that is the reason thermal overload relay is used in motor starters to protect from overload, especially when MCP is used. Per NEC (2014 Edition) Article 430.52; the maximum rating/setting of MCP is 800% of motor full load current 124 Amp (FLA for 100hp motor from NEC Table 430.250). With the 125Amp rated MCP, trip set at 1250Amp, it is 1000% of motor full load amp, but may be used for energy efficient design B motors per NEC 430.52(C)(3) Exception No.1.
SKM equipment evaluation and data visualizer are the two powerful tools that power system engineers use frequently. It can calculate connected load, demand load, design load with or without utility impedance, etc. And like you mentioned, it can calculate short circuit currents and evaluate design to comply with NEC Article 110.9 & 110.10
The reason I said the MCP was marginal on load flow is because, the full load amp of the motor is 113 Amps per your data and the MCP is rated 125Amp. The percentage load flow current is 113/125*100=90.4%. In the software, I set to flag at 90% and fail at 100%. You may use 125Amp MCP if that was the practice. We design our switchgear, switchboard, MCC & Panel boards in SKM software and evaluate the design prior to manufacturing. And if we were to design a controller for a 100hp motor, the typical protective device will be rated 200Amp MCP or 200 Amp thermal mag breaker.