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Old February 28th, 2006, 06:33 PM   #1
russrmartin
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Motor Ground Fault Short Circuit protection question

Hey guys,

I currently have 2 Powerflex 70 drives that are running 2 2HP motors. We will be replacing the current machine with another machine which has 3/4 HP fan motors, and we will use the existing drives to run them. My question is, do I need to resize the GFSC protection that is upstream of the drive? This breaker I assume is also the protection for the motor leads as well as the drive. I have never done this for a motor that was running off a drive before. I'm used to just grabbing the data out of 430-150 and sizing the breaker. I'm just unsure of how the VFD plays into the equation. Right now, for the 2 HP motors we are using an AB 140M-C2E-B40. I am thinking that I need to replace these with the -B25. Thanks in advance.

Russ

Last edited by russrmartin; February 28th, 2006 at 07:16 PM. Reason: more info
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Old February 28th, 2006, 06:42 PM   #2
randylud
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??????????

Quote:
Originally Posted by russrmartin
My question is, do I need to resize the GFSC protection that is upstream of the drive?
And what is a GFSC? Something akin to a GFCI? Also, protecting motor leads by some device ahead of the drive is not protecting the motor leads in my book. We need DickDV to put this in perspective I wager!
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Old February 28th, 2006, 07:16 PM   #3
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I wouldn't change the protection before the drive unless you check the manual. The AB manual specifies what range fuses, CB etc should be used for the different horsepower drives. The motor nameplate parameters should be changed in the drive to match the new motor. I believe they are parameters 40-50 and include motor voltage, HP, etc. The drive will most likely need to be tuned for the new motor also.

Last edited by Logix; February 28th, 2006 at 07:18 PM.
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Old February 28th, 2006, 08:47 PM   #4
DickDV
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Since the drives are not changing, nor should the protection for them. The motor protection resides in the drive so, if the motors change, the drive parameters need to be changed to get the new level of motor protection.

I am not closely familiar with the AB drives but, usually, when you put the correct motor nameplate data into the drive, the drive does the rest and configures the overload protection for the new motor.
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Old February 28th, 2006, 09:00 PM   #5
Tark
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Quote:
Originally Posted by randylud
And what is a GFSC?
Russ is referring to the branch-circuit short-circuit ground-fault protection (the circuit breaker or fuse size). It starts at Article 430.51 in the 2005 NEC.

In 2005 the NEC added section X to Article 430. This section covers adjustable speed drive systems. The first line of this section (430.120) reads – “The installation provisions of Part I through Part IX are applicable unless modified or supplemented by Part X.” There is no supplement or modification to the branch-circuit short-circuit ground-fault protection in section X of Article 430.

So your BCSCGF protection should be sized according to the 3/4 HP motor. But that doesn’t mean that the existing protection is oversized for the 3/4 HP motor.
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Old February 28th, 2006, 09:14 PM   #6
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Russ...you didnt spec what voltage you were using..That is required to resize the breaker..

Best guess..seing as you are in the states..i would guess 480v..at that voltage i would thing a -B16 would suit better..However if 240v then you are correct a -B25 would be the correct choice...

For everyone else..Russ has just asked what size of Motor protection he requires...(Albiet a little redundant with a drive)

Russ..You could also get away with just a fusable disconect switch for this application..rather than paying for this..The drive will take care of the overload..just use the fusable switch or better yet a standard circuit breaker..(Be careful becouse not all Din rail mounted breakers are rated for branch circuit)

Tark beat me to it!!

I do not know about the NEC only the CEC..Up here it would be OK to do as i said..however after reading Tarks reply it might not apply "down south!"

D

Last edited by darrenj; February 28th, 2006 at 09:17 PM.
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Old March 1st, 2006, 12:47 PM   #7
russrmartin
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Thanks guys. Tark, thanks for the info. I couldn't even find a 2002 codebook, so I was referring to 1999. I'll make sure to get an updated copy. If I read the specs on the current protection correctly, it is nowhere near oversized for the 3/4 HP, but I'd like to know everything for future reference. Thanks again.

Russ
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Old March 1st, 2006, 02:24 PM   #8
DickDV
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I'm not an electric code expert but I can assure you that if you have a 15hp motor running on a 30hp drive (this is not so unusually on heavy overload applications), and you size the branch circuit protection for the 15hp motor instead of the 30hp drive, you will not get past powering up the drive without the branch circuit protection releasing. That's due to the inrush current when first powering up the drive.

Maybe the drive instruction manuals are in conflict with the code but, if you don't follow the fusing requirements in the drive manual, you are headed for trouble.
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Old March 1st, 2006, 04:21 PM   #9
Logix
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Quote:
Originally Posted by DickDV
I'm not an electric code expert but I can assure you that if you have a 15hp motor running on a 30hp drive (this is not so unusually on heavy overload applications), and you size the branch circuit protection for the 15hp motor instead of the 30hp drive, you will not get past powering up the drive without the branch circuit protection releasing. That's due to the inrush current when first powering up the drive.

Maybe the drive instruction manuals are in conflict with the code but, if you don't follow the fusing requirements in the drive manual, you are headed for trouble.
You're right Dick. The manual specs only the 140M-C2E-B40 for the 20AD3P4 drive. (480V 2HP) I'd downsize the drive if the NEC now requires exact motor overload protection before the drive. I'm not up on the latest NEC rules. Lowering the size of the breaker will most likely give nuisance trips.

http://literature.rockwellautomation...d001_-en-p.pdf

page 26

Last edited by Logix; March 1st, 2006 at 04:35 PM.
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Old March 1st, 2006, 10:04 PM   #10
Tark
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Quote:
Originally Posted by DickDV
I'm not an electric code expert but I can assure you that if you have a 15hp motor running on a 30hp drive (this is not so unusually on heavy overload applications), and you size the branch circuit protection for the 15hp motor instead of the 30hp drive, you will not get past powering up the drive without the branch circuit protection releasing. That's due to the inrush current when first powering up the drive.

Maybe the drive instruction manuals are in conflict with the code but, if you don't follow the fusing requirements in the drive manual, you are headed for trouble.
I can prove otherwise. If I size the branch circuit protection according to the NEC for a 15HP motor, I could use this calculated size to run a 15HP motor on a 30HP drive. At least according to the AB pdf that Logix provided.

To be honest, if you follow the circuit breaker requirements in the drive manual, (the one that Logix provided) you’d be in even bigger trouble.

Here’s a perfect example as to why you don’t want to ignore the NEC. There is a mistake AB’s drive manual!!! If you look at page 26 in the pdf that Logix posted a link to, you will see AB’s table for Branch Circuit Protection Devices and Power Dissipation. There is a column for Circuit Breaker with a note 3, and a column for Motor Circuit Protector with a note 4. Go down to the bottom of the table and you will see that note 3 reads “Circuit breaker – inverse time breaker.” and note 4 reads “Motor Circuit Protector – instantaneous trip circuit breaker.” The mistake is that AB put the ampere numbers for the circuit breaker in the motor circuit protector column and the ampere numbers for the motor circuit protector in the circuit breaker column.

Take the 20AD022 (15HP drive @ 480 Volts) the ampere rating according to the table for the circuit breaker is 80 amps, the rating for the motor circuit protector (instantaneous trip circuit breaker) is 30 amps. If you put a 30 amp instantaneous trip breaker on this drive, it will trip every time. But if you put a 80 amp instantaneous trip breaker on the drive you’ll be just fine. If you will notice, all the ampere ratings for the motor circuit protectors (instantaneous trip circuit breakers) are less than those listed for the circuit breakers (inverse time breakers). Whoops.


If I size the maximum branch circuit protection for a 15HP motor @ 480 volts according to the NEC, I get the following –

Dual Element (Time Delay) Fuse – 40 amps
Nontime Delay Fuse – 70 amps
Inverse Time Breaker – 60 amps
Instantaneous Trip Breaker – 175 amps

If you look at the fuse ratings AB lists for the 15HP drive, these fall right in line with their listed ratings. The ratings for the breakers don’t, (1) because AB put the numbers in the wrong columns, and (2) because I calculated the maximum allowed.

Now if I look at the 20AD040 (30HP drive @ 480 Volts) the ampere rating for the inverse time breaker (keeping in mind that AB put the ampere rating in the wrong column) is 50 amps. So I could follow the NEC and put a branch circuit protection for a 15HP motor (which is up to 60 amps) on a 30HP drive and the drive will run fine.

Did I happen to mention that there’s a mistake in AB’s drive manual?


P.S. - I better get a gold star for this one

Last edited by Tark; March 1st, 2006 at 10:06 PM.
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Old March 2nd, 2006, 12:15 AM   #11
Vic
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Tark

Allen-Bradley's manual is correct. What you don't realize is that the Amp rating of a Motor Circuit Protector refers to the Frame Size (continuous current rating) of the breaker, not the trip current rating. The instantaneous trip current is usually adjustable as a multiple of the continuous current rating. There is no thermal or overload trip.

See this link for information on the Square D Motor Circuit Protector. The table at the bottom of page 3 shows that a 30 Amp MCP has an adjustable trip range of 90-330 Amps which meets the NEC requirements for a 15 HP @ 480 V motor.

P.S. Are you going to give me a Platinum Star for this?
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Old March 2nd, 2006, 10:46 AM   #12
Tark
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I stand corrected. I guess I'll turn in my 'you da man'.
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Old March 2nd, 2006, 10:11 PM   #13
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Ok, I’m going to do some save face here. Plus I want to hang on to my ‘You Da Man’.

First let me say, there is NOT a mistake in AB’s drive manual. The mistake was on my part for not thinking harder on how AB was coming up with the current ratings. Also there is better detail in the User Manual than the Technical Data manual.

http://literature.rockwellautomation...m001_-en-p.pdf

I was correct in saying that you should use the NEC when sizing the branch circuit protection, as a matter of fact that’s how AB came up with the ampere ratings. I even checked other drive manufactures’ manuals and they also noted that they used the NEC for the branch circuit protection sizes they had listed.

On page 127 of the User Manual reads – “Sizes listed are the recommended sizes based on 40 degree C and the U.S. N.E.C.”

If you look at AB’s User Manual, you will see that the Circuit Breaker and Motor Circuit Protector sizes are listed with Note 5 which states “Maximum allowable rating by US NEC. Exact size must be chosen for each installation.” This is on page 130 of the User Manual.

After reading that I realized why the circuit breaker size AB lists is different than what I had calculated. It appears that the reason for the difference is that AB is listing the maximum size as calculated by using an exception, which you can do but only under a certain situation. An interesting note, AB was the only drive manufacture out of those I checked that was using the exception for their calculation.

The ratings that AB is listing as the maximum for Circuit Breaker are calculated by using the exception that states if the rating specified in Table 430.52 is not sufficient for the starting current of the motor you can increase the size no more than 400% for full-load currents less than 100 amperes or less and no more than 300% for full-load currents greater than 100 amperes.

So if you install the maximum circuit breaker as listed in AB’s table right off the bat you would be in violation of the NEC, because the rating calculated with Table 430.52 might be sufficient.

As far as using the circuit protection for a 15HP motor on a 30HP drive. The maximum branch circuit protection for a 15HP motor (using the exception, after you found out that the calculated one isn’t sufficient) is 80 amps for a circuit breaker. I’m sure this is sufficient to run a 15HP motor on a 30HP drive.

Stick with the NEC, even the drive manufactures are doing so.
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Old March 3rd, 2006, 06:43 AM   #14
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By all means, stick with the NEC, whatever it requires. But you are going to have to read the fine print when the drive is much larger than the motor because the branch circuit protection is not protecting the motor. It is, instead, protecting the AC drive. The drive protects the motor for overload, overcurrent, and ground fault.
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Old March 3rd, 2006, 10:33 PM   #15
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Quote:
Originally Posted by DickDV
By all means, stick with the NEC, whatever it requires. But you are going to have to read the fine print when the drive is much larger than the motor because the branch circuit protection is not protecting the motor. It is, instead, protecting the AC drive. The drive protects the motor for overload, overcurrent, and ground fault.
I second this.

If you read the manual closely and look at the label on the side, you will find out the drive is a UL or UR as the case may be, rated thermal overload. Putting a motor thermal protection before a drive is a was of $$$. If you run 2 seperate motors simultaneously from the same drive then each motor needs its own thermal overload.

If you have an existing drive, you need not change the incoming power. You basically set the Motor volt, number of poles and FLA in the drive. The drive is quite smart and will provide all of the necessary protection to the motor.

Older 6 step drives could not run a motor smaller than about 2/3 of its rating. Todays drives do just fine up to a point. I have run 100HP drives on 5 hp motors to test the output waveforms. I do not see you having any real issues running the 3/4 hp motors on the drive that once ran 2 hp motors. The drive will be loafing.
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