Which CB do I need for these VFDs? - Electrician Talk - Professional Electrical Contractors Forum
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Old 05-15-2019, 12:05 PM   #1
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Default Which CB do I need for these VFDs?

(first post on this forum)

I am trying to decide between two circuit breakers to use for my VFDs.

My VFDs are Rockwell PowerFlex 525 VFDs (Part number: 25A-B8P0N104) and they will control three 2HP motors (the circuit is 240VAC). I only have the circuit breaker to protect the conductors and the motor (no overload or back-up fuse is installed).

The manual for the drives recommends using Rockwell 140U-D Molded Case Circuit Breaker but I already have a 3 pole square-d 30 Amp circuit breaker installed and available to use for hese VFDs.

I cannot find the main differences between these two breakers except the Square D costs $50 and the Rockwell 140U is $400.

Should I switch to the 140U CB? or is the square D sufficient? Is the 140U CB designed to have enough trip delay for the motors in-rush current? or is it something else? Thanks in advance

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Old 05-15-2019, 04:34 PM   #2
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You are reading an old document, the 140U circuit breakers are obsolete and no longer available. If you are buying one somewhere for $400, it is used or surplus. These were typically 600V free standing industrial grade High Interrupt capacity breakers, not even remotely comparable to a Q0 plug-on residential breaker.

If you need an SCCR in excess of 5kA, PF525 drives at that size are approved for use behind fuses or 140M Motor Protection Circuit Breakers. If you were planning on using a plug-in Square D breaker, it's likely that you are unaware of the need for an SCCR, or you know that your Available Fault Current is already low where you are going to install that, because Q0 breakers are only rated for 10kAIC anyway. So if you don't need an SCCR above 5kA, you can use any breaker that you like ahead of the VFD.
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Old 05-15-2019, 07:53 PM   #3
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If this is not a wye power system, at this facility, be sure the remove the MOV jumpers in the VFDs. Otherwise, the VFDs will fail very quickly.
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Old 05-16-2019, 12:23 AM   #4
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Originally Posted by varmit View Post
If this is not a wye power system, at this facility, be sure the remove the MOV jumpers in the VFDs. Otherwise, the VFDs will fail very quickly.
^^^^^This is very important.

The OP mentioned that the system is 240 and a 3 pole breaker, this leads me to believe it's a high-leg system.

If it is and the MOVs are left in the circuit, it'll very likely blow right up when power is applied.

Never seen it happen but I've seen the results of it a couple of times.
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