7 posts in this topic

Greetings,

I've been having some issues with a vacuum pump breaker tripping... I think I've figured out the problem and resolved it but I noticed that several of the other motors have current monitoring on the HMI but the person who set this up is no longer with us.  I'm wondering what I need besides an analog input for my control logix processor (which I have) to setup this project.  I'm not sure if the other motors are just monitoring one leg of the 3 phases or if they are somehow showing total current?  any ideas and recommendations would be greatly appreciated.

 

thanks,

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anyone???

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JAWOLTHUIS is correct...to monitor current draw of a device or load one needs a CT (Current Transducer) sized to the load with proper signal into your PLC (and ability to bring that type of PLC input, also). CTs often are analog current or analog voltage, and typically have a range (i.e., 0-10A or 0-50A).  If you have smart loads, such as a VFD motor drive (and your load is networked into your PLC), your current draw may already be available. 

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this particular motor isn't driven by a VFD and consistently pulls about 38amps per leg and was tripping a 50amp breaker... looking at the drawings I found that the original breaker was sized at 63amps.  I'm hoping I can monitor the current pull on the HMI like the other motors that are driven by danfoss drives and that I can add a bit of logic to latch a bit and let me know exactly when the breaker trips if it trips again with the correct breaker.

 

thanks for the help

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I would bet that in rush amps at motor start up are tripping your breaker.  NEC is pretty flexible about increasing rated capacity on overcurrent protection devices on motor starters to accomidate inrush amps.  

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The circuit breaker is only protecting the vacuum pump, right (and nothing else)?  Have you researched the manufacturer suggested primary protection for the vacuum pump?  Often, conventional wisdom says a XX amp breaker is plenty suited to protect a device that only pulls a fraction of FLA of the breaker (to find the breaker type is not correct).  If the manufacturer specified, say, a 30A Type D breaker (trips 10x to 20x rating) and you happen to have a 30A Type C breaker(trips 5x to 10x rating), the Type C breaker is very likely to cause you grief at some point.

Manufacturer cut sheets can save your bacon some days.

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