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How much random variation in motor inrush peak current due to closing

How much random variation in motor inrush peak current due to closing

How much random variation in motor inrush peak current due to closing

(OP)
We know that motor peak inrush current (first cycle) can vary randomly based on the phase angle of the voltage at the time of closiong.

Let's say I start the motor many times and each time record the single highest magintude peak (either polarity, any phase) for each start.

Assuming ideal conditions (simultaneous contact closure on all 3 phases without bounce, balanced voltage, no motor assymetries), what would you estimate the variability of these recorded numbers as as a result of random variation of voltage phase at time of closing?

From a simple R/L circuit model, I estimated around 6% variability (max peak minus min over average) at pf = 0.15.  Spreadsheet attached (don't look at the cell formula's... they are uglier than they need to be).  Seems like surprisingly low variability.  Maybe I made an error?

What do you estimate?
 

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RE: How much random variation in motor inrush peak current due to closing

But aren't you really mainly concerned about the magnitude of the peak (maximum case) versus the minimum (no transient inrush) ?  The average value is really of little interest in terms of setting the instantaneous trip.  

  

RE: How much random variation in motor inrush peak current due to closing

(OP)
I appreciate that you are looking to understand or solve whatever is the overall question/problem.   I have lots of questions/problems, but want to attack them in well-defined individual chunks.  (Many more chunks will follow, I guarantee).

This particular chunk - we recorded a multitude of starts.  I was presented summary data consisting of a single number for each start - the highest peak on any of the phases during that start.   There was considerable variability in those particular numbers.   I did an analysis (attached) to see what is expected variability under these particular assumptions (balance etc).  The results suprpised me (low variability).  I figure they might suprise others as well.   I want to make sure that particular analysis is correct.  If so, it helps us BEGIN to answer some questions I have been asked like: How many starts do I need to do to capture the worst case?  If I record 3 starts and want to set my breaker comfortably above the behavior observed during those starts, how much additional margin should I include to allow for closing time variation (yes, there is a ton of other margin to be accounted for, including the balance assumption which I think will be a big factor in explaining the variability... but it is not the question here).

So the question is unchanged, do you agree with my analysis/conclusion as described in my first post, under the assumptions given ?

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RE: How much random variation in motor inrush peak current due to closing

Hi Pete;
When you start a motor one phase is always in a position to give a large inrush.
I couldn't load your spreadsheet but I agree that there will be a relatively small variation between the maximum currents of the highest phase.
The ripple of a three phase bridge rectifier is much less than the ripple of a single phase bridge rectifier. I believe there may be a similarity.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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