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