electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Determining locked rotor current
- Thread starter electrofelon
- Start date
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gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
Isn't the rule-of-thumb 6 or 7 times FLA?
It is. Maybe the OP could offer an estimated figure. Who will test its accuracy?
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Right, that is typical. Should I just guess?
I would but state it as an estimate.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
That’s not a NEMA compliant nameplate, otherwise it would provide you with the Locked Rotor kVA code number. You could call Weg and ask them for it, but good luck with that.
On something that small, I would just guesstimate it at 600% of FLA.
On something that small, I would just guesstimate it at 600% of FLA.
gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
Since I'm in the office I took a quick peek at WEG's web site. This motor is a very odd duck and the closest I could find was this:
http://ecatalog.weg.net/redirect/index.asp?p_Categoria=490&p_Produto=471&p_Table=YES
You'll have to filter to find the 30 HP motors. I picked the one operating at 3,600 RPM.
It lists the locked rotor as "(II/In)=9.5"
http://ecatalog.weg.net/redirect/index.asp?p_Categoria=490&p_Produto=471&p_Table=YES
You'll have to filter to find the 30 HP motors. I picked the one operating at 3,600 RPM.
It lists the locked rotor as "(II/In)=9.5"
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
GeorgeB
ElectroHydraulics engineer (retired)
- Location
- Greenville SC
- Occupation
- Retired
II instantaneous current
In normal current
so 9.5x the marked 60 Hz value ... for a SIMILAR motor ... so a good estimate would be that ... but give it as an estimate unless WEG will give you a better number.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
180227-1443 EST
I posted some plots of motor starting current at
http://forums.mikeholt.com/showthread.php?t=174880&highlight=
Essentially locked rotor motor current is the initial starting current of a motor. Basically when you start a motor the rotor is not rotating, essentially locked rotor, and therefore if you measure motor starting current you can determine locked rotor current.
Note from my plots that motor starting current remains high for many cycles.
.
I posted some plots of motor starting current at
http://forums.mikeholt.com/showthread.php?t=174880&highlight=
Essentially locked rotor motor current is the initial starting current of a motor. Basically when you start a motor the rotor is not rotating, essentially locked rotor, and therefore if you measure motor starting current you can determine locked rotor current.
Note from my plots that motor starting current remains high for many cycles.
.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Oh ok. Thanks. Wow that's pretty high.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Not sure if it applies here or not, but in general "high efficiency" motors have higher inrush because of the design changes such as lower winding DC resistance made to increase efficiency.
I agree. When I had to provide system efficiency guaranteed figures at the bid stage I noted the differences. It also impacts on starting/locked rotor currents.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
there is a difference between "instantaneous / inrush current" and "locked rotor current".
A distinction not often appreciated.
MyCleveland
Senior Member
- Location
- Cleveland, Ohio
Jraef: Please expand on this difference...is instantaneous / inrush current a manufacturer design parameter ?
Regarding "LRC", I thought we are directed by 430.110 steering us to use values in back of 430. Is this not the direction for the original question ?
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
See the link gar posted. It gets into that discussion
My issue is this motor does not have a letter code
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
And I should have added, the PoCo will be interested in Locked Rotor Current because that can have an effect on voltage drop, not the instantaneous/inrush/magnetizing current that only lasts a few cycles.
gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
Per the OP, there is no information on the type of motor, B, C, or D.
E.T.A
Ooops, didn't read far enough down the thread. My bad. :ashamed1:
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