The inrush current such as of a transformer is liable to have a DC component whereas starting current of a motor usually does not have it and I think that is the principal difference between the two.
1-phase induction motor starting current
- Thread starter gar
- Start date
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kwired
Electron manager
- Location
- NE Nebraska
All he is saying is that the transient current that happens when starting a motor is not quite the same as the transient current that happens when energizing a transformer.
To most field electricians, there is still a transient current that occurs when energizing either one and they seldom need to know much more then that.
The O/L protection device of a transformer, unlike that of a motor, is likely to operate when energising the transformer.
Does the code have any safeguard against that?
- Location
- Massachusetts
Thanks for explaining to me what I already knew and was pointing out. :roll:
mivey
Senior Member
He wanted to be sure you understood your point.
kwired
Electron manager
- Location
- NE Nebraska
Well you don't have to be so sarcastic every time you are misunderstood, what is wrong with a little clarification? Beats going off in an unintended direction on a topic. I like to think my electrician skills are a stronger talent of mine then word-smithing is.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
Three phase induction motor
Three phase induction motor
160227-2140 EST
New measurements to show starting current of a three phase motor. These look nicer because there is no centrifugal switch.
The measurements were made of the current in one phase line. The other two lines can be expected to have been about the same. No real need for the simultaneous measurement of all three lines for the purpose here.
The test load is a Bridgeport type vertial milling machine. The motor is probably about 3 HP supplied by about 250 V. Source is wild leg delta. All wire is copper. About 10 ft #10, 50 ft #6, and 375 ft 0000.
The tests were gear box disengaged, low gear engaged, high gear engaged at 2800 RPM, and only a small tool in the spindle for all tests. This is a variable speed bellt drive. I failed to record high gear and the spindle set to 4200 RPM, but the steady current was about 30% of the starting current. None of the tests were with external mechanical load on the spindle. To have put full load on the motor would have required a large drill or milling cutter and cutting steel. Different ratios produce different inertia loads, and grossly different friction loads.
Note: There is no sign of any initial inrush current. We only see motor starting current.
Motor only:
Motor plus low gear:
Motor and high gear:
.
.
Three phase induction motor
160227-2140 EST
New measurements to show starting current of a three phase motor. These look nicer because there is no centrifugal switch.
The measurements were made of the current in one phase line. The other two lines can be expected to have been about the same. No real need for the simultaneous measurement of all three lines for the purpose here.
The test load is a Bridgeport type vertial milling machine. The motor is probably about 3 HP supplied by about 250 V. Source is wild leg delta. All wire is copper. About 10 ft #10, 50 ft #6, and 375 ft 0000.
The tests were gear box disengaged, low gear engaged, high gear engaged at 2800 RPM, and only a small tool in the spindle for all tests. This is a variable speed bellt drive. I failed to record high gear and the spindle set to 4200 RPM, but the steady current was about 30% of the starting current. None of the tests were with external mechanical load on the spindle. To have put full load on the motor would have required a large drill or milling cutter and cutting steel. Different ratios produce different inertia loads, and grossly different friction loads.
Note: There is no sign of any initial inrush current. We only see motor starting current.
Motor only:
Motor plus low gear:
Motor and high gear:
.
.
- Status