Timeline for Can Rpi4B power external / portable USB HDD / SSD?
Current License: CC BY-SA 4.0
10 events
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| Sep 27, 2019 at 13:59 | comment | added | tlfong01 | @Botspot, Ah, so it is the intertia + friction that makes the motor start very slow, so slows that the current is as large as it is stalled. In large drill press machines, the start or stall current might be 100A or more. But for HDD, I think the power circuit is smart to use over current limiting so there might not be any rush current at all. BTW I also read that the HDD power circuit is very smart, with a built in accelerometer, so if there is a free fall or shock, the protection immediately kick in and cut power or something, ... | |
| Sep 22, 2019 at 19:01 | comment | added | Botspot | That's why large drill press machines have a two-stage startup switch: the first setting on the switch limits the current. Once the motor reaches full speed, the operator clicks the switch again. If they didn't have this feature, the drill presses would likely trip a breaker! | |
| Sep 22, 2019 at 18:59 | comment | added | Botspot | Yes, when a motor is initially spinning up, it takes more current than when it is coasting at full speed. (This is because it has to combat inertia + friction instead of only friction). | |
| Sep 22, 2019 at 14:02 | comment | added | tlfong01 | @Botspot, On second thought, I do remember often reading that a motor starts with "a rush current", ... I don't understand why. Perhaps I made some mistake somewhere. Comments welcome! :) | |
| Sep 22, 2019 at 13:40 | comment | added | tlfong01 | @Botspot, I guess the HDD motor needs perhaps a couple of minutes (not sure) to arrive at the steady state, when the read/write heard slides on a very thin air layer, blah, blah, blah, .. Since the motor coil is an inductor, so the current starts at very small and goes up exponentially (opposite of a capacitor which the initial current is "infinite". In other words, the initial spinning current is smaller than the steady current, so no need to bother. I also read that modern HDD using BLDC motor, which I have played only for a short while, so cannot make any even quick and dirty comments! :( | |
| Sep 22, 2019 at 13:31 | comment | added | tlfong01 | @Botspot, The picture above reminds me that the voltage/current/speed characteristic is a multidimension thing, beside non linear. From the chart, I think I can only guesstimate that the max current is the stall current, which should be at most two or three times of the steady current. But now I am looking at another diagram: raspberrypi.org/forums/…. Again I need to stare at it for 3 minutes, to refresh my memory. | |
| Sep 22, 2019 at 13:18 | comment | added | tlfong01 | @Botspot, Just now I searched my old posts on a couple of forums I have been hopping around, and found that I usually started discussions with a colourful picture, and I think I shall use this picture to start another discussion: penzu.com/public/a25b1076. I need to stare at this picture for 3 minutes, to refresh my memory! :) | |
| Sep 22, 2019 at 13:02 | comment | added | tlfong01 | @Botspot, Good idea, let me search my junk posts. | |
| Sep 22, 2019 at 12:23 | comment | added | Botspot | I suggest you include the initial spin-up current, as that would seem to be the highest. | |
| Sep 22, 2019 at 6:13 | history | answered | tlfong01 | CC BY-SA 4.0 |