Timeline for Asynchronous motor with motor capacitor sometimes won't run backwards when it should
Current License: CC BY-SA 3.0
39 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Dec 30, 2019 at 9:37 | comment | added | Russell McMahon♦ | I commented: 9 years on :-). Re Zebonauts comment on TL431 Cathode voltage. Vcathode_min is indeed usually specified as Vref. In practice (based on extensive experiments with a range of brands of TL431 and TLV431) I've found that Vcathode can fall to slightly above Vref-1_diode-drop with "reasonable safety". This is of course an out of spec condition and if used in a designed design the designer needs to take responsibility for any consequences. | This "feature" can be quite useful. | If Vcathode falls to under somewhat above Vref-1-diode-drop Vref will be loaded and regulation fails. | |
| Dec 30, 2019 at 9:37 | comment | added | Russell McMahon♦ | You wrote: Be careful when allowing "Cathode" to go lower than "Ref": The diagram alone says that this might work, but remember that the internal reference needs at least 2.5 V (and 1 mA) to work, and both are supplied through the "Cathode" pin. Also remember that the internal opamp "triangle" will try to keep both of its inputs at the same voltage, i.e. 2.5 V. Therefore, the "Cathode" should, in a real application, remain at or above the voltage at the "Ref" pin, i.e. >= 2.5 V. The LT1431 has separate pins for the supply and the open collector output and can go lower than "Ref" at "Cathode". | |
| Dec 30, 2019 at 9:36 | comment | added | Russell McMahon♦ | Zebonaut on a question with a comment on a deleted answer by you (so I could not there flag you) I wrote the comment below. This concerns the IMPROPER use of a TL431 - which happens to work well in practice :-). | Question is: electronics.stackexchange.com/questions/7038/… ... | |
| Feb 27, 2018 at 6:37 | history | bumped | CommunityBot | This question has answers that may be good or bad; the system has marked it active so that they can be reviewed. | |
| Dec 30, 2017 at 17:51 | history | bumped | CommunityBot | This question has answers that may be good or bad; the system has marked it active so that they can be reviewed. | |
| Nov 29, 2017 at 9:20 | history | bumped | CommunityBot | This question has answers that may be good or bad; the system has marked it active so that they can be reviewed. | |
| Oct 27, 2017 at 0:18 | history | bumped | CommunityBot | This question has answers that may be good or bad; the system has marked it active so that they can be reviewed. | |
| Sep 23, 2017 at 3:55 | answer | added | spamsink | timeline score: 1 | |
| Sep 10, 2017 at 2:37 | answer | added | J Wisemiler | timeline score: 0 | |
| Aug 31, 2017 at 3:47 | answer | added | user161523 | timeline score: -4 | |
| Aug 6, 2017 at 0:42 | answer | added | Doctor Circuit | timeline score: 0 | |
| Jul 21, 2017 at 18:09 | history | bumped | CommunityBot | This question has answers that may be good or bad; the system has marked it active so that they can be reviewed. | |
| Jun 16, 2017 at 17:43 | history | bumped | CommunityBot | This question has answers that may be good or bad; the system has marked it active so that they can be reviewed. | |
| Apr 29, 2017 at 19:15 | history | bumped | CommunityBot | This question has answers that may be good or bad; the system has marked it active so that they can be reviewed. | |
| Mar 25, 2017 at 22:34 | history | bumped | CommunityBot | This question has answers that may be good or bad; the system has marked it active so that they can be reviewed. | |
| Feb 19, 2017 at 18:19 | history | bumped | CommunityBot | This question has answers that may be good or bad; the system has marked it active so that they can be reviewed. | |
| Jan 18, 2017 at 14:50 | history | bumped | CommunityBot | This question has answers that may be good or bad; the system has marked it active so that they can be reviewed. | |
| Dec 19, 2016 at 12:14 | history | bumped | CommunityBot | This question has answers that may be good or bad; the system has marked it active so that they can be reviewed. | |
| Nov 19, 2016 at 0:33 | history | bumped | CommunityBot | This question has answers that may be good or bad; the system has marked it active so that they can be reviewed. | |
| Oct 19, 2016 at 16:50 | history | bumped | CommunityBot | This question has answers that may be good or bad; the system has marked it active so that they can be reviewed. | |
| Sep 18, 2016 at 0:19 | answer | added | mass1351 | timeline score: 0 | |
| Sep 16, 2016 at 21:17 | answer | added | Tim Shrum | timeline score: 1 | |
| Jul 3, 2016 at 19:33 | comment | added | user16324 | @zebonaut ; Nice investigation, and an interesting difference between film caps and the foil caps I was expecting. This equipment is relatively new (less than 30 years old) then? | |
| Jul 3, 2016 at 17:06 | comment | added | zebonaut | @BrianDrummond The washing machine I mentioned above had a film capacitor (like the dishwasher now, but it hat 14 µF). It had degraded to 5 µF over time. Of course, I took it apart and looked at the film: What used to be a good layer of aluminum deposit on the film looked like thousands of little craters. I guess that every time the motor was turned off, the freewheeling spike may have caused a little spark in the film cap, and because of self-healing, the show went on until the capacitance (areas of the conductive layer on the foils) became too small. Replacing the cap brought back the torque | |
| Jul 3, 2016 at 17:00 | history | tweeted | twitter.com/StackElectronix/status/749648910092791808 | ||
| Jul 3, 2016 at 10:48 | comment | added | user16324 | If it's paper-in-oil, or most foil caps, I can't see any mechanism for it to lose its capacitance. And at 4uF it's unlikely to be electrolytic. I'd suspect a problem with the measurement rather than the cap, unless a DC resistance measurement shows leakage current. Switching direction, however, requires a complete stop. If the motor is still rotating, it will continue in the same direction. So I'd suspect a failure or wear in some mechanical part like a friction brake or a clutch, or some grease where it shouldn't be (or vice versa) | |
| Jul 3, 2016 at 9:44 | comment | added | zebonaut | @transistor Cool, yet another light bulb trick... The cap has a bit over 2 µF (but I used another hack to measure it, see edit above). | |
| Jul 3, 2016 at 9:43 | history | edited | zebonaut | CC BY-SA 3.0 |
Capacitor checks out around 2.2 µF.
|
| Jul 3, 2016 at 9:04 | comment | added | Transistor | 4 uF -> 800 Ω at 50 Hz. If you remove the capacitor and wire it in series with a 60 W filament bulb (if you're still allowed have them in Deutschland) you should see the lamp glow fairly brightly. The resistance of a 60 W lamp will also be about 800 Ω. Since the lamp is resistive it's voltage will be at right angles to the capacitor voltage on a vector diagram making a right-angled triangle with equal sides. The diagonal is 230V so the voltage across the lamp should be about \$ 230 \frac {1}{\sqrt {2}} \approx 160 V \$. That will give you an idea if the capacitor is OK. | |
| Jul 3, 2016 at 8:45 | comment | added | zebonaut | @transistor Thanks, image fixed. You're correct, the schematic is now similar to those I found about single-phase asynchronous motors. | |
| Jul 3, 2016 at 8:41 | history | edited | zebonaut | CC BY-SA 3.0 |
fixed schematic
|
| Jul 3, 2016 at 8:36 | comment | added | Transistor | You've too many windings on the motor sketch. Remove the one in parallel with the capacitor. I agree with @Mark that two triacs on simultaneously shouldn't cause any problem other than some increase in power. | |
| Jul 3, 2016 at 8:31 | history | edited | zebonaut | CC BY-SA 3.0 |
added schematic
|
| Jul 2, 2016 at 21:13 | comment | added | Mark | There are two windings in the motor, and both are capable of sustaining the full current that the AC line produces. What the capacitor does is simply cause one winding to lag (and that winding might produce reduced torque compared to the other). If both windings were connected to the AC line simultaneously, there would be no short. There would simply be no preferred direction and maybe a little more power. | |
| Jul 2, 2016 at 21:06 | comment | added | zebonaut | @Mark I have already worried about the triacs, too. What makes me suspect the capacitor is to blame is this: When the triacs (mis)fire, wouldn't they self-destruct quite quickly by shorting out the third phase that the capacitor and windings (artificially) create? As far as I understand the setup, it is very important that only one of the two triacs be on at any given time, and there even should be some extended break-before-make dead-time? | |
| Jul 2, 2016 at 21:00 | comment | added | Mark | Zebomaut - your understanding is correct. But even a weak capacitor should bias toward the correct direction. The cap could be completely dead, at which point the direction would be somewhat random (but there might be a mechanical bias toward the direction that it fails in). Note that it doesn't seem to fail when it wants to wash. Another possibility is that one triac is (mis) firing when it shouldn't. | |
| Jul 2, 2016 at 20:38 | comment | added | zebonaut | @winny I'll add this info once i have it. Do you think my general guess my be true, i.e. depending on whether you connect L1 or L2 to L, the motor gets either L1, C-L2 (lag via C on L2), N as a somewhat elliptical field, or, respectively, L2, C-L1 (lag via C on L1), N, and this would decide if it runs forward or backward? | |
| Jul 2, 2016 at 20:32 | comment | added | winny | Can you measure the capacitor value and compare against the rating? The capacitor is what creates the lag for what I suppose is your two phase motor in order to make it run backwards. | |
| Jul 2, 2016 at 20:24 | history | asked | zebonaut | CC BY-SA 3.0 |