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The long story short:

Is there any chance that an asynchronous motor with a run (or start?) capacitor fails to run backwards when the capacitor ages and loses a significant portion of its rated capacitance?

The somewhat longer story:

An asynchronous motor using a motor capacitor within a single-phase device is controlled by two triacs. The motor itself has three wires, let's call them L1, L2 and N. There's a capacitor between L1 and L2. The two triacs power either L1 or L2 and thereby allow forward or backward rotation of the motor - at least that's what I'm assuming.

Simplified schematic (switch symbols are really triacs):

enter image description here

C is labeled with 4 µF, but it tests out around 2.2 µF (checked by using a RC network with a 100 Ω resistor and finding the corner frequency with a scope and a function generator - in lack of a DMM with a C range...). So yes, C is indeed bad and it's a good idea to replace it... As long as that's in accordance with what I assume concerning the direction of the motor...

Sometimes, it seems that the motor runs forward when it should be running backward, and my guess is that this may be caused by a low capacitance in the motor's run capacitor.

The even longer story, with some background info:

I'm in the process of troubleshooting my dishwasher. My particular model has one pump motor with two wheels. When it runs forward, it pumps water into the interior, happily splish-splashing and rinsing the dishes. When it runs backwards, it pumps the water out of the dishwasher and into the sewer. After having watched it work and fail for a while now, I get the feeling that sometimes, the motor fails to run in the "dump" direction and keeps moving in the "clean" direction, often in a step of the program when it switches quite fast from "clean" (forward) to "dump" (backward). Then, some blinking LEDs on the panel say that something has failed and it stops working. The blink pattern is not in the user's manual, and it's different from other service error messages (like "water in the bottom, micro-switch above floating styrofoam 'brick' has detected leakage", and I made sure there's no leak). I can rule out pretty much everything else, and the only remaining explanation for the problem is the motor's failure to run in reverse direction (dump) every once in a while. It seems like the level switch still tells the controller that water is present (and there is), when the controller assumes that the pump should have dumped the water and, as a consequence, runs into a time-out. Is there any chance that replacing the motor's run capacitor will fix the problem?

Yes, the motivation of this question is that I'm trying to fix a household appliance. But... Asynchronous motors on single-phase systems with motor capacitors are a very common occurrence, and the question is really with a quite general focus... Once the elliptical field created by the capacitor becomes too narrow, couldn't it be that the motor starts off in the wrong direction? Even more so when it's maybe not a run capacitor but a start capacitor? I might still edit info about my particular make and model of dishwasher into the question at a later point, but as it stands, the question is relevant for any equipment with a motor capacitor and a motor that should be able to run forward and backward.

Once, I had to replace a capacitor next to the motor in a washing machine, but there, the problem was not the direction. It just didn't have enough torque once the laundry got wet and heavy in the drum. Here, the pump's torque still seems to be OK, but it appears it sometimes fails to start in the right direction...

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    \$\begingroup\$ 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. \$\endgroup\$
    – Mark
    Commented Jul 2, 2016 at 21:00
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    \$\begingroup\$ 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. \$\endgroup\$
    – Mark
    Commented Jul 2, 2016 at 21:13
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    \$\begingroup\$ 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. \$\endgroup\$
    – Transistor
    Commented Jul 3, 2016 at 9:04
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    \$\begingroup\$ @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). \$\endgroup\$
    – zebonaut
    Commented Jul 3, 2016 at 9:44
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    \$\begingroup\$ @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 \$\endgroup\$
    – zebonaut
    Commented Jul 3, 2016 at 17:06

6 Answers 6

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If the switching cycle is too short, the motor will run in the same direction. there should be a brief interlude between running forward and backward. I have seen this on other equipment, if the switch is moved to fast and the motor is still turning, it will continue to run in the same direction.

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  • \$\begingroup\$ This can definitely happen with something manually reversed - especially something with a lot of inertia like a lathe (which is part of why 3-phase motors are strongly preferred there). But I suspect the motor reverse on a dishwasher is not manually selected, but rather an automatic part of the cycle, and the timing of any gap probably hasn't changed. Capacitors can indeed fail with age. \$\endgroup\$
    – Chris Stratton
    Commented Sep 17, 2016 at 9:19
  • \$\begingroup\$ I've seen this - see my previous comment. \$\endgroup\$
    – Kevin White
    Commented Nov 19, 2016 at 0:42
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Tim Shrum got it right. If the direction reversal is instantaneous, the motor will not reverse. It needs to stop in between. Is the motor fully loaded (pumping water) when running forward, at the time of the switchover? If it is freewheeling, it may have too much inertia and not enough load to stop quickly so it can react properly to the reversal.

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Perhaps the commander of your dishwashing machine is malfunctioning?

The capacitor in single-phase motor is just starting role. What I mean is that the condenser coil with artificial help to establish a phase difference. If the capacitor has leakage, the single phase motor can't be started. (not forward or reverse rotation) But for changing rotation in single phase motors, the switch or triac changes direction instead of main and auxiliary winding. For this reason there are three wires. Your schematic diagram is correct if the switch was connected to L1.

When the motor rotates to the right (main winding) but the switch is connected to L2, the motor rotates to the left and the (auxiliary winding) capacitor is the starter. This has nothing to do with the direction of rotation.

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  • \$\begingroup\$ I expect that the motor uses the capacitor for run not just starting. My treadmill uses a similar arrangement for elevation. A long time ago I repaired a magnetic crack detector that had a motorized Variac transformer that suffered from a similar problem - it was supposed rotate up to a maximum value then reverse back to zero - often it would fail to reverse and would carry on forward until it jammed on the end-stops. \$\endgroup\$
    – Kevin White
    Commented Nov 19, 2016 at 0:42
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OK, this might not be the answer to your question, but it is just an idea. If you think the capacitor is the reason it won't run backwards, then why does it still run forwards just fine? Also, if you want to see the effect of the capacitor on the motor's operation, then remove the capacitor and see what happens. If it doesn't run at all, then the capacitor is critical. If it still runs, but not as well, then the capacitor is not as critical of a component. Also, it could be the control circuit in the dishwasher. Maybe it is not working properly and is not supply a "reversing power" of sorts to make the motor go backwards.

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Since a single phase motor has a start winding & a run winding the motor can be reversed by reversing the start windings. This has nothing to do with the capacitor since it is only used to temporarily sync the start & run windings together. Once the motor is at 60-70% of rated speed a centrifugal switch disconnects the start winding. This switching is done through the timer control & is used in many applications like dishwashers (wash/drain) & washing machines Wash/spin(drain). These motors are called capacitor start/induction run.

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If the fault is due to the hardware one of them is definitely faulty.

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