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Thread: Circulating pump - start capacitor

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    Default Circulating pump - start capacitor

    Hi - I've got a hydronic heating system and the circulating pump failed recently.

    It's a system that pumps hot water through a towel rail in the bathroom. It "interrupts" the HWS heating loop, which is supplied by boiler in the wood-fired kitchen range. It's switched by thermostat that sits on the boiler output pipe. Temp rises, thermostat switches on, and the pump circulates hot water to the towel rail until the temp drops enough to switch the thermostat off. It's been running fine since 2014, so I'm generally very happy with it.

    The pump uses an electromagnetically-driven impeller, so there's no shaft or seal to wear out and leak, and it's very quiet, but what failed was the motor start capacitor. Not surprising it let go, considering its environment is very close to hot water. I suspected the cap because I could supply power to the pump, and watch the impeller twitch as it tried to start. If I flicked the impeller manually, it would continue to run. The pump is a "Laing ecocirc" model D5Vario-38/700B.

    So I went a bought a replacement cap - 1000uF, 63V, 105C - and *carefully* soldered it in. I didn't try soldering it to the PCB (I'm not that good), but I clipped the connection wires of the previous cap and soldered the new one to those. I was careful to make sure I got the polarity right.

    Now - it works, but not as well as before. It's slower to start, and even won't start sometimes. I switch off the power and come back half an hour later and then it will start. It's taking longer to start up than before, the startup hum is definitely longer and more drawn-out than before.

    Original Cap: "SamWha" brand, 1000uF, 63V, 105C
    New cap: "SunTan" brand, 1000uF, 63V, -40 - +105C - but it's significantly physically smaller than the original - slightly smaller diameter, and about 3/4 cm shorter.

    Could it be the new cap is optimistic about its abilities? If I wanted to replace it with something better, what would I be looking for? I don't want to replace the pump if it's just a starter cap that needs replacing. Also, a new pump is about $400, so a $4 cap is a better option.

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    Would this be your original Start Cap?

    Last edited by oceanboy; 21-04-19 at 03:56 PM.
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    Close, but not quite. I see from the ebay description that one is 25mm in length. I just measured my original and it's 35mm. The replacement I bought is 30mm, so it's more like 0.5cm shorter than the original. Curious about the difference between the ebay item and the one that came out of my pump. They both have the same ratings, though.

    P.S. I never thought about trawling ebay for a replacement. I thought the shipping would greatly exceed the price of a single cap.
    Last edited by dwywit; 21-04-19 at 04:03 PM. Reason: additional

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    Is there not a processor in there too for Soft Start?

    I believe the pump you have is also very popular in Solar Water Heaters too
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    Yep - there are two PCBs in there. I think the first one, where the DC supply leads and the cap are attached, is the "soft-start" mechanism, and the second board is the one that controls the electromagnet.

    There are a few models in this range. Mine (Vario) has variable flow control via a pot, and the sticker says 8-24V so accepts input in that range with maximum flow only available at 24V (mine was supplied with a 12V power supply), another is optimised for runniing directly off PV, and so on.

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    its not a "motor start cap"
    I would has at a guess its a smoothing cap cap for a triac circuit , putting the new one across the old one could be your problem as ti has the effect of two caps in parrallel remove the old one if its bulged or swollen on the top ???? when you say clipped ??? what do you mean ? clipped with jumper leads or cut out of the circuit ?
    '
    just googled the pump , you can disconnect the motor and feed the red and black as dcv from 8 to 24 volts this will tell you if the motor is stuffed or not then you can go from there
    understand ??? cheers don
    Last edited by hinekadon; 21-04-19 at 05:20 PM. Reason: more
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    I removed the original by cutting its two connector wires. I didn't want to de-solder it from the PCB - as I mentioned, I'm not that good and I didn't want to go anywhere near the PCB with a soldering iron - and I left a bit less than 1cm of connector wires still attached to the PCB. I then soldered the connector wires of the new cap onto what was left of the original connector wires. At no point was there more than one capacitor attached to the PCB. I suspected it was a faulty cap for the reasons I mentioned above. Apply power, impeller twitches and won't start. Flick the impeller by hand and it runs. New cap soldered in and it now starts by itself, but takes longer to "spin up", and sometimes needs full power off and 30 minutes rest before it will work again.

    Additional:
    I'm not sure if I made it clear - the pump *works* with its new capacitor - only it seems to take longer to start up. It even worked with the old capacitor, except I had to give the impeller a hand-start, and it would then keep spinning.
    Last edited by dwywit; 21-04-19 at 06:00 PM. Reason: additional

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    As a wild guess - look around for resistors in the same circuit. One of them might be open now and doesn't allow the cap to do it's job properly. Hence you might see slower than usual starts and sometime no starts at all for half an hour.

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    you will have to establish there is nothing wrong with the motor as i suggested connect to a battery and check it runs ok ?????
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    I suspect the original Cap is Low ESR and the new one is not.

    Google the series number on the cap to confirm its type.

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    If I understand correctly, this is designed to run on a variable power source from an unregulated PV system. So it would use a buck converter to regulate the voltage for the pump. There should be an inductor with a ferrite core.
    The cap may be a low ESR type if the switching frequency is high, they are often dark green but not always and should have Low ESR printed on them.
    Other possible fault could be the rectifying diode, often duo diodes(have three legs) maybe one path is open circuit.
    An image of the circuit board would help.

    However if it is not always starting up there could be a problem with the motor drive circuit. This is often 3 phase and maybe one phase is dead. Having to flick it to start it up is typical for that.
    Last edited by nomeat; 22-04-19 at 12:29 AM.
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    Thanks everyone for your suggestions. I'm going to remove the pump from the loop today and see what I can come up with. This is obviously more complex than replacing a single cap.

    - just connecting to a battery won't work. The impeller is electromagnetically-driven, under the control of two PCBs. I believe there's a rotating magnetic field to drive the impeller.

    I've verified that there's 12 volts present on the supply wires when the thermostat switches on. I'll try to get photos of the control circuitry, as suggested above.

    This is what's printed on the outside of the original cap:

    SamWha (logo) WB
    63V
    1000uF
    105oC (M)

    and around the back

    E6A
    PET

    The stamp on the top is an 'X' pattern, not 'K'
    It's brown, with silver lettering and a broad silver stripe and minus sign down one side indicating polarity

    The replacement has the following:

    Suntan
    TS14
    -40+105oC

    and on the other side:
    63V
    1000uF

    It's black, with gold lettering and a broad gold stripe down one side indicating polarity, although it uses a repeating 0> (zero greater-than) to indicate polarity.

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    Isn't it a DC motor though? so it doesn't need a starting cap. As Hinekadon says, check that the motor can spin freely by hand, then apply 12VDC direct to the motor, to see if it works properly.

    Edit: OK, I just Googled it, it's a Brushless DC motor (ECM) so the cap is somewhere in the control circuit
    Last edited by bob_m_54; 22-04-19 at 10:07 AM.

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    You can't bypass the control circuitry and apply DC directly to the coils of a 3 phase motor. Brushless DC means we have a control circuit somewhere that converts the DC to a rotating AC field.

    If you can leave it on for 10-20 minutes and the cap doesn't get very warm then the cap you are using should be fine. This is just for filtering the supply voltage not creating the AC.

    Try to measure the wires that go directly to the motor's coil, there should be three and measure their resistance against each other, which should all 3 be the same. It might be very low, less than 1Ω.
    If you can turn the impeller slowly while it is switched on, try to feel if there are 3 physical resistance points per revolution that are equally strong.
    Last edited by nomeat; 22-04-19 at 10:47 AM.
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    i dont understand why you thought the cap was shit in the first place ??? was it just a guess?? so you dont know if the motor is any good or not ? I wouldnt have told you to connect it to a battery if it wasnt a dc motor the magnetic coupling has nothing to do with the with it ???
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    Default Pump update

    OK, pump's out. When I connect 12V directly to the red (+ve) and black (-ve) leads, the impeller is showing the same characteristics as before. It twitches, and only spins when I give a helping flick, then it continues to run by itself as long as the power is connected. Repeating this a few times, it even managed to start itself once or twice - probably because there was no load, i.e. no water to have to push around. I can't run it for very long this way - the installation sheet says to only run it like this for a few seconds to test it. As far as I can tell, while grabbing the impeller and turning it manually with the power applied, all points of resistance are equivalent - but that's a very uneducated assessment. What's interesting is that there's a permanent magnet in there somewhere - the impeller takes a bit of effort to remove (for something that just sits on a ceramic bead), and when replacing it there's a definite magnetic attraction.

    A couple of things I noticed that may or may not be clear from the photo - the first PCB, where the power leads connect, and where the capacitor is soldered, is about the size of a postage stamp, has a small chip on one side and a wound core on the other. It's kind of a daughterboard to the main PCB, mounted perpendicular. There is browning of the epoxy/potting at various points/junctions. Presumably long-term effects of running with water temps between 70-90 degrees? I never set the thermostat higher than 90 degrees.

    There are six pairs of soldered connectors around the periphery of the main board - you can see two pairs clearly at 10 o'clock and just past 12 o'clock, and two pairs obscured at 2 o'clock and 4 o'clock. Some of them are clearly numbered - 1 2 3 4 -but some of the numbers are obscured beneath brown epoxy or potting. Is it possible there are two 3-phase circuits?

    Another thing I've now observed is that when the thermostat switches on, there's 12V present at the junction. When the thermostat switches off, the multimeter shows *negative* 4.7 volts at the same junction. Is this normal?

    For what it's worth, I though it was the cap because it showed the same symptoms as a desktop fan that I recently fixed - weak or no start, but it would run if started by hand. I replaced the starting cap and it worked fine. I suppose it was premature to assume that the same symptoms = same cause. I can always put the original cap back in.

    Photo of the inside of the pump motor and control PCBs:

    Pump photo link not working - I'll try another way.

    Try this:
    Last edited by dwywit; 22-04-19 at 12:59 PM. Reason: add photo

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    Brushless DC motors need more power to start than to run. Usually caps are used with a starting circuit to release the starting pulse. If you cap did not charge it will have no power to release and give that kick to the motor.
    Look for components that will be connected to your capacitor.
    What you can do is measure how your capacitor is charging by measuring voltage across it from when you power on your pump.

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    this is a self comutating dc motor they are also used on hrv fans etc for ventilation if its showing the same symtoms as with out the controller then it time to bite the bullet and buy a new motor as you have no show of fixing it at all but a lot of fun to play with sorry for the bad news cheers don
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    Thanks everyone for your thoughts - I think it's "new pump" time. Still, it was worth some time and effort to explore it. Cheers.

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    Quote Originally Posted by dwywit View Post

    There are six pairs of soldered connectors around the periphery of the main board - you can see two pairs clearly at 10 o'clock and just past 12 o'clock, and two pairs obscured at 2 o'clock and 4 o'clock. Some of them are clearly numbered - 1 2 3 4 -but some of the numbers are obscured beneath brown epoxy or potting. Is it possible there are two 3-phase circuits?

    You could measure the resistance of each of the 3 pairs. They could be the coils of the motor but I don't see any power transistors, maybe they are underneath.
    Your symptom is very typical of a 3 phase motor that gets power only on two coils.

    That little board is definitely the buck regulator and the cap provides a smooth DC. It has nothing to do with a starter cap. The buck converter have to deliver the start up power and this can not be compared to a 50Hz power transformer and rectifier where a large cap can buffer a bit extra maybe for half a second but you ruled out a fault there by connecting a 12V source directly to the controller board.
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