I have a water pump motor (single phase induction motor) that used to work fine. Recently it will not start running when I close the switch but it will hum for sometime then it will start to run but very noisily. After some experiments we found that it will only run when manually turned using a screw, for example. It was water which was pushing the motor to start after sometime.

I searched on the web and most people suggested this to be a fault in the starting winding or capacitor so I opened the box of the capacitor and disconnected it and it looked fine. I isolated the wires connected to the capacitor and tried to start the motor and it will not start even after being manually turned. I then tested the capacitor with home AC supply and a test screwdriver and it was conducting AC current. I don't have a multimeter right now so I couldn't test it like tutorials on the internet recommend.

I couldn't open the motor case further to see the windings state but I plan to send the motor to an electrician to diagnose the problem.

Can I conclude from this that the main winding is the faulty part because the motor will not run (not start) without the capacitor?

  • \$\begingroup\$ It needs a capacitor to run and start. Let the electrician do the diagnosis because trying to get a diagnosis on line without multimeter numbers is not going to be anything other than guesses. \$\endgroup\$
    – Andy aka
    Commented Mar 3, 2023 at 16:17
  • \$\begingroup\$ Some motors need capacitors to start and some also need them to keep going. \$\endgroup\$ Commented Mar 3, 2023 at 16:56
  • \$\begingroup\$ @user253751 can you mention that type of IM than needs a capacitor to keep running after initial start? \$\endgroup\$
    – dev65
    Commented Mar 4, 2023 at 15:53
  • \$\begingroup\$ I don't think it's a type, I think it depends on the motor design. I would guess that motors with run capacitors are probably harder to stall. Well, the troubleshooting steps should be no different for either type. \$\endgroup\$ Commented Mar 6, 2023 at 11:35

1 Answer 1


It was the capacitor which was faulty and prevented the motor from auto starting. The capacitor AC conductivity test I done was wrong because the capacitor failed short not open, so it was conducting both AC and DC and not charging so not working. Why didn't the motor run with the capacitor removed? I don't yet know but I guess that the motor circuit connections are made so that the main windings terminals will be open circuited when the capacitor terminals are removed from their place to prevent the motor from long startup times which may cause high long lasting high starting current leading to windings burning!

  • 1
    \$\begingroup\$ "Why didn't the motor run with the capacitor removed?" That's the expected behaviour for a single phase IM - no capacitor = no rotation. "I then tested the capacitor with home ac supply and a test screwdriver" I'm going to try very hard to forget reading that. If you haven't got the proper test equipment you're limited to fault-finding by substitution. You seem to have achieved that by changing the capacitor. \$\endgroup\$
    – Graham Nye
    Commented Mar 7, 2023 at 15:37
  • \$\begingroup\$ @GrahamNye The motor should run without capacitor if given initial start torque and this is why it was running after a while when the capacitor was fault. seems that when the capacitor terminals are removed from the panel the main windings are open then. I saw on the internet people saying that they run IM without capacitor but after manually turning it \$\endgroup\$
    – dev65
    Commented Mar 7, 2023 at 16:04
  • \$\begingroup\$ "The motor should run without capacitor if given initial start torque" That's true. It's more precise to say that a single phase IM won't self-start without a capacitor. However, as I've was answering the specific query I quoted it seems you had no start torque. \$\endgroup\$
    – Graham Nye
    Commented Mar 7, 2023 at 16:16
  • \$\begingroup\$ @GrahamNye I had a starting torque! The motor was turned with a screwdriver! \$\endgroup\$
    – dev65
    Commented Mar 7, 2023 at 16:30
  • \$\begingroup\$ You asked "Why didn't the motor run with the capacitor removed?" and I answered that. I quoted the question so it was clear what I was answering. If you turn it with a screwdriver it may run, possibly roughly. Ask a different question and get a different answer. \$\endgroup\$
    – Graham Nye
    Commented Mar 7, 2023 at 16:55

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