First off I'll start by saying that I have almost no experience of single phase induction motors but I do have a reasonable knowledge of electrical & electronic stuff.

My neighbour asked me to have a look at a two post lift he has, which was not running properly/at all. First off, I found a few connections which were loose and once corrected the motor tried to run - so it seems to have cured the intermittent problem. The motor is configured to run in either direction through a couple of contactors.

The real issue is that it does not seem to want to start correctly all of the time. Sometimes you can get it to run well, but most of the time it either does not move at all, or runs very slowly and makes that horrible growling noise of a stalled AC motor.

I have checked both capacitors with a DVM and they both measure within the value marked on the side of them. He tells me the motor was sent for a checkup a while back and came back with a clean bill of health, but annoyingly he couldn't find the paperwork that he got for it and the motor has no identification on the outside. At a guess it's a 3-4kW motor. I'm in the UK so we use 240V 50Hz.

There is a starting relay, Klixon 2CR3-285. The datasheet (https://www.muekra.de/downloadfile/download/aitfile/aitfile_id/671/) for this suggests that it does not pull in the contact until a fairly high current and will drop out at 26A min. I am wondering if it may be that this is rated too high and a lower value or alternative part should be used. I wonder if depending on the rotor position sometimes there is enough current to pull the relay in, but at other times there is not. I guess I could measure the approximate resistance of the winding and use ohms to guess the operating current, but all these calcs go out the window during inrush and I don't have an LCR meter.

I've had a Google and can't seem to easily find these relays in the UK. There is some kind of overload protection relay inside the controller which is set to just above 16A continuous, with about double that peak (I'm guessing).

I think the terminals in the box are just so that you have something to screw down to. I don't think they are connected to the motor as there are 4 other wires coming from the bottom of the terminal box, but I'm not certain of this...

Here are a few pictures, not very good I'm afraid I didnt think at the time..:

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Any help is greatly appreciated.

  • 1
    \$\begingroup\$ Added the images for you. Feel free to move them in line and/or name them if needed. \$\endgroup\$ – winny Jun 20 '18 at 14:56

Because your documentation is not detailed enough, all I can do is give you general information.
Most large power motors have two rotor windings. One is the "starting" and the other is the "running" winding. It appears that your motor's starting winding is not being "activated." Based on your description, the only thing that could be causing the problem is the starting relay.

  • \$\begingroup\$ Hi, Thanks for the advice. I'll try to get the relay and test it. If it turns out to work, could it be that the relay is not suitable for the motor? As in the motor needs one which picks up at a lower current? Is there a way I can estimate what the relay current should be by measuring the DC resistance of the winding? \$\endgroup\$ – jammy86 Jun 29 '18 at 15:57
  • \$\begingroup\$ Typically the starting current is 10X larger than the running current, but it should quickly be reduced to less than the running current (or even 0). Also, keep in mind that I am talking about AC (RMS not DC) current. I recommend bypassing the frw/rev switch by connecting the wires directly in one direction. The fewer connections the fewer possibilities for contact problems. For 3KW @240V, the running current should be 10 - 15 A (AC) \$\endgroup\$ – Guill Jul 2 '18 at 3:46
  • \$\begingroup\$ The calculated value is 12.5 A, so the pull in current of 26 A seems correct. You need not only to tighten the contacts, but also you need to clean them, before you tighten them. I need you to improve the schematic you drew. Put voltage in, grnd, neutral, motor winding and its connections, etc. \$\endgroup\$ – Guill Jul 2 '18 at 4:11
  • \$\begingroup\$ The OverLoad controller needs to be in the schematic. Measuring the DC resistance of the relay winding does not help. A lower pull in current would not solve the problem. Connect a 10A circuit breaker(CB) between the wte and red relay contacts, push it "on" and then apply power to the motor. As soon as it starts, turn of the CB. If it does not start, the problem is not the relay. If it starts OK, then the relay (or connections) are the cause. \$\endgroup\$ – Guill Jul 2 '18 at 5:23

Your single phase motors appear to be using a "potential relay" (the little box in the third photo down) to switch the capacitors, rather than a centrifugal switch inside of the motors. It could be that this relay is starting to fail, they don't last forever, and is intermittently not functioning correctly. It could also be that your capacitors are bad. Your second paragraph describes exactly what it would do if the starting capacitor system failed to function.

Either way I doubt it has anything to do with the Klixon or the OL relay, unless BECAUSE the starting capacitor system is intermittent, the motor stalls and eventually trips the OL relay or Klixon.

  • \$\begingroup\$ Yet the Klixon 2CR3-285 is a Current Relay pick-up at 31.5A \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 22 '18 at 23:11
  • \$\begingroup\$ I am wondering if the relay is oversized for the motor and one with alower current should be used. \$\endgroup\$ – jammy86 Jun 29 '18 at 15:58

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