Sunday, May 20, 2012

Replacing a Motor Run Capacitor

Lately I've had a bit of trouble with a pump connected to our rain water tanks.  Not turning on, stalling, just generally not working.  Catchup on my previous attempts to fix the pump in my last post to bring yourself up to speed.  The pump in question is an Onga JM100 pump.  The specs are shown below.

Pump Specifications

Warning - This article describes equipment and circuits that operate at high voltages.  Don't attempt to repair any high voltage circuits if you're not trained to safely work with electricity.  You may be seriously injured or even killed.  For further information read the blogs Terms Of Use.

Well it turns out that I was wrong about the cooling fan cowling.  It wasn't too tight and causing the motor to stall.  How do I know?  I took it off completely and let the pump run, but after cycling a couple of times it stopped working again.  All I had left to check was the pressure switch, the motor capacitor, and the motor itself.  If it was the motor I didn't have a hope. So hoping for something easy, I decided to take a look at the pressure switch and check if the contacts were dirty. I thought the humming noise I was hearing may be the contacts chattering or the motor humming because it couldn't get enough current because of high contact resistance.

Pressure switch

The pressure switch is a mechanical switch is by a company called Square D that is now owned by Schneider Electric.  The pressure of the water controls two sets of contacts that break the neutral and active lines.

Pressure switch mechanism

In the above photo, just above the screw terminals where the wires are terminated, you can see the set of contacts that break the neutral line.  Sometimes after a long period of operation the contacts can become dirty and  increase the resistance of the switch, but after listening closely to the switch and testing it with a multi meter it was clear that the switch was working fine.

The only thing left to check that I could actually fix was the motor run capacitor.  It's housed in a box that's mounted on the top of the motor.  In a single phase induction motor a second winding that is 90 degrees (or as close as possible) out of phase with the main winding is needed to create a rotating magnetic field.  The capacitor in series with the start winding is used to produce this phase difference

Motor run capacitor housing

The capacitor is connected via standard spade connectors, so it's easy to remove, but before doing so I labelled all the wiring to make sure I knew how to reconnect the windings.

Removed capacitor

As you can see from the markings on the capacitor above it's meant to be 10 uF.  So I pulled out my multimeter to test if it really was.

Removed capacitor measurement

I know I don't have the best multimeter in the world, but come on, 2.1 uF?  To make sure, I double checked the multimeter against a know capacitor and the measurement agreed to within 5 percent.  Just to be extra sure, I set up an RC circuit, fed it with a low frequency square wave, and measured the time constant with a scope, and once again I got 2 uF.  So it looked like this capacitor was the culprit.  That was great, after all, this was the first time I could point at something that I knew was definitely wrong.  Two days later I had new capacitor in my hands from RS components.  Not exactly the same part, but its specs were equal or better.  Like the old one it's a four terminal device with two terminals connected to either side of the capacitor which allows you to jumper connections off it.

New capacitor

New capacitor measurement

Before installing the new capacitor I measured its value to make sure it agreed with the marked value of 10 uF.

New capacitor installed

Although I marked the leads on the capacitor, it doesn't hurt to double check the connections.  By measuring the resistance between the leads of the motor you can tell how the motor is wound.

Wiring diagram

On this motor if you measure the resistance between the black and blue wires you measure the resistance of the main and start windings in series.  If you measure between the brown and black wires you get the resistance of the start winding, and if you measure between the brown and blue terminals you get the resistance of the main winding.  The measurement with the lowest resistance is typically going to be the main winding.

Confident that I had reconnected all the wiring correctly I replaced the covers, turned the pump back on, primed it and got the outlet line up to pressure.  I then turned the tap on and cycled the pump about 10 times.  Each time the pressure switch cut in and the pump started.  What was immediately noticeable was the increased flow rate of the pump and a reduction in the amount of noise it produced.

The reduction in capacitance causes a drop of current in the start winding, which reduces the starting torque at certain rotor positions.  This also has a side effect of reducing the power output of the motor, and creates a fluctuating rotating magnetic field within the motor which causes increased operating noise.

So far the pump has been operating correctly for the past two days, which I think is a good indication that the problem is fixed.  I learned quite a lot during the repair process, and as usual the last thing I checked was the actual problem, but as with any problem worth solving it came down to the electronics.


  1. The step I missed on my pump was marking the original terminal positions. So your photo and explanation have helped a lot. Cheers , Howard, Brisbane.

  2. Thanks for this drawing. My Onga LT1100 pool pump cap had a melt-down and I had to replace it. After clearing out the debris, I just had 3 wires coming out of the pump assembly!. Managed to work out how to connect it referring the the drawing and to the resistances (values different as it is a larger pump, but still proportionally the same. Thanks !

    1. Great to hear I could help. It seems to be a common problem. I've replaced 3 now.

  3. Thanks Grant. That solved my problem with my JSP-120. I had 2 units with the same issue. The last one that failed was only a little over 3 years old and I kept it for spare parts. So I bought a second capacitor and now I have a backup pump.

    BTW later models of the JSP-120 now have a wiring diagramme for the capacitor on the underside of the cover, which is helpful.

    1. Glad I could help. Good to know about the wiring diagram too. Thanks.


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