Motor cap failed again

[Simon47]

I have a pressure washer that I got as part of "a good deal", actually an exceedingly good deal as I later found out. I've had it a while, I guess probably at least 25 years, but it doesn't get used all that often. Hard to say, but I reckon I've probably not done 200 hours with it in that time - when I do use it, it tends to get used for a few hours at a time. But it's had the capacitor fail twice (I have a new one on the way).

The first time it just failed - carried on running and I recall noting a change in noise, but after stopping it it didn't start and just hummed. This time it failed rather more visibly ! In case it's not obvious, it's clearly overheated and melted plastic internally - there's a blob of grey plastic that's oozed out. Part of the "bog roll" construction has been pushed outwards a bit, pushing the potted seal out, and you can see the foil on the outer wrap of the section that's moved.

I've just replaced like-for-like (capacitance wise), but out of curiosity, does 40uF sound about right for this motor - it's capacitor run ? The plate doesn't give a speed.

This time while I had it open, I decided to draw out the circuit. I was surprised the first time I had it open to find what looked like a simple rotary switch and a small contactor in the box on front of the motor terminal box. But just looking at the wiring it seemed a little more complicated. So this is what I drew out :

I've assumed that the two terminals labelled O1 and O2 are a thermal switch embedded in the windings. The windings measured around 1.6R for U and 4R for Z.

But that bit with the NO contacts (13 & 14 on the contactor) and terminals 3 & 4 on the switch had me a little stumped (something along the lines of WTF is going on here went through my mind) until I got my multimeter out and had a closer look at the switch (rotary cam type). Then it made more sense. I'd assumed it was just a 2 pole on-off switch, but it isn't.

So that's a feature to prevent automatic start - either on power on or the thermal trip resetting. Terminals 3 & 4 only close while the switch is in-between the on and off positions, so closes momentarily while switching on to allow the contactor to close, after which it's help in by it's latching contacts. If power is lost or the trip operates, the contactor drops out but can't pull in again as 3&4 are open.
Like so many things, so simple and obvious once you've seen it, but it's not a configuration I'd come across before. Easy when you can build a cam switch for the job.

 

[marconi]

Overheating of the capacitor may be caused by:

1. Frequent on/off/on/off of the motor which causes the capacitor to rise in temperature;

2. A long extension lead (and perhaps a thin one too) which causes a high voltage drop during the starting period. The starting torque of the motor is reduced - since torque roughly proportional to motor terminal voltage squared - which means start-up acceleration is lower thus it takes longer to reach operating speed so capacitor starting current is higher for longer causing more capacitor heating and rise in internal temperature. This is made even worse because there are probably two 13 A fuses in series which also cause a volt drop during starting.

3. Paras 1 and 2 together.

The plastic casing of the capacitor has a low thermal conductivity so heat generated within is only slowly dissipated away to the surroundings with the consequence the temperature of the capacitor rises quickly until it fails as you have illustrated. I'd try to buy a metal cased capacitor.

Or something along these lines...

Cornell Dubilier and Ducati are makers of high quality capacitors.

https://www.mouser.co.uk/c/passive-components/capacitors/motor-start-capacitors-motor-run-capacitors/?m=Cornell Dubilier

Ducati Energia - Motor Run and Start AC Capacitors - https://www.ducatienergia.com/product.php?lang=en&id=8&cat=5&product=9

 

[brianmoooore]

If the circuit diagram is correct, the capacitor is permanently in circuit, so a motor start capacitor might not be adequately rated, if that's what's been tried.

 

[Simon47]

It was described as a motor run cap, and so is the replacement I've ordered.

 

[Marvo]

There's a pressure switch that stops the motor running when full pressure is reached and restarts the motor when the trigger is operated and the pressure drops again. If that switch is out of adjustment or calibration it might be starting the pump under too high head pressure resulting in very high start current and capacitor failure. Also so high pressure machines have an únloader' valve that releases the pressure at the motor before it restarts, again this ensures the pressure is below a certain threshhold for motor starting. Cap failure can equally be caused by mechanical issues as well as electrical ones.

 

[Simon47]

This isn't a start-stop type. The motor just runs (while switched on) and it has a flow sensitive unloader valve.
So with trigger valve closed, unloader valve opens and reduces pressure; pull the trigger, there's a small flow from the lower pressure, valve senses flow and closes to put full.pump output through the lance; release trigger, pump momentarily pumps through pressure relief valve, until tbe unloader senses lack of flow and opens.

My other pressure washer, which is as old as me, is a bit more powerful and has no unloader valve. You make sure you have a good hold of the lance and switch on ...