How to supply a draw of > 13 amps on a socket - possible?

[digdilem]

We have a pressure washer which although sold for connection to a household supply has a motor of (I think) about 3300 watts. According to my maths, this works out at just under 14 amps.

We're using a fused, wired RCD on the lead with a 13 amp fuse.

It works okay, but every now and then the load pushes up and the fuse blows as you'd expect. Replace the fuse, you're ok for a bit longer. Sometimes it'll run all day on a single fuse, sometimes you're swapping every half an hour. (My guess is the voltage fluctuates a bit on the supply enough to make the difference, or some fuses take more of a load than others?)

Given that returning it isn't an option - nor is replacing any sockets (too big an area), can any of you clever chaps think of a /safe/ way around this? It's used by employees so slipping a nail into the fuseway just ain't going to play.

Scratched my head and just can't think of one.

 

[Spark123]

It isn't suitable to be connected to a normal 13A plugtop, it must be pulling a fair bit over 13A to blow the fuse though.
You are going into the realms of blue ceeform sockets and dedicated supplies, or buy a pressure washer suitable for the exsting supply I'm afraid.

 

[JohnD]

Blowing a 13A fuse takes quite a lot more than 13A, so see if you can have it PAT tested in case there is e.g. a water leak causing an intermittent short.

Otherwise, if returning it isn't an option, and providing a new socket isn't an option, the only options you have left yourself are (a) throw it in the skip (b) sell it secondhand and buy one with a 13A max load.

 

[ericmark]

A 13A fuse will deliver well over 13A for quite a time. To blow the fuse means it is drawing not just 13A but more like 20A so likely even with a B16 MCB it would open the device.

Motors tend to have a high inrush and where the inrush needs controlling then either a soft start or inverter is used. However these are normally fitted from day one not fitted as an after thought.

However with any motor it is the load on the motor which determines the run current. There are a number of things which could cause a problem. The first is starting up under load. There could be a pressure switch which stops starting under load which has failed and this could be why fuses are blowing. Or some one may have adjusted the pressure to higher than standard which could also cause the problem.

So I would get the machine serviced so that all pressures are returned to standard and any faulty pressure switches are replaced.

I had the same sort of problem with a compressor. It would run from start OK but as day wore on it would blow fuses. A clamp on meter showed it was drawing the correct current in star and delta and as electricians we were scratching our heads as to why the fuses should blow. In pure desperation I sat and watched the compressor and while stopped I noticed the drive wheel moving bit by bit until it was bottom dead centre of stroke. Once this had happened next start the fuses would blow. Turned out the non return valve in the cylinder head was leaking as a result it filled the cylinder with air so that the air operated de-loading valve could not open. This in turn caused the overload which caused the fuses to blow.

The story shows that fuses blowing is not always an electrical problem and in fact with motors quite often the fault is mechanical so it will be worth having mechanical side checked. I may be something quite small.

 

[ericmark]

I found the curve for the 13A BS 1362 fuse.

Untitled

• ericmark
• 19 Mar 2011
• 1

As can be seen inrush around 4 times rating and needs around 1.5 times rating to blow. There is a variation in fuses but for 30 minutes it should take between 1.6 to 1.9 times the rating to blow. So I was not far out when I said 20A it is in fact 20.8 for 30 mins.
So around 4.7 kW so if everything is correct 3.3 kW should not blow the fuse once running unless used for more than 1/2 an hour.

 

[roberta1000]

Just a quick comment that the power factor isn't to be ignored even on a lightly loaded motor so the current could be quite a bit higher than the rated power might suggest. Only sure way is to measure it.

 

[digdilem]

Thanks guys, you've confirmed what I've thought.

We actually have two of these units, both about three years old, and they've behaved in the same way from new - so I don't think they do have a fault as such. They are pat tested every six months.

The piston ending up at TDC is an interesting thought, and would explain that little bit of extra draw required to start at seemingly random times (they always fail at start, never once running)

One last question - is there any modification that can be made to reduce the power used? Excuse my electronic ignorance - but a "dimmer switch" on the incoming supply, or those power saver plug adapter things that reduce voltage or current? Might that help?

 

[ericmark]

It would depend on the start method already used. There are two basic types of motor. Those used in an electric drill or vacuum cleaner would work with DC as well as AC and as with a drill a device like a dimmer switch as found in many drill triggers can control the power and speed of the motor.

However most AC motors use the frequency of the supply to control the speed of the motor. With these motors to control the power used by the motor needs a lot of electronics. With the three phase motor the boxes to do this job are really available there are two basic types. The soft start which only controls the start current and the inverter which also controls speed. With the inverter a three phase motor can be used on a single phase supply and controlling the motor speed and ramp up and down time can be done with ease.

However likely you have a single phase motor. This is more of a problem. The motor does not like starting and to get it to start a second winding with a centrifugal switch and or a capacitor is used to give it that start kick. Too much of a kick and the fuse blows and too little and it stalls and again the fuse blows. So the balance is critical and anything which disturbs that balance is likely to make it worst rather than better.

About the only way to assist the start is with an inverter which turns the power to DC charges up capacitors then uses the capacitors to supply the electronics which turn the power back into AC to feed the motor. I would think the cost of that sort of method would be more than the units are worth so unless it is a very expensive unit if the motors have capacitors bolted onto their side then likely there is nothing you can do.

So if the units are worth less than £1000 each then likely nothing can be done economically. If they are very expensive units then three phase motors and inverters could replace the single phase motor and because of the inverter the start can be better controlled.

That is assuming it uses a capacitor start motor. I have renewed the motors with laths with three phase and used inverters but the lath was a screw cutting one and worth a lot of money in first place. You could easy be looking at £1000 plus to convert.

So to take any further you need to say what type of motor they have and what starting method is already used.

 

[digdilem]

Thanks for the detailed reply.

They cost about 500ukp each, so well beyond the hobby range, but obviously short of economic changing. (Wouldn't replacing the motor with a smaller one work out cheaper than this anyway, or is that too simplistic a view?)

This is clearly beyond my electronic experience, so I'm going to leave the things alone and keep buying fuses - but I'll also contact the manufacturer and see what they say too, got to be worth a try.

Thanks all!

 

[xr4x4]

Why can't you change it to a 16 supply?

 

[MrTinker]

Let me guess it’s a Karcher HD series?

A place I used to work for, had one that went through 1 or 2 fuses a month, when it threw its hand in, it was replaced with a newer version, and that one used to eat fuses, like you describe. The funny thing is I kept the old one repaired it and use it at home, and in the four or five years I’ve had it at home it hasn’t blown a single fuse, ok I don’t use it as much as it was but it dose get used quite a bit.

There are two things I can think of that may have caused this change.

1 Supply voltage is slightly lower at home, I don’t know if this is the case as I never measured the supply voltage in that workshop, but it could possibly explain it.

2 After I rebuilt it, the pressure switch has got a bit more of a delay on it, although it was not adjusted, so I can’t see how, but with these machines when you let go of the trigger on the lance, the pressure on the outlet side builds up and operates the switch shutting the motor off, when the pressure drops a bit the motor kicks in, if the pressure in the hose has not dropped sufficiently when the motor starts up, it will be fighting against the back pressure on the pump, causing the motor to be overloaded at start up, dramatically increasing the start up current.

 

[ericmark]

2 After I rebuilt it, the pressure switch has got a bit more of a delay on it, although it was not adjusted, so I can’t see how, but with these machines when you let go of the trigger on the lance, the pressure on the outlet side builds up and operates the switch shutting the motor off, when the pressure drops a bit the motor kicks in, if the pressure in the hose has not dropped sufficiently when the motor starts up, it will be fighting against the back pressure on the pump, causing the motor to be overloaded at start up, dramatically increasing the start up current.

As you say that would cause the fuse to blow. Maybe there is an adjustment and it can be set so it does not restart until zero pressure.

Also changing the lance so the water runs free until trigger is pulled so once started motor does not switch off.

I think these machines are designed for Europe where the domestic supply is 16A not 13A and this is it seems a problem with a few imported appliances for example ovens.

What we need is a 16A FCU so 16A outlets can be put on our ring main.
Clipsal with their 56 series did do a socket and overload in a modular manor but not sure since take over but

was the old way around the problem. Part No 56C3134ELGY is 13A version but think there is a 16A version also available.

 

[MrTinker]

Maybe there is an adjustment and it can be set so it does not restart until zero pressure.

On the one I’ve got the adjustment on the pressure switch doesn’t work like that, it would be an improvement if it did mind, the only adjustment is for the pressure at which the motor is stopped, which obviously needs to be above the normal working pressure, however I think that if it was set too high, not enough pressure would be vented when the trigger(valve) on the lance is opened, before the motor started up.
Also the one I’ve got and its replacement are both UK spec machines, the model Numbers are different to the European spec versions.

 

[digdilem]

Why can't you change it to a 16 supply?

Cost. It's intended to be used in a wide variety of locations, all of which are fitted with 13a sockets (the MK weatherseal ones) To add 16a sockets in enough places to allow it to be used would cost several thousand.

Not a Karcher - I forget the manufacturer. Both blue and red bodies.

 

[Spark123]

What we need is a 16A FCU so 16A outlets can be put on our ring main.

A ring final circuit should not have 16A appliances attached.
A dedicated 20A radial may serve a number of 16A ceeform outlets though.