Adding a flow switch to an electric pump...

[garmcqui]

Hi all,

I collect rainwater in a couple of IBC containers and use it for my micro irrigation system. Water is pumped through the system by a Karcher GP40 3.5 Bar pump, rated at 650W.

At present I have to switch it on manually, however am considering the best way to have it automatically kick in when a tap is opened and water starts flowing.

I was considering adding a flow switch of these into the pipework from the IBC containers:

https://www.ebay.co.uk/itm/AC-220V-...netic-Water-Flow-Switch-w-Sensor/303310410726

Specs:
Material:ABS/Plastic
Color:Black
No standby power requirement
Simple installation and adjustment
Self-Clean Function
Screw diameter: 22mm
Contact Form:A
Max Contact Rating:10W
Max Switching Voltage:220V
Min Breakdown Voltage:200 VDC
Max switching current:3.0 A

As you can see the max switching current is 3.0A (I calculate the 650W pump's current to be 2.82A at 230V, or 2.95A at 220V). However it also says the Max contact rating is 10W?

I am wondering if this would be suitable to use, or whether I would need to add a relay to be safe?

Thanks,

G

 

[bernardgreen]

I am wondering if this would be suitable to use, or whether I would need to add a relay to be safe?

Adding a relay would be sensible..

I would go further and say that putting mains voltage onto that device would be unwise.

An ELV ( such as 12 volt ) power supply with its output switched by the flow sensor to energise the relay coil would avoid having mains voltage in the sensor.

 

[garmcqui]

Adding a relay would be sensible..

I would go further and say that putting mains voltage onto that device would be unwise.

An ELV ( such as 12 volt ) power supply with its output switched by the flow sensor to energise the relay coil would avoid having mains voltage in the sensor.

That makes sense, and I think I was leaning towards a relay anyway. Thanks.

Next question - would you be able to suggest a suitable relay please? I see lots talking about Solid State relays, but are they suitable for inductive loads such as a motor? I was considering something like this: https://uk.rs-online.com/web/p/solid-state-relays/1025544

 

[bernardgreen]

My method would be :-

Finder_DIN_base

• bernardgreen
• 8 Nov 2015

Relay

RS Stock No.708-3131
Mfr. Part No.40.31.8.012.0000
Brand Finder

Socket

RS Stock No. 400-4129
Mfr. Part No. 95.03SPA
Brand Finder

Supply

CM TSBT2
1 Amp 8V Door Bell Transformer
https://www.tlc-direct.co.uk/Main_I...imes_Index/Door_Bells_and_Pushes_1/index.html

mounted on the DIN rail in a small

WYLEX ESE2 IP40 2-MODULE UNPOPULATED DIN ENCLOSURE

 

[garmcqui]

Thanks Bernard.

Can I ask why you would use a bell supply, rather than a simple 12v plug-in power supply?

 

[SUNRAY]

At 650W it's over the 0.37KW limit and requires protection. These have been picked at random:
https://www.cef.co.uk/catalogue/pro...a-100ka-circuit-breaker-thermal-magnetic-415v
https://www.cef.co.uk/catalogue/products/283554-6a-contactor-3-pole-3-no-contacts-24v-coil-ac3-ac5
And a small 24V transformer.
I know it seems expensive compared to a small relay but for long trouble free use...
I hope it's not too obvius I'm from a controls background.

 

[bernardgreen]

Can I ask why you would use a bell supply, rather than a simple 12v plug-in power supply?

Because the bell supply is designed to fit on the DIN rail in the enclosure, it is a wound transformer and not a Switch Mode Power Supply so inherently less likely to fail.

The mains to the transformer and the mains via relay contacts to the pump can all be wired up safely inside the enclosure.

 

[SUNRAY]

Hang on a moment, this is a flow switch to switch on a pump. Where is the flow before the pump starts?
Would it not be better to fit a non return valve after the pump and a pressure switch after that? Possible with a small pressure vessel

That way the pump will pressurise the pipework when the tap is turned off and power down then restart when the tap is opened, you may need a flow switch too depending on pressure limits.

 

[bernardgreen]

I agree with SunRay, the additional cost of good motor protection is worth paying.

 

[JohnD]

I have a transformer similar to that for an entryphone, in a 4-way enclosure since the screws are accessible otherwise. A very simple and intuitive device.

it was put in when the house was built so is probably around 30 years old. I wouldn't expect a PSU to last that long.

it is supplied from a SFCU with 3A fuse.

 

[SUNRAY]

I agree with SunRay, the additional cost of good motor protection is worth paying.

Is it not required in the regs?

 

[bernardgreen]

Where is the flow before the pump starts.

Good point, with the water butts higher than the tap a centrifugal pump would allow some flow of water through the pump when the tap opened. The Karcher GP40 3.5 appears to be a positive displacement pump that can suck water up and that will not let water past.

Is it not required in the regs?

Not sure, if the pump has internal overload / thermal protection it might be acceptable under the regs.

 

[SUNRAY]

Good point, with the water butts higher than the tap a centrifugal pump would allow some flow of water through the pump when the tap opened. The Karcher GP40 3.5 appears to be a positive displacement pump that can suck water up and that will not let water past.

Yep thems is my thoughts too

Not sure, if the pump has internal overload / thermal protection it might be acceptable under the regs.

Very possible, needs checking.

 

[garmcqui]

Thanks all, some decent discussion here

Regarding flow before the pump starts, there is a half decent gravity-induced flow through the pump before I switch it on...

In terms of protection for the pump, using a contactor etc, I’m inclined to think it’s overkill. I picked up the pump on eBay for £45! happy to be convinced!

 

[bernardgreen]

there is a half decent gravity-induced flow through the pump before I switch it on.

I picked up the pump on eBay for £45!

I hope I am wrong but that suggests the pump might not be in perfect working order.