How to limit DC water pump inrush current

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How to limit DC water pump inrush current

Postby tom rickard » Sat Jan 21, 2017 4:17 pm

I am having a problem getting my power supply (48V to 12V DC/DC Meanwell SD350) to start my DC water pump that I use for my evaporative airconditioner. The problem is with excess inrush current, I emailed the power supply people and they said they would need to modify the unit to allow me to run a capacitor to handle the inrush current as the standard unit would detect it as a short circuit and switch off.

Has anyone got other ideas as to how I can get the pump to run with this power supply?

I was originally using a smaller pump, which worked for a while but then failed. I like the larger pump as we can use it for a back-up pump for showers etc.

When I build my next shed I will mount a rainwater tank up high and use this to gravity feed the airconditioner, but who knows when i'll get around to that!
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Re: How to limit DC water pump inrush current

Postby karrak » Sat Jan 21, 2017 8:13 pm

Hi Tom, what about a small 12V battery on the output of the supply?

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Re: How to limit DC water pump inrush current

Postby tom rickard » Sun Jan 22, 2017 9:36 pm

Hi Karrak, thanks for your reply. I have a couple of little gel cells that I use for starter batteries on our quads. My concern with this was that there is no control of the charge current to the battery. With a fixed output voltage is this going to be an issue?

Cheers.
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Re: How to limit DC water pump inrush current

Postby davidg » Sun Jan 22, 2017 10:07 pm

tom rickard wrote:My concern with this was that there is no control of the charge current to the battery. With a fixed output voltage is this going to be an issue?

If your really concerned you could use a one or more high capacity electrolytic capacitors they really don't care and should offer the brief surge your after.
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Re: How to limit DC water pump inrush current

Postby karrak » Sun Jan 22, 2017 10:20 pm

tom rickard wrote:Hi Karrak, thanks for your reply. I have a couple of little gel cells that I use for starter batteries on our quads. My concern with this was that there is no control of the charge current to the battery. With a fixed output voltage is this going to be an issue?

Cheers.


There maybe a voltage adjustment on your supply. If you can increase the voltage to the battery float voltage that should work.

Regarding using capacitors, this might work, depending on what the power supply will put up with.

A couple of questions,
    Is the power supply on all the time and you just switch the load or do you switch the supply and load on at the same time?
    Do you know the running current and inrush current for the motor?
Simon
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Re: How to limit DC water pump inrush current

Postby karrak » Mon Jan 23, 2017 12:51 am

Something like this soft start motor controller https://www.motiondynamics.com.au/12v-48v-dc-speed-controller-50a-pcb-model.htmlmight also work.

Simon
Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
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Re: How to limit DC water pump inrush current

Postby tom rickard » Wed Jan 25, 2017 2:47 pm

Thanks guys, the power supply manufacturer said if I used a capacitor similar to a car audio set-up that the power supply would not function.

The output voltage isn't variable, it is specced at 12V.

I generally turn on the power supply only when I wish to use the water pump.

Running current of the pump is 7A @ 12V. The pump manufacturer said the start-up current could be as high as 50A depending on the pump situation. The way it is set-up feeding the evap aircon it pulses the pump for a couple of seconds every 30seconds or so.
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Re: How to limit DC water pump inrush current

Postby karrak » Wed Jan 25, 2017 5:54 pm

tom rickard wrote:The output voltage isn't variable, it is specced at 12V.



Which model of SD-350 power supply are you using? The ones mentioned in this data sheet http://www.meanwell.com/productPdf.aspx?i=61 are all adjustable?

Running current of the pump is 7A @ 12V. The pump manufacturer said the start-up current could be as high as 50A depending on the pump situation. The way it is set-up feeding the evap aircon it pulses the pump for a couple of seconds every 30seconds or so.


That is hard on the power supply and pump!

If the pump is only operating at such a low duty cycle, 10% (3 seconds every 30 seconds)? would it be possible to replace the pump with a smaller one that will run for a longer time, and preferably one that operates at 48 volts?

Does this pump the water out of the reservoir at the base of the cooler to the top of the pads on the sides of the unit?

Another option for limiting the inrush current might be an NTC Thermistor, https://www.ametherm.com/blog/inrush-current/dc-motor-inrush-current Only problem with this is that the thermistor needs to cool down before it will limit the current so working out which thermistor to use will be more complex.

Simon
Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger https://github.com/simat/BatteryMonitor
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Re: How to limit DC water pump inrush current

Postby tom rickard » Wed Jan 25, 2017 9:26 pm

This pump is the supply to the evap. airconditioner (not the internal pump that circulates water through the pads). It essentially pumps water until it is stopped by a float valve. The airconditioner constantly uses a small amount of water, so the pump cycles often but for a very short time.

I originally had a smaller pump, (which worked fine off the power supply). In the end the pressure cut-off switch gave up and it just stayed on all the time.. I had a larger pump that i had as a spare unit for my caravan, so I thought i'd use that. I'll try fitting a battery inline, otherwise i'll have to speed up my process of fitting a header tank to gravity feed the air conditioner sump.

I'll also check again to see if my unit output voltage is adjustable.

Thanks for your suggestions, much appreciated.
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Re: How to limit DC water pump inrush current

Postby davidg » Wed Jan 25, 2017 11:16 pm

tom rickard wrote:This pump is the supply to the evap. airconditioner .............. It essentially pumps water until it is stopped by a float valve.

Something that might be useful, pumps don't not like very short on, off cycling, ie high cycling rates.

It tends to kill the motor. Your idea of a header tank sounds good even small header tank should drastically reduce pump cycling could be very good.

Having the pump running for longer than, say 5 minutes on and say 10 minutes or longer off.

I'm sure your aware that there is always a high current start up for a motor if was a STD squirrel cage motor approx 7 to 9 times than the run current. Generally if the supply is unable to provide the full start current then there tends to be considerable voltage drop which drives up the current demand on start and so on. With a limited current power supply the old motor may have died due this, a battery may well resolve the issue, try and ensure the power supply can supply ideally the proper float voltage typically listed on the side of the battery if it's a little 12V seal lead acid battery you are considering using.
Hybrid & Off-grid Systems - You could store Sun to use later- I Do!
SELECTRONIC SPMC-482 7.5kW, 7.2kW's of Arrays

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1100Ah 48V Bank

Auto Backup Genset
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