Powering a 12V 6A DC Motor

[Yardstick]

My dad posed a question about powering a 12V, 6A DC motor for a pump with a switching power supply and I'm coming up empty in searches.  Are there any limitations or other problems doing something like that?  I assume this is a brushed motor.  Could the frequency of switching cause some strange effects with the frequency of the brush/stator interaction?  Is there a better way to power a DC motor from a household (120V) supply?

[IanB]

This is an example where you could probably re-purpose a computer power supply, especially if you can salvage one without having to buy it.

I have done this for a 12 V air compressor for car tires. It is intended to run off a car battery, but it works fine running from a PC power supply that I have adapted for the purpose. I use this as a general purpose utility 12 V power supply. There is no problem at all running a typical 12 V DC motor from such a supply.

[Ian.M]

That depends on the specifications of the PSU.

6A will be the nominal operating current at rated load i.e. full speed pumping the fluid its designed for against the specified head (back pressure), however, for a brushed motor, the startup current (initially the locked rotor stall current) can be an order of magnitude higher and if mains power is initially applied with the pump connected to the PSU, and the PSU has foldback current limiting, the over-current protection may trip and the pump fail to start.   Its also possible to trip the protection switching the 12V DC circuit to the pump, but is less likely as the reservoir capacitors in the PSU will be fully charged and will take the initial surge for a ms or two.

To determine if this is likely to be the case, you need to know the DC resistance of the pump, which lets you calculate the stall current, then consult a detailed datasheet for the PSU.   If the PSU has a constant current mode when overloaded, and has enough margin on its output current rating, it will be able to spin up the pump without tripping.   I would expect a 10A rated CC/CV PSU, to be satisfactory.   However a PC PSU will need a much higher current rating on its 12V output to avoid tripping.

If the pump has a brushless motor, it is likely to have a soft start mode. Consult its datasheet for the peak current during startup.

If you don't have data for a brushless motor pump, then you'll need to power the pump off a low impedance source like a large Lead Acid battery, with a current shunt in the low side, and capture the startup current waveform with a DSO.  You can do the same with a brushed motor, but its seldom necessary as you can directly measure its DC resistance.

The other consideration is what are the consequences of the pump supply failing?  E.g. if its a sump pump, or a circulation pump aerating a fishpond containing valuable fish, you may want to power it off a deep cycle Lead Acid battery maintained by a float charger that can supply the average operating current for the pump (allowing for its usual duty cycle), with enough Ampere hours capacity to keep it operating while you arrange an alternative supply or alternative pump.

[Yardstick]

Thank you both very much for the quick replies.  I'll see what I can find out about the pump motor.  We don't have the equipment to check the startup current waveform and I have some doubts about finding a datasheet.  Ultimately this is a low penalty for failure application - pumping homemade wine from one container to another.  Maybe that's a high penalty for failure, depending on your perspective.   

[Ian.M]

Its going to be a PITA sterilising the pump before use and cleaning it properly afterwards. Product can easily be trapped by the shaft seal and act as a resevoir of bacterial contamination when its next used. Consider using a self-priming food-safe syphon instead, preferably one with a stainless steel or plastic coated shaker valve so it can withstand sterilisation with Sodium metabisulfite or propritary sterilising solutions.

[Yardstick]

He was using a syphon.  Unfortunately with a syphon you need gravity to assist.  At 70 years old, it's becoming a bit too much for him to lift 5 gallons of wine in a glass container once it is all transferred. 

[Ian.M]

Fair enough, but as I said sterilising the pump is going to be a PITA.  To have a reasonable chance of being effective you'll need to first flush with tap water then recirculate a detergent solution long enough to remove surface contamination, then flush and recirculate the sterilising solution, then if the instructions call for rinsing after sterilisation, flush with boiled water that has been cooled under an airlock (or tapwater if you are only cleaning it for storage).

When putting it into use, you can go straight to the sterilising solution if its been cleaned as described before storage.

You may be better off setting up a vacuum pump system for vacuum assisted syphoning.  It will need a closely fitting two hole bung for the receiving vessel, with a rigid pipe through each hole he can connect the syphon tubing to and the vacuum hose to, and the vacuum pump will need to be protected by a liquid trap with either a float switch that cuts the pump off and operates a valve to vent the vacuum, or a float operated valve to close its outlet.  Beware of excess vacuum possibly imploding containers.  Its advisable to limit the maximum vacuum the system can generate to the equivalent lift of no more than twice the height of the largest container used, e.g. by an adjustable vacuum operated switch.

If you've got a compressor, consider using compressed air to operate a vacuum ejector rather than getting a vacuum pump.  To limit the ejector's vacuum, use a T to a lift pipe before the liquid trap, cable tied to a vertical stick to the desired limiting height with the bottom end in a jar of water.  The jar should be several times the lift pipe volume, but much smaller than the liquid trap volume.

[Yardstick]

I like that idea a lot!  I'll make that suggestion to him.  I know he is familiar with sterilization procedures, but he may not have considered how challenging it will be to sterilize the pump. 

[Brumby]

Alternate approach.

Use your motor to make a small lift - to raise the 5 gallons to a convenient height.

[Gyro]

A peristaltic pump would be perfect from the sterilization point of view, but I suspect the flow rate would be too slow for his needs.