| Author | Message | ||
| Gary Stadler (Boogiethecat) (68.7.217.217)
Rating: N/A |
Often times in a system without an accumulator of some sort, a pump like a sureflo will "pulse" on and off during use (annoying and not good for the pump switch) because it pumps faster than the water is used. A cheap and simple solution is to add a string of 10 amp (or preferably higher current rating) diodes in series with the pump motor... each diode you add drops .7 volts and slows the pump down a bit. For my bus I added four 20 amp diodes in series and that was just enough to slow the pump down sufficiently to stop the pulsing, 'cause now the pump can't "get ahead" of the usage. Any voltage rating of 50 volts or higher will do just fine for the diodes... Cheers Gary Stadler | ||
| woody48348 (66.82.48.141) Rating: N/A |
Just make sure you position the cathode side of each diode toward the pump, as this will forward bias the diode, and allow the voltage to pass. The problem with this idea, is you are now running your 12Volt pump at 10Volts, and drawing more current. The low voltage may shorten the life of your pump. Why not just purchase an accumulator tank? FWIW, Bob  | ||
| Jim Ashworth (Jimnh) (172.130.176.175)
Rating: N/A |
For those just building their bus, a variable speed pump, like the Aqua-Jet can eliminate the need for the accumulator tank and put out 5 gpm at pressures up to 60psi. Smoothest pump I have ever had... and it will keep a garden hose with a nozzle spraying at the maximum stream until the tank runs dry. Talk about a great shower, too!!! This may be the most ridiculous use of a conversion yet but, my wife has some flowers that are at the beginning of the driveway, way out of reach of any hose. I have used the bus with this pump to water them. That's how I found out it would keep up with the hose! Great pump. www.aquatec.com Jim | ||
| Gary Stadler (Boogiethecat) (68.7.217.217)
Rating: N/A |
Woody, just for the sake of arguement... your comments would perhaps apply to a starter motor yes, but not really to a water pump like this with a few diodes in series. For the record I did actual tests tonight and the results of my pump with a DVM: at 12.6 volts the pump draws 5.25 amps. With the diodes in series at 9.8 volts, not surprisingly, still 5.25 amps (because the load torque is constant). The motor is rated at 7 amps and even if it were running on 1 volt, as long as it stayed under 7 amps (which it would in this type of service) it would be quite OK. Dropping the voltage simply makes it run slower and it draws less wattage in doing so, which if anything is better for it. DC permanent magnet motors are very happy running from zero to their maximum rated voltage as long as you don't lock their rotors or present them with loads that are outside of their torque ratings. The way PM motors work is quite different from series wound motors such as starter motors which can be easily damaged by lower voltages. With a pump such as this, it's always running with very little load because the pressure switch only turns it on when the pressure drops. It will load down for a brief and insignificant moment as you turn off the water faucet and the pressure instantly builds up to where the pressure switch turns the motor off. But during continuous runs with a load such as a typical RV pump sees, running at decreased voltages will cause no current increase of any concern. Even if the motor did draw "more current" it's doubtful that the duty cycle encountered (what, 5-6 minutes per day maybe?) would affect a pump motor's life by any detectable amount. In answer to your last question, why don't I just buy an accumulator tank? Three reasons: (1) It's a lot of $ and installation hassle as compared to 4 diodes, and the diodes do this particular job perfectly well. (2) An accumulator tank takes a lot of space- by comparison the diodes do not. (3) This particular pump is solely for my 20 gallon drinking water tank hooked to a separate drinking faucet, independant of my main water system... an accumulator would be completely silly in this application. In fact I do have a 5 gallon accumulator on my main system and in that application it's the way to go! I was just presenting a nice alternative method to fix a common problem...surging intermittent pumps will definitely have a shorter lifetime than ones that run smoothly, and in many applications this diode trick is a very practical solution! Cheers Gary | ||
| j maxwell (66.42.92.12) Rating: N/A |
How about a flow restrictor, make it adjustable if u want high tech, into the suction side? | ||
| joe shelton (Littlewind) (67.241.228.93)
Rating: N/A |
Gary, Thanks for all that explenation. That realy helped me get a better understanding of the series vs PM motors. Now here is another question. I have a little accumulater (just a little bigger than a milk carten). some times my pump will start running and wont stop. I loosen the cap on top of my accumulater, some water and maybe a little air comes out, replace the cap and the pressure gos up and the pump stops. why is this happining? Joe 4106-2119 | ||
| Gary Stadler (Boogiethecat) (68.7.217.217)
Rating: N/A |
Joe, it may be that your pressure switch is set to stop the motor at just a slightly higher pressure than the pump actually puts out. A properly working accumulator has air above the water, usually with a rubber bladder as a separator. If you're getting water and air out the top of yours, I'd guess there's a problem with it (though not related to the pump not turning off) Consider this: If (this is my guess), your pressure switch is set slightly above the pump's maximum pressure ability, and you have some air in the accumulator, the air would tend to buffer out the pump pulses and then perhaps the pressure never gets high enough to turn the pump off. Bleed off the air and the accumulator now can't soften the pumps' pulses, so the peak presure of the pulses is now enough to kick the switch off. So my guess is that your problem might be caused by either the pump being old and not able to put out the pressure it used to, or the switch cut-off point adjustment perhaps needing to be set lower (if it's even adjustable!) Cheers Gary | ||
| JERRY (152.163.188.227)
Rating: N/A |
Sears sells a several sizes of "expansion" tanks for $35-$45. metal tanks with rubber bladder. A 2.5 gallon one works great in my bus! (repeat post from several months ago) | ||
| jmaxwell (66.42.92.4) Rating: N/A |
Joe; U have a leaking bladder, could be around the water inlet on the tank. Grainger also has a good selection | ||
| Peter (Sdibaja) (209.242.148.130)
Rating: N/A |
Try 6 feet of 2" PVC. Put cap on one end and make that end about a foot higher than the other. On the bottim end "T" it into the pressure system. All you need is a gollon or so of air, it makes an excelent "almost free" "expansion" tank. KISS! Peter | ||
| Ross Carlisle (Ross) (216.107.197.175)
Rating: N/A |
Jim.... FYI...I tried to buy an Aquatec pump. They would not sell to me direct. When I asked for a distributor, all they gave me were wholesalers who would also not sell to me. I tried one more time to go direct then gave up. I went with a high end sure-flo pump. Ross | ||
| Jim Ashworth (Jimnh) (172.165.136.40)
Rating: N/A |
There are distributors marketing them at FMCA shows. That is the easiest place to buy them. Usually at a price less than MSRP. I bought two directly from Aqua-tec as a converter (with company letterhead) but in the future, they wanted a credit application and open account status. That's fine, but I haven't needed any more of them. | ||
| FAST FRED (63.208.85.116) Rating: N/A |
Because they can "save" the instalation cost of the accumulator there are at least 3 RV pumps that vary their speeds. Sureflow makes one. The accumulator is only about $25 from the boat folks. It will need to be emptied every few months as the air in the cheap ones , with no diaphram gets absorbed. No air no Cushion ,no work. Any style needs to be DRAINED to properly winterize it , as the antifreez won't go in unless you do. Most times if the pump "runs away " you have lost the prime from low water, or a suction air leak. FAST FRED | ||
| Gary Stadler (Boogiethecat) (68.7.217.217)
Rating: N/A |
Well, I took a look at all the variable speed pumps and besides being mucho expensive, all they are basically doing is "automatically" adjusting the speed of the pump **down** to match your delivery rate. Um, how do they make the pump go slower? By using electronics to lower the voltage to the motor.... in the long run the only major difference between these and my silly diodes is that I do the adjustment "manually" and the diodes cost a LOT less... Cheers Gary | ||
| Ross Carlisle (Ross) (216.107.197.177)
Rating: N/A |
Actually...electronic motor speed control is usually done by pulse width modulation. basically, for a 12V motor you send 12V voltage pulses to the motor via a micro controller of some sort. The more pulses per second you send, the faster the motor runs. This method of speed control does not harm the motor because it is either off or on and running on its rated voltage. Amperage draw is also reduced over just using a voltage limiting device. I've used PIC and Motorola micro controllers for motor speed control. I'm sure the variable speed pumps use this method as it is the best way to control a motors speed electronically. By reading the output pressure at the pump, you could precisely control motor speed to match pressure output. Ross | ||
| Gary Stadler (Boogiethecat) (68.7.217.217)
Rating: N/A |
Well Ross, not really. What pulse width modulation does is to take advantage of the relatively high frequency of pulse width modulation vs the inductance of the motor, which basically filters out the pulses and effectively turns them into varying DC inside the motor. By varying the duty cycle of the power applied, the effective *power* delivered to the motor is exactly the same as far as the motor is concerned as it would be if you were varying the DC voltage. It has absolutely nothing to do with the current being "either off or on and running on its rated voltage". And "Amperage draw is also reduced over just using a voltage limiting device" isn't reallly true either- it has nothing to do with this. If you were to operate a DC motor on a pulse width controller at 50% duty cycle with a given load and a given supply voltage, say 12 volts, the current draw and power consumption of the motor would be exactly the same as if you ran that motor and load on a simple 6 volts. The main reason people design with pulse width modulation is that it means very little heat will be created in the controller, and it's usually a simpler circuit to implement. As my occupation before I retired, I created and owned a company that designed and manufactured OEM high power laser switching power supplies and high powered pulse width (and analog) DC motor controls over 25 years. While this doesn't mean I'm an expert, it does mean that I kinda know what I'm doing with my water pump.... Cheers Gary | ||
| John Clark (64.12.96.235)
Rating: N/A |
As to the original post, have you tried turning the adjustment screw on the end of the Shurflo pump? It will reduce the rapid pulse cycling I assume you are talking about. Don't turn it in too much though as you can damage the membrane. You may have to remove some labeling to find it. The use of a small accumulator is the best answer. Be sure to use a loop of flex hose (12 or so inches in diameter) on the inlet and outlet side of the pump to reduce vibration and noise. | ||
| Gary Stadler (Boogiethecat) (68.7.217.217)
Rating: N/A |
John, good suggestion, though I didn't try it and I still stand with my diodes. They worked immediately, perfectly, accumulatorlessly...(oops, is that a real word?) The diodes are just tooooooo simple Cheers Gary |