Submersible pump: can I replace 3/4 hp motor with 1/2 hp?

[CountryBoy19]

Hear me out before answering.

Application: pump in a cistern (easy to replace), no more than 5 feet of head other than the 20/40 pressure switch/tank setup (minimal back-pressure on the pump).

I have a 3/4 hp pump I replaced when the motor got smoked by lightning. The tag is worn but I assume it is a "standard" 3/4 hp 7 gpm bc that is a very common pump.

I also have a 1/2 hp pump that was given to me. Motor is good but steel fitting rusted and broken off in plastic pump end. I tried to get it out unsuccessfully. Once again, tag is gone but I assume it was a 1/2 hp 10 gpm because once again, that seems to be the most common 1/2 hp size.

Both motors are Franklin, the pump end of the 3/4 is slightly longer (more stages?), and they physically connect together. Considering the low back pressure do you think I can get away with running the bigger pump on the smaller motor? I'm thinking it will probably be pumping less volume if my assumptions are true. I plan to monitor current to confirm that I'm not overloading it.

Bonus question, for a cistern is solid THWN wire ok? It's what I have on hand and I've already gone above budget on this project to get more reliable water to the cattle. I've read several threads on here and it seems the insulation of THWN isn't well suited for a well but in a buried poly tank I think it is worth a try. If it fails it's an easy replacement. Thoughts?

 

[Valveman]

You can put a larger motor on a smaller pump, but not a smaller motor on a larger pump. The 3/4 pump end will make the !/2HP motor draw 7 amps and it is only designed to handle 5 amps. But it will work until the motor burns up. I don't like solid wire. But the leads on the motor are probably long enough to get out of the water in a cistern. So, the kind of wire from there up is not that important.

It is a little overkill but there is a 1HP, 33 GPM pump on the Internet for just over a hundred bucks that makes a good cistern pump. Use one of your good check valves on the Hallmark pump though, because the ones that comes with are no good.

 

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[CountryBoy19]

You can put a larger motor on a smaller pump, but not a smaller motor on a larger pump. The 3/4 pump end will make the !/2HP motor draw 7 amps and it is only designed to handle 5 amps. But it will work until the motor burns up. I don't like solid wire. But the leads on the motor are probably long enough to get out of the water in a cistern. So, the kind of wire from there up is not that important.

It is a little overkill but there is a 1HP, 33 GPM pump on the Internet for just over a hundred bucks that makes a good cistern pump. Use one of your good check valves on the Hallmark pump though, because the ones that comes with are no good.

Doesn't reducing the pressure or volume reduce load on the motor? Pumps don't always run at the max nameplate current.

My 3/4 hp in my well, 200 feet down, only runs at 3.8-4.1 amps according to the readout on my CSV pump protector. I attribute it to me running well under the rated head (it's not pumping into a pressurized system so it only has the head of the pipe loss and the depth to water). As a matter of fact, the current goes down as the well pumps the hole dry, the more pressure as the water level drops the lower the current.

Either way, I'm probably still going to try it and monitor motor current. I'm thinking it should be fine. I may have to restrict the flow rate but I think it could be made to work.

I really regret not buying more of the NOS all stainless Goulds pumps I found on ebay for $400. The guy had 4, I bought one to see if it was a scam and when I went to buy more they were all gone. DOH!

BTW, can you give me a little more info on the cheap pump you mention? How do you replace a foot valve that is in the pump?

 

[Valveman]

If the CSV keeps the amps in the 1/2HP range it might work for a long time. However, the CSV will not keep you from opening too many faucets, which will cause max flow and max amps. Submersible motors are already working with a 1.65 service factor and can't take high amperage for very long.

 

[CountryBoy19]

If the CSV keeps the amps in the 1/2HP range it might work for a long time. However, the CSV will not keep you from opening too many faucets, which will cause max flow and max amps. Submersible motors are already working with a 1.65 service factor and can't take high amperage for very long.

It's for livestock water. There is only 1 faucet to open. That being said, I don't have a CSV on this, I was just using that as a point of reference because that's how my well runs (4 amps on a 3/4 hp pump).

I have 2 wells, 1 cistern for well water, and 2 rainwater cisterns, each with a submersible pump, on my property so I'm fairly familiar with how submersible pumps operate and behave. I've just never planted a 1/2 hp motor onto a 3/4 hp pump. I know it won't last long if amps don't stay low.

That being said, you mentioned a cheap pump, I'm interested/intrigued but you didn't give much information. You mentioned that the check valve wasn't good and should be changed. Every submersible I have dealt with had an integral check valve. Is that the same type as on the pump you mentioned? If so, how do you change an integral check valve?

 

[Bannerman]

I'm interested/intrigued but you didn't give much information. You mentioned that the check valve wasn't good and should be changed.

Here's a link for the pump mentioned.
Hallmark 1HP submersible pump

how do you change an integral check valve?

You wouldn't replace the internal check valve, but could thread an additional external spring loaded CV directly on the outlet port of the pump.

 

[CountryBoy19]

Here's a link for the pump mentioned.
Hallmark 1HP submersible pump

Is it just the 1 hp model or are the other sizes of Hallmark pumps good too? I really don't even need a 1/2 hp for my purpose but they don't go smaller than that.

 

[Reach4]

For a submersible in a cistern, do use a flow inducer. The pump can be horizontal.

 

[Valveman]

Is it just the 1 hp model or are the other sizes of Hallmark pumps good too? I really don't even need a 1/2 hp for my purpose but they don't go smaller than that.

Hallmark seems to be very limited in the size of the pumps for each horsepower. I only see one 1/2HP listed and it has a max head of 150', which is the same as 64 PSI. That is just not enough head to work with a 40/60 pressure switch. If you can use a 30/50 switch the 1/2HP is fine. If you need 40/60 the 1HP is needed even though is is overkill. An over sized pump doesn't cause problems like excessive cycling or needing a much larger pressure tank when it is controlled with a Cycle Stop Valve. But without a CSV over sizing a pump is not a good thing.