pump motor overheats

3/4 hp motor with a proven 3/4 hp impeller and 238 volts under load, inground pool, clean filter, plenty of water available to suction end. Pressure gauge not working. Recent refitting of chlorine generator allows for possibility something got into return line to pool which may be causing a restriction. Would that explain motor drawing 7.5 amps vice 5.9 nameplate and thus heating up and cutting out on thermal overload. All valves have been disassembled and found clear.

Any other ideas as to what is eluding us.


Thanks, PMD

First question is how are you measuring the current? I assume a clamp on amp meter? I really have no evidence but I would question how the 7.5 compares to normal current draw as you measure it. The 5.9 on the nameplate sounds about right compared to my 1 HP but 5.9 on your pump may not be right. One thing to try is to run the pump dry and see what the current is. Just another data point. I doubt you have a motor problem as a shorted winding would smoke. Next, are you sure it is overheating and the thermal switch just hasn't gone bad? Next, a working pressure gauge would tell a lot about the water flow. I do know that a pressure side blockage, i.e. pressure rise, will cause a drop in current. Never checked what happens on a suction side block.
Hope this helps a little.
Al

Pool has been in use for several years with no problems of this nature. It is my son's, so I have not been much involved with it till now.

Yes current is being measured with an amprobe. The original motor failed, not sure how, a brand new 3/4 hp motor drew too much current so it was returned and now a slightly used 3/4 hp identical motor (A. O. Smith USQ 1072) is drawing over 7 vs. 5.9 nameplate and after a while the cutout operates. It eventually resets and it the cycle starts over. Current draw with no load (strainer open to the atmosphere) is less than 4 amps.

A new pressure gauge indicates 8 psi at the filter, so I believe we can rule out the return lines.

It must be the suction lines although hard to believe as there are two all the way to the pump, one from the two main drains and one from the two skimmers. The suction Y valve at the pump has been disassembled and is OK. We will try backflowing the suctions lines and see what happens. Thanks for the help. PMD

It sounds as though you are replacing the motor on an existing pump. Assuming you have matched the motors properly, you may need to repace some of the pump hardware. Check the impeller and the bearings to make sure they are in good condition. This could be why the original motor failed and the second one was drawing too much current.

Still laying in bed laptopping. I know...LAZY. One of the retirement perks. Going to be the last of the nice, warm, sunny days for a while here in the North Pole. Then snow later this week. Bah. Old backup pump is already set up to externally circulate water, and I like to play with water. Me feels an experiment happening a little later with suction side blockage. MAS may be on to something with the impeller but I doubt bad bearings would cause this. With enough drag on the motor to heat it up, the bearings would grind up real quick and you would need hearing protection.
Al

Did you verify that the motor and wet end are matched up, it sounds like the motor and impeller are mismatched - a common problem when not replacing the motor with an identical unit - HP is not the only spec in the equation. Seeing that it's draw is only 4A with no load, it looks like it's trying to push too much volume once under load, straining the motor - this could be because of a mismatch between the inpeller and motor.
You did not state whether the original motor was replaced with an identical unit, only that hte replacement was replaced with an identical unit - this is what's leading me to believe you're dealing with a mismatched motor.

Sorry I wasn't clearer. The motor that quit was a 1HP with, as it turns out, a 3/4 hp impeller. Took the impeller to the pool warehouse and proved that it was a 3/4 hp by comparing it part # and physically to a brand new 3/4 hp impeller.
Parts folks said they could see nothing wrong with impeller.

Pump is an Ultra-Flow and the diffuser is good from 1/2 hp to 1 1/2 hp. Both 3/4 hp motors exhibit same problem, high amps, high heat, cut out operates. Voltage while pumping is 238 at motor. New pressure gauge indicates 8 psi at filter.

We are trying to backflow suction lines. May have to find a pump to really get some flow in the lines as garden hose may not hack it.

Have you tried running the filter with the valve set to waste to see if the current draw changes or if the overheating problem goes away? Setting it to waste would eliminate a restriction (if there is one) where the new SWG was installed. When something fails and is replaced, often there was something else wrong in the system that either caused or accelerated the failure of the part.

It sounds as though the impeller may be for a 3/4 HP full rated motor or a 1 HP uprated motor, they are typically the same. You may have chosen a 3/4 HP up rated motor for which the impeller would be mismatched.

What is the service factor of each motor, including the old one?

What is important is to match the HP * Service factor.

For what it's worth. It appears as if restrictions on the suction side have the same effect on current as a restriction on the pressure side. Best I could do was plumb a ball valve in the suction side and start restricting it and watching the current. If I restrict it too much the pump basket starts getting sucked dry so I avoided that. Adding just a little restriction indeed dropped the current. The jury rig I use is airtight. Verified that years ago when I first started using this old pump to circulate before de-winterizing the real equipment. I always have air in the suction side and I think what's going on is air is coming out of solution under the vacuum. I see this more with colder water which makes sense. I can only easily use 1.25" pipe so that's a slight restriction right off.
Pump is an old Jacuzzi 1 HP SF-1 240V configuration
Running dry -- 3.7A
Full flow (best possible) -- 5.0A
Slight restriction -- 4.6A and down from there.
Al
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