New motor not reaching recommended RPM

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TexasTea2

Member
Feb 28, 2021
14
Dallas, TX
Part 1: pool motor sounded weak and pump not moving water properly. On the assumption that we had a dying motor, we bought a new one, (exact replacement Century (A.O. Smith) 1.0 HP Up Rate Motor, Round Flange 56J Frame, Single Speed - Model UST1102 Voltage 115/230, Max Amps 115V = 15.0 230V = 7.5, Single Speed, Round Flange, RPM = 3450, Threaded Shaft, Frame = 56J) and swapped it in. Same weak noise and pump not pulling water.
Part 2: took motor out of the pump housing, checked that impeller was tight, etc., and got a strobe app to check RPM because it sounded so weak. App says motor is running at ~1100 RPM instead of specified ~3450 RPM.
Part3: On the assumption that the old timer might be impeding the current to the motor, bought a new timer and swapped it in (motor still OUT of the pump housing). No improvement.
So: does our logic re impedance on the electrical circuit even make sense? Could the electrical wires from the house to the pool be shot:eek:. Or did we just have the bad luck of getting a faulty new pump? We are not sure how to proceed from here, so suggestions welcome. Again, the motor is NOT attached to the pump/plumbing so no plugs, etc.). Also, the toggle on the motor is set to 230V. Thanks in advance.
 
You need to measure the input voltage.

You can't assume anything.

If it is only 120 volts, then the motor will run slow like you are describing.

Do not change the selector unless you are 100% sure.
 
Did did not change the selector and do not plan to. Also, confirmed with the previous motor (and the one before that sitting in a closet) that the selectors were set to 230. Based on your comment, is it possible that one of the two lines is dead? Thanks.
 
Measure the input voltage with a voltmeter. Check the voltage across the two hot leads and report it here.
 

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TexasTea2 said:
Did did not change the selector and do not plan to. Also, confirmed with the previous motor (and the one before that sitting in a closet) that the selectors were set to 230. Based on your comment, is it possible that one of the two lines is dead? Thanks.
Click to expand...
If you turned a 240V dual-pole breaker "off," be sure that you feel both poles turn "on." I always use two fingers as you will feel the individual breakers set in the "on" position.
 
If you had 240 and one leg is bad, the pump won't work at all since you need the return leg.

The symptoms point to low voltage, so maybe miswired or a big voltage drop or maybe the motor is undersized for the pump or maybe the motor is binding for some reason due to bad parts or maybe the pump was assembled incorrectly.
 
"If you had 240 and one leg is bad, the pump won't work at all since you need the return leg." This seems to be the issue: we only got 120V from the 2 input lines going to the timer. As 1P1 alludes to, we have two 1-pole 20A breakers and one of them seems to be bad. Old house so working on finding the correct replacement...Struck out at HD & Lowe's, ordered something from Grainger that we looks correct. Thanks everyone.
 
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TexasTea2 said:
"If you had 240 and one leg is bad, the pump won't work at all since you need the return leg." This seems to be the issue: we only got 120V from the 2 input lines going to the timer. As 1P1 alludes to, we have two 1-pole 20A breakers and one of them seems to be bad. Old house so working on finding the correct replacement...Struck out at HD & Lowe's, ordered something from Grainger that we looks correct. Thanks everyone.
Click to expand...
Have never understood how a motor set for 240V will run with one leg dead, but have seen it many times. Just don't allow it to run for long. It will burn the windings. Have seen that, also.
 
Well, back to report on the outcome: after getting the new breaker, the problem persisted. We had an electrician come out he confirmed that one of the two lines from the house to the pool was dead. Rewiring finally completed yesterday and we got the hoped for ~3450 RPM from the new motor (which we may not have needed). System now reassembled and seems to be in working order (fingers crossed). Thanks to everyone who helped us troubleshoot this.
 
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