Why is my new pump running hot?

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Backyardchemist

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Jun 2, 2010
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Northern Vermont
I recently installed a new Pentair Whisperflo WFDS-24, replacing a very old Hayward Superpump. The first time I ran it on high to use the vacuum, after 20 minutes or so the motor housing got so hot I couldn't hold my hand on it. After turning it off, then trying to restart on low, it wouldn’t run until I let it cool off for several minutes. Even when running on low the housing is warmer than I think it should be. There appears to be decent airflow through the vents.

I'm not working this thing very hard (see signature), I don't think. I want to figure out if I have a problem with the pump that would justify a warranty return, or if I have some other problem. Detailed information follows, for those of you willing to stick with me!

WFDS-24 is a two-speed pump. 1.0/0.12 HP, SF 1.25 (therefore SFHP is 1.25/.15). This is 115V, not 230V.

Total length of electrical wiring from circuit breaker panel (20 AMP breaker) is no more than 25 feet, all with solid copper 12 AWG. I have bypassed my old mechanical timer; there is one switch on the circuit so I can turn the pump on and off. There is a two-position switch on the motor that came pre-installed, to control low/high speed, and I have verified that it's wired properly and securely.

Operating voltage is nominal (~121 V). At low speed, the pump is drawing its max rated amps of 4.7. At high, it's drawing its max rated amps of 14.6. This in itself seems odd.

I measured the vacuum and pressure at the pump (using the drain plugs) and found 7 in Hg/3.5 psi on low and 24 in Hg/14.5 psi on high. Using the formula for dynamic head that I find elsewhere on TFP, that gives 16 and 61 feet of dynamic head at low and high respectively. Looking at the pump curve for this pump, I estimate 26 gpm and 58 gpm, which are smack in the middle of the curve (which is good, right?). I’m getting good water flow, I believe. On low, it’s moving nearly as much water as the old pump did, I believe. Pool is clean and happy.

Potential issues: Suction line from pool is only 1.5", return is 2". I have a small leak on the suction side, but when running on low, it draws very little air; the filter basket has only a little air in the top and I see very few fine bubbles in the return. On high, it definitely sucks in more air, and the top of the filter basket is pretty turbulent.

So, what’s my problem? Do I even have a problem? Am I reducing the life of this motor running it hot and at max amps? I also note that the motor has developed a faint but consistent rattling sound, which is not stopped or modified by placing pressure on the switch plate or housing.

Sorry for the very long post, but if I’ve learned one thing on TFP, it’s that there will always be questions in response to a post, and I’m trying to anticipate to get all the info in one place. Any thoughts/advice/additional diagnostic strategies would be appreciated!
 
Given all your info I'd have to say you got a bad unit. A motor pulling a normal amount of power and tripping the thermal breaker is not right. That motor should run continuosly at full load amps and never overheat enough to trip the thermal breaker.

What I do find odd is that at 61' of head you're pulling FLA (full load amps).
 
The wiring is borderline at 12 AWG; it should probably be 10 AWG, but I don't think that it's enough to matter. You could check the voltage at the pump when the pump is running to verify that it is getting full voltage under load.

Your vacuum pressure is higher than what it should be at low and high speed. There could be some sort of blockage in the suction lines. Also, check the impeller to make sure that it turns freely. It should spin easily. The rattling sound might be cavitation or something stuck in the impeller (possibly a rock).

Your suction, at 1.5 inches, is too small for that pump on high speed. The max flow rate that I would recommend is about 40 gpm. I think that, for your pool, I would use a 1/2 HP single speed pump and allow it to run most or all of the time.
 
@Bama Rambler: So, should I expect that the motor runs at full load amps regardless of the load? I didn't think so, but my understanding of motor/pump physics is limited. Why is the head measurement at FLA 'odd?' You would expect more, or less?

@JamesW: I checked the voltage both at the switch, which is about 6 wire-feet from the motor, and where I bypassed my timer, and got the same 121V. Why would the voltage at the pump be different?

I understand that the suction line is smaller than desired, but can't change that at this time (it's buried under a patio). I suppose I could down-size the pump and run longer hours, but the intent with the two-speed was to do just that, while maintaining the ability to vacuum on high-speed periodically. A quick look at the pump curves for Whisperflo and SuperFlo pumps on Pentair's website shows that the smallest single-speed pumps are not going to draw a lot less, maybe 50 gpm.

The sound seems to be coming from the motor, not the pump housing, but checking the impeller is a good idea.

Anybody else have any ideas?
 
Backyardchemist said:
@Bama Rambler: So, should I expect that the motor runs at full load amps regardless of the load? I didn't think so, but my understanding of motor/pump physics is limited. Why is the head measurement at FLA 'odd?' You would expect more, or less?
Click to expand...
I think that he meant that the pump should not be pulling full load amps at only 61 feet of head.
Backyardchemist said:
@JamesW: I checked the voltage both at the switch, which is about 6 wire-feet from the motor, and where I bypassed my timer, and got the same 121V. Why would the voltage at the pump be different?
Click to expand...
My idea was to check the voltage with the pump running to see how much it dropped vs. without the pump being on.
Backyardchemist said:
The sound seems to be coming from the motor, not the pump housing, but checking the impeller is a good idea.
Click to expand...
Try turning the shaft from the back or by reaching in and turning the impeller with your finger to see if the shaft is turning freely (turn off the power at the breaker for safety).
 
@JamesW: I see. Yes, the 121V measure is with the pump running, not just the voltage on the line with the pump off.

@daddybug: No, this pump is only 115V. I've double-checked my wiring to the two-speed switch and the wiring of the switch itself.

@Bama Rambler: This, to me, is the crux of the issue. According to the technical support person I just spoke with at Pentair, the motor will draw the full amps regardless of the head. What changes with different conditions is the head and flow, not the amps. Does this match your understanding? Again, I don't have the electrical/motor knowledge to know whether this is accurate. Is the answer to this question dependent on the type of the motor?
 
The pump may indeed draw full load amps over part of the head curve because of the power factor but I would be very surprised if the draw was the same over all the head curve. I have measured my own pump's current draw over different head loss and the current usually changes some and doesn't remain constant. But the power factor also changes at the same time. I would call back and see if you can get a different tech on the phone who might have a better answer.

Based on your measurements, total head is 64' (+3 for the height of the gauge) and flow rate is 57 GPM. Power draw should be about 1550 watts but because of the power factor, the amps could be higher than 12.8 amps @121v. With 14.6 amps, the power factor is around 88% which is reasonable for an induction motor under partial load. So other than suction head being a little high, nothing on the plumbing side seems that unusual.

I also note that the motor has developed a faint but consistent rattling sound, which is not stopped or modified by placing pressure on the switch plate or housing.
Click to expand...
Have you taken the wet end off the motor and inspected the impeller?
 
@mas985: Interesting about the power factor. I might be able to measure that, I'll give it a try. FWIW, my total head is actually 61', because I measured the pressure at the pump, not up on the filter. Doesn't change the big picture, though.

I have not taken off the 'wet end' yet, and am a little hesitant to do that, having never done so before. I also don't want to mess up any potential in-warranty replacement or repair, although Pentair may determine that they're going to send me a new motor to swap in, which means I'm going to have to figure it out eventually.
 

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As Bama Rambler originally said, it could just be a bad unit. Possibly bad bearings or windings. Bad bearings could explain the noise.

If the shaft and impeller turn freely and smoothly, the next thing you could do is check the resistance of the windings, or you could take the motor to an electric motor shop to have it evaluated to support a warranty claim.

If this is going to be covered under warranty, then I think that you should get a whole new pump, not just the motor.
 
Well, they've agreed to send me a new motor under warranty. I didn't push for an entire new pump. I guess I can check the impeller and diffuser when I get the thing apart to see if there's any damage or such, and that it spins freely.

Any tips on the disassembly and reassembly? I suppose there will be some seals that will need to be lubricated? Should I use the same clear silicone stuff I use on the filter caps and strainer basket lid?
 
@JamesW: Hmmm, I don't think I want to reduce my low-speed flow rate. At 26 gpm (see OP) that's only ~1.5 turnovers per day at continuous operation (~1.7 if you go with 22,000 gallon pool, which I think is closer to the truth than 25,000). I don't want to go below 1.5 turnovers a day, I don't think. My whole objective with this two speed pump was longer run hours, lower flow, less energy, cleaner pool (from better skimming).

Thanks for the link to the video, that will be helpful in doing the motor change out.
 
If Pentair does not want the old motor back, I think that it would be worthwhile to take it to a motor shop to have it evaluated for a few reasons:

1) If the motor checks out OK, then that would indicate that there might be an unresolved issue with your system.
2) If the motor is OK, then you could keep it as a spare, sell it, or return it to Pentair.
3) If the motor tests as bad, then it would help assure you that the problem is not with your system.
4) If the motor tests as having a problem, it might be a problem that is easy to correct, such as bad bearings, and might be worthwhile having fixed to have a spare motor.
5) If the motor tests as having a problem, it might help identify a quality control issue with that motor manufacturer.
 
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