Determining RPM of a Pond Pump

In summary: In which case, you should see the commutator and brushes by removing the motor from the pump.In summary, the individual is building a small device powered by a pond pump purchased from Home Depot. They plan to use the pump's impellor shaft to rotate their device and are trying to determine the RPM of the pump. They have provided information on voltage and amperage and are considering using a stroboscope or other methods to measure the RPM. After receiving suggestions and advice, they have determined that the pump likely has 2 poles and runs at 3600 RPM. They have also identified the pump model number and brand.
  • #1
DaveC426913
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I am building a small device that operates underwater so I am powering it from this http://www.homedepot.ca/webapp/wcs/stores/servlet/CatalogSearchResultView?D=900694&Ntt=900694&catalogId=10051&storeId=10051&langId=-15&Dx=mode+matchallpartial&Ntx=mode+matchall&recN=112940%2018%2010000048&N=0&Ntk=P_PartNumber&showreviews=1". It is 11W, (120V 60Hz, .19A, 180GPH).

I will attach my rotating device directly to the impellor shaft of the pump (after removing the impellor). So effectively, I am using the pump as nothing more than a waterproof motor.

The device I'm powering is a disc about 6" in diameter that I want to spin at about 10-15RPM, which I will get by hooking up various nylon step-down gears.

What I need to do is figure out the RPM of the pond pump.
Not sure how to do this.

Yes, obviously a stroboscope would be perfect, but...

Follow my logic. Presumably, the 60Hz AC plays a role in the RPM. If there were only 2 "brushes" (I think they're actually brushless) in the motor (at 180 degrees), that should make for one AC cycle per rotation, which would produce a 3600RPM motor. But I don't know if I can count on only one set of "brushes", there could be four or even six, making for a 1800RPM or 1200RPM motor...


Any ideas about how to figure out the RPM? It doesn't have to be too accurate.
 
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  • #2
There is no way to determine the speed of the pump with the info your provided. The speed of the motor, assuming its synchronous AC, is by the number of poles the motor has, not the number of brushes(because there aren't any). If the motor has brushes then it is DC motor in which case there is no way to tell speed unless you use an encoder on the motor/pump shaft or you know the geometry of the pump.
 
  • #3
Topher925 said:
There is no way to determine the speed of the pump with the info your provided.
Yes. I'm just providing as much info as possible to help in a diagnosis.

I was hoping that members might have some ideas about how I might Dx it. Strobe lights, sound pitch, throughput volume, etc.

Topher925 said:
The speed of the motor, assuming its synchronous AC, is by the number of poles the motor has, not the number of brushes(because there aren't any).
Right. Poles. That's the term I was looking for.

In my Googling of pumps in the 180GPH ballpark, I keep coming across 1725RPM, which is about right if there are to ~4 poles (1800RPM).
 
  • #4
1725 is a widely used industrial PD pump/motor speed.
 
  • #5
It depends on the cost of your pump unit. The cheapest 11 watt motors are shaded pole (2 pole), that usually run at 3600-400 = 3200 RPM. I have seen some more expensive shaded pole motors that are 4 pole (1600 RPM). The next more expensive are AC/DC, which are brush motors designed to run on AC or DC. Induction motors are usually 1/4 HP or higher, and are usually 2 pole (3450 RPM) or 4 pole (1725 RPM). Look carefully at the motor label: if it says AC only, it is a shaded pole. If it says AC/DC, then that's what it is. Your pump is 11 W, and 23 VA so very inductive. It is probably shaded pole. If you can see the impeller head-on, look at it running with a single piece of white tape on one blade under a fluorescent light- if you see two symmetric moving spots, it is ~1600 RPM, if you see only one, ~3200 RPM.
 
  • #6
Bob S said:
It depends on the cost of your pump unit. The cheapest 11 watt motors are shaded pole (2 pole), that usually run at 3600-400 = 3200 RPM. I have seen some more expensive shaded pole motors that are 4 pole (1600 RPM). The next more expensive are AC/DC, which are brush motors designed to run on AC or DC. Induction motors are usually 1/4 HP or higher, and are usually 2 pole (3450 RPM) or 4 pole (1725 RPM). Look carefully at the motor label: if it says AC only, it is a shaded pole. If it says AC/DC, then that's what it is. Your pump is 11 W, and 23 VA so very inductive. It is probably shaded pole. If you can see the impeller head-on, look at it running with a single piece of white tape on one blade under a fluorescent light- if you see two symmetric moving spots, it is ~1600 RPM, if you see only one, ~3200 RPM.

You are freaking awesome. :!)


...

OK, I couldn't get the tape thing to work, so I tried painting the edges of the blades: 5 black and 1 white. That is harder than I thought, but...

What I found is that that the rotor only wants to be in one of two positions, 180 degrees apart. This pretty much means 2 poles, right?
 
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  • #7
So I suppose you can't visually inspect the motor casing? They virtually always have a nameplate that tells you or at the very least a model number so you can look it up.
 
  • #8
russ_watters said:
So I suppose you can't visually inspect the motor casing? They virtually always have a nameplate that tells you or at the very least a model number so you can look it up.

Absolutely I can. I linked to it in my first post: http://angelodecor.com/blog/180-gph-fountain-pump/". I bought it at Home Depot but it is labeled as Angelo Decor. The product number is AD40180.

But RPM is not something that is commonly spec'ed in pond pumps.
 
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  • #9
Sorry, missed the link...
 
  • #10
DaveC426913 said:
OK, I couldn't get the tape thing to work, so I tried painting the edges of the blades: 5 black and 1 white. That is harder than I thought, but...

What I found is that that the rotor only wants to be in one of two positions, 180 degrees apart. This pretty much means 2 poles, right?
The fluorescent lamp flickers at 120 Hz. If you see tow positions, that means about 60 RPS or 3600 RPM. If this unit has a low voltage adapter that plugs in the wall, and there are rectifiers in it, it could be a small permanent magnet dc motor.
 
  • #11
If the motor only wants to be in two positions then its almost certainly a DC brushed motor as AC motors tend to not have "spots" where they want to stay.

This also means that there is no fixed rpm that the motor will operate at. The pumps speed will be dependent upon the pressure head of the pump assuming constant flow rate. So the only way for your to actually know the speed of the pump under operating conditions is to actually measure it or ask the manufacturer for its specs.
 
  • #12
Aren’t those pool pumps variable speeds? The flow rate of 180 GPH is the max flow rate and if you turn the orange knob toward the left (ccw), it pumps less and turns slower.

As for measuring the speed, you can use a digital laser tachometer. It comes with a special tape that you install on the shaft and you just aim and read. If you do not have the tape you can use Al tape.

You can buy one at Princess Auto for 75 CAD.
http://www.princessauto.com/vmchk/tools/automotive-tools/diagnostic/8077414-digital-laser-tachometer
 
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  • #13
nucleus said:
Aren’t those pool pumps variable speeds? The flow rate of 180 GPH is the max flow rate and if you turn the orange knob toward the left (ccw), it pumps less and turns slower.

As for measuring the speed, you can use a digital laser tachometer. It comes with a special tape that you install on the shaft and you just aim and read. If you do not have the tape you can use Al tape.

You can buy one at Princess Auto for 75 CAD.
http://www.princessauto.com/vmchk/tools/automotive-tools/diagnostic/8077414-digital-laser-tachometer
Because it has a speed contol, the motor has to be a dc brush motor, probably of the permanent magnet stator type. I have special battery-operated laser tach for rotating machinery that I got on eBay for about $30 USD.
 
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  • #14
Topher925 said:
This also means that there is no fixed rpm that the motor will operate at. The pumps speed will be dependent upon the pressure head of the pump assuming constant flow rate.
I am not using it as a pump. I intend to remove the impellor and use the shaft.


Bob S said:
The fluorescent lamp flickers at 120 Hz. If you see tow positions, that means about 60 RPS or 3600 RPM. If this unit has a low voltage adapter that plugs in the wall, and there are rectifiers in it, it could be a small permanent magnet dc motor.
No, not seeing two positions, it sticks in two positions when off (there are 2 poles).

nucleus said:
Aren’t those pool pumps variable speeds? The flow rate of 180 GPH is the max flow rate and if you turn the orange knob toward the left (ccw), it pumps less and turns slower.

Bob S said:
Because it has a speed contol...

It does not have speed control, it has flow control. The flow rate is controlled by blocking the throughput, not by changing the speed. Turning the dial closes and opens the intake hole. These are cheap pumps.

nucleus said:
You can buy one at Princess Auto for 75 CAD.
http://www.princessauto.com/vmchk/tools/automotive-tools/diagnostic/8077414-digital-laser-tachometer
$75 to check the RPM of a $20 pump?? :tongue:
 
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1. How do you determine the RPM of a pond pump?

To determine the RPM (revolutions per minute) of a pond pump, you will need a tachometer, which is a tool used to measure rotational speed. Place the tachometer on the pump shaft and turn on the pump. The tachometer will display the RPM of the pump.

2. Can I calculate the RPM without a tachometer?

Yes, there are a few methods to calculate the RPM without a tachometer. One method is to use a strobe light and mark one of the pump blades with a piece of tape. As the blade passes the tape, the strobe light will make it appear stationary, and you can count the number of times the blade passes in one minute to determine the RPM. Another method is to measure the time it takes for one full rotation of the pump shaft and use the formula RPM = (60 seconds / rotation time in seconds).

3. What factors can affect the RPM of a pond pump?

Several factors can affect the RPM of a pond pump, including the size and type of the pump, the horsepower of the motor, the voltage and frequency of the power supply, and any external forces or resistance on the pump, such as clogs or obstructions in the water flow.

4. Why is it important to know the RPM of a pond pump?

Knowing the RPM of a pond pump is important for several reasons. It allows you to monitor the performance of the pump and ensure it is running at the correct speed. It also helps to troubleshoot any issues that may arise with the pump, such as a decrease in flow rate or excessive noise. Additionally, knowing the RPM can help you determine the pump's efficiency and whether it is using the appropriate amount of energy.

5. Can the RPM of a pond pump be adjusted?

Yes, the RPM of a pond pump can be adjusted by changing the speed of the motor. Some pumps have adjustable settings, while others may require a variable frequency drive to change the speed. It is essential to consult the pump's manual or a professional if you are unsure how to adjust the RPM of your specific pond pump.

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