# RPM of a pond pump

1. Jun 16, 2009

### DaveC426913

I am building a small device that operates underwater so I am powering it from this http://www.homedepot.ca/webapp/wcs/... 10000048&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.

Last edited by a moderator: Apr 24, 2017
2. Jun 17, 2009

### Topher925

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. Jun 17, 2009

### DaveC426913

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.

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. Jun 17, 2009

### FredGarvin

1725 is a widely used industrial PD pump/motor speed.

5. Jun 17, 2009

### Bob S

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. Jun 17, 2009

### DaveC426913

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?

Last edited: Jun 17, 2009
7. Jun 17, 2009

### Staff: Mentor

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. Jun 17, 2009

### DaveC426913

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.

Last edited by a moderator: Apr 24, 2017
9. Jun 17, 2009

### Staff: Mentor

10. Jun 17, 2009

### Bob S

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. Jun 17, 2009

### Topher925

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. Jun 17, 2009

### nucleus

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.