Ammeter and Motor Inrush Current

Why not just use a "Clamp-on Ammeter" when you need to check it: http://www.bing.com/search?q=clamp-on+ammeter&src=IE-SearchBox

You can get one relatively cheap at any home improvement store (Lowes, Home Depot...).

Thanks for replying. Great minds think alike. Before reading your comment I had been looking at a clamp-on multimeter available from Harbor Freight for $19.99, so that is definitely a possibility. The only problem is that currently it is difficult to access a cable to get the clamp-on around. The cable from the control panel to the motor runs in watertight flexible metal conduit. Inside the control panel it is behind the panel face and is inaccessible. The only way to get at enough free cable to do the test is to de-terminate at one end, pull the cable out of the conduit, remove the conduit, re-terminate the wires and then do the test. If it was a one time test, that is what I would do, but I'd like to be able to check it from time to time as conditions change (e.g. as the filter loads up and provides more resistance in the system). A permanently mounted ammeter would allow me to check the valve setting any time without disturbing the cable. If I end up having to use the clamp-on, I will replace the hot lead to the motor with a longer cable that I can loop around inside the control panel and pull it out to where I can get to it with the clamp-on when I want to do a test. It's not extremely user friendly (and who like to see a loose cable running around inside the control panel?) but it would work.

If you still want to fit an ammeter, I'd say it's best to go with the switch idea (to put it in circuit only when the pump is running). You could use either a biased toggle (or rotary) type or a momentary pushbutton to prevent the possibility of forgetting to switch it out when starting.

Assuming your panel contains only switching elements (i.e. current into panel = current into motor ± a few mA for indicators etc.), the meter could be on the incoming live to the panel, or even back at the (indoor) breaker panel, if it makes wiring easier.

Ideas worth considering all. The approach with a standard ammeter and a switch may be less expensive than a suppressed scale ammeter. Don't know haven't priced one yet.

Cheers

Try what's known as a "current transformer". These are small, sealed coils that are installed around the current carrying conductors and have two output terminals to connect an ammeter. The fixed ratio (often 10:1) allows easy conversion of the scale reading to the actual current flow.

True, but I don't know how that helps with the inrush. The CT doesn't protect the meter from a current spike.

Just went and bought one a Harbor Freight

Getting involved with a directly connected meter in a wet environment is never a great idea for a DIY project. The clamp on ammeter is a great idea, just put it around one conductor and you get your reading without exposing any bare conductors. I have a couple of digital ones that I have bought at HarborFreight Tools, really inexpensive and no worries about the needle pegging during the inrush current and if you want greater sensitivity simply coil the conductor around the clamp and divide the reading by the number of conductor-turns in the jaws.

It's not really a wet environment. The controls are inside a waterproof enclosure which is about 20 feet away from the pool. I would make a cut-out in the panel face and mount the meter with all teminations behind the panel face so no chance of inadvertent contact. There are already other controls inside the cabinet (timers etc.) so it would be no different from those. It is a DIY project, but I have some familiarity with this sort of thing as I spent 30 years as a professional mechanical engineer in the field of hydroelectric power, however, the controls were the province of the electrical engineer. He and I are both retired now and are no longer in contact. In a powerhouse environment current was measured with CT's but they are a bit too expensive for what I am doing. ;-) So I was looking for a way to make a simple ammeter work.

great

For the modest amperage here, I think there are compact CTs about the size of a LifeSaver candy that could drive an inexpensive panel meter; all enclosed in the existing panel. 20:5 would give good midscale indication, and not too likely to be toasted by inrush.

An interesting thought. I'll see if I can find such a thing. In the meantime, if anybody is aware of a source for this, I'd appreciate the help.

Beautiful answer. Thank you for taking ther time. All you say is true, about pumps in general and swimming pool pumps in particular. I guess my concern is not so much operating at BEP but not operating at runout. Efficiency goes to pot at runout. I was approaching this on the basis of what happens when a pump goes into runout. As noted in the Lawrence pump manual:

Run-out operation occurs when a pump is over-sized for a system, or when a downstream restriction to flow has been eliminated. As pump flow increases, pressure decreases. This is usually accompanied by an increase in power consumption for radial flow impellers and a decrease in power consumption for axial flow impellers.

The increase in power consumption does not follow the smooth power demand curve of the normal operating range. It rises more steeply. I was hoping to detect that point with the ammeter and throttle the valve so the pump operates below it. But you are correct that a certain water turn-over rate should be observed for swimming pools. Your turn-over rate of three hours may be correct for public pools, I don't know. For residential pools the suggested rate is 8 hours. At least it is now. Perhaps when my pool was designed they installed a pump capable of a three hour turn-over. Oversizing pumps has become such an issue here in California that the State of California, recently enacted a law to control how big of a pump could be placed on a swimming pool. My pump was installed before this legislation and I think it may be too large for the application. If so, it can be throttled back (with, as you say, a corresponding reduction in power demand) and still achieve the desired turn-over rate. It would be great to have a flow meter in the system but even the least expensive ones cost more than I wanted to invest in this. As to measuring with a bucket and stopwatch, the discharge enters the pool in three places, all of them underwater. It's too much flow to measure that way anyway. Having said all of that, your idea about using differential pressure has merit. I can install gauges before and after the pump and determine TDH from them. Then I can look at the manufacturer's performance table and estimate where the pump is operating. Then I can set the valve for an 8-hour turn-over rate. Perhaps that's what I'll do instead of the ammeter.

I can see what you are trying to do now, and it makes sense. In theory it ought to be possible by measuring current to identify the point at which the power curve starts to rise steeply, but I'm not sure how easy it would be in practice. As long as you can get the pump curve or performance data from the manufacturer, TDH should tell you exactly where you are with the pump. Pressure gauges can be useful to have anyway. They can tell you other things, like when the filters are getting clogged.

I see what you mean about oversized pumps and runout operation. Radial flow impellers/pumps like to generate differential pressure, and are not designed for high-flow/low-pressure situations, hence the low efficiency. However, I would be a bit surprised if the pump were running totally at runout. Unless they are even more oversized than the pump, the pipework, bends, valves, filters, etc should generate enough back-pressure to keep the pump happy. And as you know, the higher the flowrate the greater the friction losses and system head. I could be quite wrong about this, not knowing what the pump and system look like, so I'd be interested to hear what you find.

Good luck, and keep us posted.

I asked the manufacturer for a curve but the one they gave me only shows head and flow. Swimming pool pump manufacturers apparently don't operate like other pump suppliers. In any case I have enough pump curves with similar pumping charateristics from other pump manufacturers to surmise approximately where the BEP is likely to be, so I should be able to get close enough. There is already a gauge on the discharge that is used to monitor filter loading, but I don't have a gauge on the suction at present. That is needed to determine TDH because the pump is above the water level and there is system piping on the suction side. On the topic of existing backpressure, the problem is that the filter in a swimming pool system is typically the source of the largest pressure drop. Originally the system had a diatomaceous earth filter which has a fairly high back pressure, and I suppose the pump was sized for that. But I replaced it with a cartridge type filter and the back pressure at the filter is about half of what it was before, and I can see from the appearance of the discharge jets in the pool that the flow has increased quite a bit. The manufacturer's pump curve ends on the high end at 28 feet of head at a flow of 125 gpm. With the cartridge filter installed, I am past the end of the curve on the discharge pressure gauge, but because of the suction losses, the TDH is actually higher than that which may bring the operation back onto the curve. Even if it does, operating at the extreme end of a pump curve is usually not good practice. Although efficiency is not shown on the curve I have, it is usually not good at the end of the curve. Thus my desire to tune the system to match the pump curve by throttling the discharge. I have to go find an inexpensive gauge that can read negative pressure to install on the suction side of the pump to see what's really going on.

Douglas:

Please send all the details for your pump and motor, manufacturer, model number, etc.

Based on what you have said so far, I am going to guess the follow parameters:

* Motor 2 HP with running current of 9.7 amps (230v single phase)

* Pump flow rated: 170 gpm at 20 ft or 8.7 psi developed head

* Maximum Pump Head: 60 ft (or 27 psi)

If your discharge pressure is 8.7 psig or higher, then you are not in run out. Naturally, the total developed head is going to be greater than 8.7 psi because the suction is below atmospheric pressure (a negative gauge pressure). Clearly, anything a or greater than 8.7 psig discharge pressure proves that you have nothing to worry about.

You may have a higher pressure pump, but you can see what I am saying. The same thinking can be applied.

In your application with the typical pumps we are talking about, even if you have a little cavitation, the cavitation is not going to do any signifcant damage to the pump that will require any replacement for a couple of decades of continuous operation. The bearings will wear out before there is any noticable degradation due to cavitation. As you know, pump impellers wear over time anyhow even with no cavitation.

Pumps in this application that are designed for suction pressures below atmospheric pressure are designed to operate with some cavitation.

There are many applications of centrifugal pumps that operate in continuous cavitation without problems because they are designed for the servious. Some of these are many hundreds of horsepower.

When I was in the Navy (a long time ago), the ships I served on with steam electric plants had the condensate pumps (large pumps) running in continuous cavitation mode. Pumps designed for it work just fine.

The pump is a Hayward Super II SP3015EEAZ. It's 1.5 HP. I have a curve for it but it only shows head and flow. The manufacturer says that's all they can provide. There is no "rated" condition shown, but as a judgement I would say that the pump is "rated" at 80 gpm at 50 ft of head. Shut off head is 90 ft. On the other end, the curve ends at 28 ft at 125 gpm. With a clean cartridge filter, the pressure on the gauge on the filter is less than 10 psi so less than the 28 ft where the curve ends. The suction negative pressure will add to that for TDH, and I don't know how much that is yet. So the pump may squeak onto its curve with the suction added. You are correct of course about pumps being able to take a little cavitation. The question is how much? My impression of residential swimming pool equipment is that it is not built to the same quality standards as a good industrial pump is. The impeller in this pump is plastic. I started out in the steam power plant business (nuclear plant construction) and later changed to hydoelectric so I know that you are right about the condensate pumps. But my guess is that a swimming pool pump is not built with cavitation in mind. Cavitation can be nasty business. Being in the hydroelectric field I could show you photos of hydro turbines with holes the size of your fist completely through a 3" thick stainless steel vane due to cavitation. I have seen cavitation in water passages eat holes in concrete that could swallow a car. A turbine we installed in Brazil (which the manufacturer later admitted was incorrectly designed) didn't last 3 months in service before the runner was damaged. Hydro turbines are typically designed to operate right on the edge of cavitation for maximum efficiency, but they missed the boat on that one. You may be right that the pool pump could operate for a decade with cavitation, I don't know, but you can see why I might be a little worried about what cavitation might do. In any case, I don't need to be pumping 125 gpm to meet the water turn-over requirement. At 65 gpm the water would turn over in 8 hours. At what I think to be the "rated" condition (80 gpm), it would turn over in 6 hours, which is fine. I can set the timer to run the pump for 6 hours a day. But the only way to get the flow down to 80 gpm is to throttle the discharge valve. After all of the discussion in this thread, I think I may abandon the ammeter idea and just put in a suction gauge so I can set the system for a flow of 80 gpm (TDH = 50 ft.). That should be sufficient. But I never guessed that so many people would read and respond to this. It's been great.

You found an amazing amount of good information on the pump in a short time. More than I got directly from Hayward after several exchanges of e-mails. Where did the 8 feet of suction lift figure come from? I didn't see that in the manual. In any case, not mentioned up to now is the fact that I have a suction side pool cleaner in the system (a Kreepy Krauley) that has 40 feet of hose on it. So while the pump is less than 2 feet above the water, the pool cleaner and all of the underground piping bringing the water to the pump may produce a suction loss of greater than 8 feet, especially if the pump is trying to move 125 gpm. That would produce cavitation. If it isn't damaging the pump I won't worry about it too much, but the pump runs much more quietly when I throttle the discharge. As to the Shiner Bock, an admirable idea but I have never seen that brand in California. I guess a Pyramid Ale will have to do.

The figure of 8 feet suction lift is in fact the priming lift (it is a self-priming pump), and has nothing to do with the point at which cavitation might occur. In fact, under normal conditions the pump would operate at elevations well above 8 feet, it just might not prime.

Given that the vapour pressure of water at 20 deg C is about 0.023 bara (about 9 inches), and atmospheric pressure is about 1.0125 bara (almost 33 feet), you have almost 32 feet of suction head available from which to subtract static lift, suction losses and NPSHr.

Normally you shouldn't even have to think about cavitation in a swimming pool pump (unless the suction strainer gets blocked). Having said that, the pump NPSHr curve (I bet they don't even produce these for swimming pool pumps?) is also continuously rising as flowrate increases, so if you are operating off the end of the curve it could be quite high. At the same time, the higher flowrate will increase suction losses and therefore reduce NPSHa. All the same, if you throttle back a bit and are operating on the published curve, you should not have a problem (though it might still be worth checking the losses in your cleaner system).

Just to note, apart from on certain special applications centrifugal pumps are generally not designed to handle any cavitation. And the potential damage is not limited to the destructive effects of implosions on the impeller, which is what we are usually shown. It can also lead to premature failure of mech seals and bearings.

Douglas:

My 93 year old mother has a swimming pool that is now about 50 years old. All maintenance ever done on the pump has been done by me, my brother, often with my father "supervising".

We have a similar arrangement with a suction side pool cleaner in operation. That thing has about 40 feet of hose also. It has been in operation for about 30 years.

The pool has the original pump and impeller. Although it is not the same model as yours, it has a "plastic" impeller because I have taken it apart two times, the last time about 10 years ago. At that time, I replaced the gasket (cut my own) to stop it from leaking. The impeller was "chewed up" a little at that time, but it continues to pump water like a champion to this day.

The impeller on my mother's pump looks like rather low grade plastic to me. It certainly is not a quality product like Noryl.

In my circle, I am one of the few without a swimming pool, primarily because we can use my mother's any time and she also has a beach house.

From time to time, I have "consulted" with friends who are old like me and all have old swimming pools. I have never heard any of them report nor have I seen an impeller problem with any of these ancient systems, all of them more than 30 years old.

Obviously, you could have something really out of the ordinary going on with your swimming pool that makes it a lot different.

Nonetheless, there is not a crisis going on across the nation with regard to swimming pool pumps an impellers. In checking for swimming pool pump failures, there is no indication that there is any problem with any pump impeller that I can find on the Internet. Give it a search and see if you can find anything on impeller failures. The only thing that happens to them is they leak, according to the problems discussed across the web.

We can safely conclude that the manufacturers really understand the application very well and produce quality products.

Your pump is rated as one of the best available. As noted before, if you have something about your system that is really out of the ordinary, there may be an issue, but so far the description is of an "average" type of installation.

Just install a heavy-duty SPDT switch to bypass the ammeter at all times except when you want to read it. A pushbutton would be convenient. Of course, any switch you use should be rated for motor service, since a motor is a very inductive load.

Don't forget though that motor current is very much power factor dependent, especially at lower motor loads so it may not be a good index for optimising your pump curve.

penttijp

'down under'

Penttiip,

You are correct. The power factor can vary easily from 0.8 to 0.95+ depending on the load.

Our friend needs a power meter to do a "good" evaluation.

That being said, it is unlikely that your pump is so oversized that you operate it near the end of the curve. In most cases like yours, the lower the restriction, the lower the power consumption. Since this is what you want to reduce, minimize the restriction. Any throttling above the sand filter restriction is likely to increase the power consumption.

N.B. If you have time on your hands, you can use the house power meter to test this. Turn off all breakers except the one for the pool pump. Check that there aren't any other parasitic loads by keeping an eye on your meter's wheel or (blinker if electronic version). Start the pump in normal setting and measure the time to get a full wheel revolution. Change the pump setting and repeat until you are happy with the results or your wife kills because the ice cream melted in the freezer.

Don't forget to turn the breakers back on after...

Excellent and a practical idea!

penttijp

What you propose is likely a complete waste of effort.

First, check the type of valve that you propose to use for throttling at the pump discharge. If it is a gate valve, it is not suitable for throttling and should not be used for throttling. You would then need to buy and install a throttle valve.

Second, your pump is extremely unlikely to be anywhere close to a runout condition. Your pipes will have to be huge so that the system flow resistance is extremely low (or your pump has to be extremely small in comparison to the pipe sizes). What swimming pool contractor is going to pay the extra money to buy piping that large? Also, the lines have many elbows in them and that is a lot of flow resistance.

Every pump has a pressure gauge at the pump discharge. Runout conditions occur when the differential pressure across the pump is very low and the flow is maximum. If you show a healthy discharge pressure on your gauge, you are definitely NOT in runout -- and every swimming pool discharge gauge I have ever seen shows lots of pressure unless the pump has a suction problem is pumping no water.

If you have a pump curve for your pump, which is easy to get from the manufacturer, you can figure out about where on the curve you are operating from the discharge pressure. (Note: It is not a true statement that all centrifugal pumps have higher power demands at runout. Think about it for a moment. An ideal pump at runout is producing no head. Power is proportional to head times flow. Therefore, for an ideal pump, the power is low at runout. For a real pump, depending on how it is designed, runout power will range from somewhat less than at a lower flow rate to the highest power consumption of any flow rate.)

You can also get a copy of the Crane handbook and using your pipe sizes, number of elbows, changes in elevation around the system, etc., estimate the system characteristics. From this, you can apply the discharge head and estimate the flow through the system. From this, you can also estimate the suction pressure. Now knowing the differential pressure across the pump and the flow rate, and using the pump curves which tell you the pump efficiency, you can find the input power required from the motor.

Get the motor curves, and from the power you can estimate the current and the power factor.

Pencil whipping this will probably be more satisfying than wrestling with the electrical work. Electrical work for swimming pools must be done to high standards. Should you install an ammmeter, you will need a properly grounded enclosure for your ammeter with conduit connecting to it and mounted in a sturdy manner.

Thanks for taking the time. I appreciate your input.

I don't know if I am supposed to answer each person individually or if people read my replies to others. If you can read my reply to Holzfeller I won't have to repeat it here. I do have a gate valve and I agree that a gate valve is not ideal for throttling service (a globe valve would be better) but it can certainly be done and it is suitable for what I am doing. A gate valve doesn't start throttling until the gate gets near the seat so it isn't useful for precise control. But I can still set the desired pressure on the discharge gauge with it. I have done just that as I have been experimenting with the system. I know that, over time, throttling with a gate valve can cause wear on the internal parts and it may no longer seal tightly after a while, but this valve should never be closed completely anyway. It is in the main discharge line to the pool. In any case, a 2" valve is far less expensive than a swimming pool pump if I damage it or its motor by running it in an unhealthy spot. As to the runout possibility, as I meantioned to Holzfeller, I am off the end of the manufacturer's pump curve according to the discharge gauge. So as you say, the pressure is low and the flow is high. But that gauge does not give the TDH of the pump because, in the case of a swimming pool pump there is significant negative pressure on the suction side of the pump due to the fact that the pump is above the water surface and there are suction losses in the suction piping system. However I can tell that the flow has increased substantially since the installation of the cartridge filter, and it may now be so high that the suction losses may be causing pump cavitation (NPSHR not being met). The curve I have doesn't show NPSHR but the pump is noisy unless I throttle the discharge and the noise may be cavitation. Cavitation is the best way to destroy an impeller that I know of. Plus, if the pump is in the runout zone (and as you say, it may not be) one of the big dangers of operating there is overloading the motor. It is typical for motors to overload when the pump goes into runout and the extra heat generated can greatly shorten its life. Again, another expensive component to have to replace. Checking for motor overload was the reason I wanted to install an ammeter. I would much rather damage a $20 gate valve by throttling it than the pump or motor. I would love to analyze the piping system as you suggest, and I have the Crane handbook as well as computer software that does piping system analysis (after all, this is a forum for engineers, isn't it?), but unfortunately, with a swimming pool system, 90% of the piping is underground. I don't have the pool design drawings and so I have no idea what is going on down there. So I fear I will not be able to calculate flow. But by adding a suction gauge I can find TDH and I can estimate approximately where on the curve the pump would best operate. That is good enough for my purposes. As long as that meets the required water turn-over rate, that is where I will set the valve. Of course, as the filter loads the system back pressure will increase, but I can keep the pump in its happy spot by changing the valve setting.

Cheers

You should use a suppress scale ammeter. This will take care of the starting current of the motor.

S. V. Iyer

I posted my query in the instrumentation section hoping to get an instrumentation answer and finally, after a lot of replies that addressed other areas (which I nevertheless appreciated) came your reply, which was really what I was looking for in the first place. I am a mechanical engineer, not electrical, and I was not aware of supressed scale meters. However, since you told me about them I have researched them and found that they are exactly what is needed. In fact, it seems that addressing motor inrush is the main reason they are made. They provide good resolution for the normal operating range, but can accept up to 6X the rated current at the upper end of the scale. It looks like for my 9.7 FLA motor a meter with range 0-15/75 would be perfect. Thanks so much for replying. I think we can now bring and end to this thread.

Cheers