Slow down motor?

Hi, I need a new pump for my pond but I've refused to buy a 'proper' one because they are always ridiculously expensive. However today I have
bought a pump on sale, as a special, at my local supermarket at a very reasonable price, the problem is that it is, to say the least, a bit excessive.
It is 600w and 13,000 litres per hour, my little waterfall looks like Niagra falls in full flood. To say nothing of the 600w of energy that I am wasting.
I've put a dimmer switch in series with it and it slows it down but my knowledge of electronics and motors is very poor.
Can anyone tell me if using a dimmer switch is likely to have some horrible effect like burning out the windings? Or is there another way to slow it down and use less electricity?
I presume that it is an induction motor, but I can't tell without dismantling it.
You can see the pump here.... http://www.lidl.co.uk/uk/home.nsf/pages/c.o.20070903.p.Immersion_Pump.ar7 and here..... http://www.lidl.co.uk/C1256C790050C7BC/images/GB.07_0313 /$file/07_0313_b.jpg
Thanks in advance!
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snipped-for-privacy@googlemail.com wrote:

Divert part of the flow directly to the reservoir.
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snipped-for-privacy@googlemail.com wrote:

The dimmer is not a good idea. You can restrict the flow on the *output* of the pump, to some extent, with a valve. You don't want to starve the input to the pump. You can also divert the flow, in part, to a second loop. This could be just a length of tubing that feeds back into the pond, or it could include a fountain and/or pond "sculpture" (eg a statue of a frog) that sprays water in the pond. You will need to determine if these options are practical for you, and part of that includes a comparison of the old pump's capacity to the 13,000 LPH this one can deliver.
Been there, done that.
Ed
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snipped-for-privacy@googlemail.com wrote:

You might try restricting the intake of the pump. Whenever I've restricted the intake of a pump or blower, the current draw has dropped. I don't know if a restriction would cause cavitation with your pump unless you close the intake down to almost zero. If you restrict the outlet, the current draw will increase. If you have a way to measure the electrical current, a little experimenting with restricting the water flow could solve your problem.
[8!{} Uncle Monster
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wrote:

You mean decrease don't you? But the pump behavior depends on what kind of pump it is. This certainly looks like a centrifugal pump so it will draw less power when the discharge is restricted. The only concern is not to restrict it so much that there isn't enough flow to keep things cool.

I'd certainly agree to that.
daestrom
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daestrom wrote:

In my experience with air handlers, restricting the inlet lowers the electrical current draw of the motor because it's doing less work but restricting the outlet raises the static pressure and causes an increase in current draw by the motor. I don't know if this works the same way with your water pump but I assumed that the same principles would apply to a centrifugal water pump as to a squirrel cage air blower. I saw a post by someone else who suggested a bypass on the pump outlet. It makes a lot of sense as it would make it easy to regulate your water flow but I doubt it would lower the current draw of the pump. A "T" fitting with a valve on the leg going to the waterfall and the other leg going back to the reservoir should do the trick. Darn! I just remembered, I once checked a pump on a large artificial waterfall at an apartment complex and the current draw increased when I cleaned out the inlet screen. I should defrag my intracranial hard drive.
[8~{} Uncle Monster
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On 9/3/07 6:06 PM, in article fbib74$vld$ snipped-for-privacy@aioe.org, "Uncle Monster"

I certainly have the experience of observing thee effects using a vacuum cleaner. I would be very surprised if restricting the output flow instead would change things very much. I will check that out next time.
If you use water, the pressure is produced from the centrifugal of water rotated by the impeller. For small flow rates, the effective pressure is not going to be affected much as long as the impeller volume is full of water. The motor will deliver power proportional to the flow rate times the load pressure. As you reduce the flow, either by using either a restriction on the intake or output side, you reduce the flow rate and the power absorbed by the motor.
Bill -- Fermez le Bush--less than 18 months to go.
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Thanks for all the suggestions from everyone. I have restricted the outlet and as predicted the power consumption has dropped quite considerably. It is now down to 330w from 580w. I am still wondering about the dimmer switch, how does it work? Does it just switch 50 times a second slicing a bit off each cycle? Because if that is the case I can't see it doing much long term harm, I've tried it so I know that it works and the power consumption drops to 200w. I can't see how it can overheat whilst it is imersed in water. What do you think? Just to make it interesting this is the water feature,
http://www.users.waitrose.com/~zeonozo/pump.jpg
I know I said it was a pond, but that was just to make the question clear. Thanks James
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wrote:

I guess the term "using a bulldozer to crack an egg" springs to mind looking at your application :-) However since you have to deal with it, INMHO you could just about close off the outlet of the pump with little consequences (since it is submerged) without the dimmer, which in the average case does what you suggest, it restricts the power dissipated in the load by that means. Perhaps you have another application for the pump that would be more efficient rather than using it and restricting it's performance in that manner? Then you could use the dimmer for some lights or whatever :-)
Cheers .......... Rheilly P
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On 9/4/07 5:04 AM, in article snipped-for-privacy@w3g2000hsg.googlegroups.com,

Are you using a watt meter? If you are using an ammeter and multiplying by the voltage, you would be getting 330VA and much lower actual power consumption.
Bill
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wrote:

That's how most of them work. Just a 'phase-controlled' triac.

Motors are not like resistance loads. If an induction motor is supplied with a reduced voltage it doesn't draw less current it actually draws more. For the speed to actually drop when you use the dimmer makes me start to wonder if it really is an induction motor at all. If instead it is a 'universal' motor with brushes inside that casing, then a reduced voltage would slow it down and not harm anything.
But an induction motor can actually be burned out by running on low voltage because *those* types of motors will draw more current at low voltage and not really slow down very much.
daestrom
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wrote:

I have run small induction motors (typically shaded pole) with a dimmer switch (e.g. a fireplace blower which I slow down to reduce noise). The speed can be controlled if you have something like a fan or pump load where torque and speed are related. In such a case there may not be an increase in current-up to a point - but there is also the chance that the motor could stall -then it will smell funny. Generally this is not a particularly useful control at even 200-600 watts. At one time there was a device on the market for "power savings" for fridges etc where the circuitry sensed the load and adjusted voltage. Haven't heard anything about it recently- probably cost more than the savings and possible burnout risk of relatively cheaply built motors.
Another problem is the dimmer switch as lamp dimmers are designed for unity pf loads. I would not recommend them for inductive loads. There are "dimmers" intended for small motors and these also allow you to limit the lowest voltage to a safe value. The cost will be about 3 times as high as that of equivalent "wattage" lighting dimmers.
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Hi, Bill asked how I am measuring the power consumption. I am using an energy monitor which you can see here....
http://www.users.waitrose.com/~zeonozo/energymonitor.jpg
I know that it gives a correct reading under normal circumstances because I've checked it using a 100w bulb and a 3kw kettle but I do wonder how accurate it is when used in conjunction with a dimmer. I mean, how is it measuring the power? My understanding of these things has not advanced since my schooldays when we controlled power with a potentiometer and measured it with a galvanometer. I now know that the dimmer is using a triac to switch the power on and off at 50Hz. James
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     snipped-for-privacy@googlemail.com writes:

Most induction motors (of moderate size) are designed to run near-synchronous with the mains frequency with little slip. By using a dimmer, you are effectively reducing the drive to the point where the motor is no longer powerful enough to remain near-synchronous with the mains frequency, and it slows only because you are now overloading it. It will likely be drawing a higher current during the on-time of the dimmer, and the heating in it's windings is proportional to I so it will be getting hotter. Also, the higher frequency components resulting from the phase control switching can cause eddie current heating in the laminations, which are only designed to limit eddie currents at 50/60Hz. Reduced speed will mean it probably doesn't cool itself as well. You might get away with it, but on the other hand, you might not.
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     snipped-for-privacy@googlemail.com writes:

I would presume that too, and a dimmer will cause the motor to get hotter than is was designed for, and possibly fail. To slow down an induction motor, you need to reduce both the frequency and voltage supply to it. The complexity of doing this used to rule it out for most applications, but over the last few years, air conditioners with inverter technology have been doing this. If you could find an inverter module for an air conditioner, that might do what you want, but I've never looked at one so this is just speculation.
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Andrew Gabriel
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A dimmer switch has the effect of reducing the voltage (we ASSume the dimmer is intended for use with induction motors).
Were I to face the situation, I would restrict the outlet to get the flow I really wanted. This I would crank down the dimmer while monitoring the RMS current. I would set the dimmer to the level that minimizes the RMS current.

In a perfect world all inductor motors would come with VF/VV drives but ...

Many fan motors are slowed down by reducing the voltage (often by using a "tapped" coil with the longest "tap" effectively driving the motor with the least voltage.)
"Serious" pumps often come with a warning to restrict the outlet flow if there isn't much "head."

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