run dry pump / current transducer

Does anyone know if i can use a current transducer to tell if a 3-phase pump is running dry. I have done this before with a Franklin Pump Saver, but it states that it is only for single phase applications. In this case i have a 3 phase 220V pump which is by design capable of short "run dry" periods. I would just like to detect dry and cut it off.

narlin

Measure the current when the pump is running dry. If it is less (or more) than the normal condition, then you can use many brands of a current-sensing switch combined with a timer (to allow normal pump start-up period).

Automation Direct (similar to many other brands) has the AcuAmp AC Multi-Range Current Switch with Time Delay. 1-phase or 3-phase is not relevant as the switching current is what your route though the doughnut sensor and can be either. The current input range for this model is 1.5 to 200 Amps.

http://www.automationdirect.com/adc..._AC_Current_Operated_Switches_with_Time_Delay

Detects current draw changes in motors when they encounter problems such as pumps running dry or pending bearing failure.

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It seems like using a pressure switch to check for a pressure drop might be a more direct way to find out it has run dry. I guess that would depend on what you are using the pump for.

There is no convenient place to install a pressure switch. I've certainly considered it. I either already have the automation direct switch above, or can get one. A-D also makes an analog version that might be easier to use. Thanks for the tip about start-up. I didn't remember that, but it would have become obvious pretty quick.

A-D also makes an analog version that might be easier to use.

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Yes, if you have a PLC where you can send the analog signal. (You didn't say). The multi-range digital switch types can be set to produce an on/off output after some time delay and at a certain current.

Sometimes looking at current alone is adequate. Often it is not. You need to look at the total load on the motor. The power used by a motor is linear with the load on the motor, but current is not linear.



You may find it difficult to reliably detect a run dry condition, especially if the pump is oversized or operates some of the time under low flow conditions.

Load Controls manufactures and sells motor load monitors for this purpose. The module watches current on all three phases and provides an output proportional to power instead of amps. A model with an adjustable relay output is also available if you don't wan't to use an analog input to your PLC.

http://www.loadcontrols.com/products/products.html

They have an application note on using their load monitors for pump run dry detection here: http://www.dexknows.com/

I have used these before and they worked well in the applications where I used them.

Depending on the type of pump you are using, dry may not be the only low point on the current draw. Some of the centrifugal pumps draw very low current when their flow is blocked.

I did a search Hope this may help
http://www.clrwtr.com/SymCom-PumpSavers.htm

I would put in a discharge pressure switch. What you are trying to detect is if the pump is pumping or not. Even with discharge blocked it is going to pump but at shutoff head.

The main issue with running dry or at shutoff head is cooling the pump itself - mainly the shaft seals depending on type. Running dry allows little time to shut off - at shutoff you have a little more.

If is not that hard to put in a pressure switch on a pump all you need is the pilot drill and a 1/8 pipe tap. Then copper tubing assuming pressure is 400 psi or less.

Lancie has a good point with a time delay on startup - depending on circumstances it may take a little time for pump to start pumping.

Dan Bentler

On previous applications, we've gone for the 'belt and braces' approach of a flow switch (paddle switch) and current sensing on the motor to detect a running dry /broken coupling condition. I accept that this may not work in your particular application.
Tim.

Narlin doesn't say what he's pumping. Paddle switches and pressure transmitters are OK in clean water but are problems with sewage or slurries. Pressure transmitter taps can plug, even if you use isolation diaphragms. Amps isn't a bad way to go, perhaps in conjunction with one or more of the above.

You can get false readings from all of these, including amps. If you plug the inlet side of the pump you won't get flow or pressure, and amps will be low. This is what Narlin seems to be most concerned about. If you plug the discharge side you will get low amps, no flow, but you will get pressure. If you clog the impeller with rags you will get low flow, high amps, and probably normal pressure.