Water Well Journal

August 2015

Water Well Journal

Issue link: http://read.dmtmag.com/i/543740

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Page 37 of 71

W e kicked off this three-part series last month with many of the specific characteristics regarding pumps. This month we will delve into various aspects of electric motors you should understand or know how to find. Actually, we'll expand this topic somewhat and include engines in our discussion. That's why the title has been changed to what you should know about drivers. Submersible or "Regular" Motors Anybody who has worked around electric motors used for well pumps for any period of time can tell you the fit characteristics between an ordinary squirrel-cage electric motor and a submersible pump motor could not be further apart. The submersible motor is generally designed to fit onto a National Electrical Manufacturers Association (NEMA) rated frame based on the motor's diame- ter, horsepower, and speed. But wait— so is the normal squirrel-cage motor. They are each usually designed for bolt- ing and fitting onto a specific pattern established for the motor's horsepower and speed, which are also often a func- tion of the motor's diameter for a sub- mersible motor. Beyond that specific factor, though, is the next thing in common the two kinds of motors share. It is what usually ends up killing the motor. Now there can be many potential defects in a motor or operating conditions that can harm a motor. But the odds are one specific op- erating issue will become the ultimate culprit resulting in the motor's demise— heat. Heat is the great equalizer when it comes to a cause of failure for any elec- tric motor, whether it is less than ½ hp or as large as 5000 hp. If the motor is asked to operate in any condition it is not designed for and for any extended period of time, you can bet the motor will ultimately fail. Think about it. Almost all the com- mon causes of motor failure you hear about—overloading, excessive cycles (starts and stops), dusty or dirty operat- ing environments, low or high voltage, running a three-phase motor on open- delta power, inadequate bearing lubrica- tion, insufficient cooling—are simply individual factors with the same out- come. The motor dies because of exces- sive heat. As the technician on the job, it is your responsibility to investigate and eliminate any potential cause of this heat and correct it. By recognizing ex- cessive heat is the ultimate outcome arising from another situation, or even several situations, you can generally identify and possibly arrest the root cause of this heat and stop the death of the motor. One of the most common types of motor failures is operating a three-phase motor on open-delta (two transformers instead of three) power. Normally, this results in a negative sequence voltage where the motor is literally fighting against itself. Generally, the most reli- able fix in this case is to derate the load up to 10%-15% to prevent problems. For example, this means you should rate the maximum pump load on a 50 hp motor to no more than 42-45 bhp (brake horsepower) of the pump load. This leads us to another basic item about electric motors you should know. A simple fix of one potential symptom may not correct the final result. You should never consider correcting an overloading condition as enough by it- self to prevent a motor burnout, if the motor is still generating excessive heat due to inadequate cooling or low volt- age. It is incumbent upon you to verify all of the motor's operating conditions and ensure you have done all you can. One of the obvious questions sur- rounding submersible motors is how can you be expected to verify adequate water is passing by the motor in a well. This is a situation where you can only do the best you can. Know what to look for and how to correct it when you find it. Inspect the entire installation and document the well conditions. Where is the motor located within the wellbore? Is the water top feeding (coming in over the top of the pump and motor) or being bottom fed (from under and over it)? If top feeding, was there a shroud placed over the motor to induce flow past the motor and ensure there is ade- quate cooling velocity? If not, was there any type of basic flow-inducing tube placed to generate any movement of water around the motor at all? If you spend a few extra minutes with the installation and inform your client of the operating conditions that will someday lead to failure of their motor—and better yet, correct it— you will most certainly have gained a customer for life. ED BUTTS, PE, CPI ENGINEERING YOUR BUSINESS ENGINEERING continues on page 38 A simple fix of one potential symptom may not correct the final result . waterwelljournal.com 36 August 2015 WWJ There are basic characteristics of motors you need to know.

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