Measuring Current on a Well Pump Motor

 

Measuring a well pump is important for the following reason as it; provides system feedback for automation control, provides performance statics, provides life statics, provides energy information, provides safety information and it provides a method to protect the motor for running in a dry well potential resulting in damage.

When measuring current on a well pump motor it is imperative that the type of motor is identified and how the motor is being driven. It should be asked if the motor is a single phase motor or multiphase motor. If the motor is a multiphase motor, you may want to measure each of the phases and then sum the resulting current or you may chose to measure only one phase and then multiply the current by the number of phases used in the motor. The later is more economical and it is simple to perform and setup the system however some critical performance and life statics are lost. The next critical question that needs to be determined is the maximum current consumption per phase; this will help in selecting an appropriate CT to perform the task. Ideally you want to select a CT that is as close fit as possible to the maximum current rating for each phase of the motor. The more current size overage you have the lower the accuracy you will have for your measurements. This implies that a 1000A CT shouldn’t be used for a 70A current measurement for example, but rather a 75A CT would be a better fit. Current measurements are typically monitored by a PLC (programmable logic controller), Digital Panel Meters and or other digital devices that accept analog DC signals. Most CTs output a small signal AC sine wave. This sine wave needs to be converted to a ratiometric DC signal. Two common DC signals are 4-20mA and DC voltage output. Many transducers use a conversion method called “voltage averaging” this is a pseudo true RMS measurement. It is recommended that a transducer that is used, be a True RMS converter, this would ensure the best motor drive compatibility, as most motor drive types are not a perfect sine wave and a True RMS transducer would provide the best accuracy. The difference in actual current measurements between voltage averaging and True RMS conversions can be as high as 40-50% depending on wave type (Square Wave +11%, SCR ½ Power -29.3%, SCR ¼ Power -40.4 or more).  Thus if selecting high quality transformers, appropriately sized and good systems to monitor the current measurements all benefits could be lost because the wrong transducer conversion type had been selected.

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