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.

When Do I Need 3 ½ Digit or 4 ½ Digit Panel Meter

3.5_4.5_Digits_Panel Meters

First there is a cost difference between the 3 ½ digit and 4 ½ digit displays, that can sometimes be substantial.  The 4 ½ digit display provides a level of resolution for small scale inputs that may not provide much value to the viewer. The resolution can be 1/100 of an Amp for example, thus for a 50 AC Amp measurement, a value such as 50.01 AC Amp would be presented. If however a 3 ½ digit display had been used for the same measurement a resolution of 1/10 of an Amp would result in a measurement of 50.1 AC Amp. The end user may find the result is more stable and easier to read and potential record if necessary. If however a value of more than 199.1 is required, with a 1/10 resolution a 4 ½ digit display is needed. Always remember that a panel meter is displaying results only for human consumption.

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How to Calibrate the OSM neXCalibrator (LOOP-CALB)

If the unit is not calibrated properly, the OSM neXCalibrator has two resistor VR2 variable resistor 2 for voltage calibration and VR1 variable resistor 1 for current calibration. You should be able to access these resistors by removing the back cover. It is possible to calibrate the unit yourself, by connecting the output to a precision multi-meter or you can have it calibrated by a professional. Once the calibrator is connected to a multi-meter, ramp up and inspect the accuracy of the each step.  Ramp up to the last step; use either VR2 if calibrating for voltage or VR1 if calibrating for current output, to adjust the output.  The display has one LSD so it is not the ideal metric for determining the accuracy of the calibrator.

To see more information about OSM neXCalibrator follow the link:  neXCalibrator


Understanding a Loop Powered LCD Digital Panel Meter


There are a number of variants in digital panel meters is more are less and understatement. It however can be stated that there can be a generalized understanding of digital panel meters. There are number of display types and input types of digital panel meters, for the purposes of the this article we are going to look at a very simple panel meter that can be used in many process control applications, and thus has one of the highest rates of utility.
The simple LCD digital panel meter loop powered 4-20mA process meter is a good starting example to have a closer look at digital panel meter’s specifications.
How Display Digits are specified – Displays are specified first by the number of digits. Commonly you will see 3 ½ digits and or 4 ½ digits. The ½ digit is used for 1 or not one and also to provide a negative symbol or not this is referred to as “polarity”. This allows for a maximum displayable value for a 3 ½ digit display of 1999 or -1999 and for a 4 ½ digit display you have a maximum value of 19999 and -19999.
How Display Types are specified – In general you have displays that of type LCD or LED, however whether it is a LCD or LED type display, the displays are commonly 7 segment digits. Seven segment digits are used to create numbers from 0 to 9 for each digit. The dimensions of the 7 segment digits or the digit size can vary from very small to very large. Common sizes include 0.35”, 0.45”, 0.56”, 0.6”, 0.8” 1” and 1.5”
How Display Colors are specified – For LCD displays you have with our without backlight. A LCD without backlight, the digits are generally black in color on a non light positive background. With backlight you have color options for the digits that are commonly green, amber, red and blue. It is also possible to have the digits be negative (black) and have the background be backlight with the common colors of green, amber, red and blue.
How Display Units and Decimals are specified – On LCD displays the units or “Annunciators” are provided, these provide important feedback defining what is being measured. These units are ⁰F, ⁰C, PSI, %, A, V and others. These units are usually selectable in the field.
How Inputs are specified – The most common process control input is 4-20mA DC. A 4-20mA input can specify line impedance; this is usually as a result of the length of wire used in a 4-20mA twisted pair run. A specification will be listed as nominal impedance in Ohms at the max protocol current for example 300Ω nominal @20mA
How Performance is specified –
• Accuracy – This is defined as the percent error rate at full scale (experimental value – expected value) / expected value. The accuracy also specifies the least significant digit as a count. The count value would be representing for example “count +2” this would imply that beyond the 2 digits from the decimal point accuracy is not stable.
• Conversion Rate – This defines the number of conversations that the A/D in the meter, can do and display per second. Typical conversation rates may be something like 5, 3 or 1 conversations per second.
• Normal Mode Rejection – This sometime is abbreviated as NMRR, this is the ability of a panel meter to reject AC normal mode noise or signal. The value is listed in –dB @ power line frequencies; for example -30dB @60 Hz implies -30dB attenuation of a 60 Hz signal.
• Adjustments – This specifies a zero setting and a gain setting capability, this can be sometimes done with coarse and fine adjustments or just with coarse adjustments.
• Temperature Coefficient – This value is specified as ppm or parts per million per ⁰C. An example will be +/-100 ppm per ⁰C, implying that accuracy will be affected 0.0001 per degree change.
How Environmental performance is specified – Operating frequencies are the range of temperatures which a panel meter can function and maintain is stated accuracy. The range is usually a commercial, Industrial or military temperature range or a value similar to these ranges. An example of a commercial panel meter’s operating temperature is 0 to 50⁰C.
How Power Supply Requirements are specified – For a loop power device an expected range is from 8 to 32VDC. Different voltages are available; see your manufactures’ documents for more information.


What Do I Need to Know When Selecting a Current Transformer

A Current Transformer is used to measure alternating electrical currents.  The word Current Transformer is abbreviated as (CT); there are two basic physical types of current transformers Split Core Current Transformers and Solid Core Current Transformers. A current transformer is used to measure AC load currents going through a conductor such as a wire or bus-bar. The Split Core CT opens up and fits around the conductor and then is closed such that the load conductor doesn’t need to be disconnected. A Solid Core CT requires that the load conductor be disconnected and placed through the center of the Solid Core CT.

A Solid Core CT should be selected, if your CT needs are for new construction and there is a need for better cost, reliability and accuracy. A Split Core CT should be used in existing installations, which would be difficult and costly to remove the load conductor.

In general it is expected that you will see 0.5% to 2% accuracy in mid range Split Core and Solid Core CTs. It is necessary to select a CT according to your desired overall metering accuracy needs. For example some power meters installations require ANSI C-12.1-2008 this requirement sets a minimum of +/-2% accuracy, were as ANSI C12.16 requires a CT that is +/-0.6% accurate, with an overall +/-1% system accuracy. Not all installations have ANSI requirements, however be aware that there are differences in current transformer accuracy and you will usually pay a premium for higher accuracy transformers.

There a several different common outputs available from current transformers used for mid range power meters, power monitors, power quality monitors, and current meters. These outputs are 333mV, 1V, 100mA, 1A and 5A. The 333mV if becoming more and more popular in the United States were as Europe is commonly using the 5A CT. When selecting for your needs make sure that your metering device supports your current transformers output type. Remember that many current transformers can be built to order, so asker your current transformer distributor to see if they can provide you with whatever custom solution you may need. The costs of making a custom solution are usually 2-5 hours or engineering time.

Temperature range of many Current Transformers manufactures is commonly not specifically stated however most are designed to operate from -20 to 60 degrees Celsius. Check with your current transformer distributors for more details. The temperature range is of more importance for CTs that are of type current output; 5A output CTs for example will be subject to an internal temperature rise were as 333mV will not. Many CTs are not designed for outside use, it is important if outside use is needed to verify with your CT distributor that an outside weather package can be applied.

You should know your maximum load. The closer you select a CTs nominal load to the actual load the better your overall accuracy will be. For example if your loads maximum current will be 93A a 100A CT would be a much better fit than a 200A CT. Nominal load current for mid range CTs range from 1A to 5000A or more.

The tighter the fit around the conductor the better the accuracy, this implies that when you select a CT you should know the size of the conductor to ensure that you get the best fit, it would be unwise to use a CT with a 3 inch x 5 inch opening to for a 1 inch x 1 inch conductor.