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IntroductionIn order to choose the best current sensor to monitor ECM current, let’s first establish what an ECM is and how they operate. An Electronically Commutated Motor (ECM) is also known as a brushless DC motor and is driven by a specially programmed microcontroller that alternates the DC voltage signal to spin the motor shaft. With a “brain” behind the action of the motor, less current is consumed, which results in cost savings. Everyone is happy with a more efficient motor that drives the same load as a less efficient motor, until that motor needs to be monitored by a building automation system. What causes issues with the current sensor monitoring the motor?

The ProblemAs mentioned earlier, the process of controlling an ECM is an intelligent process completed by a microcontroller. The microcontroller is typically placed in the motor housing or immediately outside of the motor housing in its own compartment. This “brain” needs a constant source of power to keep it ready to receive instruction to turn the motor on. This constant flow of current may be called stand-by current, or leakage current. To monitor just the current powering the motor would require dismantling the motor to access the appropriate wires. That would be quite inconvenient or nearly impossible.

The next best solution would be to take a current sensor and monitor the main power line feeding the entire ECM unit. The ECM unit is powered by some amount of AC voltage, so an AC current sensor may be utilized. However, the stand-by current that is always flowing needs to be “overlooked” by the current sensor.

The SolutionThrough testing of ECM units, and analysis of the collected data, various RIBX current sensors prevent false signal transmission of the ECM status. Below is a list of the RIBX current sensors that have proven operable in an ECM related application. To determine the correct current sensor and then install the device, some steps to follow are suggested.

As when installing any current sensor, it is best to take a measurement of the current flowing through the electrical line intended to be monitored. In this case, measure the stand-by current being consumed by the ECM without the motor

turning. With the stand-by current measurement noted, determine whether a fixed or adjustable threshold is desired on the current sensor. This is done by comparing the current

measurement taken to the designated threshold rating.

RIBX Series Current Sensors and ECMs

• If the current measurement is markedly lower than the threshold on the fixed current sensor, that fixed current sensor will work. • If the current measurement is larger than the rated threshold of a fixed current sensor, use an adjustable current sensor to adjust the threshold above the measured current.

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List of Current Switches for ECMs Model # DescriptionRIBXKA Enclosed Solid-Core AC Sensor, .5-150Amp, adjustable, wires

RIBXKF Enclosed Solid-Core AC Sensor, .25-150Amp, fixed, wire lead

RIBXKTA Enclosed Solid-Core AC Sensor, .5-150Amp, adjustable, terminals

RIBXKTF Enclosed Solid-Core AC Sensor, .25-150Amp, fixed, terminals

RIBXGA Enclosed Split-Core AC Sensor, .75-150Amp, adjustable, wire leads

RIBXGF Enclosed Split-Core AC Sensor, .35-150Amp, fixed, wire leads

RIBXGTA Enclosed Split-Core AC Sensor, .75-150Amp, adjustable, terminals

RIBXGTF Enclosed Split-Core AC Sensor, .35-150Amp, fixed, terminals

RIBXG21A Enclosed Split-Core AC Sensor, 120-277V, .75-150Amp, adjustable, wires

RIBXG21F Enclosed Split-Core AC Sensor 120-277V .5-150Amp fixed wires

RIBXG21TA Enclosed Split-Core AC Sensor, 120-277V, .75-150Amp, adjustable, terminals

RIBXG21TF Enclosed Split-Core AC Sensor 120-277V .5-150Amp fixed terminals

RIBXGA-SCAL Enclosed Split-Core AC Sensor, 3-150Amp, self-calibrating, w/wire leads

RIBXGFL Enclosed Split-Core AC Sensor, LED, .75-150Amp, fixed, wire leads

RIBXGHA Enclosed Split-Core AC Sensor, 120V, .75-150Amp, adjustable, wire leads

RIBXGHF Enclosed Split-Core AC Sensor, 120V, .5-150Amp, fixed, wire leads

RIBXGHTA Enclosed Split-Core AC Sensor, 120V, .75-150Amp, adjustable, terminals

RIBXGHTF Enclosed Split-Core AC Sensor 240V .5-150Amp fixed terminals

RIBXGTA-SCAL Enclosed Split-Core AC Sensor, 3-150Amp, self-calibrating, w/terminals

RIBXGTFL Enclosed Split-Core AC Sensor, LED, .75-150Amp, fixed, terminals

RIBXA Enclosed Internal AC Sensor, Adjustable

RIBXF Enclosed Internal AC Sensor, Fixed

RIBXJA Enclosed Split-Core AC Sensor Adjustable

RIBXJF Enclosed Split-Core AC Sensor, Fixed

RIBXRA Enclosed Solid-Core AC Sensor, Adjustable

RIBXRF Enclosed Solid-Core AC Sensor, Fixed

RIBMXRA Panel 4in Remote Adjustable Current Sensor

RIBMXRF Panel 4in Remote Fixed Current Sensor

RIBHX24BF Enclosed Relay/AC Sensor 20Amp SPST-NO 24Vac/dc, mini RIB hsg

RIBX243PA Enclosed Internal AC Sensor, Adjustable +20Amp 3PST 24Vac/dc Relay

RIBX243PF Enclosed Internal AC Sensor, Fixed, + Relay 20Amp 3PST 24Vac/dc

RIBX24BA Enclosed Internal AC Sensor, Adjustable + Relay 20Amp SPDT 24Vac/dc

RIBX24BF Enclosed Internal AC Sensor, Fixed, + Relay 20Amp SPDT 24Vac/dc

RIBX24SBA Enclosed Internal AC Sensor, Adjustable + Relay 20Amp SPST + Override 24Vac/dc

RIBX24SBF Enclosed Internal AC Sensor, Fixed, + Relay 20Amp SPST + Override 24Vac/dc

RIBXLCA Enclosed Internal AC Sensor Adjustable +10Amp SPDT 10-30Vac/dc Relay

RIBXLCF Enclosed Internal AC Sensor Fixed +10Amp SPDT 10-30Vac/dc Relay

RIBXLCJA Enclosed Split-Core AC Sensor Adjustable +10Amp SPST 10-30Vac/dc Relay Override

RIBXLCJF Enclosed Split-Core AC Sensor Fixed +10Amp SPST 10-30Vac/dc Relay

RIBXLCRA Enclosed Solid-Core AC Sensor Adjustable + 10Amp SPDT Relay 10-30Vac/dc

RIBXLCRF Enclosed Solid-Core AC Sensor Fixed +10Amp SPDT Relay10-30Vac/dc

RIBXLSA Enclosed Internal AC Sensor Adjustable +10Amp SPST 10-30Vac/dc Relay + Override

RIBXLSF Enclosed Internal AC Sensor Fixed +10Amp SPST 10-30Vac/dc Relay+ Override

RIBXLSJA Enclosed Split-Core AC Sensor Adjustable +10Amp SPST 10-30Vac/dc Relay Override

RIBXLSJF Enclosed Split-Core AC Sensor Fixed +10Amp SPST 10-30Vac/dc Relay Override

RIBXLSRA Enclosed Solid-Core AC Sensor Adjustable +10Amp SPST Momentary 10-30Vac/dcRelay

RIBXLSRF Enclosed Solid-Core AC Sensor Fixed +10Amp SPST 10-30Vac/dc Relay Override

RIBMX24BA Panel 4in Internal Adjustable Current Sensor + Relay 20Amp SPDT 24Vac/dc

RIBMX24BF Panel 4in Internal Fixed Current Sensor + Relay 20Amp SPDT 24Vac/dc

RIBMX24SBA Panel 4in Internal Adj Current Sensor + Relay 20Amp SPST-NO+Override 24Vac/dc

RIBMX24SBF Panel 4in Internal Current Sensor Fixed + 20Amp SPST-NO + Override 24Vac/dc

Current

TimeECM Current DrawStand-by Current Trip Point

Motor Speed Trip Point 1Motor Speed Trip Point 2

Motor Speed Trip Point 3

12

3

4

5

6

CHART 02

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Current

TimeECM Current DrawStand-by Current Trip

12

3

4

CHART 01In Chart 01 the example plot, the ECM unit was measured to have a stand-by current draw of 0.25A. One RIBX current sensor was used to monitor when the ECM began to spin.

Point 1: ECM was in stand-by mode; unit was not turning and current sensor deactivated.Point 2: ECM began to turn with increasing speed; current draw was above the stand-by current draw and current sensor activated.Point 3: ECM speed reached maximum; unit fully on.Point 4: ECM slowed to stop; unit returned to stand-by mode and current sensor deactivated.

In Chart 02 the example plot, the ECM unit was measured to have a stand-by current draw of 0.25A. Four different RIBX current sensors were utilized to monitor when the ECM began to spin and when three different rotation speeds had been met.

Point 1: ECM was in stand-by mode; unit was not turning and current sensor deactivated.Point 2: ECM began to turn; current draw was above the stand-by current draw and current sensor activated.Point 3: ECM reached first desired speed to be monitored; second current sensor activated.Point 4: ECM reached second desired speed to be monitored; third current sensor activated.Point 5: ECM reached third desired speed to be monitored; fourth current sensor activated.Point 6: ECM turned off; unit returned to stand-by mode and all current sensors deactivated.