Measuring Motor Load Current with a Current Transformer

Accurately measuring the load current of a motor (to determine if the motor is operating at a light load, full load, or overload) is a common requirement for the end-user and can be done quickly using a current transformer designed for metering applications.

Determining which current transformer to use requires the installer to know the full load current (FLC or FLA) of the motor.  To find the full load current, locate the nameplate label on the motor and note the current ratio provided.  If the motor's nameplate is no longer legible or missing altogether, reference the motor load data chart from the NEC Handbook based on horsepower rating, rated system voltage and whether it is a single-phase or three-phase motor.

For example, If the rating of a three-phase 460V rated induction motor is 110A, then, according to the NEC handbook, we would select a current transformer with a 150:5A ratio.  IMPORTANT - remember to verify the outside diameter of your conductor is less than the inside diameter of the current transformer.

Using the model 180RL-151 (for the above example) with a 5A rated output (150:5A) and a 2.5" inside diameter window opening, we will:

1. 1. Assume that the outside diameter of the conductor is less than the 2.5" inside diameter of the 180RL current transformer.

1. 1. Determine the actual motor load by verifying that the meter scale matches the current transformer ratio.  In this case, the meter scale should be 0-150A.

1. 1. Select an analog panel meter to display the load current.  If the load current of the three-phases is to be monitored and read concurrently, one could use three separate 180RL-151 current transformers, each with an analog panel meter.   As an alternate option, one can use three current transformers (180RL-151), one analog panel meter (HST905A150A) and a selector switch (N25-61328-37S or N25-61325-37S) to get the current reading of each phase.

In instances where cables cannot be removed, generally in medium voltage installations, a split-core current transformer like the model FCL should be used to monitor the motor load current.

Note that window type current transformers are rated 600V but can be used on higher voltages with fully insulated cables. Caution should be exercised in the proper installation of window type 600V rated current on higher voltages. Where a low voltage window type CT is intended for use in a higher voltage application, it is the purchaser's responsibility to ensure that the operating conditions are met, and the necessary precautions are taken. This is usually verified by performing insulation tests at an appropriate level of system voltage with the low voltage current transformers installed.

For some applications, such as load motor testing setups which require high metering accuracy, it is recommended to use a higher accuracy (and more robust construction) current transformer like the JAK-0C or the JAK-0S models. These models have 0.3% and 0.15% revenue grade metering accuracy.

For applications where the metering device is housed at a separate location from the current transformer, the standard 2RL current transformer, which has a low burden, is not suitable and a higher burden CT will be required to compensate for the additional impedance of the long lead wires.  We recommend using the 60RBT metering class current transformer.

If the distance between the current sensor and the metering device is further than 100', a suitable option is to use a current transducer with a 4-20mA output signal and a metering device that has 4-20mA input signal.  Please consult with a FLEX-CORE® application engineer if you find yourself in this situation.