Wiring - Current Transformers (CTs)

Overview

The LTMR controller has three CT windows through which you can route motor leads to contactor load connections.

The CT windows enable you to wire the controller in four different ways, depending on the voltage and controller model used:

• Internal CT wiring through the windows

• Internal CT wiring using multiple passes

• External Load CT wiring

This section describes each of these options.

Internal CT Wiring Through the Windows

The following diagrams show typical wiring using the CT windows for either three-phase or single-phase motors:

Internal CT Wiring Using Multiple Passes

The controller can physically support up to a maximum of five passes of 2.5 mm² (14 AWG) wire through the CT windows. There are three looping windows located under the CT windows that physically support up to a maximum of four wire loops.

Set the parameter Load CT Multiple Passes to account for the number of times the motor wires pass through the CT window in order to display the correct current readings. For more information, refer to Load Current Transformer settings.

The following diagram shows typical wiring using two passes (one wire loop):

Multiply the current by the number of times that the motor wires pass through the CT windows to determine the amount of current passing through the internal current sensors.

Adding multiple passes allows to:

• Increase the current sensed by the internal current sensors to a level that the controller can properly detect, or

• Provide a more accurate reading by the internal current sensors.

We recommend that you select a controller with an FLC value range that includes the motor FLC. However, if the motor FLC is less than the FLC range of the controller, multiple passes can increase the current level sensed by the internal current sensors to one that the controller can detect.

For example, if using a controller with an FLC range of 5 to 100 A, and the motor FLC is 3 A, the controller cannot properly sense the current. In this case, if you pass the power wiring through the internal current sensors of the controller 2 times, the internal current sensors of the controller sense 6 A (2 passes x 3 A), a current level that falls within the FLC range of the controller.

For more information about controller types, refer to the TeSys T LTMR Motor Management Controller User Guide.

External Load CT Wiring

The controller can accept 5 A and 1 A secondary signals from external current transformers. The recommended controller model for these currents is the 0.4-8 A model. Use multiple passes through the controller CT windows, if required.

External CTs are specified with a transformation ratio. The ratio of the external CT is the ratio of the motor input current to the CT output current.

To enable the controller to adjust the FLC range and display the actual line current, set the following parameters:

• Load CT Primary (the first number of the CT ratio)

• Load CT Secondary (the second number of the CT ratio)

• Load CT Multiple Passes (the number of times the CT output wires pass through the controller’s internal CT windows.)

For more information, refer to Load Current Transformer settings.

The following diagram shows wiring using external CTs:

For a description of external CT characteristics, refer to the TeSys T LTMR Motor Management Controller User Guide.

CT Wiring in Presence of Variable Speed Drive

When the motor is controlled by a variable speed drive (VSD):

• The current transformers (external or internal) must be mounted upstream of the variable speed drive, and not between the variable speed drive and the motor. The CTs cannot be used between the drive outputs and the motor because the drive can output fundamental frequencies outside the 47-63 Hz range.

• Chokes must be mounted on the three phases, between the current transformers (external or internal) and the variable speed drive, to minimize the soft start harmonics current and the voltage disturbances generated by the variable speed drive.