Current Isolators

In many three phase systems for the control of AC or DC power, it is desirable to control the current by use of current feedback in a closed loop configuration. Because of high voltages and often high currents, the isolation of the feedback signal from the actual current is essential for protection of the low voltage circuitry and other signal sources that may be in contact with personnel.

Gemini has two assemblies that can accomplish this goal. One of these, part no. 12M04-00010, connects directly to the current to be measured by inserting two short bussbars in series with the line currents and is designed for a nominal maximum of 250 amps. It has self contained current transformers for isolation. The other, part no. 12M03-00132, uses external standard 5 amp current transformers and thus can be used for much larger currents. This version fits into the brackets described in our Control Modules page.

Both of these assemblies take advantage of the fact that in a three phase system, the line currents must always add to equal zero, so it is only necessary to measure two of them to obtain a signal representative of all three lines. With this in mind, the use of these assemblies is limited to systems that do not operate with a neutral wire that contains current, and the configuration must be such that all load current also flows through at least two of the lines at all times. Thus, any circuit that contains diodes for “free wheeling” conduction is not a candidate for this technology.

To accommodate different current levels, each of these assemblies can be “fine tuned” with a dip switch on the circuit board. The application notes for these part numbers provide the instructions for calculating the output of these assemblies and details for setting the switches.