Clamp on current transformers using permanent magnets

A clamp on current transformer, or a current clamp, is a common piece of test equipment used to measure current flow in a primary cable without making physical contact with the cable. A toroidal shaped current transformer is generally regarded as the best choice for a core shape, but a frequent dilemma is how to connect the transformer without disconnecting the cable. A spring loaded current transformer with a cut toroid shape is often employed to solve this problem. Another approach is to use two permanent magnets to clamp the split toroid back together.

We recently manufactured some split core current transformers using this idea. First an epoxy coated silicon iron tape wound core is cut in half, permanent magnets are glued to the core mating surfaces along with an alignment bracket, and a plastic hinge is taped on the toroid halves, see figure

1. Two secondary windings are wound onto the core, see figure

2. The windings are connected in series so the current from the two windings adds together. A final inductance test is performed at 100mV/1kHz to ensure the windings are connected correctly. Inductance measures about 15.5mH on properly connected parts and 9.9mH on parts with the connections reversed. A test limit of 12.5mH minimum is used. More insulation is added to help isolate the primary and secondary windings and a power cord is also assembled, see figure

3. These current transformers were coated bright yellow to make them easier to locate, see figure

4. The inside diameter of this current transformer is greater than 4.5” so current flow in very large cables can be measured.

Written by Mike Horgan
Engineering Manager
Butler Winding