Miniature current transformers are important components for energy meters, power measuring devices, and other instruments. Usually, there’s at least one coil in a CT. The busbar, which the primary current comes through, can be made together with the coil. Then, we have a busbar built-in current transformer. The busbar type current transformer can be directly installed in a device or instrument, while the through-hole type current transformer require a primary wire or connect bar come through its hole.
Magnetic Core Inspection
Usually, the production process of a current transformer starts from a magnetic core. There’s a few different kinds for magnetic cores for variety of current transformers. The amorphous & nanocrystalline dual cores are used for DC immune current transformers, the nanocrystalline single cores are used for normal miniature current transformers, the zero phase current transformers may need silicon steel cores or nanocrystalline cores, the split current transformers usually need ferrite cores.
These magnetic core should be tested before coil winding. The induction value must be within the upper and lower limitation.
Coil Winding
When we get the qualified cores, we’ll make coils by automatic winding machines. The cores should be fixed properly and the enamel wire length and gauge should be considered in advance.
Coil Inspection
Every winding coil should be inspected before further processing. If the coil is a defect one, we must remove it from the production lines. Otherwise, extra material and labor will be wasted.
Coil Assembly
As the secondary, the coils should be connected to lead wires, as the enamel wires are not strong enough. For PCB type current transformers, the pins replace the lead wires, and therefore, the enamel wires should be connected these pins.
After the coil is ready, for different types of CT, there’s different process. For plastic case CT, we put the coil inside a plastic case. Then, fill the case with liquid epoxy resin which will become harden within one or two days. For insulation tape wound coils, we’ll wrap the coil by hand.
Inspection
For each piece of CT, we should test the current error and phase shift. The test result should meet the State Standard or the customers’ requirements. According to GB/T 10665 of China, the required current error and phase shift for different accuracy class of miniature current transformer is as below. Our high accuracy current transformers usually have accuracy class of 0.1 or 0.05, and the cost will be higher than normal type.
Current Error +/-(%)
| Accuracy Class | 0.01In | >=0.05In |
| 0.5 | 1.0 | 0.5 |
| 0.2 | 0.4 | 0.2 |
| 0.1 | 0.2 | 0.1 |
| 0.05 | 0.1 | 0.05 |
Phase Shift +/-(‘)
| Accuracy Class | 0.01In | 0.05In | 0.2In | In | 2In | 3In | Imax |
| 0.5 | 90 | 45 | 30 | 30 | 30 | 30 | 30 |
| 0.2 | 30 | 15 | 10 | 10 | 10 | 10 | 10 |
| 0.1 | 15 | 8 | 5 | 5 | 5 | 5 | 5 |
| 0.05 | 8 | 4 | 2 | 2 | 2 | 2 | 2 |
Packing & Shipment
After inspection, qualified products will be placed to small paper boxes and then put these boxes inside cartons. For some occasions, we’ll use plywood case or pallet to protect these cartons from damaging during long transshipment.