How to use a multimeter to measure the polarity of a current transformer (CT)?
Current transformer, CT for short, which stands for current transformer. In the power system, electromagnetic current transformers (hereinafter referred to as current transformers) are widely used, and their working principle is similar to that of a transformer.
The characteristics of a current transformer are as follows: (1) The primary coil is connected in series in the circuit, and the number of turns is very few. Therefore, the current in the primary coil completely depends on the load current of the circuit under test and has nothing to do with the secondary current. (2) The impedance of the current coils of the instruments and relays connected to the secondary coil of the current transformer is very small. So under normal circumstances, the current transformer operates in a state close to short circuit.
The ratio of the rated primary current to the rated secondary current of the current transformer is called the rated transformation ratio of the current transformer: kn = I1n/I2n.
Since the rated primary current I1n of the primary coil has been standardized, and the rated secondary current I2n of the secondary coil is uniformly set to 5 (1 or 0.5) amperes, the rated transformation ratio of the current transformer has also been standardized. kn can also be approximately expressed as the ratio of the number of turns of the primary and secondary coils of the transformer, that is, kn ≈ kN = N1/N2, where N1 and N2 are the number of turns of the primary and secondary coils.
The function of a current transformer is to measure relatively large currents.
To put it simply, if you want to measure a current of 400A, and there is no ammeter with a range of 400A, what should you do? So you need to use a current transformer to transform the large current into a small current through the transformer and then input it into the ammeter. The transformer has a transformation ratio. For example, for a 200/5 transformer, it means that the ammeter reading needs to be multiplied by 40. That is to say, if the ammeter shows that one kilowatt-hour of electricity has been consumed, in fact, 40 kilowatt-hours of electricity have been used. There are many grades of transformers, such as 15/5, 30/5, 50/5, 75/5, and so on.
For on-site work, the best way to determine the polarity of the transformer is to use a dry battery, a pointer multimeter MF47, and some wires. This is the most direct and obvious method, and the accuracy can reach more than 90%. The method is very simple. The primary side of the transformer is marked with P1 and P2, and the secondary side is mostly marked with S1, S2, K1, K2, etc. Connect the positive pole of the battery to P1, and connect the positive pole of the secondary side to S1, and connect the negative pole to the other end. Connect the secondary side directly. Then, apply a direct current to the primary side of the transformer in a short-circuit manner using the battery. At this time, you will see the pointer of the multimeter swing. If it swings clockwise, it is a positive polarity; if it swings counterclockwise, it is a negative polarity, indicating that there is an error in the internal winding. At this time, you need to observe carefully because the swing of the pointer is not large. It is recommended to use two No. 1 dry batteries in the same way, because using too many batteries is too dangerous. And stop the test for about half an hour or replace the batteries after about 10 tests, because the batteries will heat up severely during the test. It is also recommended not to be lazy and use 12V or 6V power converted from alternating current, thinking that you can save batteries. But in this way, the error will increase and the safety will decrease.
