Application scope and functional skill description for clamp ammeters
Usually, when measuring the current with an ordinary ammeter, it is necessary to cut off the circuit and shut down the circuit before connecting the ammeter for measurement. This is very troublesome, and sometimes the motor in normal operation does not allow this. At this time, it is much more convenient to use a clamp ammeter, which can measure the current without cutting off the circuit. The working principle of the clamp ammeter is as follows:
The measured circuit wire passing through the iron core becomes the primary coil of the current transformer, and the current is induced in the secondary coil by passing the current. So that the ammeter connected to the secondary coil has an indication ----- measure the current of the line under test. The clamp meter can be changed to different ranges by changing the gear of the switch. But it is not allowed to operate with electricity when shifting gears. The clamp meter is generally not high in accuracy, usually 2.5 to 5 grades. For the convenience of use, there are switches of different ranges in the meter for the function of measuring different levels of current and measuring voltage
When checking whether the three-phase current (less than 5 amps) of a small three-phase motor is balanced, if it is measured with a clamp ammeter, the minimum meter head is 1 to 10 amps, and some meters are 2 amps at the same time. It is even more impossible to measure below 2 amps or a few tenths of amps. In addition, the ammeter usually has a large error at a low range, and it is difficult to read when the pointer is deflected. In order to measure the current value accurately, the load insulated wire can be wound 1 to N turns on the ammeter jaw core. Apply the principle of current transformer to enhance the magnetic field, so as to read a larger current value. The reading obtained in this way is enlarged, but the real current value must be subtracted from the enlarged part. That is, when adding one turn, divide the reading by 2; when winding two turns, divide by 3. The rule is that the number of turns is N turns, the current is enlarged by N+1 times, and the current value = meter reading/(N+1). In addition, to eliminate the influence of residual magnetism in the core of the ammeter, after measuring a large current, if a small current is measured immediately, the core of the ammeter should be opened and closed several times to eliminate the residual magnetism in the core.
