Introduction
Ampere-turn is the unit of magnetic potential Fm, which can be calculated according to the required core magnetic flux Ф and the total magnetic resistance Rm of the core magnetic circuit, Fm=IN=ФRm. Among them, N is the number of turns of the coil, and I is the current flowing through the coil.
If the magnetic circuit is segmented and uniform, the magnetic resistance Rm=l/(μS), where: l is the length of each section of the magnetic circuit (in meters), and μ is the material used in the magnetic circuit Permeability, S is the cross-sectional area of each section of the magnetic circuit (in square meters).
If it is an AC magnetic circuit, the magnetic flux of the iron core is generally calculated based on the voltage of the coil. If the leakage impedance of the coil is neglected, the coil voltage is approximately equal to the electromotive force induced by it, then U≈E=4.44NfФ. In other words, Ф≈U/(4.44Nf). Among them, f is the frequency of the AC power source.
If it is a DC magnetic circuit, the magnetic flux Ф will be calculated according to the suction requirements, and the shape of the core is different, and the calculation method will be different. For the π-shaped iron core, if the suction is represented by the letter Fx, then Fx=0.5×SoBo^2/μo≈400000×So×Bo^2. Among them, So is the cross-sectional area (in square meters) at the magnetic circuit break, and Bo is the magnetic flux density at the magnetic circuit break, namely Bo=Ф/So. Generally, Bo can be taken from 1.2 to 1.8 (Tesla). In this way, according to the suction force Fx, So and Ф can be calculated, and the magnetic resistance Rm of the magnetic circuit can be calculated. With Ф and Rm, the magnetic potential of the coil Fm=IN=ФRm and the number of ampere turns can be calculated.
The essence of calculation
On any closed line perpendicular to the cross section of an infinitely long energized wire, as shown in the figure "Magnetic Field Generated by an Energized Wire":
p>If there are N wires carrying I current in the closed line, there are:
It is the magnetomotive force, that is Magnetomotive force is expressed in ampere-turns.
The magnetic potential difference between the two ends of the solenoid is approximately as follows:
It can be seen that the magnetic potential difference between the two ends of the solenoid is only approximately equal to (1 /2) Nl, the unit of measurement is also A. That is to say, the magnetomotive force generated by one turn (or one turn) with 2A current is different from the magnetomotive force generated by 1000 turns (or 1000 turns) with 2mA current. The magnetomotive force is also different. It is related to the geometry of the coil. In some cases, it cannot be simply expressed in amperes (A) of the current, but expressed in ampere-turns. In other words, ampere-turn is the engineering measurement unit of magnetomotive force generated by the coil.
Balance principle
Ampere-turn balance is actually called magnetic potential balance. Generally used when analyzing the magnetic circuit of transformers and asynchronous motors. If the primary winding of the transformer is connected to the power supply, magnetic flux will be generated in the magnetic circuit, and the primary and secondary windings will induce electromotive force to balance the power supply voltage. Generally, the power supply voltage does not change much, and the magnetic flux in the magnetic circuit will basically remain unchanged.
After the load, there is a current flowing in the secondary winding, which will also generate a magnetic potential, which will affect the magnetic flux in the magnetic circuit. This affects the value of the induced electromotive force of the primary and secondary windings. When the magnetic flux in the magnetic circuit changes, since the power supply voltage does not change, the current in the primary winding will automatically change to keep the magnetic flux in the magnetic circuit unchanged, and the change in the primary winding current will cause the magnetic potential in the magnetic circuit to change. The change basically compensates for the influence of the secondary winding on the magnetic flux in the magnetic circuit caused by the load change, that is, the magnetic potential generated by the primary and secondary windings are balanced.
And because the magnetic potential is equal to the product of the amperage of the current and the number of turns of the winding. Therefore, the magnetic potential balance is also called ampere-turn balance. The situation of the magnetic circuit of the asynchronous motor is basically similar to the above analysis of the transformer.
Application
The current clamp meter calibrator basically adopts the principle of ampere-turn balance method, that is, a standard current is applied to a current coil passing through the clamp meter to correct The clamp meter is checked. The formula is:
where: I1 is the standard current applied to the coil; N1 is the number of turns of the coil; I2 is the secondary current of the clamp meter; N2 is the second current of the clamp meter The number of turns of the secondary coil is connected to the current measuring circuit.
Capacity
The through-core current transformer is a common device. Because of its simple wiring and easy installation, it is widely used in juice walls, detection and protection circuits. However, a little carelessness in use can cause great errors and cause inaccurate measurement and protection failure. All of this is related to the ampere-turn capacity of the current transformer.
The so-called ampere-turn capacity refers to the maximum rated current value of the single core on the primary side of the current transformer. That is, the product of the constant current and the number of turns through the heart.
If the model is LMZJ-0.5400 ampere-turn, it means that the maximum current of the primary side single-turn through-core is 40A. If two-turns are through-wind, the primary side rated current is 200A. It is used in conjunction with the detection ammeter. It not only shows the rated current working range of the primary side of the current transformer, but also implies the wiring method. If this problem is ignored and the wiring is simply based on the transformation ratio of the transformer, many unpredictable problems will occur. As a result, inaccurate measurement, protection failure, and even electrical accidents occur.