We already know that alternating current has the following properties:
1. The size and direction are changed periodically, and the average value is zero;There are three elements: amplitude, angular frequency and initial phase;
2. The ways to describe alternating current include instantaneous value representation, waveform, RMS and vector method;
3. Different alternating current may be in the same phase, inverse phase, orthogonal, or different Angle;
4. Alternating current through resistance, inductance, capacitance and their combination of circuits, the performance of different properties, mainly reflected in phase, impedance, power;
The above four points are different from direct current, so alternating current has its own formulas, methods and properties in calculation.
Well, after reviewing the above problems, we can proceed to the following study;The difference of resistance, inductance and capacitance lies in the following 6 aspects, we will explain and compare them one by one.
1. Instantaneous value relation: u=RI, that is, the voltage passing through the resistance is equal to the product of the current passing through and the resistance.This formula is rarely used in practice. It reflects the instantaneous value relationship between voltage and current.
2.RMS relationship: the resistance is equal to the ratio of voltage and current. Note that both voltage and current here are RMS.
3. Impedance: the impedance of a resistance is resistance. Why does it sound so awkward?As we have said before, impedance includes resistance, inductive reactance, and capacitive reactance, which are specific to different reactance elements, and which are specific to alternating current;
4. Vector diagram, from which we can see that the voltage and current applied to the resistance are in phase relation;
5. Power is equal to the product of voltage and current, or the product of current squared and resistance, P=UI, per watt (W). This power is called active power, which is the actual power that consumes electric energy.
6 power factor: we know that the power factor is the cosine of voltage and current phase difference, due to the voltage and current applied on the resistance with phase, namely phase difference is zero, then its power factor is cosine phi =1, which is the maximum;
1. Instantaneous value relationship
That is, the voltage applied to the alternating current is proportional to the rate of change of the current.This formula is worth remembering;And it's proportional to the rate of change, not the change, not the fixed value, which is different from resistance;
2.RMS relationship: the current applied to the inductance is proportional to the voltage, equal to inductive reactance.
3. Impedance: inductance ACTS as a barrier to the current, and its value is expressed by inductive reactance as XL=2 PI fL, f ac frequency, L inductance;It can be seen from the formula that the greater the ac frequency, the greater the inductive resistance of the inductance to it, this is the ac through direct isolation function, with this function can be used for filtering;
4. Vector diagram: it can be seen from the diagram that the voltage applied to the inductance is 90° ahead of the current
5. Power, active power P=0, that is, the active power of the inductor is zero. It is not an energy-consuming element, but an energy-storing element.
6. Power factor: since the voltage applied to the inductance is 90° ahead of the current, that is, the phase difference of the two 90°, then cos phi =0;
Iii. Capacitance C
1. Instantaneous value relationship
That is, the current flowing through the capacitor is proportional to the rate of change of the voltage at both ends.This formula is worth remembering;
2.RMS relationship: the current applied to the capacitor is proportional to the voltage and equals to the capacitive reactance. Please note that both voltage and current are RMS.
3. The impedance, capacitor and resistor, inductor, also hinder against the current, the size of its value in capacitive reactance, f the frequency of the alternating current, capacitance C, through the formula we can see that the frequency of the alternating current is larger, the smaller the capacitive reactance, direct current frequency is zero, the capacitive reactance is infinite, this is the capacitance of the pass-through effect.
4. Vector diagram, from which it can be seen that the voltage applied to the capacitor lags behind the current by 90°
5. Power,P=0, that is, the active power of both capacitor and inductor is zero, and it is not an energy-consuming element either. Like inductor, it is an energy-storing element, and its energy-storing capacity is expressed as QL=UI in terms of reactive power, which is equal to the product of voltage and current, or the product of current square and capacitive reactance.
6. Power factor, because the voltage applied to the capacitor is 90° ahead of the current, that is, the phase difference of the two 90°, then cos phi =0;
Through the above analysis, we can see that alternating current through resistance, inductance, capacitance showed the property is very different, it is this difference, make their combination of property diversification, show a variety of properties, in the electronic circuit has been widely used, such as frequency selection, shift equality.
Post time: Aug-17-2019