There are several types of power in ac circuit :. Consider the ac load in Figure. In terms of the rms values,. Thus we may write Equation. We notice from Equation. Also, we notice that the angle of the complex power is the power factor angle.
The complex power may be expressed in terms of the load impedance Z. The load impedance Z may be written as. Substituting this into Equation. Comparing Equation. The real power P is the average power in watts delivered to a load; it is the only useful power. It is the actual power dissipated by the load. The reactive power Q is a measure of the energy exchange between the source and the reactive part of the load. The unit of Q is the volt-ampere reactive VAR to distinguish it from the real power, whose unit is the watt.
It represents a lossless interchange between the load and the source. Complex power for a capacitive load is given as below.
To better understand, let us consider the voltage and current of a load to be V and I respectively. Assume the load to be capacitive. Mind that load is not purely capacitive rather capacitance is dominating in load. As we know that, active power P and reactive power Q is given as. Therefore, complex power can be written form 1 as shown below.
Carefully observe the above expression. This is the reason reactive power is called imaginary power. Consider following RLC circuit Figure-3 :. In this circuit, value of inductive reactance and capacitive reactance is same. If you will calculate Power Factor PF of this circuit, then you will find it unity. Energy stored by the inductor is as written earlier. It implies that in a particular quarter cycle if inductor is storing the energy, at the same time capacitor is releasing the energy.
In next quarter cycle reverse will happen, i. It is called:. Inductor consumes reactive power and capacitor generates reactive power.
But it is an energy exchange between two elements. No true power is consumed or generated; this is the reason reactive power Q is called imaginary power.
Note that:. Now you can understand easily that in this case half of the total reactive power consumed by inductor will be supplied by voltage source and half reactive power will be supplied by capacitor.
It implies that inductor will exchange the energy both with voltage source and capacitor. Consider the circuit diagram shown in Figure Value of current shown in the figure can be verified by the readers, it is —j5 A. If one ammeter is inserted in the circuit, what will be its reading?
Its reading will be 5 Amp. So, for the purpose of energy exchange, between voltage source and inductor, current flows in the circuit. You can understand that inductor is consuming reactive power imaginary power , no true power is consumed despite that flow of current is necessary. If wattmeter is inserted in this circuit current coil in series with voltage source and pressure coil across voltage source , its reading can be found to be zero. Consider Figure Synchronous generator is taking 20 KW Mechanical input through steam supply , its total losses are 2 KW and electrical output is 18 KW.
Synchronous generator is supplying 10 KVAR reactive power, but it can be seen that to generate reactive power, no mechanical input is required. With the figure-6, you can understand synchronous generator is generating reactive power if running at high DC excitation and induction motor is consuming reactive power, but in fact it is just an energy exchange between generator and induction motor.
Answer is It can be calculated as. Their addition follows the equation:. In your house, suppose you have an air-conditioner, having consumption of 2 kW power.
You may require a stabilizer voltage controller with this. Stabilizer is just a transformer. Stabilizer should be installed between voltage supply and the air-conditioner. What may be the rating of stabilizer? Rating of stabilizer maybe around 4 KVA. The reason is that air-conditioner consumes reactive power also it has a motor inside it.
So, the kVA rating of stabilizer is certainly more than 2 kVA. Though the reactive power is imaginary power i. In most of the countries, the meters installed in residential areas are KW-Hr meter; so you have to pay electricity bill of active power; reactive power is free; but any transformer or voltage controller is to be installed accommodates reactive power also, hence rated in kVA apparent power. Therefore in aforesaid example, a consumer has to pay bill of 2 kW power as it is true power , reactive power is free as it is imaginary power , but while installing any transformer or voltage controller, reactive power cannot be ignored.
Note a few more important points:. The answer is that it has field winding, which can increase the flux in the synchronous motor.
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