Why AC is preferred over DC | Why is AC power used instead of DC?

Why is Alternating current or AC preferred over Direct Current or DC

In our early days the main power distribution used to be carried out by DC or direct current. The direct current distribution was developed by Thomas Edition. 

Later on one of his employees, Nicolas Tesla developed alternating current or AC. Gradually AC or alternating current overtook the direct current distribution or DC distribution. 

Now the question arises that why direct current or DC usage got reduced and Alternating Current or AC gained popularity. There are many advantages of AC power over DC. 

In this article, we will be discussing those advantages.

The first reason Alternating Current or AC has such an application is

1. The induction phenomenon.

Due to this induction, the Transformer is introduced which can easily change the voltage level. Changing voltage is very much essential for a successful Power system. Why??? Let’s see……

We know that Power=Voltage×Current. Now if we want to transfer a huge amount of power and we don’t increase the voltage, the current will increase. The high value of current will cause high power loss, voltage drop, heating, and heavy conductor cross-section. Overall causing a poor power system. Hence we need to increase the voltage for sending the power. Again at the receiving end, we need to reduce back the voltage at our desired utilization level. So frequent changes in voltage are desired for a healthy power system. Now, this process becomes very easy with a transformer and Alternating Current or AC.

But changing in voltage with DC is very difficult as it needs a power electronics device.

So that is a very important reason we are using AC over DC.

2. Circuit Breaking Ease.

Alternating Current or AC changes its magnitude with respect to time i.e. it starts from zero, goes to its maximum value, then comebacks to zero then goes to negative magnitude (i.e. changing the direction of flow), and again goes to zero. So for completing a cycle an AC wave crosses three no zeros. At those points, the circuit has no voltage, and therefore no current or arc. So the insulation at the breakpoint can be easily established at the point zero crossing, making the circuit breaker operate with less difficulty.

AC signal with zero crossing

But in direct current or DC breaker as DC has no zero crossing it is difficult to extinguish the arc and re-establish the insulation.

DC signal with no zero crossing

So for easy circuit Breaking and fault isolation, AC is preferred over DC.

3. Low Cost of Metering instrument for AC than DC.

In the AC system, we have an instrument transformer that reduces the voltage and current at a lower level suitable for measurements. But in a direct current or DC system, there is no such Instrument transformer. So other types of sensors are required for measurements.

Also for AC measurement Moving Iron instruments are used which are less expensive than PMMC instruments that are used for DC.

To overcome these difficulties in HVDC often the circuit Breaking and measurement is made at the AC side.

So these are the three major reasons why AC power is preferred over DC power.

Apart from these, there are also many factors like 

4. DC  equipment like motors and generators are bulky in size due to huge copper winding, and competitively AC machines are robust in size.

5. DC machines are doubly excited machines whereas AC machines are singly excited(induction motor)

6. No need to replace Carbon brushes as that required in the DC machine.

7. Due to the absence of Brushes there is no commutation problem in AC machines, no spark.

So these are the main reason why we shifted to an Alternating Current system from a direct current supply system.

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