Why AC is preferred over DC

 Why is Alternating current or AC is 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 employee Nicolas Tesla developed alternating current or AC. Gradually AC or alternating current over took the direct current distribution or DC distribution. 

Now 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 about those advantages.

The first reason of Alternating Current or AC having such application is

1. The induction phenomenon.

Due to this induction the Transformer is introduced which can easily change the voltage level. Changes 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. High value of current will cause high power loss, voltage drop, heating, heavy conductor cross section. Overall causing 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 utilisation level. So frequent changes in voltage is desired for a healthy power system. Now this process becomes very easy with transformer and Alternating Current or AC.

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

So that is 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 come backs to zero, then goes to negetive 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 point of the circuit has no voltage, and therefore no current or arc. So the insulation at the break point can be easily established at the point zero crossing, making the circuit breaker to operate with less difficulty.

AC signal with zero crossing
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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
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So for easy circuit Breaking and fault isolation AC is preferred over DC.

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

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

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

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

So these are the three major reason for which AC power is preferred over DC power.

Apart from these there are also may factors like 

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

5. DC machines are doubly excited machine where as AC machines are singly excited(induction motor)

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

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

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

Hope you have enjoyed this article.

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