Why DC Voltage is preferred for Control circuit? 

Why DC Voltage is preferred for Control circuit?  Advantage Disadvantage:

In industry mostly 110 VDC uses to control all important circuit. Example Protection system, Process Control system, PLC control system, all feedback system, etc. First, all electrical engineer must know why dc voltage is preferred for these application instead of AC Voltage. There are 8 amazing advantages in DC control system over AC Control System. Let’s start,

1) DC Voltage do not suffer on inrush current, But AC power system, during switching time such as Turn on Circuit breaker, starting heavy inductive load etc at that time starting, the AC loads takes 5 to 7 times of its full load current due to this, all neighboring feeders may suffer in voltage fluctuation. All control systems are more sensitive if small milli second of voltage fluctuation may cause auto restarting equipment. Guys this is my personal experience.

My Case Study:

I was working in a distillery plant and which capacity is 95 KLPD (Kilo Liter per day). In this, we have installed new PLC automatic control system. At that time, we have taken power supply from UPS (230V AC) and we had erected and Commissioned successfully. After some days we had problem with PLC. The problem was the PLC was auto restarting whenever the grid failure. But we thought ok normally there will be a problem in PLC inner circuit. So we called PLC manufacturer, he had checked all the circuit found really good working and he had gone.
After a week, again we face the same problem, we little bit confused. So we had planned to trace the incoming supply, at that time we found the real culprit. Actually, a 28 W Emergency tube light was connected in the same tapping which was connected with the PLC supply. So, what happened is, whenever the grid supply failure, the emergency circuit will be turned on. (the circuit was arranged such as) Which means, whenever the tube light circuit is turned on at that time we faced the problem of PLC restarting due to power fluctuation. Also the UPS capacity was 10 KVA two set Parallel well synchronized and Connected load was only 28 Watts tube light.

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Solution: We Removed PLC’s UPS supply and alternative 110 DC has tapped from Battery circuit and it is still working good.

2) DC control system do not require Big UPS i.e It does not need costly inverter circuit to take supply from battery. You can use directly DC supply to your control circuit.

3) DC supply system does not suffer on harmonics but in AC control system receives more harmonics.

4) DC control system has high reliable.

5) Maintenance cost is less as compared with AC system. It consists of diode system with battery unit and maintaining Simple diode rectifier system is enough throughout a year, also It does not require skilled technicians. You do not want to provide additional AMC (annual maintenance charges)

6) AC UPS control (In AC has inverter Circuit, Inverter produces lot of heat energy during switching and the temperature of the inverter should be less than 27 deg) needs special type of cooling system such as Air conditioner, fresh air system etc. But Battery does not need such control system. Simple exhaust fan cooling is enough throughout of that life.

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7) Efficiency of the DC system is high. The output of the DC supply is directly fed to the utility, here does not requires any inverter circuit, Wave shaping circuit, etc. So we do not want to pay any switching losses or any additional loss. As per theory of electrical, If the losses decrease the efficiency of the system get increases.

8) You can feed DC supply for Long distance without dropping, and AC supply drops due to line inductance. This is the major disadvantage of AC supply system.

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DC system has some disadvantage

Let’s see..

1) You should operate DC system with safe condition otherwise you feel heavy shock, and may lead to death

Check here the human with stand able Current Limit:

2) Testing cable liveness: You can not check DC supply without millimeter or ammeter, Normal household tester does not work on DC supply.

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3) Circuit Fault Tracing: In DC supply takes longer time as compared with AC supply system. DC system need –ve point should be looped perfectly otherwise it is difficult to identify the circuit fault. But In AC system the current return loop is neutral and the neutral is nothing but a ground, you can take ground point at anywhere else.


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