Friday, April 5, 2013

Voltage Regulation



What is Voltage Regulation?

In power engineering (basically in electrical engineering), Voltage Regulation describes the ability of a system to provide near constant voltage over a wide range of load conditions. It is a measure of change in voltage magnitude between sending and receiving end in case of a transmission line or between load and no load in a power supply. Voltage Regulation is also a figure of merit used for comparison one machine with another.

For example, let’s think about a computer power supply. It has many leads that provides many different voltages like +12V(Yellow), -12V(Blue), 5V(Red), 3.3V(Orange), 0V(Black) etc. The power supply gets these voltages by converting the input 110V/220V AC. Now it is known that terminal voltage changes with the loads connected with it. But it is very important to keep the voltage levels steady in a computer as it is very sensitive machine. It won’t start up if it can’t get proper voltage. So, constant (or near constant) output voltage must be served by the computer power supply unit. If power supply unit can’t provide near constant voltage over wide range of load, computer won’t run or might not function properly.

We know voltage provided at the output under no load condition (no current is drawn from the supply) is reduced when load current is drawn from supply (under load). The amount the DC voltage changes between the no-load and load conditions is described by the factor called Voltage Regulation.

The load on a large power transformer in a sub-station will vary small value in the early hours of the morning to a very high value during the heavy peaks of maximum industrial and commercial activity. The transformer’s secondary voltage will vary somewhat with load and because motors, incandescent lamps and heating devices are all quite sensitive to voltage changes. So transformer’s voltage regulation is an important thing.

A smaller value of Voltage Regulation is usually beneficial. Because it describes a small amount of deviation from its rated voltage.

It can be seen by our own eyes too. Sometimes during peak hours we may observe lights to be a bit dim than that of normal hours.There is another way to make sure too. A simple multi-meter can be used to measure the AC main line voltages throughout the day and a difference in readings will be found. I have personally measured this and found out voltage fluctuates from 195V-227V where the line voltage should be at 220V.


How it is measured:

Voltage Regulation is given by:

Voltage Regulation = (No load voltage – full load voltage)/full load voltage



%VR is the percentage of voltage regulation,
Vnl is terminal voltage of no load and
Vfl is terminal voltage of full load.
Full load means when load start to draw transformer's (generally for power supplies) rated output current.


An Example:

Assume a DC power supply provides 120V when output is unloaded. When connected to a load, output voltage drops to 112V. Calculate the value of voltage regulation.

Solution:

%V.R. = (Vnl – Vfl)/Vfl = ((120V-112V)/112V) * 100% = 7.1%



Why this occurs? 

In transformers the effect of leakage flux and winding resistance cause internal voltage drop that results in different output voltages for different loads. From Ohm's Law we know that more current through a resistance will cause more voltage drop across it, as more voltage drops in the winding of the transformer we have less voltage on the output of it under heavy load condition.


Sources:

1. Electric Machines, Second Edition by Charles I. Hubert.
2. Wikipedia.


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