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3 Phase Calculation

Three phase current can be calculated by converting the three phase problem into an equivalent single phase problem. To do this, take the total power (kW or kVA) and divide it by three to get the power per phase. Then use the single phase formula of kVA = kW / power factor to calculate current per phase. Finally, multiply the current by three to get the total three phase current. This straightforward approach avoids complex formulas and relies on basic principles.

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sach6inspired
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299 views1 page

3 Phase Calculation

Three phase current can be calculated by converting the three phase problem into an equivalent single phase problem. To do this, take the total power (kW or kVA) and divide it by three to get the power per phase. Then use the single phase formula of kVA = kW / power factor to calculate current per phase. Finally, multiply the current by three to get the total three phase current. This straightforward approach avoids complex formulas and relies on basic principles.

Uploaded by

sach6inspired
Copyright
© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as DOC, PDF, TXT or read online on Scribd
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Three Phase Current - Simple Calculation

The calculation of current in a three phase system has been brought up on our forums and is a discussion I seem to get involved in every now and again. While some colleagues prefer to remember formulas or factors, my approach is to do resolve the problem step by step using basic principles. I thought it would be good to write this down in case it may prove useful to someone else.Three Phase Power and Current The power taken by a circuit (single or three phase) is measured in watts W (or kW). The product of the voltage and current is the apparent power and measured in VA (or kVA) . The relationship between kVA and kW is the power factor (pf): y kW = kVA x pf

Single phase system - this is the easiest to deal with. Given the kW and power factor the kVA can be easily worked out. The current is simply the kVA divided by the voltage. As an example, consider a load consuming 23 kW of power at 230 V and a power factor of 0.86: y y kVA = kW / power factor = 23/0.86 = 26.7 kVA (26700 VA) Current = VA / voltage = 26700/230 = 116 A

Three phase system - The main difference between a three phase system and a single phase system is the voltage. In a three phase system we have the line to line voltage (VLL) and the phase voltage (VLN), related by: y VLL = 3 x VLN

I find that the easiest way to solve three phase problems is to convert them to a single phase problem. Take a three phase motor (with three windings, each identical) consuming a given kW. The kW per winding (single phase) has to be the total divided by 3. Similarly a transformer (with three windings, each identical) supplying a given kVA will have each winding supplying a third of the total power. To convert a three phase problem to a single phase problem take the total kW (or kVA) and divide by three. As an example, consider a balanced three phase load consuming 36 kW at a power factor of 0.86 and voltage of 400 V (VLL) and 230 V (VLN): y y three phase power is 36 kW, single phase power = 36/3 = 12 kW now follow the above single phase method o kVA = kW / power factor = 12/0.86 = 13.9 kVA (13900 VA) o Current = VA / voltage = 13900/230 = 60 A

Easy enough. To find the power given current, multiply by the voltage and then the power factor to convert to W. For a three phase system multiply by three to get the total power. Using Formulas The above method relies on remembering a few simple principals and manipulating the problem to give the answer. More traditionally formulas may be used to give the same result. These can be easily derived from the above, giving for example: y I = kW / ( 3 x pf x VLL ), in kA

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