# 3 Phase System - Input Power and Input Current calculation

#### mishra87

Joined Jan 17, 2016
1,037
Hi All,
I want to calculate the input power and input current for forward converter.
Specification :
Output Power : 80W
Efficiency : 80% Assuming.
Line Voltage : 230Vrms (assuming single phase line voltage 230Vrms)

#### Papabravo

Joined Feb 24, 2006
21,225
Hi All,
I want to calculate the input power and input current for forward converter.
Specification :
Output Power : 80W
Efficiency : 80% Assuming.
Line Voltage : 230Vrms (assuming single phase line voltage 230Vrms)
The first order approximation would be output power divided by efficiency.

$$P_{IN}\;=\;P_{OUT}/\text{Eff}\;=\;80\text{ watts}/0.8\;=\;100\text{ watts}$$

Does that make sense?

#### mishra87

Joined Jan 17, 2016
1,037
The first order approximation would be output power divided by efficiency.

$$P_{IN}\;=\;P_{OUT}/\text{Eff}\;=\;80\text{ watts}/0.8\;=\;100\text{ watts}$$

Does that make sense?
yes, this makes sense.

Now lets say L1 = 230Vrms
L1-L2 = 230*1.73 = 398V
L1-L3 = 398V
L2-L3 =398V
In this case how to calculate the input current ?

#### Papabravo

Joined Feb 24, 2006
21,225
yes, this makes sense.

Now lets say L1 = 230Vrms
L1-L2 = 230*1.73 = 398V
L1-L3 = 398V
L2-L3 =398V
In this case how to calculate the input current ?
I'm not that familiar with 3 phase calculations but there should be an RMS current, that when multiplied by 230 Vrms gives a power of 100 watts. Could it be approximately 0.435 Amperes(rms)? Probably not, but you need to run that down.

Last edited:

#### mishra87

Joined Jan 17, 2016
1,037
I'm not that familiar with 3 phase calculations but there should be an RMS current, that when multiplied by 230 Vrms gives a power of 100 watts. Could it be approximately 0.435 Amperes(rms)? Probably not, but you need to run that down.
Looking forward some expert to jump in to the thread who has experience in 3 phase system.
Thanks !

#### Ian0

Joined Aug 7, 2020
9,817
IF you have a standard 6-pulse rectifier, then you will get a bus voltage of 565V DC, with ripple at 300Hz.
If the input power is 100W then there will be 33W supplied by each phase, so each phase current is 33W/230V = 0.143A at best but the power factor may not be too good.

#### mishra87

Joined Jan 17, 2016
1,037
IF you have a standard 6-pulse rectifier, then you will get a bus voltage of 565V DC, with ripple at 300Hz.
If the input power is 100W then there will be 33W supplied by each phase, so each phase current is 33W/230V = 0.143A at best but the power factor may not be too good.
Hey Ian0,
Thanks you so much for joining the discussion !
Yes I have 6 diode for rectification.

#### Ian0

Joined Aug 7, 2020
9,817
You should get this rather odd looking waveform for the current.

#### mishra87

Joined Jan 17, 2016
1,037
Power Factor is not considered here in calculation.

#### Ian0

Joined Aug 7, 2020
9,817
Power Factor is not considered here in calculation.
How can you avoid it? Power = Voltage * Current * Power Factor