I am looking for 3phase motor calculations..................... 1

[Bloodnock]

Its been donkeys years since I had to do this sorta thing, and needless to say I am as rusty as hell.

I want to be able to calculate the current of a three phase motor star and delta connected.
Assuming you take the details from the 'makers plate' such as Kw, and voltage.

Also if someone says they think the total load current of all motors is 237 amps on a 400v three phase circuit how do you work out what the current per phase is.

I know this should be basic stuff but all my old apprenticeship books just don't show what I need to know.

Any help would be gratefully recieved.

Thanks

In advance.

 

[Marke]

Hello
There is no difference in the current rating for a star connected motor and a delta connected motor provided that they are connected in the format for which they were designed and at the same voltage and equivilent ratings. i.e. a star designed and connected motor at 400 V will draw the same current as a delta designed and connected motor at 400V. This assumes comparable motors with the same PF and efficiency. If you take a motor designed for delta connection on 400 volts and then connect it in star on 400 volts, the current rating will drop considerably. The motor is effectively 1/3 of the rating when connected this way.

To calculate the current drawn by a motor, take the KW rating times 1000 and divide this by the line voltage (phase to phase) then divide by root 3 (1.732) then divide by the efficiency and then divide by the power factor.
i.e. a motor rated at 75KW with a voltage rating of 400V, an efficiency of 0.89 and a power factor of 0.91
75000 / 400/ 1.732 / 0.89 / 0.91 = 133.662 Amps

When you talk about the current drawn by a three phase motor, you are talking about the current in one phase. You do not add all three phase currents together to get a total current, infact if you add all currents together, you get zero current. - put a meter around all three phases and it will read zero!!

Best regards, Mark Empson

http://www.lmphotonics.com

 

[rwill19050]

Here are some useful 3 Phase fomulae.

1)
kW = kVA x power factor

2)
kW = (hp x 746) / (1000 x efficiency)

3)
kW = (line amps x line volts x 1.73 x pf) / 1000

4)
kVA = kW / pf

5)
kVA = (hp x 746) / (1000 x efficiency x pf)

6)
kVA = (line amps x line volts x 1.73) / 1000

7)
Line amps = (kW x 1000) / (line volts x 1.73 x pf)

8)
Line amps = (kVA x 1000) / line volts x 1.73)

9)
Line amps = (hp x 746)/ (Line volts x 1.73 x efficiency x pf)

10)
hp = (kW x 1000 x efficiency) / 746

11)
hp = (kVA x 1000 x feeficiency x pf) / 746

12)
hp = (line amps x line volts x 1.73 x efficiency x pf) / 746


One horse power = 550 foot pounds per second = 33,000 foot pounds per minute = 0.746 kW = 746 watts.

Power factor is usually taken by rule of thumb as 80% but this varies with size and speed of the motor. (OK for rough calcs)

The current required to supply a three phase moter is...
I = (brake horse power x 0.746 x 1000) / (1.732 x E x percent efficiency of motor x power factor)

Where E is the voltage between phases.

I trust you can get what you need from this info.

Regards
Richard Williams

 

[Bloodnock]

Thanks Guys
I was getting there slowly as it was but this brings it all back. Sheesh, how easy it is to forget

Thanks a million.

 

[dubairay]

Kw x 1000 divided by 1.732 divided by voltage divided by power factor will give you line current.