Motor Fla

[plc noob]

Quick question.Is ac motor FLA divided equal among phases ? i.e. 30 amps fla is 10 amps per phase? .

 

[panic mode]

equal yes but not like that. if the label says 30A, then it is 30A per phase. what is the motor voltage and HP rating?

P=1.73*U*I*cos(phi)

P=power
U=voltage
I=current
1.73=sqrt(3)
cos(phi) efficiency (U/I angle)

 

[plc noob]

Hp 15
rpm 1760
fla 19.4
sf 1.15
insulation classs f
duty cont

 

[DickDV]

At full load on the motor shaft, expect to find very close to 19.4 amps in each of the three motor phase leads. The motor shaft speed will be close to 1760rpm

Just as a point of interest, at no load, the shaft speed will rise to very close to 1800rpm and the current in each motor lead will drop to around 6 amps.

 

[Brownhat]

And as an example as to why you wouldn’t consider FLA as the sum of the current in each phase; consider plc_noob’s motor running at full load.

Take a clamp-on current meter and clamp one conductor. You should read 19.4 amps.
Now put the clamp around two conductors. The meter will show? 19.4 amps.
Clamp all three conductors, and now the meter reads? 0 amps.

 

[plc noob]

Why would you get a reading of 0 amps with all 3 phases clamped?

 

[plc noob]

DickDv

You answered the second part of my question before i even asked thanks.

I was qurious as to how nameplate data was gathered with a load or no load.

 

[Clay B.]

Why would you get a reading of 0 amps with all 3 phases clamped?

Kirchoff's Law:
At every node, the sum of all currents entering a node must equal zero.

−i−i1=0 (1)
i1−i2=0 (2)
i+i2=0 (3)

When you clamp all 3 leads then it must equal zero because you are looking at the sum of all the currents at the same time.

Here is a good web page that explains this:

http://cnx.org/content/m0015/latest/