3 phase induction motor

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  • Thread starter JGrecs
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  • #1
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3 PHASE INDUCTION MOTOR

A 15 hp/230V, three phase induction motor has a full load current of 42A per terminal.
15HP= 11kW

find:
a) Full load kVA input
b) If the full load power factor is lagging 0.85, what is the kW input.

My question: How do I know whether its connected Wye or Delta?


a) kVA= 1.732 X ELine X ILine

ELine= 230V ILine=IPhase (Wye) and ILine=IPhase X 1.732 (Delta)

b) P= 1.732 X Eline X ILineX PF
 
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Answers and Replies

  • #2
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A)

Wye: kVA= 1.732 X 230V X 40A

kVA= 16kVA

Delta: kVA= 1.732 X 230V X 69A

kVA= 27kVA

B)

Wye: P= 1.732 X 230V X 40A X 0.85

P= 13.5kW

Delta: P= 1.732 X 230V X 69A X 0.85

P= 23.4kW


How do these look?
 
  • #3
gneill
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The problem stated the motor current as 42A per terminal. So that would be the line current. The motor is said to be 230V, so I think you can take that as the line-to-line voltage.
 
  • #4
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Ok, so "per terminal" refers to "Line-to-Line"?
so Id go with these:

kVA= 1.732 X 230V X 40A
kVA= 16kVA

P= 1.732 X 230V X 40A X 0.85
P= 13.5kW

Also, the 15HP (11kW) stated in the original question. Would that be the motors rated power, while the 13.5kW calculated would be its input power?
 
  • #5
gneill
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Ok, so "per terminal" refers to "Line-to-Line"?
Well, at the very least it refers to the line current since the motor's terminals are where the lines connect. I'm less certain about the interpretation of the wording for the "230V, three phase...". It could refer to the "three phase voltage", making the individual line voltages about 133V. Or it could be the line-to-line voltage. [I hate these ambiguities due to topic-specific "common-knowledge" jargon!]
so Id go with these:

kVA= 1.732 X 230V X 40A
kVA= 16kVA

P= 1.732 X 230V X 40A X 0.85
P= 13.5kW
Yeah, looks good as far as I can tell.
Also, the 15HP (11kW) stated in the original question. Would that be the motors rated power, while the 13.5kW calculated would be its input power?
That would be my interpretation.
 
  • #6
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Well, at the very least it refers to the line current since the motor's terminals are where the lines connect. I'm less certain about the interpretation of the wording for the "230V, three phase...". It could refer to the "three phase voltage", making the individual line voltages about 133V. Or it could be the line-to-line voltage. [I hate these ambiguities due to topic-specific "common-knowledge" jargon!]

Yeah, looks good as far as I can tell.

That would be my interpretation.

Yeah my prof likes to throw us off with his wording heh.
Thanks again for your help.
I have 3 more questions to go. Ill post them in this thread If im stuck.
 
  • #8
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Nice! Good find :smile:
 

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