10Hp, 3-phase, 415v, 50Hz induction motor. The above motor is to run on Star-De

10Hp, 3-phase, 415v, 50Hz induction motor.
The above motor is to run on Star-Delta connection, so how
do i make following hardware selection
1) MCB, 2) Contactor, 3) Wire Gage (Depending on P-P current
or Line Current), 4) OLR,
Please tell in detail how to do the above, so as to get the
ans. of it even if the Hp rating changes.

Question Posted / girish bodhale

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10Hp, 3-phase, 415v, 50Hz induction motor. The above motor is to run on Star-Delta connection, so h..

Answer / 7anoter4

10 hp=7.5 kW. The manufacturer has to indicate the starting
current.
Let's say the motor is designed for star/delta starting
then according to IEC 60034-12 Tab2 for 6.3 kw up to 25 kw
S1=12 then maximum permissible starting current will be
Istart=12*7.5*1000/1.73/415=125.2 A.
The short-circuit protection has to be set at
1.25*125.2=156.5 A.
The circuit breaker rating has to be 150 A. The motor
overload has to be set at 1.25*Irated and also will trip
the breaker if the current of 2*Irated will last more than
the permissible stall time [let’s say 4 sec].
The supply cable allowable current [see IEC 60364-5-52]
shall be 1.25*Irated.
Usually a 7.5 kW motor at 415 V rated current will be 13-16
A then the cable permissible minimum should be 20 A.
According to the above mentioned standard 2.5 mm^2 copper
conductor PVC insulated will be ok.
You have to check the voltage drop also: no more than 3%
rated and 10% at start. If DV=1.73*I*[R*cos(fi)+X*sin
(fi)].R,X =total resistance-at 70 dgr.C- and reactance
[ohm].
cos(fi)=0.85 [approx.] rated and 0.3 at start. Sin(fi)=
(1-cos(fi)^2)^0.5.
In this case if the cable length will be 80 m 4 mm^2 copper
conductor has to be employed.

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