Home › Electrical Engineering Forum › General Discussion › Current rating calculation of any kind of motor
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2010/08/31 at 7:51 am #10203adminKeymaster
Hi………….
what is the accurate method to calculate current rating of any kind of motor.
because sometimes there is inclusion of efficiency during calculation and sometimes not.
2010/08/31 at 10:19 am #11372NV ManiParticipantFull load current in amperes = KW (Input power) / 1.732 x Volts(Input voltage)x power factor X efficiency
(if motor is 3 phase motor)2010/08/31 at 1:21 pm #11374ParadiesVogelParticipantFor 3phase motots I= KW * 1.8
1phase motor I= KW * 6
of course you can calculate it by the formulas as well.
2010/08/31 at 1:36 pm #11375monk123ParticipantHey, thanx..
@ n v mani: i knw these formulaes.
i jst want to knw in some cases efficiency is not considered , why does it so?
@paradies vogel: from which calculation 1.8 and 6 occurs in formula?
2010/08/31 at 10:03 pm #11376adminKeymasterHello,
The correct formulas were already mentioned, if you want to do it right you must consider power factor and the efficiency, allways
remember that when you read the kW in the motor plate, you are reading the output power.
In small motors the powerfactor and the efficiency are small, usually about 0,7 and 0,6.
Usually when the power factor and efficiency are unknow, people use 0,8 and 0,7 and some people forget about efficency ( very bad idea
for small motors ) and they do :
Pmec in kW
I = Pmec *10^3 / (1,73*400*0,8) = Pmec * 1/ (692 * 0,8 ) = Pmec * 1,8
2010/09/01 at 7:56 am #11377NV ManiParticipantWhen you say calculate current rating of the motor,it means current drawn by the motor from the mains is to be calculated.
For this you should know first what is the output rating in HP or KW.
From this known value of mechanical power output,you can know the electrical power input only by knowing the efficiency of conversion of electrical energy to mechanical energy.
From the known values of Mechanical power output and efficiency you can calculate the Input electrical energy in KWH.The line voltage for star connection is 1.732 times the phase value.If Power factor value is given,it is known or assumed as 0.8.
Now the formula,which you know, KWH = Line Current * Line Voltage * Power factor can be applied.
The only unknown value ‘current’ can be calculated.2010/09/01 at 6:41 pm #11328adminKeymasterHello,
What you mentioned it is not exactally correct, as kWh is energy , so as you mentioned :
“Now the formula,which you know, KWH = Line Current * Line Voltage * Power factor can be applied.
The only unknown value ‘current’ can be calculated.”
You must not use de time, as P= dW/dt, the correct is kW = Line Current * 1,73*Line Voltage * Power factor * efficiency.
2010/09/17 at 4:57 am #11460NhoyParticipanthi, can you tell me how to calculate KWH charge for charging a 2v 3000ah batteries (24 units in series), the batteries are fully drawn and need to be fully charge.
2011/01/13 at 10:55 am #11731adminKeymasterNV Mani said:
Full load current in amperes = KW (Input power) / 1.732 x Volts(Input voltage)x power factor X efficiency
(if motor is 3 phase motor)
Hi ,what the accurate method to set correct over load to protect the 3~ motor

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