Motor Starting Current and it's Time Calculation

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cmaguna Member Join Date: Oct 2010 Posts: 9	Motor Starting Current and it's Time Calculation 10/25/2010 1:48 AM
	What is the calculation of Motor starting current ? how much time current will remain high during starting ? Is there any calculation. I need equations of motor starting current and time.
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asadmawa Active Contributor Join Date: Oct 2010 Location: Libya(Bangladeshi) Posts: 15	#1 Re: Motor Starting current and it's time calculation. 10/25/2010 3:36 AM
	starting current less than 10second and starting current will be same like short circuit current,since in starting condition there is no back emf,I(starting)=(V-back emf)/Resistance,at starting back emf is zero ,so if your supply voltage more and coil resistance small then starting current will be more.always remember that in starting rotor terminal act as short terminal.normally we can consider starting current will be 4 to 6 times of nominal current,if rating is big then can use star-delta starter. __________________ asadmawa
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JRaef Guru Join Date: Sep 2006 Location: California, USA, where the Godless live next door to God. Posts: 4665 Good Answers: 804	#2 Re: Motor Starting Current and it's Time Calculation 10/25/2010 11:49 AM
	Starting current and time can only be calculated if you know almost everything about the load connected to the motor, the type of starter being used and the power system feeding it, including the acceptable voltage drop. Without that information everything is just a guess. Acceleration time however can be expressed as follows: (sorry, ANSI units is all I have available) Seconds = WK² (lb.-ft.) x Speed Change (RPM) / 308 x Avg. Accelerating Torque WK2 = Load Inertia x Load RPM² / Motor RPM² AAT = {(FLT + BDT)/2} + BDT + LRT / Motor RPM² But the AAT is seriously affected by the starting method, as is the current, so everything is tied to that. The best way to do this is to perform a Transient Motor Starting analysis, now usually done with power systems engineering software such as ETAP or SKM. __________________ All I every really wanted to be, was... A LUMBERJACK!.
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electricalexpert65 Guru Join Date: Oct 2009 Posts: 2061 Good Answers: 169	#3 Re: Motor Starting Current and it's Time Calculation 10/25/2010 9:20 PM
	Starting Current is a phenomnenon of the motor and it is normally given by the motor manufacturer is his motor catalogue/data sheet. Whereas starting time is a phenomenon of the connected load and for this you have to study the speed-torque charecteristics of the motor and of the connected load.
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