VFD Amps Line and Load Side

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eltech Power-User Join Date: Mar 2011 Location: oz Posts: 252 Good Answers: 10	VFD Amps Line and Load Side 03/16/2021 2:51 AM
	I have noticed that we have CT's on the line side of some of our VFD's, When the motor is set to 25Hz the line side amps is half the motor amps, I did say to operations team that these amps are wrong and does not indicate the actual motor amps. They are not convinced. Is there a simple explanation that I can use to back myself ?
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PWSlack Guru Join Date: Jan 2007 Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on. Posts: 32175 Good Answers: 839	#1 Re: VFD amps line and load side 03/16/2021 3:45 AM
	Yes, of course. Power factor. Motors are inductive loads. __________________ "Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
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PWSlack Guru Join Date: Jan 2007 Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on. Posts: 32175 Good Answers: 839	#2 Re: VFD amps line and load side 03/16/2021 4:10 AM
	The <...operations team ...> is correct. __________________ "Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
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Rixter Guru Join Date: Apr 2010 Location: About 4000 miles from the center of the earth (+/-100 mi) Posts: 9910 Good Answers: 1141	#3 Re: VFD Amps Line and Load Side 03/16/2021 9:24 AM
	The power input to the VFD is approximately the same as the power output. V_input x I_input = V_output x I_output. The motor is an inductive load, so the current is proportional to frequency. To limit motor current, the VFD output voltage is proportional to frequency up to 60 hz. https://controltrends.org/wp-content/uploads/2010/10/VFFundamentals.pdf Since the output voltage of the VFD is lower than the input voltage, the output current will be higher than the input current. VFD power out = VFD power in. http://www.vfds.org/vfd-current-input-side-vs-output-side-911332.html
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eltech Power-User Join Date: Mar 2011 Location: oz Posts: 252 Good Answers: 10	#4 In reply to #3 Re: VFD Amps Line and Load Side 03/16/2021 5:58 PM
	Thank you
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R_i_c_h Power-User Join Date: Dec 2019 Posts: 160 Good Answers: 14	#5 Re: VFD Amps Line and Load Side 03/17/2021 2:06 AM
	The VFD converts the line-side voltage to DC which it then turns into a pulse-width modulated signal to simulate the 3-phase AC voltage to the motor. The inductance of the motor and any inductors between the motor and drive give you an AC current waveform at the frequency of the speed the drive is running at. The conversion to DC is the reason you can wire the line-side of the VFD on single-phase and then run a 3-phase motor; with the appropriate derating, of course.
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eltech (1); PWSlack (2); R_i_c_h (1); Rixter (1)

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