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Power-User

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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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Guru
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#1

Re: VFD amps line and load side

03/16/2021 3:45 AM

Yes, of course. Power factor. Motors are inductive loads.

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Guru
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#2

Re: VFD amps line and load side

03/16/2021 4:10 AM

The <...operations team ...> is correct.

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Guru

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#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. Vinput x Iinput = Voutput x Ioutput.

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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Power-User

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#4
In reply to #3

Re: VFD Amps Line and Load Side

03/16/2021 5:58 PM

Thank you

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Power-User

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#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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