Ampere Reading Difference of VFD Incoming and Outgoing

What are some typical values you've observed?

15kw motor is reading 18A in the incoming and 24A in the VFD out which one is correct, some body told me out readings are incorrect due to the RMS values will be varied

Incoming of VFD? The configuration should be VFD followed by motor .The input power should be always higher.So logically the current on the incoming of VFD should be high unless we have something that adds power .there is something wrong.

why do you have a question mark?....your VFDs has no incomer ?..

The amperage readings for the input and output current are likely reasonably accurate. Most VFD supplies are fairly efficient modules. So the input current will be in phase with the voltage and thus you can do a quick mental power calculation with the known power voltage. In contrast the current running through any motor is a sum of the power generated to drive the motor and the power created by the back EMF.

Now I've mixed units here a little, because current is not a unit of power. But power and not current must be conserved. But electric power is a function of current, voltage and the phase angle between the two.

If this VFD runs from a single phase source, it's certain to cause a lot of distortion to the input side, regardless of how the output looks. A TRUE RMS voltmeter should get you a fairly reasonable reading (maybe +- 10%) at the input. The comments about about phase and pulse issues will make the output really tough. I'd do like they said and trust your VFDs values. If you could hook an Oscope to the in and out, you should get an idea of what everyone is saying. You'll be surprised at what a sign wave can end up looking like when you feed it to a VFD or other switching load.

In our country we do not use single phase for anyload above 3KW.

As other responses have indicated, the waveform differences between I and O is the likely reason for unexpected current values. Use a Digital Scope to acquire the two signals, and use its RMS measurement tool to get the true RMS values (a DSO uses square root of the sum-of-the-squares to calculate RMS).

Outstanding answer LAA_Lucke!

VFD's maintain a constant volts per hertz ratio. Unless f_in = f_out, V_in # V_out.

The difference between the input power and the output power is the power that is needed to operate the VFD circuits.

Then current reading should be higher at the incoming side.(motor load + power needed to VFD circuits). Situation is reverse here..Guru!!

Only looking at the current makes no sens. You have to look at the input and output power. As already stated,

• The motor voltage will not always be the same as the input voltage of the frequency drive.
• The motor current is not a sinusiodal signal, so you have to use special true rms value meters.
• The input power is used to feed the motor AND to feed the internal capacitor in the frequency drive.
• Finally power = I x U X sqrt (3) x Cos fi (cos fi from motor is not the same as cos fi for frequency drive)

It is the same as saying, these 2 boxes have the same dimensions, so they must have the same mass.... This makes no sens.

there is something in the last point. VFD power factor will be higher than motor. natually current show less... Iam getting almost near to the end.

You should have given details such as voltage, current,frequency in the incoming, and out going of vfd.

Generally the volts for VFD motors (of medium sized application) will be higher say 630 Volts, frequency will be constant say 0 to 300 Hz. where as the incoming supply the volts will be 420, frequency will be 50 or 60 Hz.

Hence there will be difference in the current value for incoming as well as out going in the vfd.

DHAYANANDHAN.S