VFD current
- Thread starter JdoubleU
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winnie
Senior Member
- Location
- Springfield, MA, USA
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- Electric motor research
The VFD also changes the output voltage. If the output voltage is lower then the input voltage, then neglecting the (small) loss for efficiency, the output current has to be higher.
The reason that this is possible is that current can circulate between the motor and the VFD electronics, so the current between the motor and the VFD can be higher or lower than the input current.
-Jon
The reason that this is possible is that current can circulate between the motor and the VFD electronics, so the current between the motor and the VFD can be higher or lower than the input current.
-Jon
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Another major reason that the input current can be lower I'd that the motor current includes the reactive current corresponding to the low power factor of the motor. This current is at the fundamental frequency of the line, but will still just circulate back and forth to the DC bus and will not appear in the input current. At full load power and speed the VFD output voltage will normally be the same as the input.
- Location
- Wisconsin
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- PE (Retired) - Power Systems
What are you using to measure the current?
Power factor.
kwired
Electron manager
- Location
- NE Nebraska
power factor exists on the output side. Input side is supplying a rectifier then is converted back to a AC wave that is able to be controlled to get desired frequency, power factor does not pass through this conversion method but the motor still draws a reactive current from the output of the drive.
You could have a drive with single phase input and three phase output - in that case the output current should be lower then input current unless maybe you have a horrible power factor at the motor.
You could have a drive with single phase input and three phase output - in that case the output current should be lower then input current unless maybe you have a horrible power factor at the motor.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
The output of the VFD is not a sine wave. It is likely that the results of a 'standard' clamp-on meter will be not be meaningful.
http://support.fluke.com/find-sales/Download/Asset/2562791_6115_ENG_C_W.pdf
Bottom of page 3
The output current is usually a pretty good sine wave.
The voltage is sine wave weighted pulse width modulated and the inductance of the motor results in the current being sinusoidal. More or less.
Addendum:
The Fluke measurement you mention relates to voltage, not current.
The topic is about current.
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