VFD Section

VFD power ratings are related to the nature of the load they are applied to. In many brands, the kW rating they assign to the VFD is based on the load being centrifugal (quadratic), such as a pump or fan, because they comprise well over 60% of all AC electric motor applications. But anything that is not a centrifugal pump or fan requires a different capacity rating of the VFD components, which generally means using a larger drive. We do not know what your load is.

As to sizing it for the absorbed power, that generally comes down to local regulations requiring that devices be rated for the motor size, not the intended use, because people have a tendency to change their intended uses after the fact. CAN you size the drive for the absorbed power? Yes, with extreme limitations. SHOULD you? No.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

Dear Mr.Histor
If you can furnish the following data/information, we may be able to suggest with better accuracy.
1. Take a photo of the motor name-plate or state the rated current (A).
2. What is your application/load profile ? e.g. a) variable torque (fan and pump etc), or b) constant torque (for conveyor etc).
3. What is the power supply 3-phase 400V ?
4. Take a photo of the VFD data sheet of a) 45kW you intended to use, b) 55kW they had selected.
4.1 Pay attention on the Voltage and current ratings. The rating is based on normal/variable torque or heavy duty/constant torque.
5. What is the a)ambient temperature >40 deg C ? b) Altitude >1000m ?
6. The VFD is going to be placed i.e. a) stand alone in the open air b) in an enclosure e.g. inside the MCC switch board.
Che Kuan Yau (Singapore)

Dear Mr. Che Kuan,
1. The rated motor current is 98A
2. It is a Constant Torque Rotary Kiln motor.
3. 3ph 400V Supply
4. We are using ATV630 Series heavy-duty VFDs.
5. >40 deg C
6. The VFD is going to be a stand-alone panel kept inside Electrical substation

Additional Informations:
Motor Rated Power : 45kW
Absorbed Power : 30.5kW

Since the absorbed power is 30.5kW and the 45kW VFD which can be 37kW heavy duty will be enough?

Dear Mr. Histor (Electrical)(OP)8 Aug 22 04:26
[COLOR=]" 1. The rated motor current is 98A
2. It is a Constant Torque Rotary Kiln motor.
3. 3ph 400V Supply
4. We are using ATV630 Series heavy-duty VFDs.
5. >40 deg C
6. The VFD is going to be a stand-alone panel kept inside Electrical substation...."[/color]
I try to answer my personal opinion, for your consideration:
1. Motor rated current is 96A. Although your present load is 30.5kW with current < 96A. It is [advisable] to select a VFD current rating = or > than motor current rating. This is to take care of future load increase.
2. VFD rated 67A (heavy duty)would be good for your present load. Take note of above 1.
3. The VFD rated for 380...480V is ok, in order.
4. ATV 630 is ok for the present load. .
5. VFD rated for 15...50deg C (without derating). If > 50deg C, derating shall be taken into account.
6. See above 5.
Summary: 45kW is ok for the present load. Attention, check the local rule and regulation. The rule [may ] required VFD current rating = or > than motor curreent rating; irrespective of the actual motor load.
Che Kuan Yau (Singapore)

Thank you Mr.che Kuan yau