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05-30-2013 10:26 PM

Select VFD for my 17kW motor

I have a motor for which i want to select variable frequency drive. Shaft power 14 KW. Motor is of 17 KW , Drive should be of 17 KW or some what higher rating ? Any rule of thumb?
05-30-2013 10:29 PM
Top #2
05-30-2013 10:29 PM
The VFD should be of equal size to the motor or greater. In some applications requiring high starting torque due to the load characteristics, the VFD may need to be oversized to suit. You need to be aware of the load characteristics and size the drive accordingly. Even a fan application which can have low starting torque requirements, still requires full power when running at full speed. The only time that a drive smaller than the motor would be acceptable would be if the motor was never going to run at or above base speed for a variable torque load, even then you have to be aware of the run up times and starting torque requirements.
05-31-2013 03:31 AM
Top #3
05-31-2013 03:31 AM
The biggest problem is that power is not the only issue in the equation. At what speed do you require this power? A 2 pole motor will have half the torque of a 4 pole motor for the same kW rating but twice the speed. Do you require full torque at start up or only once up to speed? It this a variable torque or constant torque application? It is very difficult to be able to provide advice without knowing the application as above has already correctly stated. We can only provide you with general advice as each application has different requirements which we are unaware of. Every gent above has provided what they can in general terms but as Joe Oliver said read some of the VFD manufacturers manuals which have good guidelines for drive selection and many drive manufacturers have built in application setups. You select the application and a preset of parameters are loaded into the drive which gives a good baseline from whence to start. The VFD then needs to be fine tuned to suit the specific application. I have been dealing with VFDs for many years and typically no two applications are identical, some may be similar but ultimately each has to be taken on its merits and adjustments applied accordingly. I am sorry that I can't give you a definitive answer..
05-31-2013 08:30 AM
Top #4
05-31-2013 08:30 AM
VFDs should be sized according to the application. You can get into trouble when you try to size a VFD from just the motor information alone. VFD applications will have either constant torque or variable torque load profiles. You will usually want greater overload capabilities on VFDs used in constant torque applications than in variable torque applications. Also, it's important to know the operating speed range and the speed regulation needed. Not all VFDs are capable of the same speed ranges and speed regulations.

If you had to size a VFD based on motor information alone I would select a VFD that has output current capacity equal to or greater than the full load nameplate amps of the motor. Also for variable torque applications select a drive with a minimum of 120% overload capability for 60 seconds and for constant torque applications go with 150% overload capability for 60 seconds.
08-27-2013 09:28 PM
Top #5
08-27-2013 09:28 PM
Your motor has been rated to bear 120% overload at the shaft to enable it to stabilize and not to overheat during motor operation.

Your drive can be selected either 18.5KW as it is standard rating for a drive (in case of the Variable torque where the torque delivery and ampere ratings demand less power) or 22KW (in case of Constant torque as the process demands high torque and continuous operation)

The drive is elected based to BKW which is actual requirement of load, but nonetheless it is also rated to keep additional support to the motor during high ampere / operational requirement.

Thumb rule : Choose immediate rating of drive with respect to motor rating for VT application OR one rating higher than motor for CT application (FLA will play the key role for whichever application and design accordingly)
08-27-2013 09:28 PM
Top #6
08-27-2013 09:28 PM
Good rule of thumb is to always match drive rating to load requirement. This said the motor horsepower/Kw rating is a starting point and will work for many applications. If the load is "normal duty, VT load, 110% overload capacity will work. "Heavy" duty, CT loads will typically require a larger rating to provide 150% overload for 1 minute. If a larger overload capacity time or amperage is required the drive will need to be sized appropriately with any other application issues that may be present such as ambient temperature(above 40* C), altitude(above 1000 meters), starting duty cycle(number of starts/time based). The other thing to watch is speed/number of poles of motor design. A 900rpm/8 pole design will draw more current than a 1800rpm/4 pole and the VFDs will need to be sized to accommodate the full load amperage.
Drives are a current source device and the major issue is to dissipate heat, not causing an overtemp trip.
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