Electric Motors with VFD
Electric Motors with VFD
(OP)
Is it common practice to operate a motor above it rated RPM using a VFD? The fan is a direct drive with a 900 RPM motor. The design fan RPM is 1,250 and the vendor wants to operate the motor at 1,250 using the VFD.





RE: Electric Motors with VFD
So, you need a motor that can deliver the needed torque (speed ratio^2) at the higher speed.
And you need a drive with a rating compatible with the increased power.
You also need to consider motor voltage. Going above base speed reduces V/Hz and reduces available torque. There are several ways of handling the latter problem. Make sure your vendor understands all these issues.
Otherwise, there should not be any problem. Bearings usually designed for higher speeds than 1250 RPM. May have to check balancing.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Electric Motors with VFD
Also keep in mind that a fan's power require goes up with the square of the speed. So don't make the mistake my buddy just did of deciding to shift from 1725 to 3450 on a fan. He thought he could go from a 2HP 1725 to a 3HP 3450. WRONG! Theoretically he would need about 8HP.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Electric Motors with VFD
Your VFD will/should have a display/parameter that indicates current (load). Make sure the application is operating at full speed and the system is in it's worst case condition, meaning any dampers or vanes are fully open to ensure the VFD is operating at worst(max) load condition. If the current drawn is less than the rating of the motor (remember, the VFD would typically be rated above the motor current so it is the motor current you need to know) then you can increase by increments of 1-2 Hz at a time but always monitoring the current displayed on the VFD and limit your maximum frequency at the point where the current drawn is about 10%< the motor rating.
RE: Electric Motors with VFD
While I agree current is one limiting factor in overspeeding, what about the rotational stresses on both motor and fan at overspeeds ?
*Why make the same mistake twice when there so many new ones available*
RE: Electric Motors with VFD
I agree, these are certainly factors but I figured an increase of 900-1250 rpm should not incur too much stresses on the motor. The fan maybe but this would typically seen in the load created and therefore seen in the current demanded.
However, certainly worth noting.
RE: Electric Motors with VFD
Running a motor overspeed does not increase its horsepower or kw rating so you very well might run out of hp before you reach your desired max speed especially with fans and centrifugal pumps.
Finally, you must be sure the load can safely run at those higher speed. Many gearboxes, for example, will foam their lubricating oil above rated speed and short service life is the sad result. Also, large centrifugal load like fans can reach their critical speed and literally vibrate themselves apart due to resonance. You don't want that.
As usual, our advice basically is to do anything like this with your eyes open and your brain fully engaged!
RE: Electric Motors with VFD
In a conventional VFD drive, the voltage typically stops rising at rated speed or close to it. As the frequency increases, the current drops due to the increasing inductive reactance.
However, if the motor is connected for less than rated voltage and the VFD is properly configured, the volts per hertz ratio may be maintained at increased speeds and the horsepower will increase.
There was a thread a few days ago concerning running VFD driven wye/delta motors on the delta connection at the higher voltage to take advantage of the extra horsepower thus available.
respectfully
RE: Electric Motors with VFD
RE: Electric Motors with VFD