Electric Motors with VFD

[gumper00]

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.

 

[Skogsgurra]

Taking the fan up from 900 to 1250 RPM increases power needed by a factor 2.7 (speed ratio^3).

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

You can do that but often the horsepower is fixed and so yes the motor can be commanded faster but the torque will diminish to keep the horsepower the same. If that works for your application it won't hurt anything and is quite acceptable.

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

 

[ozmosis]

If it is an existing system then more often than not you will find that the fan/motor is drawing a lot less current than it is rated for. A tip I usually offer to users wishing to overspeed a fan above base frequency:
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.

 

[edison123]

sed2developer

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*

 

[ozmosis]

edison123
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.

 

[DickDV]

The simple answer is that almost all motors under 300hp can be operated overspeed safely and without damage to their operating lifespan. The exception being two pole motors--either 3000rpm (50hz) or 3600rpm (60hz). On those, overspeed much over 15-20% should include a call to the manufacturer.

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!

 

[waross]

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.

This is the simple answer for VFDs but exceptions are starting to develope.
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

 

[CJCPE]

USA motors up to some Hp rating are usually wound for 240/480 volts. Sometimes people buy a VFD of twice the motor Hp rating and set it up for 120 Hz at 480 volts and then connect the motor for 240 volts at 60 Hz. With that connection, you can probably operate up to 90 Hz 360 volts at 1.5 X rated Hp without any problem with the motor.

 

[jraef]

That trick works out fine for small motors, but keep in mind this is a 900RPM motor which usually means it is 100HP or more. If it can be reconnected for 240V, the increased current will mean a much larger VFD, as well as the circuit breaker, wire, etc.