fan speed vs motor speed

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Designer69

Senior Member
just want to check if you guys would agree with this:

got a direct coupled centrifugal fan to a motor powered by a VFD.

fan rated speed is 1785 RPM and full load motor speed is 1775 rpm. Vendor says the torque produced by the fan at rated fan speed is much less than the motor's full load torque, so the motor will actually run at a higher speed than its full load speed of 1775 RPM.

is that accurate? do some of you guys have experience with this type of situation?

Thanks!
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Yes, a VFD can run the motor at a higher speed than the base speed, but that causes 2 immediate issues in a fan application:

  1. The fan power requirements increase at the CUBE of the speed increase. So if for example you take the motor speed up to 2662RPM, that's roughly 150% speed. That means that the fan will move more air and therefore REQUIRE 1.5 x 1.5 x 1.5 more power, or 337% more HP. So if let's say you had a 15HP motor but it was only using 10HP at full speed (just for argument's sake) and you turned the speed up by just 50%, the power requirement just jumped to 33.6HP.
  2. At the same time, increasing the frequency alone cannot increase the HP capability of the motor, because HP is a function of speed and torque. So if you increase speed you would also have to increase torque. But torque is directly related to the V/Hz ratio and if you increase Hz without increasing V, you are lowering that ratio. That means you are going to be FLAT on the HP after the motor's design base speed. The motor will be running in what's called "Constant HP mode".
So then you have a need for MORE HP by the fan, but a motor that is giving CONSTANT HP even though the speed is higher. Bad combination. It's called an "Overload".
 

sgunsel

Senior Member
One bigger gotcha may prevent fan overloading. Fans can come apart when run at excessive speed. Since they are turning at a pretty good clip, the flying parts can ruin your whole day, as they say. If a fan has a rated maximum rpm, exceeding that can result in catastrophic failure and should be avoided. If you don't know what the maximum is, do not exceed normal operating speed or you may be courting disaster. Contact the fan manufacturer before upping fan rpm.
 

Designer69

Senior Member
One bigger gotcha may prevent fan overloading. Fans can come apart when run at excessive speed. Since they are turning at a pretty good clip, the flying parts can ruin your whole day, as they say. If a fan has a rated maximum rpm, exceeding that can result in catastrophic failure and should be avoided. If you don't know what the maximum is, do not exceed normal operating speed or you may be courting disaster. Contact the fan manufacturer before upping fan rpm.

in this case the fan has a higher rated speed than the motor, only by just less than 1% though, so it doesn't sound like it will be a problem.
 

sgunsel

Senior Member
Most VFDs can operate well above 60 Hz (400 Hz max is typical) so overspeed is a real possibility. At 400 Hz, fan rpm would be almost 12,000 rpm, 670% above normal speed. Of course either the motor or the fan is very likely to fail well below that speed. That would be a problem.
 

kwired

Electron manager
Location
NE Nebraska
just want to check if you guys would agree with this:

got a direct coupled centrifugal fan to a motor powered by a VFD.

fan rated speed is 1785 RPM and full load motor speed is 1775 rpm. Vendor says the torque produced by the fan at rated fan speed is much less than the motor's full load torque, so the motor will actually run at a higher speed than its full load speed of 1775 RPM.

is that accurate? do some of you guys have experience with this type of situation?

Thanks!

If you are applying 60 Hz the base speed without considering any losses is 1800 RPM that will be the fastest the motor will ever run at 60 Hz. That is only 1% over rated speed of the fan. There will likely be enough losses that it will still be under the rated speed of the fan - just my guess.
 

GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
Fan rated speed is 1785 RPM and full load motor speed is 1775 rpm. Vendor says the torque produced by the fan at rated fan speed is much less than the motor's full load torque, so the motor will actually run at a higher speed than its full load speed of 1775 RPM.
Vendor likely said that "torque REQUIRED by the fan at rated fan speed ..."

At 60 Hz, if the fan requirements are lower than the motor capabilities, yes the speed will be higher ... a LITTLE higher. The difference in characteristics from 1775 to 1795 (as high as it is likely to ever get loaded at all) will in all likelihood be unmeasurable.

Now you said you have a VFD. It can almost run the motor above 60 Hz; all I know get to at least 90 Hz, suggesting about 2700 rpm. If the motor is rated for it, the power required, as noted by others, varies as the cube of speed (if the fan can remain coupled to the air) will be about 3.4* what it required at 1800 rpm.

Modern fan designs are often heavily optimized for the intended speed. I seriously doubt you will want to exceed 10% over nominal (about 1.35* base power).
 

Besoeker

Senior Member
Location
UK
just want to check if you guys would agree with this:

got a direct coupled centrifugal fan to a motor powered by a VFD.

fan rated speed is 1785 RPM and full load motor speed is 1775 rpm. Vendor says the torque produced by the fan at rated fan speed is much less than the motor's full load torque, so the motor will actually run at a higher speed than its full load speed of 1775 RPM.

is that accurate? do some of you guys have experience with this type of situation?

Thanks!
The given full load speed of the motor, 1775 rpm is very likely based on a 60Hz supply. In those conditions, the motor has a slip of 25 rpm, slip being the difference between synchronous speed (1800 rpm in this case) and running speed. There has to be some slip to produce torque. The amount of slip is more or less proportional to torque.
So, suppose your fan requires half the rated torque of the motor, the slip would then be 12.5 rpm (half of 25) resulting in a motor speed of 1887.5 rpm with the same 60Hz supply.
Maybe that's your vendor meant.

However, you have a Variable Frequency Drive and, within constraints of power and load torque requirements, the motor can run at at whatever speed you tell VSD to make it run at. The usual reason for using a VFD for fan applications is that you can turn the speed down to save energy and thus running costs.
 
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dkarst

Senior Member
Location
Minnesota
So, suppose your fan requires half the rated torque of the motor, the slip would then be 12.5 rpm (half of 25) resulting in a motor speed of 1887.5 rpm with the same 60Hz supply.
Maybe that's your vendor meant.

Small typo crept in... should be 1800 (synchronous speed) - 12.5 = 1787.5 rpm
 

sgunsel

Senior Member
Fotunately, you can usually program maximum and minimum frequencies as well as lockout reverse. All are highly recommended where applicable to prevent inadvertent or intentional damage.
 
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