What are the effects of running a pump that has a namplate rating of 3500 rpm being run on a drive than has the nominal rpm parameter set to 1750 rpm ?
BY "drive" do you mean the motor RPM is 1750, or by "drive" do you mean that you have a Variable Speed Drive set to run a motor at 1750 RPM? If you are speaking of a VSD, what is the motor nameplate RPM? It makes a difference.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376
The "nominal" RPM is usually also the base speed, up to which the U/f ratio stays constant, if a scalar drive. And "somewhat" constant if a vector drive.
Above the base speed, the drive can not keep U/f constant, so you get field weakening. Field weakening means that available torque goes down with speed squared and that "thermal torque" falls approximately as 1/f so that output power is constant up to a point where the maximum torque is reached and the drive stalls.
powersoff
Do you mean you are running at half speed or do you mean the VFD's parameter for showing rpm has not been set to a 2-pole motor?
If the latter: most VFD's will use Hz internally as the reference to set the V/F base frequency and use rpm to use as a display reference. So the need to set the parameter of rpm to the correct motor base rpm is typically used to ensure the display is correct to the speed. I.e. you are using a 2 pole motor instead of the more typical 4 pole motor.
One of the important nominal base settings for the VFD is the base frequency, which is typically 60hz or 50hz depending on where you are in the world. You need to have the motor frequency and voltage set properly in a VFD. Then, the VFD can produce the proper V/Hz curve to match the motor.
Now, the rpm setpoint could be either just for display or it could be used to set the nominal frequency. But, it's not likely to be used to set the base frequency because then you'd also have to enter the slip and the motor poles to get the right base frequency calculated. So, to get the right base frequency would require correctly setting 3 parameters which is a poor method for obtaining such an important motor parameter. But then, using 1800rpm/4-pole would give the same base frequency and V/Hz ratio as using 3600rpm/2-pole.
