In some instances, operating a motor past the base pole speed is feasible and provides system advantages if the design is rigorously examined. The pole pace of a motor is a operate of the quantity poles and the incoming line frequency. Image 1 presents the synchronous pole velocity for 2-pole by way of 12-pole motors at 50 hertz (Hz [common in Europe]) and 60 Hz (common within the U.S.). As illustrated, additional poles cut back the base pole speed. If the incoming line frequency does not change, the pace of the induction motor shall be less than these values by a percent to slip. So, to function the motor above the base pole speed, the frequency needs to be increased, which could be done with a variable frequency drive (VFD).
One reason for overspeeding a motor on a pump is to make use of a slower rated pace motor with a lower horsepower score and operate it above base frequency to get the required torque at a lower current. This permits the choice of a VFD with a lower present ranking for use whereas nonetheless making certain passable management of the pump/motor over its desired operating vary. Accessible of the drive can reduce the capital value of the system, relying on overall system requirements.
The applications the place the motor and the driven pump operate above their rated speeds can provide additional circulate and strain to the managed system. This may lead to a extra compact system while increasing its efficiency. While it may be potential to extend the motor’s speed to twice its nameplate speed, it is more frequent that the maximum speed is more limited.
The key to these applications is to overlay the pump pace torque curve and motor pace torque to ensure the motor begins and capabilities throughout the complete operational pace range with out overheating, stalling or creating any vital stresses on the pumping system.
Several points also have to be taken into consideration when considering such solutions:
Noise will increase with pace.
Bearing life or greasing intervals could also be lowered, or improved fit bearings may be required.
The greater speed (and variable speed in general) will improve the risk of resonant vibration as a end result of a crucial velocity within the working vary.
The larger speed will lead to further energy consumption. It is necessary to contemplate if the pump and drive practice is rated for the higher power.
Since the torque required by a rotodynamic pump will increase in proportion to the sq. of velocity, the other major concern is to guarantee that the motor can provide sufficient torque to drive the load on the elevated speed. When operated at a velocity below the rated velocity of the motor, the volts per hertz (V/Hz) may be maintained because the frequency applied to the motor is elevated. Maintaining a continuing V/Hz ratio retains torque manufacturing steady. While it will be ideal to extend the voltage to the motor as it’s run above its rated pace, the voltage of the alternating present (AC) energy source limits the utmost voltage that’s out there to the motor. Therefore, the voltage provided to the motor can not proceed to increase above the nameplate voltage as illustrated in Image 2. As shown in Image 3, the obtainable torque decreases past 100 percent frequency because the V/Hz ratio just isn’t maintained. In an overspeed scenario, the load torque (pump) must be under the available torque.
Before working any piece of kit outside of its rated pace range, it is important to contact the manufacturer of the equipment to find out if this could be done safely and efficiently. For more data on variable velocity pumping, refer to HI’s “Application Guideline for Variable Speed Pumping” at pumps.org.
Share