In some cases, working a motor beyond the bottom pole velocity is possible and provides system advantages if the design is carefully 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 pace for 2-pole through 12-pole motors at 50 hertz (Hz [common in Europe]) and 60 Hz (common within the U.S.). As illustrated, further poles reduce the base pole speed. If the incoming line frequency doesn’t change, the speed of the induction motor might be less than these values by a percent to slip. So, to function the motor above the bottom pole velocity, the frequency needs to be increased, which can be carried out with a variable frequency drive (VFD).
One reason for overspeeding a motor on a pump is to make use of a slower rated speed motor with a lower horsepower score and operate it above base frequency to get the required torque at a decrease present. This permits the choice of a VFD with a lower present score to be used while nonetheless guaranteeing passable management of the pump/motor over its desired working vary. The lower current requirement of the drive can cut back the capital value of the system, relying on general system necessities.
The purposes the place the motor and the driven pump operate above their rated speeds can provide further circulate and stress to the managed system. This could result in a more compact system whereas increasing its effectivity. While เกจแรงดัน may be possible to increase the motor’s velocity to twice its nameplate velocity, it’s more widespread that the utmost pace is extra restricted.
The key to those applications is to overlay the pump velocity torque curve and motor speed torque to ensure the motor starts and capabilities all through the entire operational speed range without overheating, stalling or creating any significant stresses on the pumping system.
Several points additionally must be taken into account when considering such options:
Noise will enhance with pace.
Bearing life or greasing intervals could also be decreased, or improved fit bearings may be required.
The larger speed (and variable pace in general) will improve the risk of resonant vibration as a end result of a crucial velocity throughout the working vary.
The higher pace will lead to further energy consumption. It is necessary to think about if the pump and drive train is rated for the higher power.
Since the torque required by a rotodynamic pump increases in proportion to the square of velocity, the opposite major concern is to guarantee that the motor can provide enough torque to drive the load on the elevated speed. When operated at a pace beneath the rated pace of the motor, the volts per hertz (V/Hz) may be maintained as the frequency applied to the motor is increased. Maintaining a continuing V/Hz ratio keeps torque manufacturing secure. While it might be best to increase the voltage to the motor as it is run above its rated velocity, the voltage of the alternating current (AC) power source limits the utmost voltage that’s out there to the motor. Therefore, the voltage supplied to the motor can’t continue to increase above the nameplate voltage as illustrated in Image 2. As shown in Image three, the available torque decreases past 100 percent frequency as a outcome of the V/Hz ratio just isn’t maintained. In an overspeed situation, the load torque (pump) have to be under the available torque.
Before operating any piece of equipment outdoors of its rated velocity vary, it is essential to contact the producer of the equipment to discover out if this could be done safely and effectively. For more data on variable velocity pumping, refer to HI’s “Application Guideline for Variable Speed Pumping” at pumps.org.
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