In some cases, working a motor beyond the bottom pole speed is feasible and provides system advantages if the design is carefully examined. The pole velocity 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 via 12-pole motors at 50 hertz (Hz [common in Europe]) and 60 Hz (common within the U.S.). As illustrated, additional poles reduce the base pole velocity. If the incoming line frequency does not change, the pace of the induction motor shall be less than these values by a percent to slide. So, to function the motor above the base pole velocity, the frequency needs to be increased, which can be carried out with a variable frequency drive (VFD).
One cause for overspeeding a motor on a pump is to make use of a slower rated velocity motor with a decrease horsepower rating and operate it above base frequency to get the required torque at a decrease current. This permits the selection of a VFD with a decrease current rating for use while still making certain passable control of the pump/motor over its desired working range. Kickstart of the drive can scale back the capital value of the system, depending on general system requirements.
The applications the place the motor and the driven pump function above their rated speeds can provide extra circulate and stress to the controlled system. This could end in a extra compact system whereas increasing its effectivity. While it might be possible to extend the motor’s velocity to twice its nameplate pace, it’s more common that the utmost velocity is extra limited.
The key to these functions is to overlay the pump pace torque curve and motor speed torque to make sure the motor starts and functions all through the entire operational speed range with out overheating, stalling or creating any vital stresses on the pumping system.
Several factors additionally need to be taken into account when contemplating such solutions:
Noise will increase with pace.
Bearing life or greasing intervals could additionally be decreased, or improved match bearings may be required.
The higher speed (and variable speed in general) will increase the risk of resonant vibration due to a crucial velocity within the working range.
The higher speed will result in further energy consumption. It is essential to consider if the pump and drive prepare is rated for the upper energy.
Since the torque required by a rotodynamic pump will increase in proportion to the square of pace, the other major concern is to guarantee that the motor can provide enough torque to drive the load on the increased pace. When operated at a pace below the rated pace of the motor, the volts per hertz (V/Hz) may be maintained because the frequency applied to the motor is increased. Maintaining a continuing V/Hz ratio keeps torque manufacturing stable. While it would be ideal to extend the voltage to the motor as it is run above its rated speed, the voltage of the alternating current (AC) energy supply limits the utmost voltage that is obtainable to the motor. Therefore, the voltage equipped to the motor can not proceed to extend above the nameplate voltage as illustrated in Image 2. As shown in Image 3, the out there torque decreases past 100% frequency as a result of the V/Hz ratio isn’t maintained. In an overspeed scenario, the load torque (pump) should be under the available torque.
Before working No risk of apparatus exterior of its rated pace vary, it’s important to contact the producer of the tools to discover out if this could be carried out safely and efficiently. For extra info on variable speed pumping, refer to HI’s “Application Guideline for Variable Speed Pumping” at pumps.org.
Share