In some cases, working a motor past the bottom pole pace is possible and provides system advantages if the design is fastidiously 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 through 12-pole motors at 50 hertz (Hz [common in Europe]) and 60 Hz (common in the U.S.). As illustrated, additional poles cut back the base pole velocity. If the incoming line frequency does not change, the speed of the induction motor will be less than these values by a percent to slide. So, to function the motor above the bottom pole pace, the frequency needs to be increased, which may be carried out with a variable frequency drive (VFD).
One purpose for overspeeding a motor on a pump is to use a slower rated speed motor with a decrease horsepower score and function it above base frequency to get the required torque at a lower current. This allows the choice of a VFD with a lower current ranking to be used whereas nonetheless ensuring passable control of the pump/motor over its desired operating vary. The lower present requirement of the drive can scale back the capital price of the system, relying on total system necessities.
The applications the place the motor and the driven pump function above their rated speeds can present additional circulate and pressure to the managed system. This may lead to a more compact system whereas increasing its effectivity. While it may be possible to increase the motor’s velocity to twice its nameplate velocity, it is extra widespread that the maximum speed is extra limited.
diaphragm seal to these purposes is to overlay the pump speed torque curve and motor speed torque to make sure the motor begins and capabilities throughout the whole operational speed vary with out overheating, stalling or creating any vital stresses on the pumping system.
Several factors additionally need to be taken into consideration when considering such solutions:
Noise will enhance with pace.
Bearing life or greasing intervals may be reduced, or improved match bearings could also be required.
The greater pace (and variable speed in general) will increase the risk of resonant vibration due to a critical velocity inside the working vary.
The higher pace will result in further energy consumption. It is necessary to suppose about if the pump and drive prepare is rated for the higher energy.
Since the torque required by a rotodynamic pump will increase in proportion to the sq. of pace, the other main concern is to guarantee that the motor can present sufficient torque to drive the load at the increased speed. When operated at a pace beneath the rated velocity of the motor, the volts per hertz (V/Hz) could be maintained because the frequency applied to the motor is increased. Maintaining a constant V/Hz ratio retains torque manufacturing steady. While it might be perfect to increase the voltage to the motor as it’s run above its rated pace, the voltage of the alternating current (AC) power supply limits the utmost voltage that is available to the motor. Therefore, the voltage provided to the motor can’t continue to extend above the nameplate voltage as illustrated in Image 2. As shown in Image three, the out there torque decreases beyond 100% frequency as a outcome of the V/Hz ratio is not maintained. In an overspeed scenario, the load torque (pump) should be under the obtainable torque.
Before operating any piece of apparatus exterior of its rated pace vary, it is essential to contact the manufacturer of the equipment to find out if this may be done safely and effectively. For more info on variable velocity pumping, refer to HI’s “Application Guideline for Variable Speed Pumping” at pumps.org.