In some cases, working a motor past the base pole velocity is feasible and provides system benefits if the design is fastidiously examined. The pole speed of a motor is a perform of the number poles and the incoming line frequency. Image 1 presents the synchronous pole speed 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 scale back the base pole velocity. If the incoming line frequency doesn’t change, the velocity of the induction motor will be less than these values by a % to slide. So, to function the motor above the base pole pace, the frequency needs to be elevated, which may be done with a variable frequency drive (VFD).
One purpose for overspeeding a motor on a pump is to use a slower rated pace motor with a decrease horsepower ranking and function it above base frequency to get the required torque at a lower current. This enables the choice of a VFD with a decrease present ranking to be used whereas still guaranteeing passable management of the pump/motor over its desired working range. The lower present requirement of the drive can reduce the capital cost of the system, depending on general system requirements.
The functions the place the motor and the driven pump operate above their rated speeds can present extra flow and strain to the controlled system. Unlock could result in a more compact system while rising its efficiency. While it could be potential to extend the motor’s pace to twice its nameplate pace, it is extra frequent that the utmost velocity is extra restricted.
The key to these applications is to overlay the pump velocity torque curve and motor pace torque to make sure the motor starts and capabilities throughout the complete operational pace vary without overheating, stalling or creating any important stresses on the pumping system.
Several factors additionally need to be taken into account when considering such solutions:
Noise will enhance with speed.
Bearing life or greasing intervals may be reduced, or improved fit bearings could additionally be required.
The higher pace (and variable velocity in general) will increase the danger of resonant vibration as a end result of a crucial pace throughout the operating range.
The higher velocity will lead to additional power consumption. It is important to consider if the pump and drive practice is rated for the upper power.
Since List required by a rotodynamic pump increases in proportion to the square of speed, the opposite major concern is to ensure that the motor can present enough torque to drive the load at the increased speed. When operated at a velocity beneath the rated pace 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 secure. While it might be best to increase the voltage to the motor as it’s run above its rated pace, the voltage of the alternating current (AC) power source limits the maximum voltage that is out there 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 three, the obtainable torque decreases past one hundred pc frequency as a outcome of the V/Hz ratio just isn’t maintained. In an overspeed situation, the load torque (pump) must be under the out there torque.
Before operating any piece of kit outside of its rated velocity range, it’s important to contact the producer of the gear to discover out if this might be done safely and effectively. For extra data on variable speed pumping, check with HI’s “Application Guideline for Variable Speed Pumping” at pumps.org.
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