In some instances, 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 perform of the quantity poles and the incoming line frequency. Image 1 presents the synchronous pole pace for 2-pole by way of 12-pole motors at 50 hertz (Hz [common in Europe]) and 60 Hz (common in the U.S.). As illustrated, additional poles scale back the bottom pole pace. If the incoming line frequency does not change, the speed of the induction motor shall be less than these values by a p.c to slip. So, to operate the motor above the bottom pole speed, the frequency needs to be elevated, which could be carried out with a variable frequency drive (VFD).
One purpose for overspeeding a motor on a pump is to make use of a slower rated speed motor with a lower horsepower score 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 current rating for use whereas still ensuring satisfactory management of the pump/motor over its desired operating range. The decrease current requirement of the drive can reduce the capital price of the system, relying on total system necessities.
The functions the place the motor and the pushed pump operate above their rated speeds can present additional flow and strain to the managed system. This might result in a more compact system while growing its effectivity. While it could be attainable to extend the motor’s velocity to twice its nameplate speed, it is extra widespread that the utmost speed is more restricted.
The key to these purposes is to overlay the pump pace torque curve and motor speed torque to ensure the motor begins and features all through the whole operational speed vary with out overheating, stalling or creating any important stresses on the pumping system.
Several points also need to be taken into account when considering such options:
Noise will increase with pace.
Bearing life or greasing intervals may be reduced, or improved match bearings could additionally be required.
The larger velocity (and variable pace in general) will enhance the danger of resonant vibration as a outcome of a critical speed inside the working range.
The larger velocity will end in extra power consumption. It is essential to consider 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 speed, the other main concern is to make sure that the motor can present sufficient torque to drive the load on the elevated pace. When operated at a velocity beneath the rated pace of the motor, the volts per hertz (V/Hz) can be maintained as the frequency applied to the motor is increased. Maintaining a continuing V/Hz ratio keeps torque manufacturing steady. While เพรสเชอร์เกจ will be perfect to increase the voltage to the motor as it is run above its rated pace, the voltage of the alternating current (AC) energy supply limits the maximum voltage that’s obtainable to the motor. Therefore, the voltage provided to the motor can’t proceed to increase above the nameplate voltage as illustrated in Image 2. As proven in Image three, the obtainable torque decreases past 100 percent frequency as a outcome of the V/Hz ratio isn’t maintained. In an overspeed state of affairs, the load torque (pump) must be below the obtainable torque.
Before working any piece of kit exterior of its rated speed vary, it is important to contact the manufacturer of the equipment to discover out if this can be done safely and efficiently. For extra information on variable speed pumping, refer to HI’s “Application Guideline for Variable Speed Pumping” at pumps.org.
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