In diaphragm seal , operating a motor past the base pole pace is possible and presents system advantages if the design is rigorously examined. The pole velocity of a motor is a function 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, extra poles cut back the bottom pole speed. If the incoming line frequency does not change, the velocity of the induction motor might be lower than these values by a % to slide. So, to function the motor above the bottom pole velocity, the frequency must be increased, which could be carried out with a variable frequency drive (VFD).
One cause 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 present. This enables the number of a VFD with a lower current score to be used whereas nonetheless guaranteeing satisfactory management of the pump/motor over its desired working vary. The lower current requirement of the drive can reduce the capital cost of the system, relying on total system necessities.
The applications the place the motor and the pushed pump function above their rated speeds can present extra circulate and pressure to the controlled system. This could lead to a more compact system whereas increasing its effectivity. While it could be attainable to increase the motor’s speed to twice its nameplate velocity, it is more frequent that the utmost speed is extra limited.
The key to those applications is to overlay the pump pace torque curve and motor pace torque to make sure the motor starts and functions throughout the complete operational speed vary with out overheating, stalling or creating any important stresses on the pumping system.
Several points also have to be taken under consideration when considering such solutions:
Noise will enhance with speed.
Bearing life or greasing intervals may be reduced, or improved match bearings could also be required.
The higher speed (and variable velocity in general) will enhance the danger of resonant vibration due to a important pace inside the operating vary.
The greater pace will end in extra energy consumption. It is important to consider if the pump and drive practice is rated for the higher energy.
Since pressure gauge 10 bar required by a rotodynamic pump increases in proportion to the sq. of speed, the other major concern is to ensure that the motor can present enough torque to drive the load on the elevated speed. When operated at a speed under the rated pace of the motor, the volts per hertz (V/Hz) may be maintained because the frequency utilized to the motor is elevated. Maintaining a continuing V/Hz ratio keeps torque production secure. While it might be ideal to increase the voltage to the motor as it is run above its rated velocity, the voltage of the alternating current (AC) energy source limits the maximum voltage that’s obtainable to the motor. Therefore, the voltage equipped to the motor can not proceed to increase above the nameplate voltage as illustrated in Image 2. As proven in Image 3, the obtainable torque decreases beyond one hundred pc frequency as a result of the V/Hz ratio isn’t maintained. In an overspeed situation, the load torque (pump) have to be beneath the obtainable torque.
Before working any piece of kit outside of its rated speed range, it’s important to contact the manufacturer of the tools to find out if this can be carried out safely and efficiently. For more info on variable pace pumping, check with HI’s “Application Guideline for Variable Speed Pumping” at pumps.org.
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