In Which Applications Can a Pump Motor Be Operated Above Base Speed?

In some situations, operating a motor past the base pole velocity is possible and presents system benefits if the design is carefully 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 speed for 2-pole through 12-pole motors at 50 hertz (Hz [common in Europe]) and 60 Hz (common in the U.S.). As illustrated, further poles reduce the base pole speed. If the incoming line frequency doesn’t change, the velocity of the induction motor will be lower than these values by a % to slip. So, to function the motor above the bottom pole velocity, the frequency must be elevated, which could be done with a variable frequency drive (VFD).
One cause for overspeeding a motor on a pump is to use a slower rated velocity motor with a decrease horsepower ranking and operate it above base frequency to get the required torque at a decrease current. This allows the number of a VFD with a lower present ranking to be used whereas nonetheless guaranteeing satisfactory control of the pump/motor over its desired working range. The lower present requirement of the drive can reduce the capital value of the system, depending on total system necessities.
The functions where the motor and the driven pump function above their rated speeds can provide further circulate and strain to the managed system. This might result in a more compact system whereas growing its effectivity. While it could be attainable to increase the motor’s velocity to twice its nameplate speed, it’s extra common that the utmost pace is extra limited.
The key to these applications is to overlay the pump velocity torque curve and motor speed torque to ensure the motor starts and capabilities all through the complete operational pace range without overheating, stalling or creating any important stresses on the pumping system.
Several factors also need to be taken into consideration when contemplating such options:
Noise will increase with velocity.
Bearing life or greasing intervals may be decreased, or improved fit bearings may be required.
The higher speed (and variable pace in general) will increase the risk of resonant vibration as a outcome of a important pace throughout the operating vary.
The higher pace will end in additional power consumption. It is essential to suppose about if the pump and drive prepare is rated for the higher power.
Since the torque required by a rotodynamic pump will increase in proportion to the square of pace, the opposite main concern is to ensure that the motor can present sufficient torque to drive the load at the increased speed. When operated at a pace below the rated speed of the motor, the volts per hertz (V/Hz) could be maintained as the frequency utilized to the motor is elevated. Maintaining a continuing V/Hz ratio keeps torque manufacturing stable. While it will be ideal to increase the voltage to the motor as it’s run above its rated velocity, the voltage of the alternating present (AC) power supply limits the maximum voltage that’s obtainable to the motor. Therefore, the voltage provided to the motor can’t continue to increase above the nameplate voltage as illustrated in Image 2. As proven in Image 3, the obtainable torque decreases past 100% frequency as a end result of the V/Hz ratio just isn’t maintained. In an overspeed situation, the load torque (pump) have to be under the out there torque.
Before working Research of equipment outside of its rated velocity range, it is essential to contact the manufacturer of the tools to determine if this may be carried out safely and efficiently. For extra data on variable speed pumping, refer to HI’s “Application Guideline for Variable Speed Pumping” at pumps.org.
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