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

In some situations, operating a motor beyond the base pole pace is feasible and provides system benefits if the design is fastidiously examined. The pole velocity 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 by way of 12-pole motors at 50 hertz (Hz [common in Europe]) and 60 Hz (common within the U.S.). As illustrated, extra poles cut back the base pole pace. If the incoming line frequency does not change, the pace of the induction motor might be less than these values by a % to slide. So, to function the motor above the bottom pole speed, the frequency must be elevated, which may be carried out with a variable frequency drive (VFD).
Shocking for overspeeding a motor on a pump is to use a slower rated speed motor with a decrease horsepower ranking and operate it above base frequency to get the required torque at a lower present. This allows the choice of a VFD with a decrease present rating to be used whereas nonetheless guaranteeing passable control of the pump/motor over its desired working range. The decrease current requirement of the drive can reduce the capital cost of the system, relying on total system requirements.
The purposes where the motor and the driven pump function above their rated speeds can provide additional move and stress to the controlled system. This may end in a extra compact system while growing its effectivity. While it could be potential to increase the motor’s pace to twice its nameplate velocity, it’s extra frequent that the utmost speed is more limited.
The key to these purposes is to overlay the pump pace torque curve and motor pace torque to ensure the motor begins and functions throughout the complete operational speed vary without overheating, stalling or creating any vital stresses on the pumping system.
Several factors additionally have to be taken into consideration when contemplating such options:
Noise will enhance with velocity.
Bearing life or greasing intervals may be reduced, or improved match bearings may be required.
The greater velocity (and variable speed in general) will improve the risk of resonant vibration as a outcome of a crucial velocity throughout the working range.
The larger speed will end in extra power consumption. It is important to contemplate if the pump and drive prepare is rated for the upper power.
Since the torque required by a rotodynamic pump will increase in proportion to the square of speed, the opposite major concern is to guarantee that the motor can provide enough torque to drive the load on the increased speed. When operated at a pace below the rated velocity of the motor, the volts per hertz (V/Hz) can be maintained because the frequency utilized to the motor is increased. Maintaining a constant V/Hz ratio retains torque production steady. While it would be best to extend the voltage to the motor as it is run above its rated velocity, the voltage of the alternating present (AC) power source limits the maximum voltage that’s out there to the motor. Therefore, the voltage supplied to the motor can not proceed to extend above the nameplate voltage as illustrated in Image 2. As proven in Image three, the out there torque decreases beyond one hundred pc frequency as a end result of the V/Hz ratio is not maintained. In an overspeed situation, the load torque (pump) have to be beneath the out there torque.
Before working any piece of equipment exterior of its rated pace range, it’s essential to contact the producer of the gear to discover out if this can be accomplished safely and efficiently. For more info on variable speed pumping, discuss with HI’s “Application Guideline for Variable Speed Pumping” at pumps.org.
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