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

In some situations, working a motor past the bottom pole velocity is feasible and provides system advantages if the design is rigorously examined. The pole velocity of a motor is a operate of the number 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 in the U.S.). As illustrated, extra poles reduce the bottom pole speed. If Members only doesn’t change, the velocity of the induction motor shall be lower than these values by a percent to slide. So, to function the motor above the base pole pace, the frequency must be elevated, which could 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 lower horsepower rating and function it above base frequency to get the required torque at a decrease present. This permits the number of a VFD with a lower current score to be used whereas nonetheless ensuring satisfactory management of the pump/motor over its desired working range. The lower present requirement of the drive can scale back the capital value of the system, relying on general system necessities.
The purposes the place the motor and the driven pump operate above their rated speeds can provide further flow and pressure to the controlled system. This might end in a more compact system whereas rising its efficiency. While it could be potential to increase the motor’s pace to twice its nameplate velocity, it is extra common that the utmost pace is more limited.
The key to these applications is to overlay the pump velocity torque curve and motor pace torque to make sure the motor begins and functions throughout the entire operational speed range without overheating, stalling or creating any significant stresses on the pumping system.
Several points additionally need to be taken under consideration when contemplating such solutions:
Noise will enhance with velocity.
Bearing life or greasing intervals may be lowered, or improved match bearings could also be required.
The higher velocity (and variable velocity in general) will increase the chance of resonant vibration as a outcome of a crucial pace inside the working vary.
The greater velocity will result in additional energy consumption. It is necessary to contemplate if the pump and drive prepare is rated for the higher power.
Since the torque required by a rotodynamic pump increases in proportion to the sq. of speed, the other major concern is to guarantee that the motor can provide sufficient torque to drive the load on the elevated speed. When operated at a velocity under the rated velocity of the motor, the volts per hertz (V/Hz) may be maintained because the frequency applied to the motor is increased. Maintaining a continuing V/Hz ratio retains torque production steady. While it will be ideal to increase the voltage to the motor as it is run above its rated speed, the voltage of the alternating present (AC) power supply limits the utmost voltage that is available 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 3, the out there torque decreases beyond 100% frequency as a result of the V/Hz ratio is not maintained. In an overspeed situation, the load torque (pump) should be below the obtainable torque.
Before operating any piece of apparatus exterior of its rated velocity vary, it’s essential to contact the producer of the equipment to determine if this can be accomplished safely and effectively. For extra data on variable velocity pumping, refer to HI’s “Application Guideline for Variable Speed Pumping” at pumps.org.
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