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

In some situations, working a motor past the base pole speed is possible and offers system benefits if the design is carefully 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 velocity. If the incoming line frequency doesn’t change, the velocity of the induction motor might be lower than these values by a percent to slip. So, to operate the motor above the base pole pace, the frequency needs to be elevated, which can be accomplished with a variable frequency drive (VFD).
One purpose for overspeeding a motor on a pump is to make use of 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 choice of a VFD with a lower present ranking for use while nonetheless making certain passable management of the pump/motor over its desired working vary. The lower present requirement of the drive can cut back the capital cost of the system, relying on overall system requirements.
The applications the place the motor and the driven pump operate above their rated speeds can present additional circulate and strain to the managed system. This could result in a more compact system whereas increasing its efficiency. While it may be attainable to increase the motor’s speed to twice its nameplate pace, it is more frequent that the maximum pace is extra limited.
The key to those purposes is to overlay the pump speed torque curve and motor velocity torque to make sure the motor begins and features all through the entire operational pace vary without overheating, stalling or creating any vital stresses on the pumping system.
Several points also have to be taken into account when contemplating such options:
Noise will increase with velocity.
Bearing life or greasing intervals could also be reduced, or improved match bearings may be required.
The larger pace (and variable velocity in general) will increase the chance of resonant vibration as a end result of a critical velocity inside the working vary.
The larger speed will lead to extra energy consumption. It is necessary to consider if the pump and drive train is rated for the upper energy.
Since spmk700 required by a rotodynamic pump increases in proportion to the square of velocity, the other main concern is to make sure that the motor can provide enough torque to drive the load at the increased pace. When operated at a velocity beneath the rated velocity of the motor, the volts per hertz (V/Hz) could be maintained as the frequency applied to the motor is increased. Maintaining a constant V/Hz ratio keeps torque manufacturing secure. While it would be ideal to increase the voltage to the motor as it’s run above its rated pace, the voltage of the alternating present (AC) energy supply limits the utmost voltage that is obtainable to the motor. Therefore, the voltage equipped to the motor can not continue to increase above the nameplate voltage as illustrated in Image 2. As shown in Image 3, the obtainable torque decreases beyond 100% frequency as a end result of the V/Hz ratio just isn’t maintained. In an overspeed scenario, the load torque (pump) have to be below the obtainable torque.
Before operating any piece of apparatus exterior of its rated speed range, it is essential to contact the producer of the gear to determine if this might be done 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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