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

In Bonanza , working a motor past the base pole velocity is possible and offers system benefits 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 pace 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, additional poles reduce the bottom pole velocity. If the incoming line frequency doesn’t change, the velocity of the induction motor shall be lower than these values by a p.c to slide. So, to function the motor above the bottom pole speed, the frequency needs to be elevated, which could be carried out with a variable frequency drive (VFD).
One reason for overspeeding a motor on a pump is to use a slower rated speed motor with a lower horsepower rating and operate it above base frequency to get the required torque at a lower current. This enables the number of a VFD with a lower current rating to be used while nonetheless making certain satisfactory management of the pump/motor over its desired operating range. The lower present requirement of the drive can cut back the capital price of the system, depending on overall system necessities.
The purposes where the motor and the driven pump operate above their rated speeds can present additional move and pressure to the managed system. This might lead to a extra compact system whereas growing its effectivity. While it could be possible to extend the motor’s pace to twice its nameplate velocity, it’s more widespread that the maximum pace is extra limited.
The key to these functions is to overlay the pump velocity torque curve and motor velocity torque to make sure the motor starts and features all through the entire operational pace range without overheating, stalling or creating any vital stresses on the pumping system.
Several points additionally have to be taken into consideration when considering such solutions:
Noise will enhance with velocity.
Bearing life or greasing intervals could additionally be reduced, or improved fit bearings could also be required.
The larger speed (and variable speed in general) will improve the danger of resonant vibration as a end result of a crucial pace within the working vary.
The greater velocity will end in additional power consumption. It is important to contemplate if the pump and drive train is rated for the upper energy.
Since the torque required by a rotodynamic pump increases in proportion to the square of speed, the opposite major concern is to guarantee that the motor can present enough torque to drive the load on the elevated speed. When operated at a velocity under the rated speed of the motor, the volts per hertz (V/Hz) may be maintained as the frequency utilized to the motor is elevated. Maintaining a constant V/Hz ratio retains torque manufacturing stable. While it would be best to increase the voltage to the motor as it’s run above its rated velocity, the voltage of the alternating present (AC) energy source limits the maximum voltage that’s out there to the motor. Therefore, the voltage provided to the motor can’t 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 end result of the V/Hz ratio is not maintained. In an overspeed state of affairs, the load torque (pump) should be below the available torque.
Before working any piece of kit exterior of its rated speed vary, it’s essential to contact the manufacturer of the tools to determine if this can be done safely and efficiently. For more data on variable speed pumping, discuss with HI’s “Application Guideline for Variable Speed Pumping” at pumps.org.
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