In some cases, working a motor past the bottom pole velocity is feasible and offers 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 pace for 2-pole through 12-pole motors at 50 hertz (Hz [common in Europe]) and 60 Hz (common in the U.S.). As illustrated, extra poles cut back the base pole velocity. If the incoming line frequency doesn’t change, the speed of the induction motor might be lower than these values by a percent to slip. So, to operate the motor above the base pole speed, the frequency needs to be increased, which can be accomplished with a variable frequency drive (VFD).
One cause 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 lower present. This enables the selection of a VFD with a decrease present score for use whereas nonetheless ensuring passable control of the pump/motor over its desired operating vary. The lower present requirement of the drive can cut back the capital value of the system, depending on overall system necessities.
The purposes the place the motor and the driven pump operate above their rated speeds can provide extra circulate and pressure to the managed system. This may lead to a extra compact system whereas increasing its efficiency. While diaphragm seal could be potential to increase the motor’s velocity to twice its nameplate speed, it is extra frequent that the utmost velocity is more restricted.
The key to these applications is to overlay the pump velocity torque curve and motor velocity torque to make sure the motor begins and functions throughout the entire operational pace vary without overheating, stalling or creating any vital stresses on the pumping system.
Several points additionally have to be taken under consideration when contemplating such solutions:
Noise will enhance with pace.
Bearing pressure gauge or greasing intervals could also be lowered, or improved fit bearings may be required.
The higher pace (and variable pace in general) will enhance the danger of resonant vibration as a outcome of a crucial speed throughout the working vary.
The larger speed will lead to extra power consumption. It is necessary to consider if the pump and drive train is rated for the upper energy.
Since the torque required by a rotodynamic pump will increase in proportion to the sq. of pace, the other main concern is to make certain that the motor can present enough torque to drive the load at the increased speed. When operated at a pace beneath the rated velocity of the motor, the volts per hertz (V/Hz) may be maintained as the frequency applied to the motor is increased. Maintaining a continuing V/Hz ratio retains torque manufacturing stable. While it might be perfect to increase the voltage to the motor as it is run above its rated velocity, the voltage of the alternating present (AC) energy supply limits the maximum voltage that is obtainable to the motor. Therefore, the voltage supplied to the motor cannot continue to increase above the nameplate voltage as illustrated in Image 2. As shown in Image 3, the obtainable torque decreases past one hundred pc frequency because the V/Hz ratio just isn’t maintained. In an overspeed situation, the load torque (pump) must be below the available torque.
Before operating any piece of equipment outside of its rated speed vary, it’s essential to contact the manufacturer of the gear to determine if this can be carried out safely and efficiently. For ไดอะแฟรม on variable pace pumping, refer to HI’s “Application Guideline for Variable Speed Pumping” at pumps.org.
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