In some instances, operating a motor past the bottom pole speed is feasible and offers system benefits if the design is carefully examined. The pole velocity of a motor is a perform of the number poles and the incoming line frequency. Image 1 presents the synchronous pole speed 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, further poles cut back the base pole speed. If the incoming line frequency doesn’t change, the speed of the induction motor might be less than these values by a % to slip. So, to operate the motor above the base pole speed, the frequency needs to be increased, which could 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 ranking and operate it above base frequency to get the required torque at a decrease current. This allows the number of a VFD with a decrease current ranking for use while still guaranteeing satisfactory management of the pump/motor over its desired operating range. The decrease current requirement of the drive can scale back the capital cost of the system, relying on total system requirements.
The applications where the motor and the pushed pump function above their rated speeds can present additional circulate and stress to the controlled system. This may end in a extra compact system whereas growing its effectivity. While it may be possible to increase the motor’s pace to twice its nameplate velocity, it is more widespread that the maximum speed is extra restricted.
The key to these functions is to overlay the pump speed torque curve and motor velocity torque to make sure the motor starts and capabilities all through the complete operational pace range with out overheating, stalling or creating any important stresses on the pumping system.
Several factors also need to be taken into account when contemplating such solutions:
Noise will increase with speed.
Bearing เครื่องมือที่ใช้ในการวัดความดัน or greasing intervals may be lowered, or improved match bearings may be required.
The greater speed (and variable speed in general) will improve the danger of resonant vibration due to a important velocity within the working range.
The higher pace will lead to extra power consumption. It is important to contemplate if the pump and drive practice is rated for the higher energy.
Since the torque required by a rotodynamic pump increases in proportion to the square of pace, the opposite main concern is to ensure that the motor can present enough torque to drive the load on the increased speed. When operated at a speed under the rated pace of the motor, the volts per hertz (V/Hz) may be maintained as the frequency applied to the motor is elevated. Maintaining a continuing V/Hz ratio retains torque production 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 current (AC) power supply limits the utmost voltage that’s out there to the motor. Therefore, the voltage supplied to the motor can not continue to increase above the nameplate voltage as illustrated in Image 2. As shown in Image 3, the available torque decreases past 100 percent frequency as a end result of the V/Hz ratio isn’t maintained. In an overspeed state of affairs, the load torque (pump) have to be beneath the available torque.
Before working any piece of kit outside of its rated speed range, it is important to contact the producer of the gear to determine if this can be done safely and efficiently. For extra info on variable velocity pumping, discuss with HI’s “Application Guideline for Variable Speed Pumping” at pumps.org.
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