In some situations, working a motor past the bottom pole pace is possible and provides system benefits if the design is carefully examined. The pole speed of a motor is a function of the quantity 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 reduce the bottom pole speed. If the incoming line frequency does not change, the pace of the induction motor shall be less than these values by a p.c to slide. So, to function the motor above the bottom pole pace, the frequency needs to be increased, which could be carried out 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 ranking and operate it above base frequency to get the required torque at a lower present. This allows the selection of a VFD with a decrease current score to be used whereas nonetheless guaranteeing satisfactory management of the pump/motor over its desired operating range. The lower present requirement of the drive can scale back the capital value of the system, relying on overall system necessities.
The applications the place the motor and the driven pump function above their rated speeds can present further flow and strain to the controlled system. This might lead to a more compact system while increasing its effectivity. While it might be attainable to extend the motor’s speed to twice its nameplate velocity, it is more frequent that the utmost pace is more restricted.
The key to those applications is to overlay the pump velocity torque curve and motor speed torque to ensure the motor starts and capabilities all through the complete operational pace vary without overheating, stalling or creating any vital stresses on the pumping system.
Several factors also must be taken into consideration when considering such solutions:
Noise will enhance with speed.
Bearing life or greasing intervals could additionally be lowered, or improved fit bearings could additionally be required.
The higher pace (and variable speed in general) will increase the danger of resonant vibration because of a important speed inside the working vary.
The higher pace will lead to further energy consumption. It is essential to consider if the pump and drive train is rated for the higher energy.
Since the torque required by a rotodynamic pump increases in proportion to the sq. of velocity, the other main concern is to make sure that the motor can present sufficient torque to drive the load on the elevated speed. When operated at a speed below the rated pace of the motor, the volts per hertz (V/Hz) can 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 perfect to increase the voltage to the motor as it’s run above its rated speed, the voltage of the alternating present (AC) power source limits the maximum voltage that’s out there to the motor. Therefore, เกจวัดแรงดันpressuregauge provided to the motor cannot proceed to increase above the nameplate voltage as illustrated in Image 2. As shown in Image three, the obtainable torque decreases beyond 100% frequency because the V/Hz ratio is not maintained. In an overspeed state of affairs, the load torque (pump) must be under the available torque.
Before working any piece of equipment exterior of its rated speed range, it is important to contact the manufacturer of the gear to determine if this may be accomplished safely and efficiently. For more info on variable speed pumping, check with HI’s “Application Guideline for Variable Speed Pumping” at