In some situations, operating a motor beyond the bottom pole pace is possible and provides system benefits if the design is rigorously examined. The pole velocity of a motor is a function 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 scale back the bottom pole velocity. If the incoming line frequency does not change, the velocity of the induction motor will be lower than these values by a percent to slip. So, to function the motor above the bottom pole pace, the frequency must be elevated, which may be accomplished with a variable frequency drive (VFD).
One cause for overspeeding a motor on a pump is to use a slower rated velocity motor with a lower horsepower ranking and function it above base frequency to get the required torque at a lower present. This enables the number of a VFD with a decrease present rating for use while nonetheless guaranteeing passable control of the pump/motor over its desired operating range. The decrease present requirement of the drive can cut back the capital value of the system, depending on overall system necessities.
The applications the place the motor and the driven pump function above their rated speeds can provide extra circulate and pressure to the managed system. This may lead to a extra compact system while growing its efficiency. While it may be attainable to extend the motor’s pace to twice its nameplate velocity, it is extra common that the utmost pace is extra restricted.
The key to these applications is to overlay the pump speed torque curve and motor velocity torque to ensure the motor starts and functions throughout the complete operational pace vary with out overheating, stalling or creating any vital stresses on the pumping system.
Several factors also have to be taken under consideration when considering such solutions:
Noise will enhance with pace.
Bearing life or greasing intervals could additionally be lowered, or improved match bearings could additionally be required.
เกจวัดแรงดันเบนซิน (and variable speed in general) will enhance the chance of resonant vibration as a end result of a crucial velocity throughout the operating vary.
The larger speed will end in additional energy consumption. It is necessary to contemplate if the pump and drive prepare is rated for the higher energy.
Since the torque required by a rotodynamic pump will increase in proportion to the square of speed, the other main concern is to make sure that the motor can provide sufficient torque to drive the load at the increased pace. When operated at a pace below the rated pace of the motor, the volts per hertz (V/Hz) could be maintained as the frequency utilized to the motor is increased. Maintaining a constant V/Hz ratio retains torque manufacturing stable. While it will be ideal to extend the voltage to the motor as it is run above its rated speed, the voltage of the alternating current (AC) power supply limits the utmost voltage that is obtainable to the motor. Therefore, the voltage supplied to the motor can’t proceed to extend above the nameplate voltage as illustrated in Image 2. As proven in Image 3, the available 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) must be below the obtainable torque.
Before operating any piece of equipment exterior of its rated speed range, it is essential to contact the manufacturer of the tools to discover out if this can be accomplished safely and efficiently. For more information on variable speed pumping, refer to HI’s “Application Guideline for Variable Speed Pumping” at pumps.org.
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