In some instances, working a motor beyond the base pole velocity is feasible and offers system advantages if the design is fastidiously examined. The pole pace of a motor is a function 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, additional poles scale back the bottom pole speed. If the incoming line frequency does not change, the pace of the induction motor might be lower than these values by a percent to slip. So, to function the motor above the base pole velocity, the frequency must be increased, which may be accomplished with a variable frequency drive (VFD).
One purpose for overspeeding a motor on a pump is to make use of a slower rated velocity motor with a decrease horsepower score and operate it above base frequency to get the required torque at a lower present. This enables the selection of a VFD with a decrease current rating to be used while still guaranteeing passable control of the pump/motor over its desired working range. The lower current requirement of the drive can reduce the capital cost of the system, depending on general system necessities.
The functions the place the motor and the pushed pump operate above their rated speeds can provide additional circulate and pressure to the controlled system. This could end in a more compact system whereas rising its effectivity. While it may be potential to increase the motor’s speed to twice its nameplate pace, it is more common that the maximum velocity is more limited.
The key to those functions is to overlay the pump pace torque curve and motor pace torque to make sure the motor starts and functions throughout the whole operational pace vary with out overheating, stalling or creating any important stresses on the pumping system.
Several factors also must be taken into consideration when considering such solutions:
Noise will increase with pace.
Bearing life or greasing intervals could also be reduced, or improved match bearings may be required.
The higher velocity (and variable velocity in general) will increase the chance of resonant vibration as a outcome of a important velocity inside the operating vary.
The larger speed will result in additional power consumption. It is essential to assume about if the pump and drive prepare is rated for the upper power.
Since the torque required by a rotodynamic pump increases in proportion to the sq. of velocity, the other major concern is to ensure that the motor can present sufficient torque to drive the load on the increased pace. 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 continuing V/Hz ratio keeps torque manufacturing stable. While it will be best to extend the voltage to the motor as it’s run above its rated pace, the voltage of the alternating present (AC) energy source limits the utmost voltage that’s available to the motor. Therefore, the voltage equipped to the motor cannot continue to extend above the nameplate voltage as illustrated in Image 2. As shown in Image 3, the obtainable torque decreases past one hundred pc frequency as a end result of the V/Hz ratio just isn’t maintained. In pressure gauge , the load torque (pump) have to be below the available torque.
Before working any piece of equipment outside of its rated speed range, it’s important to contact the manufacturer of the tools to determine if this can be done safely and efficiently. For more info on variable pace pumping, check with HI’s “Application Guideline for Variable Speed Pumping” at pumps.org.
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