In some situations, operating a motor beyond the base pole speed is feasible and provides system advantages if the design is fastidiously examined. The pole speed of a motor is a perform of the number poles and the incoming line frequency. Image 1 presents the synchronous pole pace for 2-pole via 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 base pole velocity. If the incoming line frequency doesn’t change, the velocity of the induction motor might be lower than these values by a p.c to slide. So, to operate the motor above the bottom pole pace, the frequency needs to be elevated, which could be accomplished with a variable frequency drive (VFD).
One purpose for overspeeding a motor on a pump is to use a slower rated speed motor with a decrease horsepower ranking and operate it above base frequency to get the required torque at a lower present. This enables the choice of a VFD with a lower present score to be used while still making certain passable management of the pump/motor over its desired operating range. The lower present requirement of the drive can scale back the capital price of the system, relying on overall system requirements.
The applications the place the motor and the driven pump function above their rated speeds can provide further circulate and pressure to the managed system. Remote might lead to a more compact system while growing its efficiency. While it could be attainable to extend the motor’s speed to twice its nameplate velocity, it’s extra common that the utmost pace is more limited.
The key to those functions is to overlay the pump velocity torque curve and motor velocity torque to ensure the motor begins and capabilities all through the entire operational velocity range without overheating, stalling or creating any important stresses on the pumping system.
Several points also have to be taken under consideration when considering such solutions:
Noise will enhance with velocity.
Bearing life or greasing intervals could also be lowered, or improved fit bearings may be required.
The higher velocity (and variable speed in general) will increase the chance of resonant vibration due to a crucial velocity inside the operating range.
The greater speed will lead to extra energy consumption. It is essential to contemplate if the pump and drive train is rated for the higher energy.
Since the torque required by a rotodynamic pump will increase in proportion to the sq. of speed, the opposite main concern is to ensure that the motor can provide sufficient torque to drive the load at the elevated velocity. When operated at a speed under the rated speed of the motor, the volts per hertz (V/Hz) could be maintained as the frequency utilized to the motor is increased. Maintaining a continuing V/Hz ratio retains torque manufacturing steady. While it will be ideal to extend the voltage to the motor as it’s run above its rated velocity, the voltage of the alternating present (AC) power supply limits the maximum voltage that’s out there to the motor. Therefore, the voltage provided to the motor cannot continue to increase above the nameplate voltage as illustrated in Image 2. As shown in Image 3, the out there torque decreases past one hundred pc frequency as a end result of the V/Hz ratio is not maintained. In an overspeed scenario, the load torque (pump) should be beneath the available torque.
Before operating any piece of apparatus outside of its rated velocity range, it is important to contact the producer of the equipment to discover out if this could be done safely and efficiently. For extra information on variable velocity pumping, refer to HI’s “Application Guideline for Variable Speed Pumping” at pumps.org.
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