Welcome to the inaugural post of Nidec Motor Corporation’s Driving Force blog. Each month, we’ll be posting educational articles dealing with motors and drives. By blending timely industry topics with the exceptional expertise of our engineers and other insiders, we plan to provide readers a valuable resource for staying in touch with innovations related to motors and drives.
Considerations for a more reliable motor and VFD driven pumping system
April 2018
As many pumping industry engineers know, both the U.S. Department of Energy (DOE) and end users are demanding increasingly higher efficiency levels when it comes to pump systems and their supporting components. This makes taking a system approach to design and installation of electric pump motors and variable frequency drives (VFDs) even more important. Such an approach can help verify improved efficiency and reliability.
VFDs are often added to pump motors because they allow variable speed operation to maximize efficiency of the pumping system. These adjustable speed drives let users change the motor input frequency and voltage at the touch of a finger or with an input from a process controller, which changes the motor speed to get a desired output from the driven equipment. One unfortunate risk that using a VFD raises, however, is that of voltage spikes caused by the VFD’s pulse width modulated (PWM) waveforms that power the motor.
PWM Waveforms: Benefits and Risks
PWM waveforms provide highly effective control of motor speed but can also cause voltage spikes well above the limits that a standard motor winding is rated to withstand. This makes it important to protect motors paired with VFDs from these harmful voltage spikes. Methods for such protection include the use of additional insulating materials and winding processes to avoid damage.
The interaction of the VFD and motor can also have a negative effect on motor bearings, potentially resulting in excess noise, heat or premature failure over an extended period. In the worst cases, the bearing may be severely damaged. This danger can be alleviated by using a proper shaft grounding device on the motor, along with the correct grounding of not only the motor, but utilizing a stranded grounding cable to ground the motor back to the drive along with insulating at least one bearing on motors above 100 HP.
NEMA MG-1 Part 31.4.4, Insulation Considerations, covers maximum voltage spike limits that a dedicated VFD motor should be able to withstand as well as the minimum rise time to hit these voltage peaks. This section also covers insulation considerations for bearing protection.
Reducing the PWM Risks
Adding motor protection features like additional insulating materials and grounding devices is helpful for protecting pump motors from PWM power waveforms, but such features are more a method of treating the symptoms without treating the root cause of the problem. To achieve the most reliable variable speed pumping systems with the lowest risks of damaging effects, the motor and drive need to be installed as a system. A simple component installation can lead to issues. Correct installation of a motor drive system includes: proper grounding of each component, optimal setting of the VFD switching frequency, ensuring the appropriate distance between components, correct cabling, as well as conduit selection and installation.
It’s important to understand that each variable frequency drive operates a little differently. When a motor manufacturer understands the exact outputs of the VFD, it is easier to more fully grasp the effects it can have on the motor. There may still be an element of risk here because the motor and VFD could be installed differently, but looking at the motor and drive as a system and validating their functionality together will greatly increase the reliability of the drive system. Another benefit to designing the motor and VFD as part of an integrated system is that the manufacturer will also be able to provide superior service for both components. However, these components can still be installed incorrectly (such as placing them too far apart or using incorrect cabling or grounding) even when using a system approach.
Ultimately, this comes down to a motor manufacturer understanding what is required to create the most reliable pump and drive system, and the installer acting to ensure correct installation. Too often corners are cut for the sake of lowering initial price, but this is what puts the reliability of the system at risk. By investing the same or a little more money up front, end users can see noticeable improvements in efficiency and reliability, which will often cover or even exceed that initial investment.
This starts at the engineering specification stage and moves through to the end user making sure that the installation meets the system’s specific requirements. Those requirements may be simple, but they’re also simple to miss. This explains why motor manufacturers like Nidec Motor Corporation go above and beyond to protect the pump motor from issues that can be caused from operating on PWM waveforms. Doing so helps ensure that an integrated pump motor and VFD drive system operates with as much efficiency and reliability as possible.
For further reading, check out this article in the February 2018 edition of Pumps and Systems.
Installer sets VFD min/max speeds for ag irrigation application.