The longer the cable, the greater the wave reflection.
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Reflected waveforms can build to a point of damaging the VFD and motor. Technologies that allow for greater distances to occur in the field can be added to the system. Premature bearing failure can also be attributed to fast-rising voltage pulses. The high switching frequency inverters used to achieve the required PWM waveform causes capacitive energy to build between the rotor and stator. If this energy is released on a path through the motor bearings, it can cause electrical discharge machining EDM , also known as fluting, which can cause the bearing to overheat and potentially fail prematurely.
VFDs use pulsing DC voltage to create an imitation sinewave, which has the effect of building up a differential charge between the rotor and stator. Three-phase motors driven by standard sinewave power have a balanced charge. The PWM waveform, however, is not a true sinewave.
Rather, VFDs use pulsing DC voltage to create an imitation sinewave, which has the effect of building up a differential charge between the rotor and stator. Known as common mode voltage CMV , the differential charge that builds up between the rotor and stator must be mitigated through shaft grounding and correct system installation to avoid damaging the bearing. Aware of these challenges, many motor manufacturers have created product lines specifically designed to operate with a VFD. These motors include specially designed winding and bearing protections that mitigate some of the effects a PWM waveform can have on a motor.
The shift to VFDs has led to the addition of inverter wire as a standard feature in integral horsepower random wound motors. Electric motor design and manufacturing techniques have also undergone changes to accommodate VFD applications, as well as to eliminate shaft currents. To protect against voltage spikes that exceed their rated voltage, inverter duty motors are designed for use with VFDs by typically including inverter duty insulation systems rated to handle the voltage spikes that occur.
Insulation design standards are regulated by the National Electrical Manufacturers Association NEMA , which specifies the maximum peak voltage each type of insulation must withstand, as well as the minimum rise time for power waveforms. Motors with a voltage rating greater than volts must protect, at minimum, against a voltage spike of 2.
To prevent CMV, bearings on these motors require protection that provides a low-resistance path to ground to mitigate any charge buildup. This typically requires mounting a shaft grounding device — either externally or internally, per local codes for the motor. For additional protection, plus horsepower motors isolate one of the bearings from the motor shaft to interrupt circulating currents that would otherwise add extra energy to the motor shaft.
This is typically accomplished by coating the bearing mount with an insulating material, thereby isolating the shaft from the bearing. If the motor will be located in a hazardous environment, the manufacturer may insulate both bearings, but more often, will insulate the coupling instead. While motor protections like these are helpful, they do not address the root causes of PWM waveform damage. The fact is, manufacturers often add these protections because they cannot predict where, how or when these components will be installed. Refer to the manual to understand the proper grounding requirements of the motor and VFD.
Most distributors of VFDs offer a product that meets these requirements. Grounding: VFDs produce high frequency noise. To reduce the damaging effects of that noise, motors must be grounded back to the drive.
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That requires a braided type of grounding wire that grounds the motor to the drive. At minimum, that wire should be of the same size as a single power lead and be located in the power lead conduit. To obtain the best mitigating effects, installers should consider using multiple grounding wires.
VFDs produce high frequency noise. Conduit selection: Power leads and grounding wire should be housed in metal conduit, which should be connected to both the motor and drive without isolating either. PVC, plastic or other insulating materials are not recommended for connecting metal conduit to a drive or motor terminal box.
If these materials are used, the metal conduit carrying the power leads and ground cable must be properly connected to the grounding circuit. For existing systems, it is harder to get to the root causes of PWM. Still, its damaging effects can often be reduced with system add-ons, such as:. Capitalizing on the benefits of adding a VFD to a pump system is possible when the entire system is reviewed for the effects of PWM power before the project is started.
To achieve the greatest protections, it is necessary to design and install these components as an integrated system. Understanding both the issues and operational requirements of a VFD system will lead to years of successful operation. Patrick Hogg Motion Industries. Jim Sanderson Motion Industries. For more information, visit motionindustries. Patrick Hogg. Courtesy of Nidec.
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