As a Kaman Electrical Specialist, I am asked almost daily, “What effect will putting a Variable Frequency Drive have on my existing motor?”
There are many factors that need to be taken into consideration when making a recommendation concerning VFDs. Ideally, providing motor Nameplate data will help answer this question, but in the event that this data is not available, the following rules will prove helpful. If applied correctly and all expectations are met, motors will run long, hard-working lives on variable frequency power.
1. If the Motor is pre-EPAct (1992), it should be run on VFDs only with careful consideration. Motors made during that time were not made for VFD use, but if they are VFD rated, will be okay. Class F insulation or higher is suitable for VFD use, but VFDs may have no more than a 2:1 Constant Torque ratio. Existing condition of motor insulation systems may be the determining factor in this case, and conducting a High Pot Test is recommended.
2. In most cases, EPAct Motors with 2:1 Constant Torque ratings and 4:1 to10:1 Variable Torque ratings can be run on VFDs. It is important to note, however, that many fractional and 56-frame motors are not rated for use with VFDs. Additionally, washdown motors (including those with stainless steel) have reduced capabilities compared to non-washdown versions.
3. NEMA Premium Motors with 4:1 to 20:1 Constant Torque ratings 10:1 to 20:1 Variable Torque ratings can be run on VFDs. Please note that larger-frame motors are rated for smaller Constant Torque ratings of 2:1-4:1.
4. All of the above rules can be affected by the motor lead length from the VFD to the motor. Typically, lead lengths of 100m or less should be okay. Lead lengths greater than 100m require bearing current mitigation efforts; including using Aegis SGR ground rings, Output Line Reactors, and dV/dT Filters.
5. If speed ratios above these Inverter Duty motors listed need to be used, please call your local Kaman branch for application assistance.
Please note that these are generalizations; each motor manufacturer has their own recommendations based on the model number of the motor and the application. Ratings also can change from Motor type with in a manufacturer’s product offering.
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