The peak voltage, or Vpeak, indicates the amount of stress imposed on the motor’s stator insulation. The Pulse Width Modulation (PWM) digital signal can be measured in a number of ways. If the peak voltage generated by this oscillation escalates too high, it can break through the motor’s insulation and cause the motor’s windings to short. This overshoot interacts with the inductance and capacitance of the motor and cables, causing the voltage to oscillate. However, when a fast-rising voltage pulse is applied, the back-voltage generated by the motor’s coils can cause a voltage spike. Gradual changes in current flow are typically unproblematic. VFD-Induced Motor Insulation BreakdownĪn AC induction motor’s stator coils possess an electrical property called “inductive reactance,” that produces a back-voltage that opposes changes in current flow. And sometimes, the insulation in the motor’s windings may breakdown due to the interaction between the VFD and the motor’s stator windings. In other cases, loose wire strands in the motor’s conduit box, nicked insulation in the motor cables, or moisture in the motor or in the conduit between the VFD and the motor can result in a malfunction. The motor’s stator windings may overheat and burn out due to insufficient cooling of the motor, an overload, improper wiring, incorrect motor data supplied to the VFD, or running the motor for an extended time at low speed in a constant torque application. Motors may fail for many reasons unrelated to the VFD itself. However, VFDs may also lead to degradation of the coils that produce the magnetic field in the motor’s stator and their insulation. VFDs improve control, increase efficiency, and extend the motor’s operating life by reducing mechanical and thermal stresses.
A variable frequency drive (VFD), sometimes known as a variable speed drive, is used to control AC induction motor speed and torque by varying the motor input frequency and voltage.
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