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How to Solve Overcurrent of Variable Frequency Drive?

Causes for Overcurrent of Variable Frequency Drive
1. Overcurrent in VFD working refers to the overcurrent of the driving system occurs in the working. The causes may include:
  • The motor suffers an impact load or the transmission mechanism is stuck, which leads to sudden increase of the motor current.
  • Variable frequency drive output short circuit. For example, short circuit occurs between the output and motor, or short circuit inside the motor.
  • Abnormal operation of VFD itself, for example, two inverters with the same arm in the inverted bridge are abnormal in the continuous alternating operation. Because of high ambient temperature or the aging of inverters themselves, parameters of VFD change, which makes one device connected while the other has not yet been turned off in the alternating process. As a result, two components of a same bridge arm are directly connected, and short circuit occurs between the positive and negative poles of DC voltage.
2. Inertia of over-current with load is larger when the speed rises, however the speed rising time is set too short, that means the working efficiency of VFD increases too fast in the process of speed-up and synchronous speed of the motor rises rapidly, but the speed of motor rotor can not keep up with due to load inertia, thus resulting in larger rising current.

3. Inertia of over-current with load is larger when the speed decreases while the speed decreasing time is set too short, as a result, over-current also occurs. The time for speed decrease is too short and the synchronous speed drops rapidly, however the motor rotor maintains a higher speed for larger load inertia, which can leads to large speed of the rotor winding cutting the magnetic line, thus resulting in the over current.

Treatments
1. VFD tripping occurs at starting. It’s a serious phenomenon of overcurrent, inspections shall be made:
Whether the working machine is stuck, whether the load is short circuited, checking whether the grounded is short circuited with megger, whether the power module of the VFD is damaged, starting torque of the motor is too small, and the drive system can’t be operated.

2. Tripping occurs in running rather than starting. Inspections shall be made:
Speed-up time is set too short, please increase acceleration time. Deceleration time is set too short, please increase the deceleration time. Torque compensation (U/f ratio) is set too large, leading to excessive load-free current of low frequency space-time; Inappropriate setting of electronic thermal relay and the too small setting of operating current cause wrong operation of VFD.

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