Overcurrent protection is defined as a protective measure that is designed to perform corresponding protective actions (such as tripping a circuit breaker) when the measured current increases beyond the allowable value.
It mainly includes two types: short-circuit protection and overload protection.
Short-circuit protection is characterized by large setting current and instantaneous action. Electromagnetic current trip units (or relays) and fuses are commonly used as short-circuit protection components.
Overload protection is characterized by a small setting current and inverse-time operation. Thermal relays and time-delay electromagnetic current relays are commonly used as overload protection components.
When the current exceeds a certain limit, the circuit will be disconnected to protect electrical appliances from being burned out by the excessive current. There are automatic reconnection and manual reconnection options. After the overcurrent protection trips, the cause of the increased current should be checked and the fault eliminated before continuing to use the appliance.
The overcurrent protection of the OKE frequency converter not only protects the motor and ensures stable operation of the motor series, but also can quickly trip to protect the frequency converter itself when the peak inrush current is too large, effectively avoiding short circuits on the output side and ensuring the life of power electronic devices.
Causes of overcurrent:
1. Overcurrent during operation refers to the occurrence of overcurrent in the drive system during operation. The causes generally come from the following aspects:
① When the motor encounters an impact load, or when the transmission mechanism "jammed", it causes a sudden increase in the motor current.
② Short circuit on the output side of the frequency converter, such as a short circuit between the connection lines from the output terminal to the motor, or a short circuit inside the motor.
③ Malfunction of the inverter itself, such as abnormal operation of two inverter devices in the same bridge arm during the continuous alternating operation of the inverter bridge. For example, due to excessively high ambient temperature or aging of the inverter devices themselves, the parameters of the inverter devices may change, causing one device to be turned on while the other has not yet been turned off during the alternation process, resulting in a "shoot-through" between the upper and lower devices in the same bridge arm, causing a short circuit between the positive and negative terminals of the DC voltage.
2. Overcurrent during speed increase: When the load inertia is large and the speed increase time is set too short, it means that during the speed increase process, the inverter's working efficiency increases too quickly, the synchronous speed of the motor increases rapidly, and the speed of the motor rotor cannot keep up due to the large load inertia, resulting in excessive speed increase current.
3. Overcurrent during deceleration: When the load inertia is large and the deceleration time is set too short, overcurrent can also occur. This is because if the deceleration time is too short, the synchronous speed drops rapidly, while the motor rotor, due to the large inertia of the load, still maintains a relatively high speed. In this case, the rotor windings may cut the magnetic lines of force too quickly, resulting in overcurrent.
