The power supply for the drive circuit of the frequency converter
The power supply for the drive circuit ofthe frequency converter.
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The following figure shows the power supply
diagram for the drive circuit.
(1) Except for the drive circuits of a few high-power frequency converters that use independent power supplies, most drive circuits are powered by switching power supply circuits. Since the drive circuit is also one of the load circuits of the switching power supply circuit, when the drive IC (or the post-stage power amplifier circuit of the drive IC) is short-circuited, the switching power supply may experience intermittent oscillation due to overload. Additionally, when the switching power supply lacks sufficient load capacity, it can also cause the drive circuit to frequently report OC faults, resulting in phenomena such as single-phasing operation. Especially when the negative power supply (the cut-off voltage of the IGBT) is lost and the IGBT trigger circuit is in an open state, it is highly likely to cause the IGBT module to explode! The drive circuit is closely related to the switching power supply circuit.
What is the OC fault of a
frequency converter?
The OC fault of a frequency
converter refers to its overcurrent fault, which is one of the most frequent
alarm phenomena in frequency converters. It usually indicates that the motor
load is too heavy, or the rated current setting of the frequency converter is
improper, and it may also be caused by various other factors.
Specifically, the OC fault
may be caused by the following reasons:
· Overload: The
load exceeds the carrying capacity of the frequency converter, resulting in
excessive current.
· Too
Short Acceleration/Deceleration Time: During acceleration or deceleration,
due to the short setting time, the frequency converter cannot adjust the output
current in time, leading to overcurrent.
· Inverter
Module Damage:
The inverter module of the frequency converter malfunctions and cannot work
normally, causing abnormal output current.
· Drive
Circuit Damage:
Damage to the drive circuit prevents it from correctly controlling the
operation of the inverter module, which may also lead to an overcurrent fault.
· Improper
Parameter Settings:
For example, setting the current limit too low or setting the torque
compensation (V/F) too high may both cause overcurrent faults.
· In addition, external
factors such as motor winding shorts (including phase-to-phase shorts and
ground shorts) may also cause OC faults. When dealing with OC faults, it is
necessary to troubleshoot and repair based on specific circumstances, including
checking the load condition, adjusting the acceleration/deceleration time, and
inspecting the status of the frequency converter, motor, and cables
(2) The inverter power circuit of the frequency converter is composed of six IGBTs, which some refer to as a three-phase inverter bridge. Each phase circuit consists of upper and lower arm IGBTs. The drives for the upper three-arm IGBTs (V1, V3, V5) require independent power supplies provided by three separate windings (N1, N2, N3) of the switching transformer, because their emitters are connected to the three-phase output terminals (U, V, W) and are not at the same potential. For the drives of the lower three-arm IGBTs, since their emitters are commonly grounded (N), they can share a power supply provided by the N4 winding. Some frequency converter circuits even adopt six isolated power supplies, with each of the six windings of the switching transformer providing a separate power supply. However, when connected to the main circuit of the frequency converter, they still form a common N point. This provides a measurement basis for us to determine whether a circuit is the drive circuit for the upper arm IGBTs or the lower arm IGBTs.
(3) For common single or dual IGBT modules or integrated inverter circuits, a positive and negative dual power supply mode is often adopted to consider the inductive effect of the drive signal leads and improve the reliability of IGBT operation. A few inverter functional circuits utilize IPM (Intelligent Power Module) smart modules, where the drive circuit and IGBT protection circuit are integrated within the module. Therefore, the power supply for the drive circuit becomes a single power supply mode, and sometimes only the drive power supply is provided.
The inspection of the drive circuit always needs to be accompanied by the testing of the drive power supply and the IGBT. Ultimately, the testing has decisive significance only when the drive circuit forms a complete signal passage. When the pulse output terminal is in an open state (disconnected from the IGBT), the output of the drive circuit may appear normal (no-load state is normal), but this does not guarantee that the load capacity of the drive power supply is normal. The drive circuit, switching power supply circuit, and inverter output circuit are closely related during the maintenance of the drive circuit. When there is a fault in the drive circuit, it is necessary to simultaneously check the working states of these three circuits.
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