Maintenance principles and safety precautions of frequency converter

Maintenance principles and safety precautions of frequency converter

The basic principle of AC-DC-AC frequency conversion: first, the three-phase AC power is filtered and converted into DC power through a bridge rectifier circuit. After filtering, the DC power is converted into three-phase AC power output of different frequencies by a bridge inverter circuit.

1. Determine the fault scope of the inverter

In actual experience maintenance, generally without the inverter circuit schematic diagram, the inverter is mostly caused by the damage of the main circuit power electronic components. For the main circuit part, the fault range should be determined first, the inverter is powered on, and the DC bus voltage value is measured to see if it is equal to 1.35 times the effective value of the input voltage. If the voltage is normal, the inverter part can be judged as faulty. Otherwise, it may be damaged by the rectifier power component, pre-charge circuit or filter capacitor.

For a few inverters with internal contactors, the contactor is the pre-charging part of the DC bus. The contactor is started only after the inverter is powered on, and there is no fault alarm signal and a given "start" signal. If the contactor is not started and there is no DC bus voltage, the fault range cannot be determined. First, simulate the given "no fault" feedback signal of the inverter part and the external start signal, and artificially close the contactor. The DC bus voltage can be measured, and the fault range can be determined according to the DC voltage. The method is the same as above. Note that before starting the pre-charging contactor, the given signal is sometimes a pulse trigger signal instead of a level signal.

2. Static detection of rectifier unit

The method to judge whether a power component of the rectifier part is damaged is to use the unidirectional conductivity of the rectifier component. The forward and reverse resistance values ​​should be different when they are normal in static state. The specific method is as follows:

The three-phase bridge rectifier circuit of the rectifier part may be diode rectifier, thyristor half-controlled rectifier, thyristor fully controlled rectifier or IGBT rectifier. No matter which method is used, the three-phase rectifier circuit is symmetrical, so the static test resistance value result should comply with the symmetry principle, that is, the positive and negative test values ​​of the three-phase input or output end relative to the positive and negative poles of the DC bus should be symmetrical in static state. Select the "diode" position of the multimeter.

(1) The first step is to connect the red test lead to the positive pole of the DC bus and the black test lead to the three-phase terminals of the power input. The three test values ​​should be the same. Conversely, connect the black test lead to the positive pole of the DC bus and the red test lead to the three-phase terminals of the input power. The three test values ​​should also be the same. If a diode rectifier bridge is used for rectification, the multimeter displays 0.4~0.6V when it is turned on and displays infinity when it is reversed. If the three-phase measurement values ​​have a large deviation, or the positive and negative measurement values ​​of a phase are similar or the same, the diode element is damaged.

(2) In the second step, connect the red test lead to the negative pole of the DC bus and the black test lead to the three-phase connection of the input power supply. The three test values ​​should be the same. Conversely, connect the black test lead to the negative pole of the DC bus and the red test lead to the three-phase connection of the input power supply. The three test values ​​should also be the same. For the pre-charging circuit designed after the rectifier bridge, this operation can also determine the quality of the three rectifier components in the negative half cycle of the rectifier bridge (the test method for the 12-pulse rectifier bridge is the same as above).

Note: For the pre-charging circuit designed before the rectifier circuit, a thyristor half-controlled or fully-controlled bridge rectifier is used. The test result should have one phase with a different forward and reverse resistance test value from the other two phases, that is, one phase is actually the resistance value of the diode pre-charging circuit being tested.

3. Inverter unit static detection

The principle of the 6-pulse triggered three-phase inverter bridge is to use a freewheeling diode in parallel in each inverter IGBT module. There is unidirectional conductivity in static state. The measurement method is the same as the rectifier bridge detection method, that is, the positive and negative poles of the DC bus compare the test values ​​of the three-phase output points, and the three-phase test values ​​should be the same. When the component is single-phase turned on, the multimeter displays 0.3~0.4V, and when it is reversed, it displays infinity. The short circuit fault of the main circuit may also be caused by the abnormality of the varistor protecting the power component, which often damages the power component.

4. Control circuit detection

The detection method of the control circuit takes acs800-04 as an example. After the inverter is powered on, observe that the signal light v204 on the aint mainboard is green, indicating that +5v is normal, v309 is red, indicating that the anti-false start protection is in the on state, and v310 is green, indicating that the igbt gate drive is normal. The red light on the rmio external signal interface board indicates a fault, and the green light indicates that the power supply +24v is normal. Finally, use an oscilloscope to detect whether there is a trigger signal at the trigger pole of each power component. Generally, there is a 5v voltage trigger. Circuit boards without signal lights (rint main circuit interface board, rrfc filter board, rvar varistor board, etc.) can be statically tested for the resistance value of components that may be damaged, and a rough judgment can be made. It is also possible to replace a circuit board of the same model to try. Under normal circumstances, the green light on the control board should be normal, and the red light indicates a fault.

5. Common fault detection

The power board is the most common fault in the control circuit. Check its output and it should have +24V, +5V, ±15V or ±12V power supply. If the voltage of a phase is abnormal, check its power supply load and the power board itself carefully; if the "overcurrent alarm" signal appears, check the IGBT module or current sensor part. The Hall current sensor power supply is generally dual power supply, and its output is 0~10V or 4~20AM standard signal, which changes with the load current; if there is a "high temperature alarm", it is usually a fan failure or temperature measurement element damage. The temperature measurement element is generally installed on the radiator or built into the IGBT module. It usually uses a negative temperature coefficient (NTC) resistor and bears a high resistance value when tested at normal temperature and static; if the "DC bus overvoltage" signal appears, check the power supply voltage, voltage transformer and brake chopper part, because the load is often unstable; some inverters have two DC power supplies, one is generated by input voltage step-down rectification, and the other is obtained by sampling the DC bus voltage through series resistance step-down or DC chopping.

Before maintenance work, attention should be paid to safety. It is best to have a dedicated person to monitor to ensure personal and equipment safety, and do not artificially expand the fault. Do not reverse the input and output terminals of the inverter, otherwise the inverter will be directly damaged; during the maintenance process, pay attention to the high voltage on the DC bus after the inverter is powered off. Wait for more than 5 minutes before touching it, or discharge the capacitor manually, and operate safely according to the capacitor discharge standard. Only after the discharge can the operation continue; when the inverter is in the power-on standby state or has been started at a given zero speed, its output terminal has a DC high voltage of about 200V relative to the ground, so pay attention to personal safety; when testing the control board, it is best not to touch the pins of the integrated chip on the board with your hands to prevent static electricity from damaging the integrated chip and causing unnecessary losses.