Protection function and fault treatment of the hot

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Protection function and fault handling of general frequency converter

1. Introduction

the frequency converter itself has quite rich abnormal fault display and protection functions. If the protection function acts, the frequency converter trips immediately, and the LED displays the fault code, or the fault information is stored in a parameter of the program, so that the motor is in a free running state until it stops. Before eliminating the cause of the fault and resetting with the tesset key or RST input of the control circuit terminal, always maintain the tripping state for maintenance and inspection. The abnormal faults of the frequency converter are divided into two categories: soft faults and hard faults. The former is mostly caused by improper operation or parameter setting, and the hard faults are caused by the damage of the components of the frequency converter itself, which may be inconvenient to maintain. Before dealing with the fault, pay attention to check the operation records of the frequency converter before the fault, mainly including current, speed, winding and bearing temperature, so as to facilitate the analysis and inspection of the fault. When the frequency converter shows a certain kind of fault, but the corresponding fault does not occur in the process of troubleshooting, carefully check whether there is a problem with the fault detection element or the fault information processing system

during fault inspection or maintenance, pay attention to cut off the power supply first, shake out the circuit breaker of the high-voltage cabinet on the incoming side of the input transformer of the frequency converter, and disconnect the main switches of the A1 and A2 incoming cabinets of the frequency converter. The cabinet door can be opened for maintenance only after the power is cut off for 8min and the capacitor is discharged. Do not check immediately after shutdown. Because the DC busbar voltage of the frequency converter can reach about 1000V when it is rated, and the number of electrolytic capacitors used for filtering is up to 120, with a single capacity of 6800 μ F. A large amount of electric energy is stored. After shutdown, the voltage balance resistance in front of the capacitor module must discharge it. After the voltage is reduced (the discharge time is 8min), the cabinet can be opened for inspection. Generally speaking, the common protection functions of frequency converter include the following aspects

2. Overcurrent protection function

the object of overcurrent protection in the frequency converter mainly refers to those with sudden change, and the peak value of the current exceeds the overcurrent detection value (about 200% of the rated current). The frequency converter shows that OC indicates overcurrent. Due to the poor overload capacity of the inverter device, the overcurrent protection of the frequency converter is a crucial link

2.1 cause analysis of overcurrent

overcurrent faults can be divided into acceleration, deceleration and constant speed overcurrent. It may be caused by too short acceleration and deceleration time of the frequency converter, sudden load change, uneven load distribution, output short circuit and other reasons. At this time, it can generally be solved by extending the acceleration and deceleration time, reducing the sudden change of load, adding energy consumption braking elements, carrying out load distribution design, checking the line, etc. If the load converter is disconnected or the overcurrent fault occurs, the inverter circuit of the converter is broken and the converter needs to be replaced. According to the display of the frequency converter, the causes can be found from the following aspects:

(1) overcurrent during operation, that is, overcurrent occurs in the driving system during operation. The reasons are as follows:

● first, the motor encounters impact load or the transmission mechanism appears "stuck", causing a sudden increase in motor current

● second, there is a short circuit on the output side of the frequency converter, such as the connecting lines between the output end and the motor are shorted to each other, or there is a short circuit inside the motor, etc., grounding (contact, grounding, etc. caused by motor burnout, insulation deterioration, cable damage)

● third, the inverter itself does not work normally, such as two inverter components of the same bridge arm in the inverter bridge are abnormal in the continuous alternating working process. For example, the environmental temperature is too high, or the inverter components themselves are aging, which changes the parameters of the inverter. As a result, in the alternating process, one device has been turned on, but the other device has not been turned off in time, causing the "direct connection" between the upper and lower devices of the same bridge arm, so that the positive and negative poles of the DC voltage of inorganic sand paddle coated on both sides of the flammable polyurethane are in a short circuit state

(2) overcurrent during acceleration and deceleration: when the inertia of the load is large and the acceleration time or deceleration time is set too short, overcurrent will also be caused. In the process of speed increase, the working frequency of the frequency converter rises too fast, the synchronous speed of the motor rises rapidly, and the speed of the motor rotor cannot keep up due to the large load inertia, resulting in too large speed increase current; In the process of speed reduction, the speed reduction time is too short, and the synchronous speed drops rapidly, while the motor rotor still maintains a high speed due to the large inertia of the load. At this time, it can also make the rotor winding cut the magnetic line of force too fast and produce overcurrent

2.2 treatment method

(1) when starting, it trips as soon as the speed is increased, which is a very serious phenomenon of overcurrent. The main inspection is:

● whether the working machinery is stuck

● check whether there is a short circuit at the load side, and check whether there is a short circuit to the ground with a megger

● whether the inverter power module is damaged

● the starting torque of the motor is too small, and the driving system cannot rotate

(2) do not trip immediately when starting, but trip during operation. Main inspection:

● the speed-up time is set too short, and the acceleration time is extended

● the deceleration time is set too short, and the deceleration time is extended

● the setting of torque compensation (u/f ratio) is too large, resulting in excessive load current in low-frequency space-time

● the electronic thermal relay is improperly set, and the action current is set too small, causing the wrong action of the frequency converter

3. Overload protection and cause analysis

the motor can rotate, but the running current exceeds the rated value, which is called overload. The basic reflection of overload is that although the current exceeds the rated value, the exceeding range is not large, and generally does not form a large impulse current. The output current exceeds the overload current rating of inverse time limit characteristics, the protection function acts, and the capacity of the frequency converter is small

3.1 main causes of overload

(1) heavy mechanical load: the main feature of heavy load is that the motor heats up and can be found by reading the running current from the display screen. The main reason is that the load of frequency converter is too large, and the acceleration and deceleration time and operation cycle time are too short; The voltage of v/f characteristic is too high; The power of frequency converter is too small

(2) three phase voltage imbalance: it causes the operating current of a phase to be too large, resulting in overload tripping. Its characteristic is that the motor heating is unbalanced, and it may not be found when reading the operating current from the display screen (because the display screen only displays one phase current)

(3) maloperation: the current detection part inside the frequency converter fails, and the detected current signal is too large, resulting in tripping

3.2 inspection method

(1) check whether the motor is heated

if the temperature rise of the motor is not high, first check whether the load size, acceleration and deceleration time, and operation cycle time settings are reasonable, correct the v/f characteristics, and check whether the electronic thermal protection function of the frequency converter is preset reasonably. If the frequency converter has margin, the preset value of the electronic thermal protection function should be relaxed; If the allowable current of the frequency converter has no allowance and cannot be relaxed, and according to the production process, the overload is normal overload, it indicates that the selection of the frequency converter is improper, and the capacity of the frequency converter should be increased and the frequency converter should be replaced. This is because when the motor drives variable load or intermittent load, as long as the temperature rise does not exceed the rated value, it is allowed to overload for a short time (a few minutes, or even dozens of minutes), while the frequency converter is not allowed. If the temperature rise of the motor is too high, and the overload is normal overload, it means that the motor is overloaded. At this time, we should first consider whether we can appropriately increase the transmission ratio to reduce the load on the motor shaft. If it can be increased, increase the transmission ratio; If the transmission ratio cannot be increased, the capacity of the motor should be increased

(2) check whether the three-phase voltage at the motor side is balanced

if the three-phase voltage at the motor side is unbalanced, check whether the three-phase voltage at the output end of the frequency converter is balanced. If it is unbalanced, the problem is inside the frequency converter, check the inverter module of the frequency converter and its driving circuit; If the voltage at the output end of the frequency converter is balanced, the problem occurs in the line from the frequency converter to the motor. Check whether the screws of all terminals have been tightened. If there are contactors or other electrical appliances between the frequency converter and the motor, check whether the terminals of relevant electrical appliances have been tightened, and whether the contact condition of the contacts is good

if the three-phase voltage on the motor side is balanced, you should understand the working frequency when tripping: if the working frequency is low and vector control is not used (or there is no vector control), first reduce the u/f ratio. If the load can still be driven after reduction, it indicates that the original preset u/f ratio is too high and the peak value of excitation current is too large. You can reduce the current by reducing the u/f ratio; If there is no live load after reduction, consider increasing the capacity of the frequency converter; If the frequency converter has vector control function, vector control mode shall be adopted

(3) check whether there is malfunction

after the above checks, if the cause is not found, check whether there is malfunction. The judgment method is to measure the output current of the frequency converter with an ammeter under light load or no load, and compare it with the running current value displayed on the display screen. If the current reading displayed on the display screen is much larger than the actual measured current, it indicates that the error of the current measurement part inside the frequency converter is large, and the "overload" trip may be a malfunction

4. Undervoltage protection lu

undervoltage is also a problem we often encounter in use. After the power supply voltage is reduced, if the DC voltage of the main circuit falls below the undervoltage detection value, the protection function acts. In addition, if the voltage drops to a level that cannot maintain the operation of the frequency converter control circuit, all protection functions will be reset automatically (detection value: dc400v). When there is undervoltage fault, you should first check whether the input power supply is in phase failure. If there is no problem with the input power supply, you should check whether there is a problem with the rectifier circuit. If there is no problem, it depends on whether there is a problem on the DC detection circuit. If the voltage of the main circuit is too low (380V series is lower than 400V), the main reason is that one circuit of the rectifier bridge is damaged or one phase of the three-phase circuit of the thyristor is abnormal, which may lead to the occurrence of undervoltage fault. Secondly, the circuit breaker and contactor of the main circuit are damaged, resulting in the loss of DC bus voltage above the charging resistance, which may lead to undervoltage. Undervoltage occurs due to the failure of the voltage detection circuit, and the DC terminal (between P and N) of the main circuit exceeds the detection value due to the malfunction of the frequency converter or the misoperation caused by noise, so it is necessary to contact the manufacturer

5. Overvoltage protection oud

the regeneration current from the motor increases. If the DC voltage of the main circuit exceeds the voltage detection value, the protection function acts when the excessive voltage is applied incorrectly (detection value: DC750V). There are three main phenomena of overvoltage protection: overvoltage during acceleration, overvoltage during deceleration, and overvoltage at constant speed. Overvoltage alarm usually occurs during shutdown. The main reason is that the deceleration time is too short or there is a problem with the braking resistance and braking unit

the overvoltage of the frequency converter is concentrated on the voltage of the DC bus. Under normal conditions, the DC current of the frequency converter is the average value after three-phase full wave rectification. If calculated with 380V line voltage, the average DC voltage ud=1.35ul = 513v. In case of overvoltage, the energy storage capacitor of the DC bus will be charged. When the voltage rises to about 760v, the inverter overvoltage protection will act.

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