Overcurrent fault and overvoltage fault of servo drive, common fault repair of servo drive

At present, the mainstream servo drivers adopt digital signal processor (DSP) as the control core, which can realize more complicated control algorithms, realize digitalization, networking and intelligence. The power device generally adopts the driving circuit designed with the intelligent power module (IPM) as the core. The IPM integrates the driving circuit internally, and has fault detection and protection circuits such as overvoltage, overcurrent, overheat and undervoltage, and adds soft in the main loop. Start the circuit to reduce the impact of the startup process on the drive. The power driving unit first rectifies the input three-phase electric power or the commercial power through a three-phase full-bridge rectifying circuit to obtain a corresponding direct current power. After rectifying the three-phase electric or commercial power, the three-phase permanent-magnet synchronous AC servo motor is driven by the three-phase sinusoidal PWM voltage inverter. The entire process of the power drive unit can be simply the process of AC-DC-AC. The main topology circuit of the rectifier unit (AC-DC) is a three-phase full-bridge uncontrolled rectifier circuit.

With the large-scale application of servo system, servo drive use, servo drive debugging and servo drive repair are all important technical issues of servo drives. More and more industrial control service providers have carried out in-depth technical research on servo drives.

The over-voltage and over-current of the driver debugging process are two common faults. The following two faults are analyzed to better help the debugger to master the fault and the cause, and to quickly understand the fault point. So that the equipment can be put into operation as soon as possible.

1. Overvoltage fault: The voltage referred to here usually refers to the DC bus voltage. Figure 1 shows the main circuit circuit of the common market driver. The voltage between P and N is the DC bus voltage.

The reading of the DC bus voltage, the driver CPU can not read a very high voltage, so it is necessary to convert the high voltage into a low voltage that the CPU can read through circuit conversion. Commonly, there are transformer output reading method and resistance buck reading. Law, see Figure 2, Figure 3.

From the above schematic analysis, the first type of overvoltage generation is caused by various reasons. The voltage between the driver C and D is higher than the rated voltage. When the overvoltage occurs, the storage capacitor of the DC bus will be charged. When the voltage rises to 760V. (This value is adjustable for some drivers), the driver overvoltage protection action, the second case is the detection circuit failure, the normal voltage value is read by the detection circuit as overvoltage or read as undervoltage.

The DC bus voltage is too high for the following reasons. The first type of input voltage is too high. The second type of deceleration is too short. The motor is affected by external force (fan, rope puller) or potential load (elevator, crane). For these reasons, the actual speed of the motor is higher than the commanded belt of the drive. At this time, the slip of the motor is negative, the electromagnetic torque generated is the braking torque that hinders the rotation, and the motor is in the power generation state. The kinetic energy is regenerated into electrical energy, and the regenerative energy is charged by the freewheeling diode of the IGBT to charge the capacitor, so that the DC bus voltage rises, which is the regenerative overvoltage.

The solution to the overvoltage problem in application debugging is different because of the different causes of overvoltage. For the overvoltage phenomenon generated during the parking process, if there is no special requirement for the parking time or position, it can be solved by extending the drive deceleration time or free parking. If there is a certain requirement for parking time or parking position, DC braking function or regenerative braking can be used.

In the application debugging, if the DC bus voltage is normal, and the driver reports overvoltage or undervoltage fault, this time is to consider the driver itself, whether it is caused by any problem in the detection circuit, the voltage detection circuit can be detected and excluded.

2. Overcurrent fault: The overcurrent fault of the drive is the most common and complicated fault. When an overcurrent fault occurs, the drive protection circuit will immediately act and stop, and the drive will display the fault code or fault type. In most cases, the cause of the fault can be quickly found and corrected according to the fault code displayed by the drive. However, there are some reasons for the overcurrent fault, which are not single, but include acceleration, deceleration, constant speed overcurrent, A variety of factors such as sudden load changes, output short circuits, etc., which may cause overcurrent protection. The following is an analysis of the cause of the driver overcurrent fault and the proposed overcurrent fault handling method. Driver overcurrent internal circuit analysis, as shown in Figure 4 is a typical IGBT drive protection circuit, 14 feet monitor IGBT saturation voltage drop, when the foot 14 detects the IGBT collector voltage ≥ 7V, regardless of whether the input drive signal continues, The 11-pin output will be forcibly turned off. At the same time, the sixth pin outputs an overcurrent fault signal to the CPU. Figure 5 is another overcurrent detection circuit that uses a comparator op amp circuit to determine if an overcurrent occurs when the current signal detected by the sensor is compared to a standard signal.

Figure 5

According to the display of the drive, the reasons for overcurrent are as follows:

(1) The overcurrent in the working of the drive, that is, the overcurrent of the motor drag system during the working process, the reasons are as follows:

a. The motor drive mechanism is "stuck" or encounters an impact load, and the motor operating current suddenly increases and an overcurrent occurs.

b. The output end of the driver is short-circuited, such as the short circuit of the output connection line, or the internal short circuit of the motor, grounding (burning of the motor winding, deterioration of winding insulation, short circuit caused by cable damage), etc., the current at the output of the driver increases greatly and overcurrent occurs. .

c. The drive itself works abnormally. For example, two inverter devices of the same bridge arm in the inverter bridge are abnormal in alternate work.

If the ambient temperature is too high or the inverter components are aging, etc., the parameters of the inverter are changed, resulting in an alternation process.

One device is still turned off in the future, and the other device is turned on, causing the upper and lower devices of the same bridge arm.

At the same time, the two poles of the DC voltage are short-circuited, so that the internal current of the driver is greatly increased and overcurrent.

(2) Overcurrent when the drive is speeding up or slowing down.

If the inertia of the load is large and the drive sets the acceleration time or the deceleration time is too short, it will cause overcurrent. In the process of speed increase, if the operating frequency of the drive rises too fast, the synchronous speed of the motor also rises rapidly, and the speed of the rotor of the motor is not able to keep up due to the large inertia of the load. As a result, the speed of the rising speed is too large, causing the drive to pass. Flow protection; during the speed reduction process, if the drive set deceleration time is too short, the synchronous speed of the motor drops rapidly, and the motor rotor maintains a high state due to the inertia of the load, and the rotor winding cuts the magnetic field lines. The speed is too high and an overcurrent is generated, causing overcurrent protection of the driver.

(3) Overcurrent occurs when the drive is powered up or starts running. This overcurrent protection is generally caused by internal faults in the driver. If the load is normal, the driver still has overcurrent protection, most of which is caused by an overcurrent detection circuit, such as a current detection circuit, a sampling resistor or a sensor.

Steps for checking the overcurrent fault of the drive: first, determine whether the load meets the normal operating conditions; second, determine whether the drive itself is normal; third, determine whether the setting parameters of the drive match the process conditions of the acceleration/deceleration process or the load operation; Fourth, determine if the drive wiring is normal.

Overcurrent fault handling method, the drive shows overcurrent faults, there are two types: one is the overcurrent fault display during operation; the other is the overcurrent fault after the drive is powered on, or it still appears after the stop The overcurrent fault is displayed and cannot be reset. During the operation, the drive has an overcurrent fault display. When it is confirmed that the running current and the actual current of the drive are the same, it is mostly caused by an external cause or an unreasonable setting parameter. For example, the motor cable is damaged or the motor coil is short-circuited due to the short circuit to the ground; the motor overload is very serious, causing overcurrent; the acceleration or deceleration time is set too short, and the driver is in the process of acceleration or deceleration due to excessive load current. A drive overcurrent display and so on.

Whether the motor self-learning parameter and the encoder zero position are correct will affect the current running of the motor. When the external fault is eliminated, press the reset button to reset, or automatically reset, the drive is normal. The other is that the overcurrent fault is displayed after the drive is powered on. After the drive stops automatically, the overcurrent fault cannot be reset. It is a false overcurrent fault: because the drive shows an overcurrent fault when there is no output current at all. of. This is a fault in the current sense protection circuit of the driver: usually due to current sampling devices, such as sampling resistors, current transformers and Hall element damage or parameter values change, amplifier circuit damage and comparison circuit operation is not normal. From the links, you can check, analyze and find the point of failure.

Common fault repair

The servo drive is a kind of controller used to control the servo motor. Its function is similar to that of the inverter acting on the common AC motor. It is part of the servo system and is mainly used in high-precision positioning systems. Generally, the servo motor is controlled by three modes: position, speed and torque to realize high-precision transmission system positioning. At present, it is a high-end product of transmission technology. The following are seven methods for servo drive repair.

1. When the oscilloscope checks the current monitoring output of the driver, it is found to be all noise and cannot be read.

Cause: The current monitoring output is not isolated from the AC power supply (transformer).

Treatment method: It can be observed with a DC voltmeter.

2. The motor runs faster in one direction than the other.

(1) Cause of failure: The phase of the brushless motor is wrong.

Treatment: Detect or find the correct phase.

(2) Cause of failure: When not used for testing, the test/deviation switch is in the test position.

Treatment method: Put the test/deviation switch in the deviation position.

(3) Cause of the fault: The position of the deviation potentiometer is incorrect.

Processing method: Reset.

3, motor stall

(1) Cause of failure: The polarity of the speed feedback is wrong.

Solution: You can try the following methods.

a. If possible, turn the position feedback polarity switch to another position. (can be on some drives)

b. If using a tachometer, connect the TACH+ and TACH- on the drive.

c. If using an encoder, connect ENC A and ENC B on the drive.

d. If in HALL speed mode, reverse the HALL-1 and HALL-3 on the drive, and then connect the Motor-A and Motor-B pairs.

(2) Fault cause: When the encoder speed feedback, the encoder power supply is de-energized.

Solution: Check the power supply of the 5V encoder. Make sure that the power supply provides enough current. If using an external power supply, make sure that the voltage is signaled to the drive.

4, the LED light is green, but the motor does not move

(1) Cause of failure: The motor in one or more directions is prohibited from operating.

Solution: Check the +INHIBIT and –INHIBIT ports.

(2) Cause of failure: The command signal is not signaled to the drive.

Processing method: connect the command signal ground and the driver signal ground.

5. After the power is turned on, the LED of the driver is not lit.

Cause of failure: The supply voltage is too low, less than the minimum voltage requirement.

Treatment: Check and increase the supply voltage.

6. When the motor rotates, the LED light flashes.

(1) Cause of failure: HALL phase error.

Solution: Check if the motor phase setting switch is correct.

(2) Cause of failure: HALL sensor is faulty.

Treatment method: Detect the voltage of Hall A, Hall B, Hall C when the motor rotates. The voltage should be between 5 VDC and 0.

7, LED lights always keep red

Cause of failure: There is a fault.

Solution: Cause: Overvoltage, undervoltage, short circuit, overheat, drive disable, HALL invalid.