Servo motors are prone to failure due to prolonged continuous use or improper operation, and their repair is relatively complex. This article compiles repair methods for 13 common servo motor malfunctions for your reference.
I. What steps need to be taken before starting the servo motor?
1) Measure the insulation resistance (for low-voltage motors, it should not be less than 0.5MΩ).
2) Measure the power supply voltage and check whether the motor wiring is correct and whether the power supply voltage meets the requirements.
3) Check if the starting equipment is working properly.
4) Check if the fuse is suitable.
5) Check if the motor grounding and neutral connection are good.
6) Check the transmission device for defects.
7) Check if the motor environment is suitable and remove flammable materials and other debris.
II. What are the causes of overheating in servo motor bearings?
The motor itself:
1) The inner and outer rings of the bearing are too tightly fitted.
2) There are problems with the form and position tolerances of the parts, such as poor coaxiality of parts such as the base, end cover, and shaft.
3) Improper bearing selection.
4) Poor bearing lubrication or unclean bearing, with impurities in the grease.
5) Axis current.
In terms of usage:
1) Improper installation of the unit, such as the motor shaft and the shaft of the driven device not meeting the coaxiality requirements.
2) The pulley is pulled too tight.
3) Poor bearing maintenance, insufficient or expired grease, or dry and deteriorated grease.
III. What are the causes of three-phase current imbalance in servo motors?
1) Three-phase voltage imbalance.
2) Poor welding or poor contact in a certain phase branch inside the motor.
3) Short circuit between turns of the motor windings or short circuit between phases to ground.
4) Wiring error.
IV. How to control the speed of a servo motor
A servo motor is a typical closed-loop feedback system. The reduction gear set is driven by the motor, and its terminal (output) drives a linear proportional potentiometer for position detection. The potentiometer converts the angular coordinates into a proportional voltage and feeds it back to the control circuit board. The control circuit board compares this voltage with the input control pulse signal, generates a correction pulse, and drives the motor to rotate in the forward or reverse direction, so that the output position of the gear set matches the desired value, causing the correction pulse to tend to 0, thereby achieving the purpose of precise positioning and speed control of the servo motor.
5. Observe whether sparks are generated between the carbon brushes and the commutator when the motor is running, and determine the degree of sparking for repair.
1. There are only 2 to 4 very small sparks. If the commutator surface is flat, repair is usually not necessary.
2. It produces no sparks and requires no repair;
3. If there are more than 4 very small sparks and 1 to 3 large sparks, it is not necessary to disassemble the armature. Just use sandpaper to polish the carbon brush commutator.
4. If more than four large sparks occur, the commutator needs to be sanded, and the carbon brushes and armature must be removed. Replace and sand the carbon brushes.
VI. Commutator Repair
1. The commutator surface is noticeably uneven (can be felt by hand) or sparks occur during motor operation as in the fourth case. In this case, the armature needs to be disassembled and the converter machined using a precision machine tool;
2. The surface is basically flat, with only minor scratches or sparks. In cases like the second scenario (l口1), it can be manually ground with wet sandpaper without disassembling the armature. The grinding sequence is as follows: First, according to the outer radius of the commutator, process a wooden tool. Cut several different grits of wet sandpaper into strips the same width as the commutator. Remove the carbon brushes (note that you should mark the handles and grooves of the removed carbon brushes to ensure they are not swapped left or right during installation). Use the wooden tool wrapped with sandpaper to firmly press against the commutator. With your other hand, gently rotate the shaft of the commutator in the direction of motor rotation to grind it. For servo motor repair, the order of sandpaper grit should be from coarse to fine. When a piece of sandpaper is worn out and unusable, replace it with a finer one until the finest wet sandpaper (or metallographic sandpaper) is used up.
VII. How to align the encoder phase with the rotor magnetic pole phase zero point of a servo motor.
1. Phase alignment method of incremental encoder
The alignment method between the phase of the UVW electronic commutation signal and the rotor magnetic pole phase, or electrical angle phase, of the incremental encoder with commutation signal is as follows:
1) Use a DC power supply to apply a DC current less than the rated current to the UV winding of the motor, with U input and V output, to orient the motor shaft to a balanced position.
2) Use an oscilloscope to observe the U-phase and Z-phase signals of the encoder;
3) Adjust the relative position of the encoder shaft and the motor shaft;
4) While adjusting, observe the transition edge of the encoder's U-phase signal and the Z signal until the Z signal stabilizes at a high level (assuming the normal state of the Z signal is low level), and lock the relative position relationship between the encoder and the motor.
5) Twist the motor shaft back and forth. If the Z signal remains stable at a high level each time the motor shaft freely returns to the equilibrium position, then the alignment is effective.
2. Phase alignment method of absolute encoder
For absolute encoders, the phase alignment makes little difference between single-turn and multi-turn operations; essentially, it involves aligning the encoder's detected phase with the motor's electrical angle phase within one revolution. A currently very practical method is to utilize the encoder's internal EEPROM to store the measured phase after the encoder is randomly mounted on the motor shaft. The specific method is as follows:
1) The encoder is randomly installed on the motor, that is, the encoder shaft is fixed to the motor shaft, and the encoder housing is fixed to the motor housing;
2) Use a DC power supply to apply a DC current less than the rated current to the UV winding of the motor, with U input and V output, to orient the motor shaft to a balanced position.
3) Use a servo driver to read the single-turn position value of the absolute encoder and store it in the EEPROM inside the encoder that records the initial phase of the motor's electrical angle;
4) The alignment process is complete.
8. Servo motor axial movement during maintenance
If swaying occurs during feeding and the speed measurement signal is unstable, it may be due to cracks in the encoder or poor contact at the wiring terminals, such as loose screws. When swaying occurs at the moment of reversal between forward and reverse motion, it is generally caused by the backlash in the feed transmission chain or excessive gain in the servo drive.
IX. Creeping Phenomenon in Servo Motor Repair
This mostly occurs during the starting acceleration phase or low-speed feed, generally due to poor lubrication of the feed drive chain, low servo system gain, and excessive external load. Of particular concern is the coupling connecting the servo motor and the ball screw. Loose connections or defects in the coupling itself, such as cracks, can cause the ball screw and servo motor to rotate out of sync, resulting in inconsistent feed speeds.
10. Vibration phenomenon during servo motor repair
When a machine tool is running at high speed, it may vibrate, which will trigger an overcurrent alarm. Machine tool vibration problems are generally speed-related, so the problem should be investigated by checking the speed loop.
11. Servo motor torque reduction phenomenon during maintenance
When a servo motor transitions from its rated stall torque to high-speed operation, a sudden drop in torque may occur. This is due to heat dissipation failure in the motor windings and overheating of the mechanical components. At high speeds, the motor's temperature rises significantly; therefore, it is essential to verify the motor's load before using a servo motor correctly.
12. Servo Motor Repair Position Error Phenomenon
When the servo axis movement exceeds the position tolerance range (KNDSD100 factory standard setting PA17:400, position tolerance detection range), the servo drive will generate a position tolerance alarm "4". The main causes include: the system's set tolerance range is too small; the servo system gain setting is improper; the position detection device is contaminated; and the cumulative error of the feed drive chain is too large.
13. Servo motor not turning during repair
In addition to the pulse and direction signals, the connection between the CNC system and the servo drive also includes an enable control signal, typically a DC +24V relay coil voltage. Common diagnostic methods for a servo motor not turning include: checking if the CNC system outputs a pulse signal; checking if the enable signal is active; observing the system's input/output status on the LCD screen to ensure the feed axis starts; confirming the brake is engaged for servo motors with electromagnetic brakes; diagnosing a driver malfunction; a servo motor malfunction; or a failure or key disconnection in the coupling between the servo motor and ball screw.
