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In industrial production or daily equipment operation, DC motors often experience abnormal speed fluctuations (speed varying from high to low). This not only affects the processing accuracy and operational stability of equipment but also may shorten the service life of motors. What are the core causes of abnormal speed fluctuations in DC motors? What systematic solutions should be adopted for these causes?
I. Analysis of Core Causes
The speed of a DC motor follows the formula n = (U – IaRa)/(CeΦ) (where n is the speed, U is the armature voltage, Ia is the armature current, Ra is the armature circuit resistance, Ce is the motor constant, and Φ is the excitation flux). Speed fluctuation is essentially caused by abnormal changes in one or more parameters in the formula, which can be specifically divided into three categories: electrical system failures, mechanical structure problems, and external operating environment influences.
1. Electrical System Failures: This is the most common cause of speed fluctuations. Firstly, unstable power supply in the armature circuit—such as ripples in the output voltage of the DC power supply, poor wire contact, or increased resistance due to line aging—will cause instantaneous changes in U and Ia, directly leading to speed fluctuations. Secondly, abnormal excitation systems: in separately excited DC motors, open circuits, short circuits, or poor contact in the excitation winding will cause a sudden change in Φ; for shunt-excited motors, if the resistance of the excitation circuit suddenly increases, the magnetic flux will also decrease, resulting in a sharp increase in speed. Thirdly, armature winding failures: inter-turn short circuits, open circuits in the winding, or poor contact of the commutator segments will cause uneven Ia, damaging the speed stability.
2. Mechanical Structure Problems: Sudden changes in mechanical resistance will indirectly cause speed fluctuations. Worn, under-lubricated, or damaged motor bearings will increase rotational resistance and cause periodic fluctuations. Coupling installation deviations (such as misalignment, looseness) will result in uneven load transmission and form intermittent load impacts. If the load driven by the motor itself has the risk of jamming or stalling (such as material accumulation in conveying equipment), it will cause sudden changes in load torque. According to the speed formula, when the load increases, Ia rises and the speed decreases accordingly; when the load decreases, the speed rises again.
3. Influences of External Operating Environment: Firstly, high ambient temperature will increase the armature winding resistance Ra due to thermal expansion and contraction; at the same time, the magnetic properties of the excitation winding will weaken, leading to a decrease in Φ. These two effects together cause speed fluctuations. Secondly, environmental factors such as dust and humidity will corrode the motor terminals or commutators, resulting in unstable contact resistance, which in turn affects the current and speed. Thirdly, external electromagnetic interference: strong magnetic fields generated by high-power equipment nearby will interfere with the flux stability of the excitation system.
II. Systematic Solutions
1. Optimize the Electrical System to Ensure Stable Parameters: First, inspect the power supply system, replace aging wires, and adopt low-resistance connection methods such as copper bars to ensure good contact in the armature circuit. If the power supply ripple is large, filter capacitors or voltage stabilizers can be installed to stabilize the armature voltage U. Second, inspect the excitation system: use a multimeter to measure the resistance of the excitation winding, check for short circuits and open circuits, and replace damaged windings; for shunt-excited motors, calibrate the excitation circuit varistor regularly to prevent abnormal resistance changes. Finally, inspect the armature winding: determine the inter-turn short circuit through the armature voltage drop test, repair or replace the winding in a timely manner, and at the same time clean the commutator segments and polish the oxide layer to ensure smooth current conduction.
2. Overhaul Mechanical Structure to Reduce Resistance Fluctuations: Regularly maintain the motor bearings, add lubricating oil, replace worn bearings and seals to ensure flexible rotation. Re-calibrate the coupling, adjust the concentricity, and fasten the connecting bolts to avoid uneven load transmission. At the same time, inspect the load equipment, clean the material accumulation in the conveying channel, and repair jammed components to ensure stable load torque, thereby reducing the impact on motor speed from the source.
3. Improve the Operating Environment to Eliminate External Interference: Install the motor in a well-ventilated location, add cooling fans or cooling devices to control the ambient temperature within the rated operating range of the motor. Provide sealed protection for the motor terminals and commutators to prevent corrosion by dust and moisture. If there is electromagnetic interference, a shield can be installed in the motor excitation circuit, or the motor installation position can be adjusted to keep it away from high-power interference equipment.
In addition, establishing a regular inspection system to monitor the motor speed in real-time with a tachometer, record operating parameters, and detect abnormal trends in advance; conducting comprehensive maintenance on the motor regularly, including cleaning, fastening, and calibration, can effectively reduce the probability of speed fluctuations and ensure the stable operation of the DC motor.
