The Cause and Solution of Spark of Brushed DC Motor

When DC Motor is in operation, a spark is easily generated between brush and commutator, which is harmful for the operation of the motor. When the spark level is too high, it will burn out the motor.

when the range of spark on the brush is very small, it has very little effect on the operation of the motor. But when the scope of the brush is very large, it will affect the operation of the motor.

Especially, the electro-discharge red arc spark will accelerate the wear of brush and commutator, which is very easy to cause a ring fire accident and damage the motor. So, we must check and prevent it in time.

1, The Cause of Spark Generation:

1.1 Electromagnetic Reason:

When the Commutating element is changing-over, it will be under the influence of magnetic fields, and due to the self-induction and mutual inductance, three kinds of potential will occur.

1.1.1 Reactance Potential:

This is the potential generated by self-induction and mutual inductance, mutual inductance is the induced potential which is generated in the current change of commutation element. Self-induction potential is generated in the commutation element when the change of current. For two potentials, we called reactance potential.

1.1.2 Armature Potential:

There are two kinds of magnetic fields in the DC Motor, one is the magnetic field generated by adding direct current to excitation windings, which we call the main magnetic field. The other is that when the motor has loaded, the current passes armature windings, which will produce the magnetic field, which we call the armature magnetic field. The main magnetic field and armature magnetic field will exist in the loaded motor, which will affect each other, which is called armature reaction. The commutation element cuts the armature reaction flux(the generated potential is called armature reaction potential), whose direction is also the same as that of the current in the element.

1.1.3 Commutation Pole Potential

From the previous analysis, the direction of reactance potential and armature are the same, they are superposed to each other, which will produce a large additional current in the commutation element circuit. The function of the commutation pole is to decrease the impact of additional current on the motor. The commutation pole is installed on the geometric neutral line between the main magnetic pole, which makes the commutating element cut the commutating pole to generate commutating pole potential. The direction of potential is opposite to the former two and plays the role of counteracting.

1.2 Mechanical Reason:

In terms of mechanical aspect, there are many reasons such as the flatness and roughness of the surface of the plane commutator, and the perpendicularity between the commutator surface and the motor axis during assembly; the insulation between the commutator pieces is protruding or commutator pieces are protruding; the contact surface of the brush is not well ground, and the brush and commutator surface only have local contact; the spring pressure on the brush is not appropriate; the brush is too tight or loose in the brush box; The distance between each brush rods is not equal, which causes that the commutating element that the short circuit of some brushes is not in the geometric centerline. The commutator surface is not clean, etc.

2, The main measures of Prevent Sparks

2.1 Select brush reasonably:

The contact surface between brush and commutator surface is as large as possible, and the wear resistance of the brush is better. Commonly, the electric graphite brush is used in the DC Motor. For low voltage high current motor, the metal graphite brush is used. For motor with difficulties in commutation, the split brush is used.

2.2 The Pressure Adjustment of Brush

When the pressure of the brush is too tight, it will accelerate the wear of the brush and the commutator. Too loose pressure will cause loose contact between the brush and the commutator, resulting in sparks, ablation of the commutator. In serious cases, ablation of the collector ring will produce pitting, even the over-heating. So it is necessary to adjust the pressure of the brush, to make the brush and brush box match well. The inner surface of the brush holder box should be smooth and flat without burr. The roughness of the inner hole of the brush box is less than 2um, brush in the brush box can slide evenly without jamming. In addition, it shall be noted that the braided wire of the brush can not collide with shells or other brushes.

2.3 Choose Reasonable Brush Material and Type

The material and type of brush should be chosen by the working condition of the motor. In the process of selection, it can be chosen by the type, model, resistance rate, hardness, contact voltage drop, friction coefficient, rated current destiny, maximum circumferential velocity, application scope.

2.4 The Number of Positive and Negative Brushes Shall be Equal

If the number of brushes is relatively small, the unit's current carrying capacity of each phase should be calibrated. Check whether the current destiny of motor in the operation exceeds allowable scope, compared with allowable range whether there is a certain margin. Then the collector ring should add reasonable the number of brushes. During the operation, it shall be noted that the installation of the brush should be correct, the distance between brushes should be equal, the contact between brush and collector ring should be good, the number of negative and positive brushes shall be equal.

3. The Solution of Solving the Spark

From the above analysis, we can know that to solve the phenomenon that the motor produces a spark, we can take action from these aspects:

3.1 Shortening the period of measuring the DC resistance between excited rotor windings can quickly find the change of rotor winding resistance value. Comparing each group of windings with each other can effectively detect the defects of DC resistance. If the period is too long, and not be detected until the overhaul, it will not find the slight change of DC resistance between chips. In addition, it should notice the change of tested value over the years, which can shorten the test period.

3.2 For various fault types of DC motor brush spark, we should be good at comprehensive induction and accumulating rich data. At the same time, the analysis and judgment should be based on the actual condition of the equipment.

3.3 Regular planned disassembly and overhaul of DC motor also have a negative impact, after the disassembly of equipment, the status is still or local deficit, which can operate a period of time. So, it is necessary to gradually transition from planned maintenance to status maintenance.

4, Conclusion

Take appropriate measures to make the spark size meet the requirements, to prolong the operation time of DC motor. According to DC motor working status, establishing reasonable inspection time, maintenance cycle, which can ensure the service of equipment and provide a strong guarantee for production.