Method Of Improving Commutation Of DC Motor

There are several ways to improve the commutation of DC motors:
1. Install the reversing pole
At present, the most effective way is to install the reversing pole. In order to eliminate the adverse effect of reactance electromotive force and cutting electromotive force on commutation in commutation element, commutation pole is used, its iron core is called commutation pole iron core, commutation pole is set on the geometric neutral line between the N and S main magnetic poles, commutation winding and armature winding in series, excited by armature current, the direction generated by commutation pole is opposite to the direction of armature magnetomotive force, in addition to overcoming armature reaction magnetomotive force, The commutation magnetic field Bk is also established in the air gap where the commutation element is located, the commutation element cuts Bk, and the commutation electromotive force ek is induced, and the direction of ek is opposite to the direction of er+ea, which counteracts the effect of er+ea and improves the commutation.
Since er+ea is proportional to the armature current, the commutating pole magnetic field that generates ek should also be proportional to the armature current, so the commutating winding is in series with the armature winding, and the commutating magnetic circuit should not be saturated. Usually, a non-magnetic regulating plate of appropriate thickness is inserted between the yoke and the commutator pole core to maintain the unsaturation of the commutator magnetic circuit.
At present, DC motors above 1kW are equipped with commutator poles.
2. Choose the right brush
The presence of contact resistance between brush and commutator can reduce additional current and improve commutation. Dc motors do not use metal brushes with small contact resistance, but use carbon and graphite brushes. However, you can not choose a brush with large resistance at will, otherwise the contact voltage drop between the brush and the commutator will increase, the commutator will generate more heat, and the energy loss will be large.
3. Compensating winding
The commutating pole overcomes the influence of the armature reaction magnetic field where the commutating element is located, and the armature reaction outside of this point still exists. If its effect is very strong, the magnetic flux on both sides of the magnetic pole will have various effects. If the magnetic flux density of the pole shoe is high, the voltage of the armature winding under this part is too high, and the voltage difference between the commutator plate connected to it is too large and sparks are generated. At this time, the compensation winding is set in series with the armature winding at the pole shoe of the main magnetic pole, and the armature current flows through the compensation winding, which can completely eliminate the armature reaction. Compensation windings are actually only used in large DC motors.
4. Move the brush position
In small-capacity DC motors with no commutator pole installed, the commutation can be improved by moving the brush away from the geometric neutral line at an appropriate Angle, for the generator, the brush moves along the armature and vice versa for the motor. Therefore, the commutating element leaves the geometric neutral line and enters the main magnetic pole, and the main magnetic pole replaces the commutating magnetic pole. The induced electromotive force ef and er are equal in size and opposite in direction and cancel each other to achieve the purpose of improving the commutation.
The disadvantage of this method is that after the brush leaves the geometric neutral line, the direct-axis armature magnetomotive force which demagnetized the main magnetic field will be generated. Secondly, since er changes with the size of the load, ef should also change with the size of the load, which requires the Angle of brush movement to change with the size of the load, which is impossible. Therefore, this method is only suitable for motors with little change in load.
The above analysis is based on the complete contact between the commutator surface and the brush. But the actual surface of the commutator is always impossible to be very round, and there is dust, the brush of the high-speed DC motor still has pulsation, especially the railway motor itself also has vibration. For these occasions, it is often hoped that the commutation electromotive force design is larger. The DC motor with sharply changing load should also be designed with a larger commutating electromotive force.