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Normal Operation Of Coreless Motor Life
- Sep 19, 2018 -

Operating Point: In most applications, the torque and speed demands placed upon a DC motor determine its overall operational lifetime. As the torque requirements on the motor increase, the current through the armature increases proportionally, thus increasing the current density at the brush-commutator interface. High current densities promote electro-erosion of brush and commutator materials, a limiting factor in motor service life. In addition, high rotational speeds shorten motor service life by accelerating mechanical wear.

 

Noise Generation

  Audible noise in brush motors comes from bearings, brushes and rotor imbalance. In brushless designs, the noise generation from brushes is eliminated and, generally, brushless designs are quieter running. Electrical noise generation from brush motors is often quite large due to the white noise produced between the brushes and the commutator interface which is not present in the brushless designs. However, the brush motor noise is often quelled

 

 

Life Expectancy

  As a general rule, brushless units last longer than brush motors. The primary limiting feature of a brush motor are its brushes and commutator. Different application specific brush materials may be available and should be discussed with the motor manufacturer prior to ordering. Typical brush life of 2000 to 5000 runtime hours is common but, should not be considered a guarantee for all applications. Brushless units typically exceed 10,000 hours and are limited by bearing life and environmental conditions.

 

Cost

  More often than not, the cost of a product is the deciding factor as to which one of these technologies is chosen. The initial cost of a brushless motor design tends to be higher than a brush motor. Add to this the cost of the external commutation controls, then the brushless motor price becomes significantly higher unless the application requires

a four quadrant servo controller anyway. However, when operating life becomes an important factor, such as in high duty cycle applications, the long range cost of having to replace a brush motor can be considerable. In addition to the cost of the motor, technician expenses and lost revenue from machine downtime should be accounted for in the

selection process.

 

Although each application has its own specific requirements to be addressed, it is usually advisable to operate a DC motor with precious metal brushes and commutator continuously at no more than 1/3 of its rated stall torque. Motors with graphite on copper commutation systems should be run continuously at no more than 1/2 of the motor's rated stall torque. These recommendations attempt to maximize motor service life. Some applications may not require the maximum lifetime that the motor has to offer.

 

 

Rotor Inductance: One of the factors limiting brush and commutator life is the inductance of the motor armature. During commutation, when current flows through a particular coil winding there is storage of energy in the form of a magnetic field. When the motor commutates and the current flow is switched to another winding, the magnetic field collapses and the resulting discharge of energy causes an arc between the commutator and brush. This arcing accelerates electro-erosion and decreases motor life. One could, theoretically, reduce the armature inductance of the motor windings by decreasing the number of turns in each armature segment. This lowers the torque constant of the motor, however, which increases the motor current for a given torque and, therefore, increases the current density at the brush-commutator interface. This is not recommended. To reduce the affect of inductance and arcing on motor lifetime a capacitor ring is being mounted to the commutator. The ring provides the equivalent effect of each winding connected in parallel with a small capacitor and resistor. The collapse of the magnetic field during commutation then serves to charge the capacitor rather than creating an arc between brush and commutator. The stored energy is released and dissipated back into the next coil phase in the commutation sequence. This technique, while slightly increasing the electrical time constant of the motor, dramatically increases motor service life.



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