Mechanical commutation Graphite brushes In combination with copper commutators for the most rigorous applications. More than 10 million cycles were attained in different applications.
CLL concept With precious metal commutation, the wear on commutators and brushes is caused mainly by sparks. The CLL concept suppresses spark generation to a large extent, thus greatly ex- tending service life. When driven with a pulsed power stage (PWM) higher no load currents occur and an unwanted motor heating can result. Additional induc- tance (chokes) in the motor supply lines help.
Precious metal brushes and commutator Our precious metal combinations ensure a highly constant and low contact resistance, even after a prolonged standstill time. The motors work with very low starting voltages and electromagnetic interferences. Precious metal brushes are typically used: − In small motors
Graphite brushes are typically used: − In larger motors − With high current loads − In start/stop operation − In reverse operation − While controlling at pulsed power stage (PWM)
− In continuous operation − With small current loads − With battery operation − In DC tachometers
The commutation pattern is uniform and free of spikes, as opposed to that of other motors. The combination of precious metal brushes and maxon rotor system results in minimum of high-frequency interference, which otherwise leads to major problems in electronical circuits. The motors need practically no interference suppression.
The special properties of graphite brushes can cause so-called spikes. They are visible in the commutation pattern. Despite the high- frequency interference caused by the spikes, these motors have become popular in applica- tions with electronic controls. Please note, that the contact resistance of the graphite brushes changes dependent on load.
For further explanations, please see page 90 or “The selection of high-precision microdrives” by Dr. Urs Kafader.
Commutation pattern with precious metal brushes
Commutation pattern with graphite brushes
Commutation pattern The commutation pattern shows the current pattern of a maxon DC motor over one motor revolution. Please place a low-ohm series resistor in series with the motor (approx. 50 times smaller than the motor resistance). Observe the volt- age drop over the resistor on the oscilloscope. Legend 1 Ripple, actual peak-to-peak ripple 2 Modulation, attributable mainly to asymme- try in the magnetic field and in the winding. 3 Signal pattern within a revolution (number of peaks = twice the number of commutator segments)
Technology – short and to the point 61
März 2022 edition / subject to change
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