Detect and reduce carbon brush wear in current controller operation
Question:
How can an increased carbon brush wear be detected and reduced?
Answer:
The carbon brush wear depends on different factors:
Shocks
Quantity and quality of cooling air (collector temperature should not exceed 90 ... 110 °C)
Penetration of aggressive gases or vapours
Insufficient or excessive humidity (optimal 8...15 g water per m³ air)
Grinding dust
Incorrect commutation
Incorrect brush pressure
Excessive current rise speed (should be smaller than 200 x rated curent / second)
Oscillation in the armature current (permissible difference in the armature current amplitudes > 10 %
Insufficient current density (try different carbon quality for frequent partial load operation)
Assessment:
Optimum collector:
After approx. 100 operating hours the commutator is coated with a blue-brown skin. Under the running surfaces of the carbon brushes this skin is slightly darker. If collector and carbon brushes are evenly bright they are in a normal state under normal conditions.
Bad collector:
Collector bars have stripes
If the worn edges of the bars are black, major commutation problems and brush sparking may be the cause.
Measures:
All carbon brushes should be replaced at the same time with the same type of brushes. This lowers the risk of a different current load due to a different current-voltage characteristic of the brushes.
Improve cooling (cleaning filters, check direction of rotation of separately driven fan)
Use an armature choke, if necessary, to reduce the current rise speed
Lenze recommends brush quality F49 in case of low humidity.
Check the neutral position of the carbon brushes and the contact between brushes and brush holder.
Use an oscilloscope to check the current balance. If necessary, adjust the current control circuit at the controller (see 20010966). Caution: Observe current peaks occuring a few seconds!
