Why Carbon Brushes Are Required
Variable-speed drive (inverter) operation generates asymmetric currents in the motor shaft. Without a discharge path, these currents travel through the motor bearings to complete the circuit — a phenomenon known as bearing current. Over time, bearing current causes electrical pitting and erosion of the bearing races, leading to premature bearing failure.
On VSD-equipped STSD, STSP, and STSP2 models rated at 148 HP (110 kW) or above, a carbon graphite brush (coal brush) is mounted on the motor shaft between the motor and the airend coupling housing. This brush provides a low-resistance path for shaft currents to dissipate safely without passing through the bearings.
Replacement Interval
Replace the carbon brush every 8,000 operating hours or 12 months, whichever comes first. This interval applies to all affected VSD models and is listed in the STSD/STSP/STSP2 maintenance schedule.
Replacement Procedure
- Stop the compressor by pressing the STOP button. Allow it to fully stop.
- Press the Emergency Stop button.
- Disconnect electrical power at the mains panel. Wait for the inverter drive to fully discharge.
- Remove the connection rod housing between the motor and the screw assembly coupling.
- Disconnect the carbon brush cable from the connection apparatus.
- Remove the worn carbon brush from its slot. Note the brush length and compare to specification — if the brush is nearly or fully worn, confirm all current components are replaced.
- Insert the new OEM carbon brush into the brush holder slot.
- Reconnect the carbon brush cable to the connection apparatus. Ensure the connection is secure.
- Reinstall the connection rod housing and tighten all fastening bolts to the correct torque.
- Restore power and perform a test run. Monitor for abnormal vibration or noise.
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For parts or technical support, contact South-Tek Systems: (910) 415-1880 or support@southteksystems911.zendesk.com