Carbon Brush Replacement

Applies to: STSD (VSD) · STSP (VSD) · STSP2 (VSD) — motors rated 148 HP (110 kW) and above only

Does NOT apply to: Fixed-speed (star-delta) STSD, STSP, STSP2 models · STS Series · STSH Series

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.

Inspection and Replacement Intervals

Inspect the carbon brush every 2,000 operating hours and replace it if it is worn or nearly depleted. Replace the brush on a scheduled basis every 16,000 operating hours, and always before it is fully worn. These intervals apply to all affected VSD models and are listed in the STSD/STSP/STSP2 maintenance schedule.

⚠ Warning Never skip or delay carbon brush inspection and replacement. A worn or missing brush allows full bearing current to resume, causing serious — and non-warranty-covered — motor bearing damage. The carbon brush must be replaced before it is fully worn.

Replacement Procedure

⚠ Warning — De-energize First Ensure power is fully disconnected at the mains before beginning. Press Emergency Stop, disconnect the main disconnect, and wait for the inverter drive to fully discharge before accessing any internal components.

1. Stop the compressor with the STOP button and allow it to fully stop, then press the Emergency Stop button and disconnect power at the mains (see the warning above).
2. Unscrew and remove the grounding (carbon) brush holder.
3. Remove the worn brush and insert a new OEM brush.
4. Ensure the new brush makes firm contact with the motor shaft, then tighten its screws.
5. Reattach the grounding brush holder and tighten its screws.
6. Restore power and perform a test run. Monitor for abnormal vibration or noise.

▲ Attention — Use OEM Parts Only Only original OEM carbon brushes approved for your specific motor may be used. Non-original brushes may have incorrect conductivity, size, or wear characteristics that reduce their effectiveness or damage the shaft surface. Damage caused by non-original brushes is not covered under warranty.

ℹ Note Carbon brush replacement is included in the STSD/STSP/STSP2 maintenance schedule as a schedulable service item. South-Tek recommends tracking brush replacement hours alongside oil and filter service to avoid missed intervals.

Related Articles

• STSD/STSP/STSP2 Maintenance Schedule
• STSD / STSP / STSP2 Series Product Overview
• Spare Parts Ordering

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