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.

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.

⚠ 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 by pressing the STOP button. Allow it to fully stop.
2. Press the Emergency Stop button.
3. Disconnect electrical power at the mains panel. Wait for the inverter drive to fully discharge.
4. Remove the connection rod housing between the motor and the screw assembly coupling.
5. Disconnect the carbon brush cable from the connection apparatus.
6. 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.
7. Insert the new OEM carbon brush into the brush holder slot.
8. Reconnect the carbon brush cable to the connection apparatus. Ensure the connection is secure.
9. Reinstall the connection rod housing and tighten all fastening bolts to the correct torque.
10. 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

For parts or technical support, contact South-Tek Systems: [phone] or [e-mail]