Audible mechanical shock through the drive train when the starter transitions from wye to delta.
1
Transition timed too early (motor hasn't reached sufficient speed)
CheckVerify transition timer setting; motor should reach roughly 75–80% of synchronous speed before closing delta contactor.
FixAdjust timer per OEM starter documentation; consider closed-transition starter to eliminate the open-circuit transient.
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2
Standard open-transition starter inherently causing a transient (by design)
CheckConfirm starter type — open-transition designs briefly disconnect the motor and can produce an audible/electrical transient at reconnection even when timed correctly.
FixThis is a known characteristic of open-transition starting; if unacceptable, upgrade to closed-transition or soft-start/VFD starting.
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3
Coupling or belt drive slack taking up shock load at transition
CheckCheck for excessive coupling backlash or belt slack.
FixAdjust/replace coupling or belts; verify proper tension.
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4
Loose motor or compressor mounting amplifying the transient noise
CheckCheck mounting bolt torque and isolator condition.
FixRe-torque and service mounts.
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Safety
Lockout/tagout before opening enclosures. Verify zero energy. Bleed system pressure before opening any pneumatic connection.