To prevent shaft currents in an alternator, the outboard bearing shell or outboard bearing pedestal is insulated. If the methodology used is the insulated bearing pedestal, how is the pedestal insulation evaluated?
• Shaft currents and insulation in alternators – why you want high resistance between pedestal and bedplate • Difference between a megohmmeter (megger) and a digital multimeter (ohmmeter range) – especially test voltage and ability to detect high insulation resistance • Effect of testing an alternator assembled vs. disassembled on whether you might accidentally measure through other parallel metal paths instead of just the pedestal insulation
• Should the test instrument be suited to measuring very high resistance values or just low/medium resistances? How does that affect your choice between a megger and a standard ohmmeter? • If you want to verify only the insulation of the bearing pedestal to the bedplate, does having the machine fully assembled increase or decrease the chance of getting a misleading reading due to additional conductive paths? • For a pedestal that is supposed to be insulated from the bedplate, what test configuration will most clearly tell you whether that insulation is intact and not bypassed by other components?
• Verify which instrument is normally specified for insulation resistance testing (look at typical shipboard electrical maintenance procedures). • Check whether testing in an assembled condition could allow current to bypass the pedestal insulation through other connected parts such as piping, bonding straps, or cable trays. • Confirm whether a 500‑Volt test level is more appropriate for detecting breakdown or weakness in insulation than the low voltage used by a standard digital multimeter ohmmeter.
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