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Motor failure (very rare) Have motor tested and replace if required. 9—UNDERVOLTAGE Insufficient voltage supply to machine. Check power supply and ensure correct voltage. Power supply to variable speed drives or frequency converters has been turned off. Re-connect power to grinder. 11—OUTPUT PHASE SUPERVISION Short circuit on output side of variable speed drives or frequency converters. Check wiring in plugs connected to motor cables or inside connection boxes on motors. Motor failure (very rare