Backward rotation indicates that air or gas is being “pulled” or “pushed” through the blower causing rotation. This typically indicates that a check valve or isolation valve in the system is not operating properly.
Check valves are required in systems where more than one blower is connected to a header.
This may also occur on vacuum systems where a vessel is being evacuated and once the blower is de-energized, air will rush back into the discharge of the unit (assuming it is discharging to atmosphere) to equalize the system pressure.
This backward rotation can be detrimental to the life of the equipment.
Some units are designed to operate in one direction only (unidirectional). Backward operation may cause contact internally depending on the internal clearances.
Slow rotation may cause damage to the bearings. If the unit is splash lubricated, a minimum speed is required to splash the lubricant to the bearings. If the bearings don’t get adequate lubrication, they will gall and eventually will cause the bearings to seize.