The present invention relates in general to the control of two speed motors, which can be used in fans for moving air or gas through an industrial plant, and in particular, to a new and useful motor control arrangement which permits the manual or automatic startup of a motor from an at rest position to a low or high speed condition and the automatic or manual transfer between the high and low positions.
Industrial boilers having large output capacities generally require the use of fans for moving air or other gases through the boiler and out the stack of the boiler.
Forced draft (FD) fans are utilized to force air at above atmospheric pressure into the combustion chamber of a boiler. Such FD fans must have a discharge pressure which is high enough to equal the total resistance in air ducts, air heaters, burners and fuel beds as well as other resistance between the fan discharge and the furnace. The furnace is generally the point of balanced draft, that is the point at which the pressure in the furnace equal atmospheric pressure. The volume output of the forced draft fan must equal the total quantity of the air required for combustion plus air heater leakage.
Induction draft (ID) fans are utilized on the stack side of the boiler for discharging combustion products and other gases into the stack. The ID fans operate to establish a pressure below atmostpheric pressure at the inlet end of the fan to draw off the gases and also to establish the balanced draft.
Before an ID fan can be started up or have its speed changed, care must be taken that certain conditions exist in the boiler. This includes the correct setting of any control mechanisms such as dampers which are associated with the FD fan as well as outlet dampers for the ID fan, the correct setting of secondary air dampers for the burners in the boiler and the setting of dampers in a baghouse of the boiler. It is also desirable to control the ID fan both manually and automatically.
The speed control of large motors such as those used for induction and force draft fans as well as those used for pumps or other large equipment, has hitherto required large mechanical speed changers such as the ESCO oil filed speed changer switch for two seed motors. This speed changer switch was manually operated and had electromechanical trip circuits.