The basic principles of variable frequency speed control and the advantages of using variable speed control for forced draft fans of utility boilers have been briefly described. A comparison through tests by using the variable frequency speed control and the inlet damper control respectively has been made on two G4-73-11 NO 29.5 type forced draft fans of boiler NO.4 in Xinxiang Thermal Power Plant. The test results show that the operating efficiency of said fans after using variable frequency speed control has been enhanced from about 55% to 75%～80%

having remarkable effectiveness of energy-saving and agility in control operation

being helpful in adaptability to the boiler load and in protection of the equipments.