Electronic speed control for a variable speed fan drive

An electronic speed control system for a variable speed fan drive for use with a material to be cooled by a fan, said control comprising:

• a temperature sensitive detector positioned to sense the temperature of the material, said detector having means for developing an electrical signal having a value determined by the temperature of the material;
• a fan driver coupled to the fan to rotate the fan;
• means for varying the coupling between the fan and said fan driver in response to said electrical signal to thereby vary the speed of the fan;
• means for sensing the speed of the fan and for developing a speed signal having a frequency which is determined by the speed of the fan;
• a frequency converter coupled to receive said speed signal and providing a converted output indicative of fan speed;
• and means for comparing said converted output with a predetermined reference level and for providing an override signal when said converted output exceeds said predetermined reference level, said override signal being coupled to said coupling varying means for overriding said electrical signal, thereby limiting the maximum speed of the fan.