The consumption of industrial power network depends of rotation speed of rotary machines, for example the mechanical output of fans is proportional with cube of their rotation speed. The simplified form of the most commonly used fan laws include.

•CFM varies directly with RPM

CFM

_{1}/CFM

_{2}= RPM

_{1}/RPM

_{2}
• SP varies with the square of the RPM

SP

_{1}/SP

_{2}= (RPM

_{1}/RPM

_{2})

^{2}
•HP varies with the cube of the RPM

HP

_{1}/HP

_{2}= (RPM

_{1}/RPM

_{2})

^{3}
Indeed, we front to saving of energy (Undesirable reduction of consumption) with decreasing of system frequency. Of course as you know according to electrical basic conceptions, the power consumption are independent of frequency just in ohmic consumers (in sinusoidal conditions).Also we must not forget the measuring errors of energy meters due to voltage or frequency variation. Generally displayed consumed energy with induction type energy meters is lower of actual amount when they front of frequency or voltage decreasing( by comparison their rated parameters); because crawl torque which considered for overcoming of friction is according to rated system voltage and frequency.

Author : Hamid - From: IRAN