Vari­able fre­quency drives for pump ap­pli­ca­tions

Electrical Monitor - - TECH POINT -

vari­able fre­quency drive (VFD) is also known as ad­justable speed drive which is used to con­trol the speed of elec­tric mo­tor. Speed con­trol of mo­tor is achieved by chang­ing the fre­quency and volt­age ap­plied to the mo­tor. There are dif­fer­ent con­trol meth­ods such as V/f con­trol, VVC+, and Flux con­trol tech­niques that are avail­able for VFDs to con­trol the mo­tor speed and torque of stan­dard AC mo­tors. AC mo­tors are be­ing ex­ten­sively used in ap­pli­ca­tions like fans, pumps, and air com­pres­sors. For ex­am­ple Flow and pres­sure are reg­u­lated through the use of a throt­tling de­vice, such as a valve, damper, or by­pass. VFD + Mo­tor pro­vide more ef­fi­cient way to con­trol vary­ing flow rates and pres­sures. VFDs are best known for pro­vid­ing pre­cise process con­trol and it has been widely used in in­dus­trial ap­pli­ca­tions like con­trol­ling heat­ing, ven­ti­la­tion and air con­di­tion­ing (HVAC) sys­tems which re­sults in sig­nif­i­cant en­ergy sav­ings.

ABy Dr. S. Para­ma­si­vam & Su­jith Ku­mar Nee­lam loads that re­quires con­stant torque ir­re­spec­tive of the mo­tor speed. i. e., torque re­quire­ments at very low speed and very high speed are same. Con­stant torque loads can be cat­e­go­rized ap­pli­ca­tions like con­vey­ors, ex­trud­ers, pos­i­tive dis­place­ment pumps, and com­pres­sors. Ap­pli­ca­tion of VFDs in this type of loads re­sults in en­ergy sav­ings from low speed op­er­a­tion. The pre­cise speed con­trol of this load types could be achieved with the ap­pli­ca­tion of VFDs. One more ad­van­tage with VFDs in this type of ap­pli­ca­tion is soft start and stops which re­duces the tran­sients and im­proves the life of the mo­tors.

Con­stant power loads: Con­stant power loads means the loads that draw con­stant power ir­re­spec­tive of mo­tor speed. Re­la­tion be­tween power (P), speed (N), and torque (T) is given as P pro­por­tional to prod­uct of N and T Hence, to keep the power as con­stant, torque here varies in­versely pro­por­tional to the mo­tor speed. An ex­am­ple of con­stant power loads in­cludes lathes, drilling, and milling. Ap­pli­ca­tion of VFDs can­not re­sult in en­ergy sav­ings in this type of loads, be­cause the power is kept con­stant. But ap­pli­ca­tion of VFDs for this type of loads could re­sult in bet­ter process con­trol.

Vari­able torque loads: Vari­able torque loads means the loads that re­quires vari­able torque for the op­er­a­tion. The re­la­tion be­tween the torque and speed for this vari­able torque loads is torque varies di­rectly with the speed squared. The re­la­tion be­tween the power and speed for this vari­able torque loads is power varies di­rectly with the speed cubed. From th­ese re­la­tions, it is ob­served that a re­duc­tion in speed sig­nif­i­cantly de­creases mo­tor en­ergy use and de­mand. For ex­am­ple, de­creas­ing mo­tor speed by 10 re­duces re­quired power by 19 Ex­am­ples of vari­able torque loads in­clude cen­trifu­gal fans, blow­ers, and pumps. Ap­pli­ca­tion of VFD in this type of load could re­sult in very high en­ergy sav­ings at lower speeds. Hence, VFDs are most com­monly used for this load type. Ad­di­tional ben­e­fits with VFDs are more pre­cise process con­trol and re­duced stress on the sys­tem.

From the above three dif­fer­ent loads, it is clear that en­ergy sav­ings are more in vari­able torque loads at re­duced speeds. Now let us cal­cu­late the amount of en­ergy sav­ings with VFD in pump ap­pli­ca­tion at dif­fer­ent speeds with a nu­mer­i­cal ex­am­ple.

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