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 A vector AC drive uses feedback from what is happening at the motor (measuring) to make changes in the output of the drive in order to affect the desired outcome with the motor, such as speed or torque accuracy.

 With no feedback mechanism, the AC drive has no idea if the motor is responding correctly to that command. The drive just keeps humming along, blissfully ignorant even if changes in the load cause the motor to slow down, or if the line voltage drops. 

 Below we give one example for you to understand better.

 The drive should always be sized equal to or greater than FLA (full load amps). Drive operation will enable all motors to accelerate / decelerate and maintain a constant relationship of speed to one another. 

Even so, in some cases, special attention should be given to the motor lead lengths. 

 Regulating the motor speed has the added benefit of easily accommodating capacity rises without extra investment, as speed increases of 5-20% is no problem with an AC variable speed drive (variable frequency drive, VSD, VFD, AC drive, frequency inverter) as long as there is enough spare capacity in the system.

Figure 2: Fan applications are extremely well suited for variable speed drives (VSDs, VFDs)

 As a small reduction of the speed can make a big difference on the energy consumption in fan and pump systems. In fact, as many fan and pump systems run at less than full capacity a lot of the time, a variable frequency drive (variable speed drive, VSD, VFD, AC drive, frequency inverter) can make huge savings on energy, compared to a motor driving an application under other mechanical controls. 

 Below is the figure of the comparison of AC drive (VSD) and other methods on pumps.

 Running a motor at full speed while throttling the output is just like driving a car with one foot on the accelerator and the other on the brake. Sounds ridiculous? Yes. it's absolutely a part of the produced output immediately goes to waste. 

 The energy savings for operating point A1 over P1 is 17.8 KW (Refer to figure 2 below). If the cost of electricity is $0.06 per KWH, and the system operates for 1,000 hours at 4,000 GPM, the savings utilizing an VFD (AC drive, VSD drive, frequency inverter), is $1,068. If the flow is 3,000 GPM for 1,000 hours, the savings is $2,376. As you can see, the savings on energy bill can be significant over the life of the installation.

Figure 2