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Many fixed-speed motor load applications that are supplied direct from AC line power can save energy when they are operated at variable speed by means of VFD. Such energy cost savings are especially pronounced in variable-torque centrifugal fan and pump applications, where the load's torque and power vary with the square and cube, respectively, of the speed. This change gives a large power reduction compared to fixed-speed operation for a relatively small reduction in speed. For example, at 63% speed a motor load consumes only 25% of its full-speed power. This reduction is in accordance with affinity laws that define the relationship between various centrifugal load variables.
In the United States, an estimated 60-65% of electrical energy is used to supply motors, 75% of which are variable-torque fan, pump, and compressor loads. Eighteen percent of the energy used in the 40 million motors in the U.S. could be saved by efficient energy improvement technologies such as VFDs.
Only about 3% of the total installed base of AC motors are provided with AC drives. However, it is estimated that drive technology is adopted in as many as 30-40% of all newly installed motors.
An energy consumption breakdown of the global population of AC motor installations is as shown in the following table:
|Small||General Purpose - Medium-Size||Large|
|Power||10W - 750W||0.75 kW - 375 kW||375 kW - 10000 kW|
|Phase, voltage||1-ph., <240V||3-ph., 200V to 1kV||3-ph., 1kV to 20kV|
|% total motor energy||9%||68%||23%|
|Total stock||2 billion||230 million||0.6 million|
AC drives are used to bring about process and quality improvements in industrial and commercial applications' acceleration, flow, monitoring, pressure, speed, temperature, tension, and torque.
Fixed-speed loads subject the motor to a high starting torque and to current surges that are up to eight times the full-load current. AC drives instead gradually ramp the motor up to operating speed to lessen mechanical and electrical stress, reducing maintenance and repair costs, and extending the life of the motor and the driven equipment.
Variable-speed drives can also run a motor in specialized patterns to further minimize mechanical and electrical stress. For example, an S-curve pattern can be applied to a conveyor application for smoother deceleration and acceleration control, which reduces the backlash that can occur when a conveyor is accelerating or decelerating.
Performance factors tending to favor the use of DC drives over AC drives include such requirements as continuous operation at low speed, four-quadrant operation with regeneration, frequent acceleration and deceleration routines, and need for the motor to be protected for a hazardous area. The following table compares AC and DC drives according to certain key parameters:
|Drive type||DC||AC VFD||AC VFD||AC VFD||AC VFD|
|Control platform||Brush type DC||V/Hz control||Vector control||Vector control||Vector control|
|Control criteria||Closed-loop||Open-loop||Open-loop||Closed-loop||Open-loop w. HFI^|
|Typical speed regulation (%)||0.01||1||0.5||0.01||0.02|
|Typical speed range at constant torque (%)||0-100||10-100||3-100||0-100||0-100|
|Min. speed at 100% torque (% of base)||Standstill||8%||2%||Standstill||Standstill (200%)|
|Multiple-motor operation recommended||No||Yes||No||No||No|
|Fault protection (Fused only or inherent to drive)||Fused only||Inherent||Inherent||Inherent||Inherent|
|Feedback device||Tachometer or encoder||N/A||N/A||Encoder||N/A|
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