Copyright Goodheart-Willcox Co., Inc. Chapter 13 Electric Motors 267 The single-phase power supplied to the motor must be converted to three-phase in order to feed each of the three run windings. First, the single-phase ac is rectified to dc. The dc is processed through an inverter to produce three 120° out-of-phase signals that represent alternating current that rises and falls and changes direction. Since the three-phase signals are produced electronically, the frequency can be con- trolled. See Figure 13-27. 13.7 Variable Frequency Drive Motors (VFDs) A variable frequency drive (VFD) motor provides infinite variable motor speed instead of a few fixed speed values. See Figure 13-28. The VFD is not limited to fractional horsepower applications. It is used in high-efficiency large blowers and refrigeration and air conditioning compressors. The VFD controller, called a drive, is applicable to single- and three-phase motors. However, the windings must be rated for VFD use since they are more robust in design. The VFD drive receives single- or three-phase power and rectifies it to dc. The dc is processed through an inverter to produce single- or three-phase signals that can vary in frequency. Inverters High-end high-efficiency air conditioning compressors use VFD motors called inverters. Inverters allow for cooling capacity control by regulating the motor speed to maximize efficiency. Pro Tip Pulse width T1 ECM stator Inverter-generated three-phase Inverter-generated sine wave T3 T2 Low frequency High frequency Goodheart-Willcox Publisher Figure 13-27. Electronically commutated signal to drive an ECM motor. Rectifies and inverts using solid-state circuitry L2 L3 L1 Standard 60 Hz incoming ac signal Variable frequency signal output with permission from Carel Industries - all rights reserved Figure 13-28. A variable frequency drive (VFD).