25.7.2 Dual-Voltage Three- Phase Motors Many three-phase motors are of the dual- voltage type. For example, a motor data plate may specify the voltages of 220/440 V can be applied. However, instead of three wires to connect, this dual-voltage motor has nine or more wires that are tagged and numbered for identification. Never remove these numbers! See Figure 25‑49. A dual-voltage three-phase motor contains an extra set of three poles. In addition to the three original wires, two wires for each of these extra poles are connected to wiring terminals in the motor’s electrical box. See Figure 25‑50. All nine wires must be connected. The motor windings can be connected in parallel for low voltage and in series for high voltage. Instructions for connecting the nine wires for each voltage are included on the motor data plate. Refer to manufacturer directions, which may use different methods to explain the connections. A simple Y-type diagram can be constructed for connecting the motor for either voltage, Figure 25‑51. Draw a “Y,” and put extensions on each of the branches, allowing a small space between the extensions. Each line represents one of the stator windings. Next, number all the ends. Start by numbering 1 on any outside branch and continue numbering around the “Y” clockwise, working constantly toward the inside, until all ends are numbered. any two supply wires, causing the zero to rotate in the opposite direction. Resistance of Windings All the windings in a three-phase motor are identical in resistance. One end of each winding is factory-connected inside the motor, providing an ideal location for the internal overload. When the overload opens, all three wires are discon- nected at the same time. See Figure 25‑48. When reading resistance between any two motor leads, the ohmmeter is actually reading through two windings. The resistance reading between any two wires should be the same. N N 0 0 S S S N 0 0 S N 1 2 3 1 2 3 Goodheart-Willcox Publisher Figure 25‑47. The three poles change polarity from north to south to zero, producing a rotating magnetic field for the rotor to “chase.” Overload L2 L3 L1 Goodheart-Willcox Publisher Figure 25‑48. All three windings are connected to the internal overload so when the overload opens, all the windings are disconnected. Goodheart-Willcox Publisher Figure 25‑49. This dual voltage three-phase motor has 11 wires. Each wire has its number on its insulation as shown. Chapter 25 Motors 511 Copyright Goodheart-Willcox Co., Inc.