Copyright Goodheart-Willcox Co., Inc. 266 Essential Electrical Skills for HVACR: Theory and Labs Many three-phase motors have a dual voltage capability. A single-voltage motor has only three connections—T1, T2, and T3. A dual-voltage motor typi- cally has nine connections, so a second set of windings can be wired in series or in parallel with the first set of windings. See Figure 13-24. Three-phase motors are manufactured by using the wye or delta design, Figure 13-25. A delta wired dual voltage motor is shown in Figure 13-26. Note that L1, L2, and L3 always connect to TI, T2, and T3. 13.6 Electronically Commutated Motors (ECMs) Fractional horsepower ac induction motors, or motors that deliver less than 1 hp, are inefficient. Their efficiency is in the 60% to 70% range. That means 30% to 40% of the energy is wasted. To improve efficiency, the electronically commutated motor (ECM) was pro- duced. This motor uses the benefits of high-efficiency dc motors and the bene- fits of the distributed ac power sources. Recall that frequency is a factor that controls motor speed. The result is pre- cise and efficient speed control. Since the electronically produced ac signal is made from pulses that vary in amplitude, the pulse duration can be controlled. The length of the pulse determines the amount of time current is flowing. This makes it possible to control torque. Long pulses result in higher torque. The ECM is used in fractional horsepower applications where speed and torque control is required. This includes evaporator blowers, condenser fans, and flue gas inducer blowers. While the ECM seems complex to troubleshoot, there are special testers to simplify the task. The stator is a three-phase stator that is made of three run windings. The rotor is a permanent magnet. To generate the rotating magnetic field required to turn the rotor, the stator windings are sequentially powered to produce magnetic poles. 4 7 1 5 8 2 6 9 3 4 7 5 8 1 2 6 9 3 9 6 3 7 4 1 8 5 2 HIGH VOLTAGE LOW VOLTAGE LINE LINE Goodheart-Willcox Publisher Figure 13-24. Dual voltage three-phase motor connections for the wye configuration. Delta Configuration Wye Configuration T3 T2 L1 L2 L3 L1 L2 L3 T1 T3 T2 T1 Goodheart-Willcox Publisher Figure 13-25. Wye and delta design wiring configurations. 4 7 1 5 8 2 6 9 3 4 7 5 8 1 2 6 9 3 9 6 3 1 8 4 7 5 2 HIGH VOLTAGE LOW VOLTAGE LINE LINE Goodheart-Willcox Publisher Figure 13-26. Dual voltage three-phase motor connections for the delta configuration.