Copyright Goodheart-Willcox Co., Inc. 268 Essential Electrical Skills for HVACR: Theory and Labs 13.8 DC Motors DC motors provide the highest torque and efficiency. However, they are more expensive to build as they require more parts. It is too expensive to rectify ac for the purpose of operating moderate to large torque dc motors. DC does not rise and fall or change direction. Thus, a rotating mag- netic field is not possible without the use of a commutator. The commutator creates a change in current through the rotor winding. See Figure 13-29. The stator is a perma- nent magnet in a simple dc motor, and the rotor contains windings. The rotor windings are connected to slip rings that rub against the brushes that connect to the dc source. The brushes conduct current between the stationary power source to the rotating slip rings. The winding ter- minal slip rings contact opposite source brushes for each half-turn of the rotor. This creates alternating magnetic fields needed to rotate the rotor, Figure 13-30. There are many types of dc motors designed to meet specific application requirements. Brushless dc motors, servo motors, and stepper motors are common ones used in the field. These will be discussed in the next few sections. 13.8.1 Brushless DC Motors A brushless dc motor is a smaller version of an ECM motor that uses a permanent magnet rotor. The stator windings are energized sequentially to produce rotor movement. These motors are used in modern refrigera- tors for the evaporator fan and condenser fan. The motor speed can vary from a few to thousands of rpm. They are very efficient and operate at low voltages of about 12 Vdc. The motors are driven by a microprocessor and typically require four wires connecting three windings and one common terminal. 13.8.2 Servo Motors Servo motors are similar to the brushless dc motor, but they have added feedback to the controlling micropro- cessor. The added signal, called feedback, provides rotor position back to the processor, so it can move the rotor in either direction. The feedback is used to position the rotor to an exact position as required by operating conditions. These motors are used for dampers and louvers in duct systems. They are also used in some electronic expansion valves. 13.8.3 Stepper Motors The stepper motor is another variant of the brushless dc motor. It is driven by pulses that, in turn, move the rotor a certain number of degrees in either direction. The movement can be a little as 1°. Some can provide position feedback. This motor is used in some electronic expansion valves. Commutator Brush Connections to rotor windings + – Brush Connections to power source Goodheart-Willcox Publisher Figure 13-29. A dc motor commutator. Windings (rotor) Commutator Permanent magnet (stator) Brush Driveshaft Fouad A. Saad/Shutterstock.com Figure 13-30. A dc motor rotor, commutator, and stator Rotors have many windings but only one was shown for simplicity.