Copyright Goodheart-Willcox Co., Inc. 270 Essential Electrical Skills for HVACR: Theory and Labs Summary • Electric motors convert electrical energy into mechanical energy. • Motors drive compressors, fans, blowers, pumps, circulators, dampers, and valves in the HVACR industry. • Torque is rotational force. Motors generate torque by the repelling and attracting forces of magnetic fields. • Basic induction motor consists of a stator, rotor and shaft, housing, end bells, and bearings. • The stator is the stationary part of the motor and the rotor is the rotating part. • Electric motors require continuously changing magnetic field polarities to produce rotating motion. • The stator contains a low-resistance run winding and a high-resistance start winding. • Split-phase motors operate on single-phase power. • The run winding is called the main winding and the start winding is called the auxiliary winding. • The start winding must be removed from the circuit when the motor reaches about 75% of rated speed. • Split-phase motors have a run and start winding and include the RSIS, CSIR, PSC, and CSR motors. • Start capacitors improve starting torque and run capacitors improve running efficiency. • Motors draw LRA current during start-up, which can be two to five times greater than running current. • Motor speed is given in RPM (revolutions per minute). • A motor’s speed depends upon frequency and number of poles. • The difference between the synchronous and the actual speed is called the slip. • HVACR motors use between two and eight poles depending on required speeds and torque. • The shaded-pole motor is not a split-phase motor because it only has a run winding. • The current magnetic relay or CMR is made with a low-resistance coil and a normally open set of contacts. • The PTC relay, or PTC, uses a PTC thermistor in series with a start winding or both start capacitor and start winding to control the current. • The PTC can also be used on a PSC motor as an assist start-up device. • The potential relay has a high resistance coil and normally closed contacts. The coil is energized by the bemf produced by the start winding. • The electronic relay has no moving parts. There are types that sense changes in current and voltage to determine when to stop current flow between terminals. • Three-phase motors have higher starting and running torque and higher efficiency than single-phase motors. • Three-phase motors do not require start windings or capacitors. • Three-phase motors use three run windings. • ECM motors use a three-phase stator and a permanent magnet rotor. • The ECM is used in fractional horsepower applications where speed and torque control is required. • The VFD provides infinite variable motor speed instead of a few fixed speed values. • High-end, high-efficiency air conditioning compressors use VFD motors called inverters. • DC motors provide the highest torque and efficiency. However, they are more expensive to build as they require more parts. • Motors use internal and external overloads that react to heat or current.