Copyright Goodheart-Willcox Co., Inc. 264 Essential Electrical Skills for HVACR: Theory and Labs A potential relay relies on a bemf (back electro- motive force or counter electromotive force) generated by inductive reactance to operate. This relay is used mostly on high-torque CSR motors. The relay has a high resistance coil, typically 8 kΩ to 15 kΩ between terminals 2 and 5, and a normally closed set of con- tacts between terminals 1 and 2. See Figure 13-21. Terminals 3 and 4 are false terminals used for making splices and are not internally connected. The relay coil is wired in parallel to the start winding, and terminal 1 is connected to the start capacitor. When power is turned on, current feeds both the run and start wind- ing. An insignificant amount of current flows through the relay coil because of its high resistance, leaving the majority of the current to flow in the start winding. As current increases through the start circuit, bemf is produced due to inductive reactance. The bemf rises to a level greater than the supplied voltage and causes enough current to flow through the high resistance coil and become energized. The energized coil opens the NC contacts and removes the start capacitor from the starting circuit. Potential relays are rated by pickup voltage, dropout voltage, and continuous voltage: • The pickup voltage is the required bemf applied across the coil to open the NC contacts and remove the start capacitor. • The dropout voltage is the minimum bemf applied across the coil to keep the armature pulled in and keep the NC contacts open. • The continuous voltage is the maximum bemf that can be applied across the coil without causing damage to the coil. The pickup voltage occurs when the rotor reaches about 75% of rated speed. The initial LRA current that produced the bemf decreases to FLA. This lower current must produce a bemf that is above the dropout voltage. Recall that relays, solenoids, and motors require greater power to move from a standing position due to inertia. The electronic starting relay has no moving parts. There are types that sense changes in current and voltage to determine when to stop current flow between ter- minals. For example, the transition between LRA and FLA can create high resistance between two terminals used to remove a start capacitor. Another type uses a timer to create high resistance between terminals based on the predictability that a motor starts within one second. 13.5 Three-Phase Motors Three-phase motors have higher starting and running torque than single-phase motors. They also have higher efficiency. Since the phases are already split, these motors do not require a start winding or capacitors. Electronic (Timer Type) Starting Relays Electronic timer type starting relays are not always the best replacement choice for some refrigeration compressors. A compressor can likely need more starting time due to varying cooling loads. The better choice, in this case, is the potential relay. Pro Tip Common terminal Run winding terminal Relay coil L1 L2 Potential relay Run capacitor Start capacitor Terminal 1 Terminal 5 Start winding terminal Terminal 2 NC contacts Goodheart-Willcox Publisher Figure 13-21. Potential relay.