Both single-phase and three-phase motors are used in residential and light commercial systems. However, because of their good running characteristics and high efficiency, single-phase motors are used almost exclusively as indoor and outdoor fan motors and as blower motors. Single-speed and multi-speed permanent split capacitor (PSC) motors are typically used in these applications. Use of electronically -commutated motors (ECMs) is also on the increase in fan and blower motor applications. Because of their simplicity and efficiency, shaded pole motors are also used in some low-torque applications, such as small direct-drive fan and blower motors.
Single and Multi-Speed P SC Single-Phase Motors – The operation of basic P SC motors is described in Service Procedure SP-9. As previously described, PSC motors have at least three external terminals leading to two internal windings. The main or run winding (R) contains relatively few turns of heavy wire. The start winding (S) contains a greater number of turns of lighter wire. The point where the two windings meet internally is called common (C).
Multi-speed PSC motors capable of operating at two speeds (high and low) or three speeds (high, low, and medium) are commonly used to drive the fans and/or blowers in HVAC equipment. The motor’s speed can be changed by switching the motor leads or terminal taps, or by using speed control switches or relays. In many heating/cooling units, the motor speed is selected automatically by the control circuits, as determined by the mode of operation. Normally, slower fan speeds are used with heating modes of operation and higher speeds for cooling operation.
There are many types of multi-speed motors- Figure SP-10-3 shows a schematic o f a typical multi-speed blower motor (BLWM) used to run at high speed for cooling and at low speed for heating. A s shown, the speed is changed by connecting the line voltage either to the low speed tap (LO), medium-low speed tap (MED LO), medium high-speed tap (MED HI) and/or high speed tap (HI) of the motor. The specific taps used are selected when installing the unit. For the example shown in Figure SP-10-3, the MED LO tap is used for heating operation and the HI tap is used for cooling operation. The HI/LO relay prevents both windings from being energized at the same time, a condition that would destroy the motor.
Multi-speed motors use tapped windings, series-connected winding sections, or other wiring configurations that enable operation at different speeds. They can fail so that the motor will not run in one or more speeds, but runs at other speeds. When troubleshooting multi-speed motors, it is important to eliminate the speed selection circuits external to the motor as the cause of a problem, before condemning the motor.
Electronically -Commutated Motors — Electronically – commutated motors (ECMs) are variable-speed, high-efficiency motors. These motors can vary their speed based on predetermined control instructions programmed into a built-in microprocessor. Typically, they are programmed to provide the speed needed to provide a constant airflow based on system requirements. The use of ECMs in HVAC equipment is rapidly increasing. They are typically used in the same applications where conventional shaded-pole and P SC motors are used, such as fan motors and draft inducer motors in high-efficiency furnaces. In the same application, ECMs tend to operate with a 60% to 75% energy savings over conventional motors. A brief description of ECM motor operation is given here.
A functional diagram for an ECM is shown in Figure SP-10-4. As shown, single-phase A C power is used to power the ECM circuit. The A C power is first filtered in the electromagnetic interference (EMI) module. It is then rectified into pulsating DC by the diode module. After rectification, the power is smoothed by the capacitor placed across the line and fed to the motor drive. The smoothed DC voltage is fed sequentially to the three stator windings of the motor. The rotor is permanently magnetized.
When the ECM drive signal is applied to the A -B windings, magnetic attraction causes the rotor to advance clockwise. The signal is then sequenced to the B -C windings, causing the rotor to advance further. Energizing the C-A windings completes one revolution. The frequency of the drive signal determines motor speed, while the current level determines the motor torque.
When troubleshooting ECMs, it is important to confirm that the input signals and voltages supplied to the motor are correct before condemning the motor.
When troubleshooting ECMs, you must treat the motor as a “black box” like you would when troubleshooting a circuit with an electronic control. As a first step, always check the equipment for any fault codes that may be indicated by the built-in diagnostic function (if equipped) and consult the troubleshooting guide. If the correct input voltages or signals are available and the motor does not run, the motor is probably defective. If the input voltages are present but are the wrong value, there is a good chance the problem is not in the motor but somewhere else. Unlike conventional motors, resistance checks cannot be used to check the motor windings of an ECM. Resistance readings taken across the leads of an ECM would make no sense. This is because of the built-in electronic components that are an integral part of the ECM.
Three-Phase Motors — Three-phase motors are generally used when the motor requirements are greater than 7 HP or higher starting torque is required. All three-phase motors have at least three internal windings of equal resistance and the same number of wire turns. Three-phase motors have good starting and running characteristics and higher running efficiency. Unlike single-phase motors, three-phase motors (Figure SP-10-5) require no external starting relays or capacitors. Multi-speed three-phase motors are seldom used in residential or light commercial equipment.