HVAC Motor Types (RSES NATE Prep)

One of the first major motor types is the shaded pole motor. This motor supplies low torque and is generally used in applications below 35 watts. You can typically see the stator extending from the motor. They tend to be low-efficiency motors for small appliances and don’t require a capacitor because the shading ring assists with the startup. These motors can only rotate in one direction.

Split-phase motors are a common category of residential HVAC motors. These motors are used in applications that use split-phase power from a center-tapped transformer. They have medium starting torque and create a phase shift from high resistance on the start winding; a centrifugal switch could take the start winding out of the circuit. Otherwise, capacitors may be used to provide a phase switch.

Compressors are also motors; they are the largest motors in the system and may come in open-drive, semi-hermetic, or hermetic compressors. The latter two types have motors inside the compressor in the refrigerant circuit, and open-drive compressors have motors external to the refrigerant circuit. Compressors may be cooled by suction gas; otherwise, hermetic and semi-hermetic compressors may experience burnout if they overheat or experience arcing, which can contaminate the system. Nowadays, we have variable-speed or multi-stage compressors to match the load to the capacity more easily. Compressors should also NEVER be run into a deep vacuum.

Compressors and motors have ratings, including the RLA (rated load amps) and MCC (maximum continuous current). MCC refers to the maximum amount of current that won’t trip the overload, whereas RLA represents what we would expect during normal operation; the RLA can be determined by taking the MCC and dividing it by 1.6.

A compressor may have a permanent split capacitor (PSC) motor, which is a single-phase motor that connects to a run capacitor between the start and run windings. The current that moves in and out of the start winding is dictated by the run capacitor; the capacitance restricts the current based on the capacitor’s microfarad rating.

On the other hand, capacitor start capacitor run (CSCR) motors use both start and run capacitors. A relay takes the start capacitor out of the circuit when the motor gets up to speed, but the run capacitor stays in the circuit.

ECMs are electrically commutated motors, which have permanent magnet rotors and are controlled by a brushless DC motor. These motors are common on blowers and have a wide operating envelope for superior humidity control, efficiency, and operating temperatures.

Motor speed is determined by synchronous speed and slippage. The synchronous speed is the speed at which the magnet rotates (and is determined by the number of poles), and slippage is the difference between the synchronous speed and the actual speed.

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