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How Air Conditioner Motors Work (2024)


Air conditioner motors are some of the crucial components that are required in the operation of the air conditioning in your house. They work by converting electricity to mechanical energy to spin the blower in your air handler and compressor in your outdoor unit. We will discuss the various types of single-phase motors that are used in your equipment at home. Here are four types of electric motors that are commonly used. 

Related: What are Air Conditioner Motor Applications?


What Does an AC Motor Do?

The motor is the heart of your air conditioning system. It powers the compressor, condenser fan, and blower. Without a properly functioning motor, your AC unit won’t be able to circulate cool air throughout your home effectively.


How Air Conditioner Motors Work in General

Air conditioner motors are powered by alternating current (AC) electricity from your home and use electromagnetic components to convert the electricity from AC to DC (direct current) using transformers and then (or directly from AC) into mechanical energy. This mechanical energy drives the various components of your AC system, allowing it to cool and circulate air.

Depending on the type of AC unit you have, the way its electric motor works will differ — I’ll get to this next.


How Common Type of AC Motors Work

Each type of AC motor has its own unique characteristics and applications. There are four main types of single-phase AC motors used in residential air conditioners:

  • Split-Phase Motors
  • Capacitor-Start Motors
  • Capacitor-Start, Capacitor-Run Motors
  • Permanent Split Capacitor (PSC) Motors

Let’s take a closer look at how these motors function in your air conditioning system.

Split-Phase Air Conditioner Motor

This is the simplest design where the RUN winding and START winding are connected in parallel and 90° electrically apart. It is usually used in small pumps, fans, and blowers where the capacity is below 1 horsepower. It has a low starting torque but a high starting electric current. Since the torque is low, the ability to start the motor is only practical for low-load conditions.

The RUN winding is made from bigger diameter wire and shorter turns for lower resistance and high inductance properties. The START winding is made from smaller diameter wire for higher resistance and low inductance properties. 

When AC supply is connected to the synchronous motor, both the windings will be energized with the current in the RUN winding lags the current in the START winding by about 30° electrically. This out-of-phase effect on the stator produces a starting torque and causes the rotor to start rotating, which creates a rotating magnetic field.

Typically, the speed of the motor is 1800 rpm or 3600 rpm when running without any load. When the load is connected, the speed can go down to 1725 rpm and 3450 rpm, respectively.

The no-load speed of the motor is given by:

Speed(rpm) = (Frequency of AC power X 120)/number of poles

For example, if your supply is 60 Hz and the motor is using two-pole, the synchronous speed = (60X120)/2 = 3600 rpm

There is a switch known as the centrifugal switch which is connected in series with the START winding. This mechanical switch will open when the motor speed reaches 75% of the rated speed, typically within 2 seconds. Once the switch opens, the START winding in the circuit is disconnected.

This is to protect the START winding from overheating. When the motor is powered off, the switch will close the circuit to get ready for the next starting of the motor.

These days, electronic relay is also being widely used to disconnect the START winding.

Capacitor-Start Air Conditioner Motor

This AC induction motor is similar to the split-phase motor and variable frequency drive, except that there is an external capacitor that is connected in series with the START winding. This air conditioner capacitor will cause the current in the START winding to lead the voltage.

The current in the RUN winding lags the voltage. When this happens, the phase difference between the two windings is 90° electrically; hence, a true two-phase starting is achieved through electromagnetic induction.

The starting torque of this air conditioner motor is very high, making it suitable to drive a small compressor that needs to start under full load. The capacity of this motor can go up to 1 horsepower.

Once the motor has reached 75% of the rated speed, the capacitor and the START winding will be automatically disconnected from the circuit by using the centrifugal switch, potential, or current relay. 

After the capacitor and START winding have been removed from the circuit, the magnetic field being generated continuously will cause the motor to continue running. This type of motor is also known as CSIR motor or Capacitor-Start-Induction-Run motor.

Related: How Much Does an AC Capacitor Cost?

Capacitor-Start, Capacitor-Run Air Conditioner Motor

This type of induction motor design is similar to the capacitor-start design except that there is a second capacitor known as the RUN capacitor, which is connected in parallel with the START capacitor and the switch. It is a type of asynchronous motor.

These capacitors are effectively connected in series with the START winding. During the starting of the motor, both the capacitors are connected to the circuit. The START winding and the RUN winding will remain connected to the circuit at all times.

Usually, the capacitance of the RUN capacitor is lower than the START capacitor. During starting, the effective capacitance is the combination of both capacitors causing a greater phase angle shift between the windings.

This provides a higher starting torque and can be used to drive the compressor as well as in belt-driven motors.

As the rotor speed reaches 75% of the rated speed, the switch will be automatically open to disconnect the START capacitor from the circuit. The START winding remains in the circuit.

The RUN capacitor helps to correct the power factor of the circuit, making it more efficient. The capacity of this type of motor can go up to 10 horsepower, and it is one of the most efficient motors with the best speed control used in the HVAC industry.

Permanent Split Capacitor(PSC) Air Conditioner Motor

This motor has a similar design to the phase-split motor except that there is a RUN capacitor connected to the START and RUN windings. This motor does not have any switch, and the START winding, RUN winding, and the RUN capacitor are active whenever the motor is ON.

This type of motor has low starting torque and is suitable to be used in small fan motors such as the fan coil unit of a split air conditioning system.

Variable-speed PSC is achieved by changing the winding resistance. If high speed is required, the terminal is connected to the least winding resistance. If low speed is required, the terminal is connected to the highest winding resistance.

By utilizing relays to choose the terminal to be connected to the line voltage, different speeds of the fan motor can be achieved depending on where the current flows.

In the diagram above, there are 4 motor speeds that can be selected. Super High(SH), High(H), Medium(M), and Low(L). The selection can be done by using electronic relays to connect L2 to one of the four terminals, depending on the speed required.


Final Thoughts

Air conditioner motors are the power supply behind your AC system, converting electrical energy into the mechanical energy needed to drive moving parts, including the compressor, condenser fan, and blower. While there are several types of AC motors, such as split-phase, capacitor-start, capacitor-start/capacitor-run, and permanent split capacitor motors, the specific type isn’t crucial for most homeowners to know. What’s more important is getting regular maintenance and timely repairs to extend the life of your AC motor and ensure your home remains comfortable.


Frequently Asked Questions

What causes an AC motor to burn out?

Overheating due to insufficient lubrication, electrical issues, or a seized compressor can cause your AC motor to burn out.


What does the phase of an AC motor mean?

The phase of an AC motor refers to the number of alternating currents that power the motor. Single-phase motors are most common in AC units for homes, while three-phase induction motors are typically found in larger commercial systems.


How long do AC motors typically last?

AC motors can last between 10–20 years, depending on usage, maintenance intervals, and environmental factors.


Can I replace an AC motor myself?

Replacing an AC motor is a complex process — I don’t recommend doing this as a DIY project. Instead, hire an HVAC technician to make sure it’s done right.


Article Update Log

Reviewed for accuracy, cost data, industry best practices, and expert advice by Jonathon Jachura.



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