What Are The 5 Types of Single-Phase Motor [How single phase AC motors work]

Single-phase AC induction motors come in many classifications and are the most commonly used appliance on the market today. They come in a huge range of shapes and sizes depending on the application. In this article, we will cover what the 5 different types of induction motors, and we will also take a look at the advantages and disadvantages of using a single-phase induction motor compared to a three-phase or DC motor.

The 5 types of single-phase motors are a split phase motor, a capacitor start motor, a capacitor start capacitor run motor, a permanent split capacitor motor (PSC), and a shaded pole motor. Each motor comes with its advantages and disadvantages so choosing the correct one is important.

To find out exactly what each type of motor does and how to choose the correct one for your application, continue reading to be confident you are making a confident, informed decision.

How a split phase induction motor works

Have you ever wondered how those appliances around you magically come to life with the flip of a switch? One crucial component behind their operation is the split-phase motor.

motor maintenance pdf

Split-phase induction motors are usually small and only produce between 40 and 250 watts of power depending on the size of the motor. Split-phase induction motors are only suitable for small loads such as oven fans, fridge fans, extract fans, etc.

To best understand how split-phase motors work, you first need to understand the basic principles of electromagnetism and motors in general. So if you are not already familiar with these topics, I would suggest checking out this article on how motors work before continuing, just to be sure you get the most out of the information.

For any AC induction motor to start it first needs a rotating magnetic field. In a three-phase induction motor, the rotating magnetic field is created by the three stator windings having an alternating current 1200 apart which produces the starting torque needed to rotate the rotor.

When talking about single phase motors we will refer to the stator windings as the main (run) winding and the auxiliary (start) winding, due to there being only two windings in a single phase motor.

In a single phase motor you only have one supply phase which would mean only one stator or run winding, and with only one winding, no magnetic flux lines would be consistently created, meaning no rotating magnetic field would be possible, this means a second artificial phase must be created.

This is done by wiring an auxiliary (start) winding in parallel with the run winding. The auxiliary winding consists of a very fine wire with a small cross-sectional producing a much higher resistance than the run winding, it must also then be wound in a way as to minimize inductive reactance XL to a minimum.

Now that you have two windings both with different value inductive reactance’s XL (resistance), when AC power supply is switched on, current will flow through each winding at different times because of the difference in resistance, which in turn simulates an artificial phase and rotating magnetic field, allowing the rotor to rotate in a certain direction.

To start most single-phase AC motors a capacitor is needed to lower the inductive reactance XL caused by the high resistance auxiliary winding (coil) but because a split-phase type motor is only designed to operate very small loads, it has the ability to start without using a start capacitor.

The advantage of this is that there is less maintenance to do on the motor over time, capacitor failure is the most common problem with a single-phase motor so not having one is always a good thing.

Because the auxiliary winding is wound with such fine wire it is susceptible to overheating and burning out, for that reason the start winding is short-time rated (S.T.R) and must be disconnected from the supply once the motor starts.

The way the start winding is disconnected from the electricity supply is by using a centrifugal switch, wired in series with the start winding. The centrifugal switch is mounted on the rotor shaft and once the motor reaches 75-80% of its normal operating speed the centrifugal switch opens under the centrifugal force (momentum) created by the rotating rotor and open circuits the start winding from the supply.

centrifugal switch
Centrifugal switch

If the centrifugal switch fails to open the auxiliary winding of a single-phase motor will soon burn out, that’s why it is extra important to regularly maintain single-phase motors and ensure the centrifugal switch is not sticking or corroded.

Once the power supply is switched off and the motor speed drops below 75-80% the centrifugal switch will then return to its original closed position.

Advantages of a split phase motor

  • No capacitor is needed to start the motor
  • Suitable for small to medium size applications
  • Cost effective
  • Little maintenance

Disadvantages

  • Only suitable for small loads
  • Risk of overheating and burning out the windings
  • High starting current

How capacitor start induction motors work

The capacitor start type motors are one of the most commonly used motors. To comprehend how a capacitor start motor operates, we must first grasp the concept of phase shifting. Our aim is to create a phase difference between the main and auxiliary windings, enabling the motor to develop sufficient torque during startup. This is where the magic of the capacitor start motor lies.

single phase ac induction motor capacitor start

A carefully selected capacitor plays a pivotal role in this motor’s operation. Its purpose is to introduce a phase shift between the currents flowing through the starting and run windings. The capacitor’s capacitance value, carefully determined during the motor’s design, ensures the desired phase difference and optimal performance.

Upon initiating the motor, both windings receive electrical voltage, thus allowing current to flow. The capacitor, connected in series with the auxiliary winding, generates a leading current, which creates an artificial phase shift. This phase shift gives rise to a rotating magnetic field that interacts with the motor’s rotor, setting it in motion.

The generated rotating magnetic field produces a torque that overcomes the motor’s initial inertia, enabling it to start rotating. The high starting torque offered by the capacitor start motor is ideal for applications requiring instantaneous and robust acceleration.

As the motor gains speed, typically around 75-80% of its rated speed, a centrifugal switch comes into play. This switch, often located on the motor shaft, detects the rotational speed and disconnects the starting winding and its associated capacitor from the power supply. This disconnection prevents excessive current flow through the starting winding during continuous operation.

Advantages of a capacitor start motor

  • High starting torque
  • Simple design
  • Relatively cost effective
  • Comes in a range of sizes to suit most applications

Disadvantages

  • The capacitor will need replacing at some point
  • Requires more maintenance than a three phase induction motor
  • Louder running noise than a three phase motor

How a capacitor start capacitor run (C.S.C.R) motor works

The CSCR AC motor represents an evolution from its predecessor, combining the benefits of a high starting torque with enhanced running efficiency. To comprehend its operation, we must grasp the interplay of two capacitors: the starting capacitor and the running capacitor.

The CSCR motor ingeniously incorporates two capacitors, each serving a distinct purpose. The starting capacitor, akin to its counterpart in the capacitor start motor, initiates the motor’s rotation by creating a phase shift in the auxiliary winding. On the other hand, the running capacitor remains connected throughout the motor’s operation, contributing to improved efficiency and performance.

Upon energizing the motor, both the starting and running capacitors receive electrical current. The starting capacitor, connected in series with the auxiliary winding, introduces a leading current that creates the necessary phase shift. This phase shift generates a rotating magnetic field, propelling the motor’s rotation with a high starting torque.

Once the motor reaches a predetermined speed, a centrifugal switch comes into play, as observed in the capacitor start motor. However, in the CSCR motor, the centrifugal switch exclusively disconnects the starting capacitor, allowing the running capacitor to remain within the circuit during continuous operation.

The incorporation of the running capacitor provides distinct advantages to the CSCR motor. Firstly, it optimizes the power factor, leading to increased running efficiency. Additionally, the running capacitor enhances the motor’s torque-speed characteristics, ensuring smooth and stable operation across a wide range of loads.

The versatility of the CSCR motor finds applications in various fields, including pumps, fans, compressors, and HVAC systems. Its remarkable efficiency, robust performance, and adaptability make it a preferred choice for demanding environments. However, meticulous capacitor selection and matching are imperative to maximize performance and prevent potential motor damage.

Advantages of a capacitor start capacitor run motor

  • Highly efficient
  • High starting torque
  • Comes in a range of sizes to suit most jobs
  • High performing power band to more consistency under load

Disadvantages

  • Requires more maintenance
  • Two capacitors that will likely fail at some point
  • Slightly more expensive to purchase
  • Bulky due to having two capacitors

How permanent split capacitor (PSC) motors work

The permanent split capacitor motor, also known as a capacitor start run motor, is renowned for its efficiency and reliability and builds upon the concept of a split-phase motor. By incorporating a permanent capacitor, the PSC motor achieves remarkable performance while maintaining simplicity and longevity.

The PSC motor operates on the basis of a split-phase design. It comprises a main running winding and a secondary starting winding positioned within the motor’s stator. The inclusion of a permanent capacitor sets it apart, providing continuous operation and improved performance.

The permanent capacitor, connected in parallel with the main running winding, plays a crucial role in the PSC motor. Unlike other motor types, the capacitor remains connected at all times, hence the name “permanent split capacitor.”

8 micro farad capacitor

Upon powering the motor, both the main winding and the auxiliary winding receive electrical current. The permanent capacitor ensures a constant phase shift, generating a rotating magnetic field within the motor. This rotating field interacts with the motor’s rotor, initiating smooth and reliable rotation.

But the big question with the (CSR) motor is will it run with a bad capacitor? The answer is it depends on a couple of different factors such as load etc. To get a full explanation check out the article, I think you will be surprised.

The PSC motor excels in efficiency and power factor optimization. The permanent capacitor helps optimize the power factor, reducing reactive power and enhancing overall motor efficiency. This makes it an excellent choice for energy-conscious applications.

The PSC motor finds diverse applications, including fans, blowers, HVAC systems, and small appliances. Its reliability and efficiency make it highly valued in these domains. Careful selection and matching of the permanent capacitor are vital to ensure optimal performance and prevent motor issues.

Advantages of a permanent split capacitor (PSC) motor

  • Highly efficient
  • High starting torque
  • Reliable
  • Simple design
  • Low inductive reactance XL
  • Cost effective

Disadvantages

  • Requires more maintenance than a split phase motor
  • Noisy

How a single phase shaded pole motor works

At the heart of the shaded pole motor lies the shaded pole—a small, strategically placed electromagnet. This electromagnet consists of a copper or aluminum winding encircling a portion of the motor’s pole. The purpose of this shading is to create a time-delayed magnetic field, inducing the desired rotation.

Upon applying voltage to the motor, an alternating current flows through the main winding. As a result, a magnetic field is generated. However, due to the shading effect, a time-delayed magnetic field is induced in the shaded pole. The interaction between these magnetic fields results in a rotational force on the motor’s rotor.

The time-delayed magnetic field generated by the shaded pole creates an imbalance in the magnetic forces acting on the rotor. This imbalance causes the rotor to start rotating in the direction dictated by the design of the motor. The continuous interaction between the main winding and the shaded pole sustains the rotation of the motor.

The shaded pole motor finds extensive use in a wide range of applications, including small fans, refrigeration systems, record players, and various household appliances. Its low cost, simplicity, and reliable operation make it a preferred choice for applications that require low torque and rotational speeds.

Advantages of a shaded pole motor

  • Simple design
  • Cost effective
  • Reliable
  • Compact

Disadvantages

  • Low starting torque
  • Lower efficency
  • Limited speed control

Advantages and disadvantages of single phase AC motors

Single-phase AC motors are a very useful piece of equipment and without them, we would not have the technology, products or processes that we have today.

Even though a three-phase induction motor would be the preferred choice in most situations, a single-phase motor will do the same job when a three-phase supply is not in place. The reason for choosing a three-phase motor over a single-phase is purely due to having little to no maintenance and being relatively trouble-free.

So now let’s go through some of the advantages and disadvantages of single-phase AC motors.

Advantages

  • Simplicity and Cost-Effectiveness: Single-phase motors offer a simpler design compared to three-phase motors, resulting in lower manufacturing costs. The absence of additional phases and associated control systems reduces complexity, making single-phase motors a cost-effective choice for various applications.
  • Availability and Accessibility: Single-phase power is more widely available in residential and small-scale commercial settings compared to three-phase power. This accessibility makes single-phase motors more convenient for use in household appliances, small machinery, and light industrial applications.
  • Flexibility in Installation: Single-phase motors require less complex wiring and can be easily installed without the need for a dedicated three-phase power supply. This flexibility allows for convenient retrofitting and makes single-phase motors suitable for applications where three-phase power is unavailable or impractical.
  • Smaller Size and Lower Weight: Single-phase motors typically have a smaller physical footprint and lower weight compared to their three-phase counterparts. This compactness makes them suitable for applications with space constraints or where portability is important, such as in portable tools or household appliances.
  • Wide Range of Applications: Single-phase motors find extensive use in a broad range of applications, from household appliances (e.g., refrigerators, air conditioners, and washing machines) to small machinery (e.g., fans, pumps, and compressors). Their versatility and availability make them a preferred choice for numerous residential, commercial, and light industrial applications.

Disadvantages

  • Lower Power Output: Single-phase motors generally have lower power output compared to three-phase motors. The absence of additional phases limits the maximum power capacity of single-phase motors, making them less suitable for high-power applications that require substantial torque and horsepower.
  • Lower Efficiency: Single-phase motors tend to be less efficient than their three-phase counterparts. The asymmetrical power delivery in single-phase systems results in increased losses, lower power factor, and reduced overall efficiency. This can lead to higher energy consumption and increased operating costs.
  • Reduced Starting Torque: Single-phase motors exhibit lower starting torque compared to three-phase motors. The absence of additional phases can result in reduced torque during motor startup, which can be a limitation in applications that require high starting torque, such as heavy machinery or equipment with high inertia loads.
  • Limited Load Handling Capacity: Single-phase motors have limitations in handling heavy or demanding loads. The reduced power output and lower torque capabilities make them less suitable for applications that require continuous operation under heavy loads, such as large industrial machinery or equipment that requires constant high torque.
  • Unbalanced Power Distribution: In single-phase systems, power distribution is inherently unbalanced due to its single-phase nature. This can lead to uneven loading and potential voltage drops, which can affect the performance and lifespan of equipment connected to the motor. It may require additional measures, such as voltage regulation or the use of compensating devices, to mitigate these issues.

Conclusion

In conclusion, our exploration of various single-phase induction motors has provided valuable insights into their operational principles, advantages, and disadvantages. However, it is important to consider the limitations of each motor type. Split-phase and capacitor motors may have lower power output, efficiency, starting torque, load handling capacity, and speed control compared to their three-phase counterparts.

Nevertheless, they excel in residential, commercial, and low-power industrial applications. Remember, choosing the right motor involves careful consideration and an understanding of the unique characteristics of each type. If you found this article interesting or helpful I would strongly suggest checking out this article on how motors actually work if you haven’t already read it.

Gavin

I'm Gavin and Iv been teaching electrical science to apprentice electricians in a local technological university since 2022. I hold an Electrical Level 6 QQI Qualification along with several NZEB Certifications.

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