Universal Motor (Complete Guide)

What is a universal motor?

A universal motor is a type of electric motor that is designed to operate on either alternating current (AC) or direct current (DC) power sources.

It is called “universal” because it can function effectively on both AC and DC electrical supplies, making it versatile and suitable for a wide range of applications.

Universal motors are commonly used in various household appliances and power tools where the power source may be AC or DC. The universal motor is called so because it can run on both AC and DC sources.

Universal motor
Marrrci, CC BY-SA 3.0, via Wikimedia Commons

Universal Motor Working Principle

Universal motors work on the same principle as DC series motors, regardless of whether AC or DC powers them.

Depending on the supply, a universal motor can be either a DC motor or a single-phase AC motor.

With the application of a DC power source to the universal motor, it works just like a DC series motor. In the field winding of the motor, the flow of current produces an electromagnetic field.

In addition, the armature conductors also carry a current of the same magnitude. Conductors encased in electromagnetic fields undergo a mechanical force. As a result of this mechanical force, or torque, the rotor begins to rotate. Fleming’s left-hand rule determines the direction of this force.

When an AC supply powers a universal motor, it will produce the same unidirectional torque it can produce. The armature winding is connected in series with the field winding, so both of them are in the same phase.

Since the polarity of AC constantly changes, the current flowing in the field winding and the armature also reverses simultaneously as the polarity changes. As a result, the magnetic field and armature current are reversed so that the direction of force experienced by armature conductors does not change.

applications

There are various applications where a universal motor is used, such as:

  • portable power tools and equipment
  • washing machines
  • Vacuum cleaner
  • small fans
  • kitchen appliances
  • Table fans
  • Sewing machines

Why are Universal Motors So Loud?

Universal motors are known for being relatively loud during operation for several reasons:

  1. Brushes and Commutator: Universal motors operate using brushes that make physical contact with a commutator (a rotary switch). This direct physical contact creates friction and generates noise as the brushes slide against the commutator. The mechanical interaction between the brushes and the commutator can produce a buzzing or humming sound.

  2. Rapid Commutation: Universal motors switch the direction of the current in the motor windings rapidly to generate rotational motion. This rapid commutation causes the brushes to bounce and create additional noise as they make and break electrical contact with the commutator segments.

  3. Vibration: The physical contact between the brushes and the commutator, as well as the rapid changes in the current direction, can result in vibrations within the motor. These vibrations can be transmitted to the motor housing and other components, leading to audible noise.

  4. High Rotational Speeds: Universal motors are often designed for high rotational speeds, which can inherently produce more noise compared to slower-speed motors. The higher the speed, the more turbulent the air movement and the louder the motor can be.

  5. Air Displacement: In some applications, such as vacuum cleaners or blowers, universal motors may be used to move air. The rapid movement of air can also contribute to the overall noise level of the device.

  6. Design Considerations: In some cases, the design of the motor and the components used in it may prioritize cost and simplicity over quiet operation. This can lead to louder motor performance.

Efficiency

Universal motors are known for their versatility and high torque-to-power ratio, but their efficiency can vary depending on the application and operating conditions.

Generally, universal motors are less efficient than some other motor types, particularly at high speeds and under heavy loads. Here are some key factors that influence the efficiency of universal motors:

  1. Brush Friction: Universal motors use brushes that make physical contact with a commutator to switch the direction of current in the motor windings. This contact generates friction, which leads to energy losses in the form of heat and reduces overall efficiency.

  2. Heat Generation: Due to the friction between the brushes and the commutator and the rapid commutation, universal motors tend to generate heat. Excessive heat can reduce efficiency and, in some cases, necessitate cooling mechanisms to prevent overheating.

  3. Copper and Iron Losses: Universal motors have copper windings and iron cores, both of which contribute to energy losses. Copper losses (I²R losses) occur as current passes through the windings, and iron losses (eddy current and hysteresis losses) occur due to the changing magnetic field within the iron core.

  4. Mechanical and Aerodynamic Losses: Universal motors often operate at high speeds, which can result in mechanical and aerodynamic losses as a result of friction within the bearings and air resistance. These losses further reduce efficiency.

  5. Load Variation: The efficiency of universal motors can vary significantly with changes in load. They tend to perform more efficiently at lower loads and may become less efficient as the load increases. This is due to the nonlinear relationship between current and torque in universal motors.

  6. Power Factor: Universal motors may have a lagging power factor, which means they draw more current from the power supply than what is used for useful work. A lower power factor can result in increased energy consumption and reduced overall efficiency.

  7. Design and Size: The design and size of a universal motor can influence its efficiency. Smaller, less robust universal motors may have lower efficiency compared to larger, better-designed models.

Despite these efficiency challenges, universal motors are still widely used in various applications because of their high starting torque and ability to operate on both AC and DC power sources.

In applications where energy efficiency is a critical concern, other motor types, such as brushless DC motors or AC induction motors, may be preferred.

Advances in motor technology and materials continue to improve the efficiency of universal motors, making them more suitable for energy-conscious applications.

Advantages & Disadvantages of Universal motors

Universal motors have specific advantages and disadvantages, which make them suitable for certain applications while less appropriate for others. Here’s a breakdown of their advantages and disadvantages:

Advantages:

  1. Versatility: Universal motors can operate on both AC and DC power sources, making them highly versatile. This versatility allows them to be used in a wide range of applications.

  2. High Starting Torque: Universal motors provide high starting torque, making them suitable for applications that require rapid acceleration or high torque at startup. This characteristic is valuable in power tools, kitchen appliances, and vacuum cleaners.

  3. Compact Size: Universal motors are relatively compact and lightweight, making them suitable for portable and space-constrained applications.

  4. Variable Speed: Universal motors offer variable speed control, which is valuable in applications where adjustable speed is required, such as power tools and kitchen mixers.

  5. Reversibility: Universal motors can easily change the direction of rotation by reversing the polarity of the applied voltage or by changing the direction of the current in the windings. This reversibility is a valuable feature in many applications.

Disadvantages:

  1. Noise and Vibration: Universal motors tend to be noisy during operation due to the physical contact between brushes and the commutator. They can also generate vibrations, leading to a less pleasant operating environment.

  2. Brush Wear: Universal motors rely on brushes that make contact with the commutator. These brushes experience wear over time and may require periodic replacement, which can be considered a maintenance drawback.

  3. Lower Efficiency: Universal motors are less efficient than some other motor types, particularly at high speeds and under heavy loads. They may generate heat and consume more energy.

  4. Limited Lifespan: Due to brush wear and other factors, universal motors typically have a limited lifespan compared to brushless motor types. This can result in the need for motor replacement or maintenance.

  5. Limited Use in High-Precision Applications: Universal motors may not be suitable for high-precision applications due to their inherent brush and commutator design, which can lead to slight variations in speed and torque.

  6. Heat Generation: Universal motors tend to generate heat, especially under heavy loads or extended operation, which can impact efficiency and may require additional cooling mechanisms.

In summary, universal motors are valuable for applications that prioritize high starting torque, variable speed control, and compact size.

However, their noise, maintenance requirements, and lower efficiency can be drawbacks in some situations.

The choice of motor type depends on the specific requirements and constraints of the application in which it will be used.Carbon brushes
Stan Zurek, CC BY-SA 3.0, via Wikimedia Commons

Why Universal Motor Has High Speed?

Universal motors achieve high speeds due to their design, variable voltage capability, and high torque-to-inertia ratio.

Their lack of synchronous speed limitations, efficient commutation, and compact size contribute to rapid acceleration.

However, high speed may come with increased noise and heat, and maintenance may be required due to brush wear.

Several factors contribute to their high-speed performance:

  1. Construction: Universal motors are designed with a wound rotor (armature) and a stator, which contains the field windings. The rotor is typically connected to the shaft of the motor. This configuration allows for a compact and lightweight design.

  2. Variable Voltage: Universal motors are designed to operate on both alternating current (AC) and direct current (DC) power sources. Because the speed of a universal motor is directly proportional to the voltage applied, it can achieve high speeds when exposed to higher voltage levels.

  3. High Torque-to-Inertia Ratio: Universal motors are known for their high torque-to-inertia ratio, which means they can accelerate quickly when a voltage is applied. This characteristic is especially valuable in applications where rapid acceleration and variable speeds are required, such as power tools and kitchen appliances.

  4. No Synchronous Speed Limitation: Unlike synchronous motors, which have a fixed synchronous speed determined by the supply frequency, universal motors have no inherent synchronous speed limitation. This means they can operate at speeds well above synchronous speed, allowing for greater flexibility in speed control.

  5. Commutator and Brushes: The physical contact between the brushes and the commutator in a universal motor allows for efficient switching of current direction in the rotor windings. This rapid commutation enables the motor to maintain high rotational speeds.

  6. Reversibility: Universal motors can easily change the direction of rotation by reversing the polarity of the applied voltage or changing the direction of the current in the windings. This reversibility adds to their versatility.

  7. Compact Size: The compact size and relatively simple design of universal motors contribute to their ability to achieve high speeds. Their small physical footprint allows for reduced rotational inertia, which aids in quick acceleration.

It’s important to note that while universal motors can achieve high speeds, their efficiency and performance characteristics may vary depending on the applied voltage, load, and operating conditions.

The trade-off for their high-speed capability is often increased noise, heat generation, and the need for maintenance due to brush wear.

Manufacturers may implement design and engineering techniques to optimize the performance of universal motors in specific applications.

Speed Control

Speed control of a universal motor is easy compared to an induction motor because an induction motor is a constant-speed motor.

Two types of controlling speed of universal motor:

  • Using a thyristor circuit to obtain continuous speed control.
  • Stepped speed control using multiple taps on the field coil.

Universal Motor vs. Induction Motor

Feature Universal Motor Induction Motor
Brushes Uses brushes No brushes
Efficiency Low efficiency (30 – 75%) High efficiency
Speed control Easy speed control Poor speed control
Speed Higher speed Lower speed
Power Source Runs on AC and DC power source Runs on AC source only
Work at Hazardous atmosphere Not suitable for hazardous explosive atmosphere Suitable for hazardous explosive atmosphere
Applications Light hand tools and home appliances Used for heavy industrial applications
Maintenance Needs more maintenance due to brush issues Lower in maintenance
Noise Noisy working Not so loud

I have written a detailed article about three phase induction motor, you can find it here.

Universal Motor is not an Induction Motor. A universal motor is a series-excited motor that uses a single energy phase. They are pretty different from each other.

Induction motors are incredibly efficient, using less current per horsepower. Universal motors require more amperage per horsepower than induction motors.

Why Can A Universal Motor Work on AC and DC?

A universal motor can work on both alternating current (AC) and direct current (DC) power sources due to its unique design and characteristics. The key reasons why a universal motor can operate on both AC and DC are as follows:

  1. Construction: Universal motors are designed with a wound rotor (armature) and a stator. The rotor is the rotating part of the motor, while the stator is the stationary part. Both the rotor and the stator have coils of wire wound around them.

  2. Brushes and Commutator: The critical component that allows a universal motor to work on both AC and DC is the commutator and brushes. The rotor (armature) of a universal motor has a commutator, which is a split, cylindrical ring that rotates with the armature. Carbon brushes make physical contact with the commutator segments.

  3. The direction of Current: In a universal motor, the direction of the current through the rotor windings can be reversed by changing the polarity of the applied voltage or current. When the voltage is applied one way, the current flows in one direction through the rotor windings, and when the voltage is reversed, the current flows in the opposite direction.

Here’s how a universal motor works on both AC and DC power:

  • On AC Power: When AC voltage is applied to a universal motor, the voltage regularly alternates in polarity, causing the current to change direction with each cycle. The brushes and commutator ensure that the current direction in the rotor windings also changes with the alternating voltage. This reversibility allows the motor to run on AC power.

  • On DC Power: When DC voltage is applied to a universal motor, the current flows consistently in one direction through the rotor windings. The brushes and commutator maintain this direction of current. This allows the motor to operate on DC power.

The ability to reverse the current direction in the rotor windings makes the universal motor highly versatile and capable of working on both AC and DC power sources.

This versatility is advantageous in applications where the type of power source may vary or where variable speed control and high starting torque are required, such as in power tools, kitchen appliances, and portable equipment.

However, it’s essential to note that the motor’s efficiency and performance characteristics may vary depending on the power source and load conditions.

Read my detailed article about AC vs DC power. You can find it here.

How to use a universal motor as a dC generator?

There is no doubt that Universal Motors can also be used as DC generators. For this purpose, the rotor will need to be separated from the stator windings, and one of the coil sets will need to be powered by a DC current.

However, they are DC series motors, and general-purpose generators are not what they are made for. Getting it started is difficult since the magnetic field is insufficient. It will take a significant portion of your generated energy to support that magnetic field with a brushed rotor.

To produce electrical energy, magnetic flux lines need to be cut with coils. As a result, you have several different types to choose from, but a universal motor makes a poor generator.

Why is the universal motor used in home appliances and power tools?

Universal motors are commonly used in home appliances and power tools for several reasons:

  1. High Starting Torque: Universal motors provide high starting torque, which is essential for applications that require rapid acceleration or the ability to start under load. This characteristic makes them well-suited for power tools like drills and saws, where the tool needs to quickly overcome resistance when starting.

  2. Variable Speed Control: Universal motors offer variable speed control, allowing users to adjust the motor’s speed to meet specific requirements. This feature is valuable in power tools and kitchen appliances, where different tasks may require varying speeds for optimal performance.

  3. Compact and Lightweight: Universal motors are relatively small and lightweight, making them suitable for portable appliances and power tools. Their compact size allows for easy integration into handheld devices.

  4. Versatility: Universal motors can operate on both AC and DC power sources, making them versatile and adaptable to different electrical systems. This versatility simplifies the design and manufacturing process for appliances and tools intended for a global market.

  5. Reversibility: Universal motors can easily change the direction of rotation by reversing the polarity of the applied voltage or current. This reversibility is beneficial in power tools where forward and reverse operation is required.

  6. Cost-Effective: Universal motors are often more cost-effective to manufacture than some other motor types, making them a cost-efficient choice for consumer appliances and power tools.

  7. Suitable for High Torque Loads: Universal motors can handle high torque loads, making them ideal for tasks that require significant cutting, drilling, or grinding force, as seen in various power tools.

  8. Compact Design: The compact design of universal motors allows for space-saving integration into appliances and tools, ensuring that the devices remain portable and easy to handle.

  9. Availability: Universal motors are widely available and have a long history of use in home appliances and power tools. This availability simplifies the sourcing and replacement of motors when needed.

However, it’s important to note that universal motors also have some drawbacks, such as noise, lower efficiency at high speeds, and the need for periodic maintenance due to brush wear.

Despite these limitations, their unique combination of high torque, variable speed control, and versatility makes them a popular choice for a wide range of applications where performance and affordability are key considerations.

Manufacturers often use design and engineering techniques to mitigate their drawbacks while maximizing their benefits in consumer products.

Can We Run a Universal Motor Without A Load?

It’s not recommended to run a universal motor at no load. Due to the very small flux on very light loads or when there is no load, the motor runs at a dangerously high speed, which may damage it mechanically.

If you have to run a universal motor without a load, there are a few considerations to keep in mind:

  1. No-Load Operation: Running a universal motor without a load means that the motor is operating with no mechanical resistance or work to perform. In this state, the motor will typically run at its maximum unloaded speed, which can be quite high. The high speed can generate more heat and may cause the motor to operate less efficiently.

  2. Risk of Overheating: Operating a universal motor without a load for an extended period can lead to overheating. Universal motors rely on air circulation for cooling, and without a load, they may not achieve adequate airflow to dissipate the heat generated by the motor windings and core. Prolonged overheating can damage the motor.

  3. Wear and Tear: Unloaded operation can contribute to additional wear and tear on the motor’s brushes and commutator. The brushes may wear down more quickly when the motor is running without a load.

  4. Noise: Universal motors tend to be noisy when running, and this noise can be more pronounced when the motor is running without a load due to the high rotational speed.

If you need to run a universal motor without a load for a short period, it should generally be safe, but it’s important to monitor the motor’s temperature to prevent overheating.

However, extended or continuous operation of a universal motor without a load is not recommended, as it can lead to overheating and premature wear of motor components.

If you find it necessary to operate a universal motor without a load frequently, you may consider implementing load simulation or using a variable-speed control to reduce the motor’s speed, which can help mitigate some of the issues associated with unloaded operation.

Additionally, be sure to follow the manufacturer’s guidelines and recommendations for motor operation and maintenance.

Why does a Universal Motor Has No Capacitor?

A universal motor doesn’t require a capacitor because of its inherent design and operation, no phase shifting is required by a capacitor because both the field winding and the armature winding are energized in the same circuit.

Unlike some types of AC motors that use capacitors to create the necessary phase shift for starting and running, a universal motor operates differently.

A universal motor is essentially a DC series motor, which means it’s internally self-excited. This self-excitation occurs because both the field winding and the armature winding are energized in the same circuit, creating a magnetic field that drives the motor’s rotation. This setup eliminates the need for an external capacitor or any additional components to create the necessary phase shift in the current.

In other words, a universal motor doesn’t rely on the same principles as single-phase AC induction motors that use capacitors to create a rotating magnetic field for starting.

Instead, it’s more akin to a DC motor in terms of its electrical characteristics, with both field and armature windings being powered directly from the same source.

This self-excitation and the direct connection of windings make universal motors well-suited for applications where variable speed control and the ability to operate on both AC and DC power sources are needed, such as in vacuum cleaners, power tools, and appliances with variable speed settings.

Why does Universal Motor Have a Spark?

Universal motors can produce sparks due to the nature of their operation and the physical contact between the brushes and the commutator. Here’s why sparks can occur in a universal motor:

  1. Brush and Commutator Contact: In a universal motor, carbon brushes make direct physical contact with the commutator. The commutator is a split, cylindrical ring mounted on the motor’s rotor (armature). This contact is necessary for the motor’s operation, as it enables the transfer of electrical current from the stationary stator windings to the rotating armature windings.

  2. Rapid Commutation: Universal motors rely on rapid commutation, which means that the direction of current in the armature windings changes frequently as the motor rotates. This is achieved through the segments of the commutator, which switch the armature windings’ connection to the power source. As the brushes slide across the commutator segments during rotation, they make and break electrical contact, resulting in rapid changes in current direction.

  3. Arcing and Sparks: The contact between the brushes and the commutator can generate electrical arcing and sparks during the switching process. This arcing occurs because, for a brief moment, there is a small air gap or interruption in the electrical connection between the brushes and commutator as they transition from one segment to the next. This momentary interruption leads to a spark, as the electrical current “jumps” the gap and reestablishes the connection.

  4. Voltage and Current Peaks: The rapid changes in current direction and the resulting sparks can lead to voltage and current peaks in the motor’s electrical circuit. These peaks can cause localized heating and stress on the commutator segments and brushes.

While the sparking in a universal motor is a normal part of its operation, it is also a source of concern in terms of motor wear and tear. The sparking can lead to the gradual erosion of the commutator segments and the brushes

Over time, this can affect the motor’s performance and efficiency and necessitate maintenance, including periodic replacement of brushes and, occasionally, commutator maintenance.

In applications where sparking is a concern, such as in tools and appliances with sensitive electronic components, additional measures may be taken to reduce or mitigate the sparks, such as the use of spark suppression techniques or improved brush and commutator materials.

How Much DC Voltage Do I Need to Run A Universal Motor?

The voltage required to run a universal motor depends on the motor’s design and rating. Universal motors are designed to operate on a range of voltages, typically listed on the motor’s nameplate or in its technical specifications.

The specific voltage needed can vary widely depending on the application and the motor’s intended use.

Here are some common voltage ranges for universal motors:

  1. Low-Voltage Universal Motors: Some universal motors are designed for low-voltage applications and may operate on DC voltages as low as 12 volts. These low-voltage motors are often used in battery-powered devices, such as cordless power tools or small appliances.

  2. Medium-Voltage Universal Motors: Universal motors that operate in the range of 100 to 240 volts are commonly found in household appliances like blenders, vacuum cleaners, and kitchen mixers. These motors are designed to run on standard household AC voltage, but they can also operate on DC within this voltage range.

  3. High-Voltage Universal Motors: In industrial applications, universal motors designed for higher voltages may be used. These motors can operate on voltages exceeding 240 volts, depending on the specific requirements of the equipment.

It’s important to note that while universal motors can operate on both AC and DC power sources, the voltage rating specified should be compatible with the power source you intend to use. Ensure that the voltage supplied to the motor matches its rated voltage to prevent damage and ensure proper operation.

If you are unsure about the voltage requirements of a specific universal motor, consult the motor’s documentation, nameplate, or the manufacturer’s specifications for guidance. Using the correct voltage is crucial to the motor’s performance, efficiency, and safety

Is Universal Motor Self-Starting?

Yes, a universal motor is self-starting, which means it can initiate rotation on its own when the appropriate voltage is applied to it. The self-starting capability of a universal motor is one of its key characteristics.

Here’s how the self-starting process works in a universal motor:

  1. Voltage Application: When the motor is connected to a power source and voltage is applied, it energizes both the field winding and the armature winding.

  2. Magnetic Field Creation: The field winding produces a magnetic field, and the armature winding, mounted on the rotor, interacts with this field. As a result, a torque is generated, which causes the rotor (and the attached load) to start rotating.

  3. Rotor Movement: Once the rotor begins to move, it continues to rotate in the same direction as long as the voltage is applied and the load does not oppose the motion significantly.

  4. Self-Excitation: The self-exciting nature of the universal motor is due to the physical contact between the brushes and the commutator. As the rotor rotates, the brushes maintain continuous contact with the commutator segments, ensuring that the armature winding remains energized and the magnetic field persists. This self-excitation allows the motor to continue running without external assistance.

The self-starting characteristic makes universal motors suitable for a wide range of applications, including power tools, home appliances, and other devices where the motor needs to start quickly and efficiently.

However, it’s important to note that the speed and performance of a universal motor can be affected by the load it is driving and the applied voltage, so it may not always start at its maximum speed under certain conditions.

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