How Does the Number of Magnets Affect the Speed of a Motor

A common misconception I see is that people think that by increasing the number of magnets in an electric motor will increase its speed when in fact the opposite is true.

When you increase the number of magnets in an electric motor the speed of the motor will decrease, likewise if you decrease the number of magnets the motor speed will increase. A two-pole induction motor is the fastest spinning motor at 3600 RPM.

After reading this article you will know exactly why and how the number of magnets affects the speed of a motor.

motor poles

The number of magnets in an electric motor plays a crucial role in determining its rotational speed. Whether it’s an AC motor or a DC motor, the interaction between magnetic fields and electric currents is essential for a motor to initiate rotation and generate motion.

motor maintenance pdf

In AC motors, the number of magnets affects the speed through the concept of magnetic poles. The stator typically contains electromagnets that create a rotating magnetic field when powered by an alternating current. when current is applied to stator windings and the rotating magnetic field is set up, this is known as the synchronous speed (Ns) of the motor and is measured in RPM. The speed of rotation is inversely proportional to the number of magnetic poles. More magnets or poles result in a slower speed, while fewer magnets or poles lead to a higher speed.

A two-pole motor (1 pair) is the highest rpm motor available with a rotational speed of approximately 3600 RPM as can be seen in the table of motor speeds in relation to the number of poles.

In DC motors, the number of magnets influences the speed as well. Permanent magnets or electromagnets in the stator generate a stationary magnetic field. The rotor, with its conductive coils or bars, carries an electric current. The interaction between the magnetic field and the current in the rotor produces a torque that drives the rotation. Generally, a higher number of magnets or poles in the stator results in a slower speed, while a lower number leads to a faster speed.

When there are more magnets or magnetic poles in the motor, the magnetic field generated by the stator becomes denser and more complex. As a result, the interaction between the stator and the rotor becomes more intricate, requiring more time for the magnetic forces to exert their effect on the rotor.

Consequently, a greater number of magnets means that the magnetic forces need to act over a larger distance as the rotor rotates. This extended interaction time and distance translate into a slower rotational speed.

Conversely, when there are fewer magnets or magnetic poles, the magnetic field is less dense and simpler. The interaction between the stator and the rotor becomes more direct and immediate, allowing for faster rotation.

The formulae used to calculate motor speed is Ns = f x 60 / P.

Ns = synchronous speed
f = frequency of the supply measured in seconds
60 = converting seconds to minutes to determine revolutions per minute
P = pairs of poles, meaning a four pole motor would have two pairs

The relationship between the number of magnets and motor speed may vary depending on the motor type, design, and other factors. Engineers consider these factors carefully to achieve the desired speed and performance characteristics for a given motor application.

cross section motor

By understanding the impact of the number of magnets on motor speed, engineers can optimize motor designs to meet specific requirements, whether it be for high-speed applications, precise control, or torque demands.

It’s important to note that factors like the frequency of the alternating current, the design of the motor, and the overall construction affect the relationship between the number of magnets and the motor’s speed. These factors are carefully used to optimize the motor’s performance for specific applications, balancing speed requirements with torque characteristics.

Understanding how motors work along with the reason they rotate and the influence of the number of magnets on motor speed is vital in motor design, ensuring efficient and reliable operation across a wide range of industries and applications.

Do all DC motors have magnets?

Not all DC motors have magnets. There are two primary types of DC motors: brushed and brushless.

Brushed Dc motors

Brushed DC motors are commonly used in various applications and operate through the principles of electromagnetic induction and the interaction between a stator and rotor. The stator, the stationary part of the motor, houses permanent magnets that create a fixed magnetic field.

The rotor, the rotating part, consists of wire coils or bars wrapped around a cylindrical core, connected to an output shaft.

A commutator, a split-ring device, and brushes provide electrical contact on the rotor. The wire coils, known as armature windings, become electromagnets when current flows through them. As the rotor spins, the brushes maintain contact with different segments of the commutator, reversing the current direction in the armature windings.

dc commutator and rotor

This alternating current generates changing magnetic fields. The interaction between these changing fields and the fixed magnetic field of the stator produces a force called the Lorentz force, driving the rotor to rotate.

The speed and direction of rotation can be controlled by adjusting the voltage or current supplied to the motor. Brushed DC motors are valued for their simplicity, cost-effectiveness, and ease of control, finding applications in electric vehicles, home appliances, power tools, and robotics.

Brushed DC motors were the most commonly used DC motor used in the industry until recent years when brushless technology has surpassed its predecessor in efficiency and reliability.

Brushless DC motors

Brushless DC motors, also known as electronically commutated motors, operate through a combination of permanent magnets on the stator and electromagnets on the rotor. The stator’s fixed permanent magnets generate a magnetic field, while the rotor’s electromagnets respond to electronic control signals.

An electronic control system, often aided by Hall effect sensors or encoder feedback, determines the rotor’s position relative to the stator’s magnetic field. Based on this information, the electronic control system commutates the stator windings in a specific sequence, creating a rotating magnetic field.

As the rotating magnetic field interacts with the rotor’s magnetic poles, a force is exerted, initiating rotation. The electronic control system continuously monitors the rotor’s position and adjusts the current flowing through the stator windings, enabling precise control of speed and torque.

The absence of brushes and commutators in brushless DC motors contributes to their advantages, such as higher efficiency, longer lifespan, reduced maintenance, and accurate control. Consequently, these motors find widespread use in applications ranging from electric vehicles and drones to computer cooling fans and industrial automation.

Can you change the RPM of an electric motor

The speed of an electric motor, typically measured in revolutions per minute (RPM), can be adjusted by several methods depending on the motor type and its control system. Here are a few common ways to change the RPM of an electric motor:

Voltage Control

For motors operating on a fixed frequency power supply (such as most AC motors), adjusting the input voltage can affect the motor’s speed. By increasing or decreasing the voltage supplied to the motor, the RPM can be changed. However, it’s important to note that altering the voltage may also impact the motor’s torque and efficiency.

Frequency Control

In some cases, particularly with adjustable frequency drives (AFDs) or variable frequency drives (VFDs), the motor’s speed can be controlled by adjusting the frequency of the power supply. By changing the frequency, the motor’s RPM can be increased or decreased while maintaining a constant voltage.

Gearbox or Pulley System

Another approach to altering motor speed is by using a gearbox or a pulley system. By changing the gear ratio or adjusting the size of pulleys, the rotational speed can be modified. This method is commonly used in applications where different speed outputs are required.

Electronic Speed Control

Modern motor control systems, such as those used in brushless DC motors or electronically commutated motors (ECMs), often employ electronic speed controllers (ESCs). These devices use advanced electronics to regulate the motor’s speed by precisely adjusting the input power and commutation signals.

Motor Rewinding

In certain cases, particularly with DC motors, it is possible to modify the motor’s winding configuration. By rewinding the motor with different winding parameters, such as changing the number of windings or wire gauge, the RPM can be adjusted.

It’s important to note that the specific method for changing motor RPM depends on the motor type, its control system, and the application requirements. Consulting the motor’s technical documentation or seeking guidance from a qualified professional is recommended to ensure proper and safe RPM adjustments.

Conclusion

In conclusion, the number of magnets in an electric motor significantly influences its rotational speed. Whether it’s an AC motor or a DC motor, a higher number of magnets generally leads to a slower speed, while a lower number results in a faster speed.

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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