Why Do Electric Motors Rotate? [Motor magnetism explained]

Electric motors are an incredibly simple machine, once you get an understanding of how electromagnetism works and the effects it has on motors and generators alike. But the most common question I get asked is, why do electric motors rotate?

An electric motor rotate by the interaction of magnetic fields & currents. When current flows in the stator windings it creates an alternating magnetic field which induces a current in the rotor, also producing its own magnetic field. Both magnetic fields then interact, producing torque or a twisting rotation.

Even though it sounds simple, there are still some caveats to consider, such as if the motor is AC or DC, single phase or three phase, and so on. In this article, we will cover everything there is to know about why motors rotate along with which motor does what.

dc motor rotor

Electric motors work on the principle of electromagnetism, which was discovered in 1820 and has been used in everything from electric motors to traffic lights and wireless phone chargers, to name just a few.

motor maintenance pdf

According to the principle of electromagnetism, when a current-carrying conductor is placed in a magnetic field, it experiences a force perpendicular to both the current direction and the magnetic field lines. This force acts on the motors rotor, causing it to rotate.

Electric motors rotate due to the interaction of magnetic fields and electric currents. This principle is known as electromagnetism. The fundamental components of an electric motor include a stator (stationary part) and a rotor (rotating part).

When an electric current flows through the motors stator windings, it creates a magnetic field. This magnetic field induces a current in the ferromagnetic rotor, which consists of conductive bars or coils which are made up of high-grade silicon steel or aluminum.

Once the stator and rotor windings of the motor have induced current in them, both of them will then produce two opposing magnetic fields. When both of these magnetic fields interact with each other, a force will be produced causing them to push away from each other. This is known as torque, and it is the driving force needed to initiate the rotation of the motor’s rotor.

Since the stator is stationary, it means the rotor must move due to the force being exerted upon it.

Because the rotor is suspended on both ends by bearings, it is free to spin and rotate smoothly and efficiently with little to no friction, heat, or resistance being produced.

The efficiency and performance of an electric motor depend on factors such as the design of the stator and rotor, the strength of the magnetic fields, and the quality of the electrical connections.

By carefully engineering these components, electric motors can provide reliable and efficient rotational motion for various applications, ranging from industrial machinery to household appliances. To fully understand how induction motors work I would highly recommend checking out that article which delves deeper into the workings of both AC and DC motors.

How AC motors rotate

AC motors rotate due to the principle of electromagnetic induction. AC motors are widely used for their efficiency and versatility.

AC motors consist of two essential components: a stationary stator coil and a rotating rotor. The stator comprises of a series of wire windings arranged in specific patterns around an iron core. These windings are connected to an alternating current (AC) power source.

ac motor stator windings
Stator windings

When AC current flows through the stator windings, it generates a changing magnetic field. The alternating current causes the magnetic field to constantly reverse its direction. As a result, the magnetic field expands and contracts, pulsating in synchrony with the AC power supply frequency (typically 50 or 60 Hz).

To control the motor speed, the supply frequency may be adjusted by using a variable speed drive on three phase motors.

The rotating rotor, which is typically made of conductive bars or coils, is placed within this changing magnetic field. According to Faraday’s law of electromagnetic induction, when a conductor is exposed to a changing magnetic field, it induces an electric current within the conductor.

In the case of an AC motor, the changing magnetic field created by the AC power supply induces an alternating current in the rotor. This induced current generates its own magnetic field, which interacts with the stator’s magnetic field.

open view of motor
Rotor & Stator

The interaction between the rotating magnetic field produced by the rotor and the pulsating magnetic field generated by the stator creates a torque or twisting force. This torque causes the rotor to rotate in the same direction as the rotating magnetic field.

To maintain continuous rotation, the AC power supply continually alternates the direction of the current in the stator windings, which in turn sustains the rotating magnetic field. The rotor follows this rotating magnetic field and continues to rotate accordingly.

In the case of three-phase induction motors rotation is achieved by placing the three stator windings at 120o apart around the stator frame. Once an alternating current is applied to the windings, the rotating magnetic field will synchronize in a way that the rotor will follow the synchronized magnetic field around the stator frame.

motor stator
Stator winding

Single-phase electrical motors differ slightly due to only receiving energy from one single phase. The problem here is that no rotation can be achieved with a single phase, meaning an artificial phase must be created which is done by adding an auxiliary winding to the motor.

A capacitor is the most common method of starting a single-phase motor, depending on the type of motor, but once the motor is running, the capacitor is switched out and continuous rotation is achieved by momentum and one single-phase winding.

For capacitor-start single-phase motors, the initial start rotation cannot be achieved if you have a bad capacitor unless you manually spin the rotor by hand to give it the extra boost it needs to overcome the inertia of the rotor.

The precise design and construction of AC motors, including factors such as the number of stator poles and the arrangement of windings, are carefully engineered to optimize performance, efficiency, and reliability.

To find out exactly how single-phase motors work check out this article, I think you will find it interesting.

How a DC motor rotates

DC motors consist of two primary components: a stationary stator and a rotating armature or rotor. The stator typically includes permanent magnets or electromagnetic windings that create a static magnetic field. The rotor comprises of a commutator and conductive coils.

Unlike an AC motor, in DC motors the electric current typically flows in the rotor rather than the stator windings.

When an electric current flows through the coils of the rotor, it generates a magnetic field around the conductive wires. This magnetic field interacts with the static magnetic field produced by the permanent magnets of the stator.

According to the left-hand rule of electromagnetism, when a current-carrying conductor is placed in a magnetic field, a force is exerted on the conductor perpendicular to both the current direction and the magnetic field lines. In the case of a DC motor, this force creates a torque on the rotor.

Because with DC power there is no rotating or alternating magnetic field present, it means that the direction of the supply must constantly be changed in order to keep the rotor spinning. This is done by means of commutator.

Function of a commutator

The commutator, which is a segmented ring, ensures that the current flow in the rotor coils is constantly reversing as the rotor rotates. As a result, the magnetic force and torque on the rotor remain in the same direction, enabling continuous rotation.

dc commutator
Commutator

The direction of rotation is determined by the application of the right-hand rule. The interaction between the magnetic fields of the stator and the rotor causes the rotor to align itself with the stator’s magnetic field and rotate in the same direction.

By controlling the magnitude and direction of the current flow through the rotor coils, the speed and torque of the DC motor can be regulated, allowing for precise control in various applications.

The efficiency and performance of DC motors depend on factors such as the design of the stator and rotor, the strength of the magnetic fields, and the quality of the electrical connections. These factors are carefully engineered to ensure reliable and efficient rotation in DC motors

How to reverse the rotation of a single phase AC electric motor

To change the rotational direction of a single-phase induction motor isn’t always as simple as a three-phase motor but there is a couple of options you have to choose from, with some being more simple than the others to carry out.

Reversing the Start and Run windings of the motor

To reverse the rotation of a single-phase motor you can swap the connections of the start and run windings. To do this is simple and you will only need a 8mm nut spinner. First, locate the motor’s terminal box and identify the start and run winding leads, 99% of the time the start winding is labeled Z1 and Z2, and the run winding is U1 and U2. Interchanging these leads will reverse the magnetic field and subsequently change the rotation direction of the motor.

Using a reversing switch

Install a reversing switch in the circuit. This switch allows you to change the flow of current in the start winding, effectively reversing the rotation direction. When the switch is in one position, the motor rotates in one direction, and when flipped to the other position, the rotation direction is reversed.

Modifying capacitor connections in the motor terminal box

If your single-phase motor uses a capacitor, similar to reversing the windings, you can also alter the rotation direction by changing the capacitor connections. Swapping the connections of the start and run capacitors or reversing the connection of the start capacitor can achieve the desired rotation change.

It’s crucial to note that when modifying the motor’s connections that proper safety precautions should be followed, and the power supply must be disconnected before performing any adjustments.

How to reverse the rotation of a three phase induction motor

Changing the rotational direction of a three-phase induction motor is simple, you just need to change the direction in which the magnetic field travels around the stator. The way you achieve that is by:

  1. Identify the Motor Connections: Begin by locating the motor’s terminal box or connection diagram. Identify the three leads or terminals labeled as L1, L2, and L3, representing the three phases of the power supply which will be color coded, L1 Brown, L2 Black, L3 Grey in Europe or Black, Red and Blue in the US.
  2. Swap any Two Phase Leads: To reverse the rotation, interchange any two of the three-phase leads. For example, if the motor is initially connected with L1, L2, and L3, you can swap L1 and L3 to change the rotation direction.
  3. Verify Proper Connection: Ensure that the motor’s leads are securely tightened and properly connected after swapping. Double-check that the connections are correct according to the motor’s documentation or wiring diagram.
  4. Test the Motor: Once the connections are modified, restore power to the motor and test its operation. Observe the rotation direction to confirm if it has changed according to your desired direction.
three phase motor terminal block
Three phase motor terminal block

It’s important to note that reversing the rotation of a three-phase motor requires a careful understanding of electrical connections and safety precautions. If you are uncertain or unfamiliar with electrical work, it is recommended to seek assistance from a qualified electrician or motor technician to ensure proper and safe modification of the motor’s rotational direction.

Conclusion

In conclusion, the rotation of electric motors is a fascinating result of the interplay between magnetic fields and electric currents. Whether it’s an AC or DC motor, the underlying principles remain consistent. Both types work off the principle of electromagnetism only in slightly different ways. For more motor information covering motor speeds I think you should check out this article.

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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