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NewsSeptember 2023
What is Back EMF in a Motor?
Ian Matthews Golledge, Technical Manager, explains ...
Back electromotive force (back EMF), often abbreviated as BEMF, is a phenomenon that occurs in electric motors and generators. It is a voltage that opposes the current and voltage applied to the motor's terminals, and it is generated as a result of the motor's motion. Back EMF plays a crucial role in the operation and performance of electric motors, especially in DC (direct current) motors and brushless DC motors
Here's a more detailed explanation of back EMF
- Motor Operation When an electric motor is in operation, it generates mechanical motion by applying a force to a rotor (the rotating part of the motor). This motion causes the rotor to spin, which is the desired output of the motor
- Electromagnetic Induction As the rotor rotates, it cuts through the magnetic field created by the motor's stator (the stationary part of the motor). According to Faraday's law of electromagnetic induction, a changing magnetic field induces a voltage in a conductor. In this case, the rotor's winding (coils of wire) acts as the conductor
- Back EMF Generation The rotation of the rotor causes a change in the magnetic field strength and direction within the coils. This change induces a voltage in the coils that opposes the voltage applied to the motor's terminals. This opposing voltage is the back EMF
Determining Back EMF Value
- Motor Speed The speed of the motor has a direct impact on the magnitude of the back EMF. Faster rotation generates a higher back EMF
- Magnetic Field Strength The strength of the magnetic field generated by the stator also affects the back EMF. A stronger magnetic field will produce a higher back EMF
- Number of Turns in the Coil The number of turns in the rotor's winding affects the back EMF. More turns generate a higher back EMF
- Rotor Size and Design The physical characteristics of the rotor, such as its size, shape, and materials used, can influence the back EMF
The presence of back EMF is essential for the proper operation of motors, especially in applications where speed control is required. By opposing the applied voltage, back EMF limits the current flowing through the motor windings, preventing them from burning out due to excessive current. It also allows for precise control of motor speed since the back EMF is directly proportional to the motor's rotational speed
In summary, back EMF in a motor is a voltage generated due to the motor's motion and the interaction between the rotor and the stator's magnetic fields. Its value is determined by factors such as motor speed, magnetic field strength, coil turns, and rotor design
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