Back electromotive force (back EMF) is a critical factor that influences efficiency, control, and overall performance in electric motors. Whether you're designing motors for electric vehicles (EVs), industrial automation, or renewable energy applications, understanding and managing back EMF is essential.
Advanced material solutions are helping engineers push the boundaries of motor performance. One such innovation is soft magnetic composites (SMCs)—a revolutionary material that offers a way to reduce back EMF, optimize efficiency, and enhance design flexibility.
Let’s take a closer look at how traditional laminated materials can be enhanced by SMCs and why they are emerging as the ideal solution for next-generation electric motors.
Back EMF is the voltage generated that opposes the applied voltage when a motor is in motion. It can be explained by Faraday’s Law of electromagnetic induction, which states that a changing magnetic field induces a voltage in a conductor.
Back EMF can be expressed by the following equation:
Eb = ke*Φ*ω
Eb = Generated back EMF
ke = Back EMF constant for the particular motor configuration
Φ = Magnetic flux per stator pole
ω = Angular velocity of the motor
As a motor speeds up, back EMF increases and opposes the applied voltage, reducing the available current for torque production. This limits performance, requiring careful design considerations to manage its effects.
Back EMF affects multiple aspects of motor operation, including the following:
·Speed Control—High back EMF can limit maximum speed, requiring higher input voltages to maintain performance.
·Efficiency Losses—Uncontrolled back EMF can lead to power dissipation and heat buildup, reducing efficiency.
·Torque Limitations— In applications requiring high torque at low speeds, excessive back EMF can negatively impact startup performance.
·Voltage Requirements— Motors with higher back EMF need higher drive voltages, increasing system complexity and cost.
Managing back EMF is particularly crucial in high speed motors, where excessive EMF can cause instability and voltage spikes, potentially damaging motor components.
One way to control back EMF is via field weakening. Field weakening is a technique in which the magnetic flux in the air gap between the rotor and stator is reduced, thus enabling greater motor speeds and more constant torque over a larger speed range. There are a couple of different methods to create this reduced back EMF:
· Supplying a reverse current in the stator poles, effectively reducing the induced magnetic flux.
· Creating a larger air gap between the rotor and stator, which also limits the magnetic flux in the air gap.
Both methods effectively reduce the back EMF, enabling higher rotational speeds and a flatter torque curve vs. RPM. The problem with a fixed larger air gap, however, is that it lowers the output torque at low motor speeds, thus reducing efficiency. The question is how to have the best of both high starting torque and high speed without field weakening. This can be accomplished by a variable air gap motor – a small air gap at low speeds for high starting torque, and a larger air gap for higher rotational speeds.
This concept is very similar to the variable compression ratio internal combustion engine.
Most electric motors today use laminated steel cores to direct magnetic flux and minimize eddy current losses. While laminations have served the industry well for decades, they introduce several limitations when dealing with back EMF:
Even with thin laminations, eddy currents still flow within each sheet, generating heat and reducing efficiency—especially at high operating frequencies. It should also be noted that the way in which the laminations are held together has a significant impact on the eddy currents created. Specifically, welding or nesting of laminations results in inter-lamination shorting, thus increasing the eddy currents.
In traditional radial flux motors, laminated sheets can cause magnetostriction, leading to mechanical vibrations and audible noise. This is particularly problematic in consumer and industrial applications where quiet operation is critical.
Laminated materials are stacked in 2D layers, making it difficult to create complex 3D magnetic flux paths. This limits the ability to optimize motor topologies that could otherwise help reduce back EMF naturally.
Laminations do not dissipate heat efficiently, which can result in hotspots and reduce motor lifespan—especially in high power applications like EVs and industrial drives.
Given these constraints, engineers are now looking beyond laminations for alternative materials that offer greater design freedom and lower back EMF.
SMCs are powder-based magnetic materials with unique isotropic properties, meaning they allow for 3D magnetic flux control—something laminated materials cannot achieve. Recently, Park, Moon, Kang and Su published a paper on enhancing the efficiency of axial flux motors. The authors compared the maximum torque output of an axial flux motor constructed with steel laminations, SMC, and two hybrid SMC / laminations with differing tooth configurations (square and trapezoidal).
Figures 1 and 2 below presents their findings.
Figure 1
We can make the following observations of this data:
· Comparing the output of this motor made from 100% SMC material to 100% laminations, we see a 10% drop in output for the SMC design. The 100% SMC motor also shows greater torque ripple.
· Optimizing the hybrid SMC / lamination hybrid using a “T” style lamination core embedded in an SMC outer shell showed identical performance to the all-steel lamination design but with the added benefit of reduced back EMF.
· Figure 2 below shows the increase in core losses as the SMC material is reduced.
Figure 2
All tooth geometries in Figure 1 and Figure 2 are of the trapezoidal design, which provided a 10% improvement in output torque over the square tooth, which showed magnetic fringing at the square corners. It should also be noted that the material used in the above modeling had a maximum permeability of approximately 500. Higher permeability materials that can enhance the torque output of both the 100% SMC and the hybrid SMC/ lamination design are commercially available.
The question is, how do SMCs impact the back EMF of the motor? Unlike laminations, which are essentially stacks of 2D plates, SMCs are compacted via powder metallurgy technology into a 3D shape. As such, the tooth detail, profile, and overall geometry can be fully optimized, leading to the following benefits:
· Flux Distribution—The shape of the teeth affects the distribution of the magnetic flux in the motor. An optimized tooth shape can ensure a more uniform flux distribution, which can lead to a smoother back EMF waveform.
· Minimized Air Gap Variation—The tooth shape influences the air gap between the stator and rotor. Creating an arced tooth shape via powder metallurgy can minimize variations in the air gap and the subsequent induced magnetic field. A consistent uniform air gap helps in establishing a stable back EMF.
· Harmonic Content—The design of the teeth affects the harmonic content of the back EMF. Unique tooth shapes can reduce the higher-order harmonics, leading to a cleaner, more sinusoidal back EMF.
· Induced Voltage—The tooth shape impacts the rate of change of the magnetic flux linkage with the stator windings. This rate of change is directly related to the induced voltage (back EMF) in the windings. An optimized tooth shape will lower back EMF.
· Optimizing Motor Topology to Reduce Back EMF
Motor topology plays a crucial role in managing back EMF. Some key considerations include the following:
· Axial Flux Motors—These can be designed without a yoke, reducing losses and offering higher power density. Modifying the shape of the stator tooth has a significant impact on the torque generated. For example, a trapezoidal tooth design can create a 10% increase in output torque for the same motor size. This result is inherent due to the reduced back EMF generated from the trapezoidal design vs. the square design.
· Advanced Designs—Combining radial and axial principles allows for optimized back EMF control, especially when paired with new magnetic materials like SMCs.
What if the material itself could help solve the back EMF challenge? That’s where SMCs come in. As industries shift toward high efficiency electric motors, SMCs are proving to be a game-changing material for engineers who are looking to overcome design challenges and push motor performance further.
Back EMF is a fundamental challenge in motor design, influencing speed, efficiency, and control. While traditional laminated materials have been the standard, they come with limitations that restrict motor performance. With SMCs, engineers now have a better alternative: a material that enables lower core losses, quieter operation, enhanced flexibility, and superior heat dissipation.
At Horizon Technology, we are at the forefront of SMC-based motor innovation. Whether you're designing EV drivetrains, industrial motors, or high efficiency generators, our advanced material solutions can help you achieve higher performance with fewer limitations.
🔍 Want to learn more? Explore how SMCs can revolutionize your motor designs!