Why Can't Electric Motors Be Easily Scaled?

Electric motors power many of the devices and machines we use every day, from small motors in our phones that cause them to vibrate to large industrial motors used in factories.

At first glance, it may seem that electric motors are easily scalable - you can take a small electric motor and make it bigger to generate more power, or take a large electric motor and make it smaller for use in smaller devices. However, in reality, scalability is not so straightforward with electric motors due to some key physics and engineering challenges.

In this article, we’ll explore the factors that make scaling electric motors difficult, including magnetic strength, rotor size, cooling, efficiency, torque, speed control, and more. While workarounds exist in some cases, there are limits to how much electric motors can be scaled while maintaining performance. Understanding these scaling challenges provides insight into electric motor design tradeoffs and why motors tend to be custom-engineered for specific power, size, and performance needs.

The Strength of Magnetism

One of the most basic factors affecting the scalability of electric motors is the strength of magnetism. Electric motors work by using magnetic fields to generate rotational force on a rotor. The stronger the magnetic field, the more torque the motor can produce to drive a load. Smaller electric motors use tiny permanent magnets or electromagnets to create a magnetic field strong enough to drive the motor at a small scale.

Simply making these magnets bigger may increase the size of the magnetic field, but the magnetic field strength does not increase proportionally.

Factors like the saturation of the core material in electromagnets and the intrinsic magnetic strength of permanent magnet materials limit magnetic field strength. This means if you scale up a small motor by doubling its size, the magnetic field strength won’t double, so the torque doesn’t increase linearly.

There are workarounds like using higher strength magnets in larger motors, adding more magnets or coils, or increasing current. But this adds complexity and has side effects we’ll discuss later. The intrinsic non-linear relationship between size and magnetic field strength remains a key barrier to directly scaling electric motors.

Forces on the Rotor

The rotor in an electric motor experiences various mechanical forces that scale with size. Small rotors can spin freely on small bearings. But as rotor size increases, the forces acting on the rotor like inertia, friction, and centrifugal forces grow rapidly.

For example, the inertia of a rotor scales with its mass and the square of its size. So doubling the size of a rotor increases its inertia fourfold.

Larger bearings and mechanical components are needed to handle the higher forces. This sidesteps simple miniaturization or scaling up that might work for non-moving electrical components.

Again, workarounds exist like using higher-strength materials to allow smaller bearings. But the physical reality remains that larger rotors experience much greater mechanical forces, necessitating specialized engineering. Simply taking a small motor and making the rotor bigger without considering these higher forces would lead to failure.

Cooling and Heat Dissipation

Another key difference between small and large electric motors is heat generation and cooling. Smaller motors generate less waste heat in their windings and magnets. This allows passive air cooling without the need for vents, fans or heat sinks. But in larger motors, waste heat increases rapidly.

Thermal output does not scale linearly with size but is closer to the cube of motor volume due to factors like increased current and eddy currents. Double the linear size of a motor and its heat output could increase eightfold.

Without active cooling methods, larger motors can easily overheat leading to failure or permanent damage. Large motors thus require engineering for active cooling like air or liquid cooling systems. Again, simply taking a smaller motor and making it bigger would result in overheating issues without considering thermal management.

Torque and Speed Considerations

Electric motors are engineered to operate within a suitable torque-speed range for their applications. Small rapid-rotation motors operate at high speed and low torque. Large industrial motors run at lower speeds but with higher torque. Mid-sized motors offer a compromise.

If an electric motor were scaled up in size without changing the basic magnetic and coil design, it would produce higher torque but at a lower speed. The large high-torque low-speed output may be unsuitable. Conversely, simply shrinking a motor could raise its speed substantially while cutting torque too much for the application.

Re-engineering is required to rebalance torque and speed when scaling electric motors up or down significantly. The economics of applications that need a different torque-speed profile also constrain scalability.

It may be more cost-effective to design custom motors than modifying an existing design for different outputs.

Application-Specific Designs

Given all these considerations around scaling electric motors, it becomes clear that direct scaling is often impractical. Motors engineered for specific applications, power levels, and sizes tend to work substantially better than trying to scale up or down a motor arbitrarily.

The engineering design tradeoffs made around torque needs, speed range, efficiency targets, size constraints, and performance goals are unique to each application. Optimal motor designs are rarely one-size-fits-all. A motor can’t simply be made bigger or smaller and expect it to work well across a wide size range.

There are cases where modular platforms using shared components are possible, like scaling a basic frame size up or down incrementally. But even then, factors like cooling, controls, and materials need to be re-engineered for each size variant. Overall, electric motors remain highly application-specific with limited flexibility for scaling.

Motor Design Made Easy with EMWorks Simulation Software

Designers can use electromagnetic simulation software by EMWorks | Electromagnetic Simulation Software to design and optimize the motor's design. This software allows designers to model the behaviour of the motor under different conditions and to experiment with different designs and materials. By using simulation software, designers can identify potential issues and optimize the motor's performance before it is built, reducing the risk of costly mistakes.

Conclusions

While electric motors may seem simple and scalable at first glance, many interrelated factors make scaling motors up or down in size far from straightforward. The physics of magnetics, thermal management, efficiency, torque generation, inertia, control responses, and material properties all change in complex ways with motor size scaling.

Thank you for taking the time to explore the intricacies of electric motor scalability with us.

- Sumeet Singh, PhD