Induction vs Permanent Magnet Motors: A Breakdown of EV Motor Design

As electric vehicles continue to evolve, so too do the motors that power them. One of the most common questions we receive at Munro & Associates relates to the electric motors that drive modern EVs: What’s the difference between induction motors and permanent magnet motors? Which is better? And why are both often used together?

This article explores the engineering decisions behind these two motor types—offering teardown insights, performance analysis, and cost comparisons—to help automotive engineers, EV enthusiasts, and investors better understand the design logic powering the EV revolution.

AC Motors: The Foundation of EV Propulsion

At the heart of every electric vehicle lies an alternating current (AC) motor. While battery packs deliver direct current (DC), EVs rely on inverters to convert this into AC power suitable for driving the motor. Whether it’s an induction motor or a permanent magnet motor, both operate on this principle of converting electrical energy into mechanical torque using rotating magnetic fields.

But the similarities end there. The design of the rotor—the rotating component inside the motor—differs drastically between these two motor types.

Induction Motors: Cost-Effective and Rugged

Induction motors, sometimes referred to as “squirrel cage motors,” use a rotor made of stacked steel laminations filled with molten aluminum or copper to form conductor bars. These bars are shorted at both ends with conductive rings to form a closed-loop circuit. When the stator creates a rotating magnetic field, it induces a current in the rotor bars (thanks to Faraday’s Law), turning them into electromagnets that follow the rotating field.

Strengths of Induction Motors:

• Lower Cost: Without rare earth magnets, induction motors are significantly cheaper to produce. Using molten aluminum or copper is more economical than sourcing high-grade permanent magnets.

• Low Spin Loss: When powered off, induction motors exhibit minimal resistance to motion. This makes them ideal for scenarios where the vehicle is coasting or gliding—common in EV applications seeking to maximize efficiency.

• Durability and Simplicity: Their rugged construction and lack of magnets make them robust and resilient in demanding environments.

Limitations:

• Lower Efficiency at Low Speeds: Induction motors consume energy to generate their own magnetic fields. This energy is not recovered, reducing efficiency at lower speeds and torque loads—precisely where city driving often occurs.

• Limited High-Speed Power Output: At highway speeds, the impedance of the rotor works against the stator’s magnetic field, limiting current flow and reducing torque.

Permanent Magnet Motors: Efficient and Compact

In contrast, permanent magnet motors use rotors embedded with rare earth magnets—typically configured as interior permanent magnet synchronous machines (IPMSM). These magnets inherently produce a magnetic field, eliminating the need to induce one with electric current.

Strengths of Permanent Magnet Motors:

• High Torque Density: They produce more torque for a given size, enabling compact and lightweight packaging.

• Superior Efficiency: Because the magnetic field is always “on,” less electrical energy is wasted—especially at low speeds.

• Extended Constant Power Band: After initial torque delivery, these motors can maintain near-constant power across a wider speed range, ideal for maximizing gear ratios and top-end performance.

Limitations:

• Higher Material Costs: Rare earth magnets are expensive and subject to geopolitical supply risks.

• Permanent Drag Torque: When not in use, the ever-present magnetic field generates eddy currents in the stator, leading to unavoidable drag—also known as spin loss.

• Cannot Be Fully Turned Off: Unlike induction motors, you can’t simply “switch off” a permanent magnet motor’s magnetic field.

Performance Curves and Real-World Use Cases

When plotted on torque-speed graphs, both motor types deliver constant torque at low RPMs—a key factor in EV acceleration. However, their behaviors diverge significantly beyond this point.

• Induction motors deliver constant torque up to moderate RPMs (typically translating to ~45 mph), then shift into a constant power mode with a drop in torque. However, efficiency begins to taper off at higher speeds.

• Permanent magnet motors deliver constant torque in the same low-speed range but sustain constant power output well into higher RPMs—limited only by rotor strength. This allows for smaller, high-ratio gearboxes and optimized performance across a wider range.

Why Use Both Motors? Engineering Synergy

Many EV manufacturers, including Tesla, leverage both motor types within the same vehicle—an approach known as dual-motor or hybrid architecture. By doing so, OEMs can exploit the strengths of each motor while minimizing their weaknesses.

Highway Driving

At steady-state speeds (e.g., 70 mph), torque requirements are modest. The permanent magnet motor’s efficiency shines here—but only if it’s the sole motor in use. If both motors deliver torque simultaneously, each operates below its optimal point, reducing efficiency.

However, keeping the induction motor active and the permanent magnet motor passive is not ideal. Even when inactive, the permanent magnet rotor continues generating back EMF and eddy currents—creating drag torque.

The optimal solution? Turn off the induction motor (which has nearly no spin loss) and let the permanent magnet motor do the work. This setup aligns better with peak efficiency points on the torque-speed-efficiency curve.

City Driving

At lower speeds and higher torque demands—think stop-and-go traffic or uphill climbs—the permanent magnet motor excels. Its high low-speed efficiency and torque density deliver immediate response and battery savings.

Aggressive Acceleration and Performance Modes

During rapid acceleration, both motors may be activated to deliver peak torque and all-wheel traction. However, even in this mode, the vehicle’s control unit constantly monitors which motor should bear the brunt of the work to optimize thermal performance and electrical draw.

Real-Time Motor Management

Today’s EVs feature advanced drive controllers that make split-second decisions—up to every hundredth of a second—about which motor (or combination) to engage. These algorithms consider speed, torque demand, temperature, battery state of charge, and road conditions. The result is seamless performance, maximum range, and extended motor life.

EV Motor Comparison Takeaways

• Choose induction motors where ruggedness, cost savings, or high-speed deactivation are priorities.

• Opt for permanent magnet motors where low-speed efficiency and torque density are mission-critical—especially for city-focused EVs.

• For best of both worlds, dual-motor systems with live optimization deliver superior range, responsiveness, and drivability.

Conclusion

EV motor design isn’t a binary choice—it’s a carefully engineered balance. At Munro & Associates, we see firsthand how smart integration of induction and permanent magnet motors reflects the best in lean design, cost-conscious engineering, and performance-driven innovation. Whether you’re designing the next breakthrough EV or simply want to understand what drives your ride, understanding the strengths and weaknesses of each motor type is essential.

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