Stall Current in EV Motor Controllers: Why Efficiency Drops at Low Speed

Understanding Stall Current and PWM Control in EV Motor Controllers

You’ve probably experienced it: you’re in a fully loaded electric vehicle, trying to pull away on an uphill slope, and the response feels weaker than you expected. Or you’re in a performance EV, and the acceleration from a standstill isn’t as sharp as it should be. It’s not a failure, but it’s not the vehicle’s full potential either.

This limitation, known as stall current, affects how effectively the motor controller and inverter can deliver peak power at low speeds and high torque. In this article, we’ll look at how motor controller design, PWM control strategies, and stall current work together to influence efficiency. And what you can do to optimize performance in these challenging scenarios.

 

What Causes Efficiency Loss in EV Powertrains at Low Speed and High Torque?

At low speeds, electrical signals take longer to complete their cycles. As a result, the current remains at peak levels for longer periods. This creates more heat in the system at that precise moment in time, especially in the motor controller. The more time the motor controller operates at peak current, the more thermal stress it experiences. For a broader, DOE-backed overview of how current, losses, and heat affect motor-drive efficiency, see the U.S. Department of Energy’s motor and drive system performance sourcebook.

This picture is better explained in the following graph. These aim to depict the same time window for 2 operating points: on the top with higher fundamental frequency, and the bottom with low fundamental frequency. It can be seen how in the same time window for one phase the effective current is higher on a lower frequency.

 

 

While the electric motor is also affected, it generally handles heat better due to its higher thermal inertia. The motor controller, however, becomes the main limiting factor in these conditions.

Uneven heat distribution adds another challenge: since the heat isn’t spread uniformly across the system, temperature sensors (placed in fixed positions) can show fluctuating readings that don’t reflect the full picture.

This uneven distribution of heat leads to oscillations in temperature  measurements. An example is shown in the picture below.

Going into detail, this oscillation in temperature measurement is closely tied to the way current flows in a three-phase system. In such systems, the sum of the three phases must be zero in the absence of stray currents. This means that when one phase reaches its maximum, the other two must each carry half of the opposing value. This uneven distribution creates localized heating which moves as slow as the fundamental frequency dictates, making accurate thermal model-based control and protection strategies essential to ensure long-term performance and prevent premature wear.

 

Why Is Stall Current a Limitation in Real-World EV Applications?

Because these conditions aren’t rare, they happen in everyday driving. Whether it’s starting on a hill, towing a load, or accelerating from a stoplight, EVs frequently operate in the high-torque, low-speed range where efficiency drops and thermal stress increases. If not addressed properly, this can lead to reduced performance, accelerated component wear, and limited responsiveness just when it’s needed most.

Managing stall current and heat distribution isn’t just a technical concern, it’s key to delivering a reliable, smooth, and consistent driving experience in real-life situations.

 

How Can Inverter and PWM Control Strategies Improve Efficiency in These Conditions?

To address this efficiency loss and unlock greater inverter capability, VarioSwitch integrates a set of advanced functionalities designed specifically for demanding operating zones:

• Discontinuous PWM (DPWM) is activated precisely in the high-torque, low-speed range, often the most thermally stressful. By reducing the switching periods, the inverter spends at peak current levels, DPWM lowers switching losses and limits heat buildup, helping the system remain stable and efficient during these critical moments.
• Dynamic PWM mode switching allows the inverter to adapt in real time. Depending on speed, torque demand, and thermal conditions, the system shifts seamlessly between modulation strategies. This flexibility ensures that the inverter is always operating as efficiently as possible—delivering performance when needed, and conserving energy when possible

Take a look at the following graphs to see how these control strategies come into play across the mentioned operating zones (top picture with DPWM, bottom picture without), improving efficiency, extending system lifespan, and enhancing vehicle performance when it counts.

 

 

 

We could go on, exploring how different PWM strategies apply across the speed-torque map, but that’s a story for another article. If you have any questions or need further assistance, our technical team will be happy to help. Request more info.

In the end, it’s all about giving you a smoother, safer, and more efficient drive, without overworking the parts that keep your EV moving.