Back EMF and electric motors: From fundamentals to real-world applications

Let us begin this session by revisiting a nostalgic motor control IC—the AN6651—designed for rotating speed control of compact DC motors used in t…

Let us begin this session by revisiting a nostalgic motor control IC—the AN6651—designed for rotating speed control of compact DC motors used in tape recorders, record players, and similar devices.

The figure below shows the AN6651’s block diagram and a typical application circuit, both sourced from a 1997 Panasonic datasheet. These retouched visuals offer a glimpse into the IC’s internal architecture and its practical role in analog motor control.

Figure 1 Diagrams outline the block diagram and application circuit of the AN6651. Source: Panasonic

Luckily, for those still curious to give it a try, the UTC AN6651—today’s counterpart to the legacy AN6651—is readily available from several sources.

Before we dive deeper, here is a quick question—why did I choose to begin with the AN6651? It’s simply because this legacy chip elegantly controls motor speed using back electromotive force (EMF) feedback—a clever analog technique that keeps rotation stable without relying on external sensors.

In analog systems, this approach is especially elegant: the IC monitors the voltage generated by the motor itself (its back EMF), which is proportional to speed. By adjusting the drive current to maintain a target EMF, the chip effectively regulates motor speed under varying loads and supply conditions.

And yes, this post dives into back EMF (BEMF) and electric motors. Let’s get started.

Understanding back EMF in everyday motors

A spinning motor also acts like a generator, as its coils moving through magnetic fields induce an opposing voltage called back EMF. This back EMF reduces the current flowing through the motor once it’s up to speed.

At that point, only enough current flows to overcome friction and do useful work—far less than the surge needed to get it spinning. Actually, it takes very little time for the motor to reach operating speed—and for the current to drop from its high initial value.

This self-regulating behaviour of back EMF is central to motor efficiency and protection. As the mechanical load rises and the motor begins to slow, back EMF decreases, allowing more current to flow and generate the required torque. Under light or no-load conditions, the motor speeds up, increasing back EMF and limiting current draw.

This dynamic ensures that the motor adjusts its power consumption based on demand, preventing excessive current that could overheat the windings or damage components. In essence, back EMF reflects motor speed and actively stabilizes performance, a principle rooted in classical DC motor theory.

It ‘s worth noting that back EMF plays a critical role as a natural current limiter during normal motor operation. When motor speed drops—whether due to a brownout or excessive mechanical loading—the resulting reduction in back EMF allows more current to flow through the windings.

However, if left unchecked, this surge can lead to overheating and permanent damage. Maintaining adequate speed and load conditions helps preserve the protective function of back EMF, ensuring safe and efficient motor performance.

Armature feedback method in motion control

Armature feedback is a form of self-regulating (passive) speed control that uses back EMF and has been employed for decades in audio tape transport mechanisms, luxury toys, and other purpose-built devices. It remains widely used in low-cost motor control systems where precision sensors or encoders are impractical.

This approach leverages the motor’s ability to act as a generator: as the motor rotates, it produces a voltage proportional to its speed. Like any generator, the output also depends on the strength of the magnetic field flux.

Now let’s take a quick look at how to measure back EMF using a minimalist hardware setup.

Figure 2 The above blueprint presents a minimalist hardware setup for measuring the back EMF of a DC motor. Source: Author

Just to elaborate, when the MOSFET is ON, current flows from the power supply through the motor to ground, during which back EMF cannot be measured. When the MOSFET is OFF, the motor’s negative terminal floats, allowing back EMF to be measured. A microcontroller can generate the required PWM signal to drive the MOSFET.

Likewise, its onboard analog-to-digital converter (ADC) can measure the back EMF voltage relative to ground for further processing. Note that since the ADC measures voltage relative to ground, a lower input value corresponds to a higher back EMF.

That is, measuring the motor’s speed using back EMF involves two alternating steps: first, run the motor for a brief period; then, remove the drive signal. Due to inertia in the motor and mechanical system, the rotor continues to spin momentarily, and this coasting phase provides a window to sample the back EMF voltage and estimate the motor’s rotational speed.

The reference signal can then be routed to the PWM section, where the drive power is fine-tuned to maintain steady motor operation.

Still, in most cases, since the PWM driver outputs armature voltage as pulses, back EMF can also be measured during the intervals between those pulses. Keep note, when the transistor switches off, a strong inductive spike is generated, and the recirculation current flows through the antiparallel flyback diode. Therefore, a brief delay is demanded to allow the back EMF voltage to settle before measurement.

Notably, a high-side P-channel MOSFET can be used as a motor driver transistor instead of a low-side N-channel MOSFET. Likewise, discrete op-amps—rather than dedicated ICs—can also govern motor speed, but that is a topic for another day.

And while this is merely a blueprint, its flexibility allows it to be readily adapted for measuring back EMF—and thus the RPM—of nearly any DC motor. With just a few tweaks, this low-cost approach can be adapted to support a wide range of motor control applications—sensorless, scalable, and easy to implement. Naturally, it takes time, technical skill, and a bit of patience—but you can master it.

Back EMF and the BLDC motor

Back EMF in BLDC motors acts like a built-in feedback system, helping the motor regulate its speed, boost efficiency, and support smooth sensorless control. The shape of this feedback signal depends on how the motor is designed, with trapezoidal and sinusoidal waveforms being the most common.

While challenges like low-speed control and waveform distortion can arise, understanding and managing back EMF effectively opens the door to unlocking the full potential of BLDC motors in everything from fans to drones to electric vehicles.

So, what are the key effects of back EMF in BLDC motors? Let us take a closer look:

Design influence: The shape of the back EMF waveform—trapezoidal or sinusoidal—directly affects control strategy, acoustic noise, and how smoothly the motor runs. Trapezoidal designs suit simpler, cost-effective controllers, while sinusoidal profiles offer quieter, more refined motion.
Position estimation: Back EMF is widely used in sensorless control algorithms to estimate rotor position.
Speed control: Back EMF is directly tied to rotor speed, making it a reliable signal for regulating motor speed without external sensors.
Speed limitation: Back EMF eventually balances the supply voltage, limiting further acceleration unless voltage is increased.
Current modulation: As the motor spins faster, back EMF increases, reducing the effective voltage across the windings and limiting current flow.
Torque impact: Since back EMF opposes the applied voltage, it affects torque production. At high speeds, stronger back EMF draws less current, resulting in lower torque.
Efficiency optimization: Aligning commutation with back EMF waveform improves performance and reduces losses.
Regenerative braking: In some systems, back EMF is harnessed during braking to feed energy back into the power supply or battery, a valuable feature in electric vehicles and battery-powered devices where efficiency matters.

Oh, I nearly skipped over a few clever tricks that make BLDC motor control even more efficient. One of them is back EMF zero crossing—a sensorless technique where the controller detects when the voltage of an unpowered phase crosses zero, presenting it to time commutation events without physical sensors. To avoid false triggers from electrical noise or switching artifacts, this signal often needs debouncing, either through filtering or timing thresholds.

But this method does not work at startup, when the rotor is not spinning fast enough to generate usable back EMF. That is where open-loop acceleration comes in: the motor is driven with fixed timing until it reaches a speed where back EMF becomes detectable and closed-loop control can take over.

For smoother and more precise performance, field-oriented control (FOC) goes a step further. It transforms motor currents into a rotating reference frame, enabling accurate torque and flux control. Though traditionally used in permanent magnet synchronous motors (PMSMs), FOC is increasingly applied to sinusoidal BLDC motors for quieter, more refined motion.

A vast number of ICs nowadays make sensorless motor control feel like a walk in the park. As an example, below you will find the application schematic of the DRV10983 motor IC, which elegantly integrates power MOSFETs for driving a three-phase sensorless BLDC motor.

Figure 3 Application schematic of the DRV10983 chip, illustrating its function as a three-phase sensorless motor driver with integrated power MOSFETs. Source: Texas Instruments

That wrap up things for now. Talked too much, but there is plenty more to uncover. If this did not quench your thirst, stay tuned—more insights are brewing.

T. K. Hareendran is a self-taught electronics enthusiast with a strong passion for innovative circuit design and hands-on technology. He develops both experimental and practical electronic projects, documenting and sharing his work to support fellow tinkerers and learners. Beyond the workbench, he dedicates time to technical writing and hardware evaluations to contribute meaningfully to the maker community.

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