Again EMF and electrical motors: From fundamentals to real-world purposes

November 6, 2025

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Again EMF and electrical motors: From fundamentals to real-world purposes

Allow us to start this session by revisiting a nostalgic motor management IC—the AN6651—designed for rotating velocity management of compact DC motors utilized in tape recorders, report gamers, and related units.

The determine beneath exhibits the AN6651’s block diagram and a typical software circuit, each sourced from a 1997 Panasonic datasheet. These retouched visuals provide a glimpse into the IC’s inner structure and its sensible function in analog motor management.

Determine 1 Right here is the block diagram and software circuit of the AN6651 motor management IC. Supply: Panasonic

Fortunately, for these nonetheless curious to provide it a strive, the UTC AN6651—right this moment’s counterpart to the legacy AN6651—is available from a number of sources.

Earlier than we dive deeper, here’s a fast query—why did I select to start with the AN6651? It’s just because this legacy chip elegantly controls motor velocity utilizing again electromotive pressure (EMF) suggestions—a intelligent analog approach that retains rotation steady with out counting on exterior sensors.

In analog techniques, this strategy is particularly elegant: the IC displays the voltage generated by the motor itself (its again EMF), which is proportional to hurry. By adjusting the drive present to take care of a goal EMF, the chip successfully regulates motor velocity underneath various masses and provide situations.

And sure, this submit dives into again EMF (BEMF) and electrical motors. Let’s get began.

Understanding again EMF in on a regular basis motors

A spinning motor additionally acts like a generator, as its coils transferring by way of magnetic fields induce an opposing voltage known as again EMF. This again EMF reduces the present flowing by way of the motor as soon as it’s up to the mark.

At that time, solely sufficient present flows to beat friction and do helpful work—far lower than the surge wanted to get it spinning. Really, it takes little or no time for the motor to succeed in working velocity—and for the present to drop from its excessive preliminary worth.

This self-regulating behaviour of again EMF is central to motor effectivity and safety. Because the mechanical load rises and the motor begins to sluggish, again EMF decreases, permitting extra present to circulate and generate the required torque. Beneath gentle or no-load situations, the motor hurries up, rising again EMF and limiting present draw.

This dynamic ensures that the motor adjusts its energy consumption primarily based on demand, stopping extreme present that would overheat the windings or harm elements. In essence, again EMF displays motor velocity and actively stabilizes efficiency, a precept rooted in classical DC motor concept.

It ‘s value noting that again EMF performs a important function as a pure present limiter throughout regular motor operation. When motor velocity drops—whether or not as a result of a brownout or extreme mechanical loading—the ensuing discount in again EMF permits extra present to circulate by way of the windings.

Nonetheless, if left unchecked, this surge can result in overheating and everlasting harm. Sustaining sufficient velocity and cargo situations helps protect the protecting perform of again EMF, guaranteeing protected and environment friendly motor efficiency.

Armature suggestions technique in movement management

Armature suggestions is a type of self-regulating (passive) velocity management that makes use of again EMF and has been employed for many years in audio tape transport mechanisms, luxurious toys, and different purpose-built units. It stays broadly utilized in low-cost motor management techniques the place precision sensors or encoders are impractical.

This strategy leverages the motor’s skill to behave as a generator: because the motor rotates, it produces a voltage proportional to its velocity. Like every generator, the output additionally depends upon the energy of the magnetic subject flux.

Now let’s take a fast take a look at the best way to measure again EMF utilizing a minimalist {hardware} setup.

Determine 2 The above blueprint presents a minimalist {hardware} setup for measuring the again EMF of a DC motor. Supply: Writer

Simply to elaborate, when the MOSFET is ON, present flows from the facility provide by way of the motor to floor, throughout which again EMF can’t be measured. When the MOSFET is OFF, the motor’s destructive terminal floats, permitting again EMF to be measured. A microcontroller can generate the required PWM sign to drive the MOSFET.

Likewise, its onboard analog-to-digital converter (ADC) can measure the again EMF voltage relative to floor for additional processing. Be aware that because the ADC measures voltage relative to floor, a decrease enter worth corresponds to the next again EMF.

That’s, measuring the motor’s velocity utilizing again EMF includes two alternating steps: first, run the motor for a quick interval; then, take away the drive sign. On account of inertia within the motor and mechanical system, the rotor continues to spin momentarily, and this coasting part supplies a window to pattern the again EMF voltage and estimate the motor’s rotational velocity.

The reference sign can then be routed to the PWM part, the place the drive energy is fine-tuned to take care of regular motor operation.

Nonetheless, normally, because the PWM driver outputs armature voltage as pulses, again EMF will also be measured in the course of the intervals between these pulses. Maintain notice, when the transistor switches off, a powerful inductive spike is generated, and the recirculation present flows by way of the antiparallel flyback diode. Due to this fact, a quick delay is demanded to permit the again EMF voltage to settle earlier than measurement.

Notably, a high-side P-channel MOSFET can be utilized as a motor driver transistor as a substitute of a low-side N-channel MOSFET. Likewise, discrete op-amps—relatively than devoted ICs—may also govern motor velocity, however that could be a subject for an additional day.

And whereas that is merely a blueprint, its flexibility permits it to be readily tailored for measuring again EMF—and thus the RPM—of almost any DC motor. With only a few tweaks, this low-cost strategy may be tailored to assist a variety of motor management purposes—sensorless, scalable, and simple to implement. Naturally, it takes time, technical talent, and a little bit of persistence—however you may grasp it.

Again EMF and the BLDC motor

Again EMF in BLDC motors acts like a built-in suggestions system, serving to the motor regulate its velocity, increase effectivity, and assist clean sensorless management. The form of this suggestions sign depends upon how the motor is designed, with trapezoidal and sinusoidal waveforms being the most typical.

Whereas challenges like low-speed management and waveform distortion can come up, understanding and managing again EMF successfully opens the door to unlocking the total potential of BLDC motors in the whole lot from followers to drones to electrical automobiles.

So, what are the important thing results of again EMF in BLDC motors? Allow us to take a better look:

Design affect: The form of the again EMF waveform—trapezoidal or sinusoidal—instantly impacts management technique, acoustic noise, and the way easily the motor runs. Trapezoidal designs go well with less complicated, cost-effective controllers, whereas sinusoidal profiles provide quieter, extra refined movement.
Place estimation: Again EMF is broadly utilized in sensorless management algorithms to estimate rotor place.
Pace management: Again EMF is instantly tied to rotor velocity, making it a dependable sign for regulating motor velocity with out exterior sensors.
Pace limitation: Again EMF finally balances the provision voltage, limiting additional acceleration until voltage is elevated.
Present modulation: Because the motor spins sooner, again EMF will increase, lowering the efficient voltage throughout the windings and limiting present circulate.
Torque influence: Since again EMF opposes the utilized voltage, it impacts torque manufacturing. At excessive speeds, stronger again EMF attracts much less present, leading to decrease torque.
Effectivity optimization: Aligning commutation with again EMF waveform improves efficiency and reduces losses.
Regenerative braking: In some techniques, again EMF is harnessed throughout braking to feed vitality again into the facility provide or battery, a beneficial function in electrical automobiles and battery-powered units the place effectivity issues.

Oh, I almost passed over a couple of intelligent methods that make BLDC motor management much more environment friendly. One among them is again EMF zero crossing—a sensorless approach the place the controller detects when the voltage of an unpowered part crosses zero, presenting it to time commutation occasions with out bodily sensors. To keep away from false triggers from electrical noise or switching artifacts, this sign usually wants debouncing, both by way of filtering or timing thresholds.

However this technique doesn’t work at startup, when the rotor is just not spinning quick sufficient to generate usable again EMF. That’s the place open-loop acceleration is available in: the motor is pushed with fastened timing till it reaches a velocity the place again EMF turns into detectable and closed-loop management can take over.

For smoother and extra exact efficiency, field-oriented management (FOC) goes a step additional. It transforms motor currents right into a rotating reference body, enabling correct torque and flux management. Although historically utilized in everlasting magnet synchronous motors (PMSMs), FOC is more and more utilized to sinusoidal BLDC motors for quieter, extra refined movement.

An enormous variety of ICs these days make sensorless motor management really feel like a stroll within the park. For example, beneath you’ll discover the applying schematic of the DRV10983 motor IC, which elegantly integrates energy MOSFETs for driving a three-phase sensorless BLDC motor.

Determine 3 Software schematic of the DRV10983 chip, illustrating its perform as a three-phase sensorless motor driver with built-in energy MOSFETs. Supply: Texas Devices

That wrap up issues for now. Talked an excessive amount of, however there’s a lot extra to uncover. If this didn’t quench your thirst, keep tuned—extra insights are brewing.

T. Ok. Hareendran is a self-taught electronics fanatic with a powerful ardour for revolutionary circuit design and hands-on know-how. He develops each experimental and sensible digital initiatives, documenting and sharing his work to assist fellow tinkerers and learners. Past the workbench, he dedicates time to technical writing and {hardware} evaluations to contribute meaningfully to the maker neighborhood.