Asynchronous Motors

Asynchronous (induction) motor control presents a different set of engineering challenges to permanent magnet motor control, and they become significantly harder again at high frequency. We design and manufacture asynchronous motor controllers capable of running well beyond 100,000 rpm, including combined controllers supporting both sensorless BLDC and sensorless asynchronous operation from a single platform — a capability proven in production for a European client's spindle application.

Why Asynchronous Control Is Different

Unlike a permanent magnet motor, an induction motor has no fixed rotor flux to reference — rotor flux has to be induced electromagnetically, and the rotor always rotates slightly slower than the applied stator field (slip). Getting accurate, efficient control at high frequency means correctly modelling and compensating for this behaviour in real time, rather than applying control techniques designed for permanent magnet motors and hoping they transfer across.

Control Strategy

• V/f (Volts-per-Hertz) control — a simpler, open-loop approach suitable for applications where dynamic performance requirements are modest and cost/complexity need to stay low
• Slip compensation — accurately estimating and compensating for slip across the operating range, which becomes increasingly critical to control accuracy as frequency and speed increase

High-Frequency Operation

Running an induction motor at very high electrical and mechanical frequency — the kind of speed range relevant to CNC spindles and similar high-speed applications — introduces problems that don't show up at lower speeds:

• Switching frequency and PWM resolution requirements increase substantially, since the control loop has far less time per electrical cycle to sense, calculate, and act
• Iron and switching losses both scale with frequency, so thermal management and drive-stage efficiency become first-order design constraints rather than secondary considerations
• Current and flux estimation accuracy has to hold up at high electrical frequency, where sensorless estimation techniques that work comfortably at low speed can become significantly less reliable
• Mechanical considerations — bearing life, rotor balance, and vibration — become tightly coupled with the electrical control strategy at these speeds, since electrically-induced torque ripple or current harmonics can directly excite mechanical resonances

Sensorless Operation

For high-frequency asynchronous applications, sensorless control is often preferred over encoder feedback — fewer components, no encoder to fail or contaminate at high speed, and no additional mechanical coupling to introduce vibration or reliability risk. Our sensorless asynchronous control estimates rotor flux and speed from the motor's own electrical characteristics, tuned specifically to remain accurate at both the low end of the range (where induction motor sensorless estimation is traditionally weakest) and at sustained high frequency.

Combined BLDC/Asynchronous Platforms

Where an application or product family needs to support both permanent magnet and induction motor variants — for example, where a client offers multiple motor options across a product range — we've developed combined controller platforms capable of driving both sensorless BLDC and sensorless asynchronous motors, reducing the number of distinct controller designs a client needs to support, stock, and maintain across their product line.

Voltage and Power Range

High-frequency asynchronous controllers have been delivered across a 36–48V range for spindle-type applications, with power and current ratings scaled to the specific motor and speed range required.

Manufacturing and Validation

Every high-frequency asynchronous controller is validated on our own test bench, with dynamometer loading and precision power analysis used to characterise real efficiency, thermal behaviour, and speed range under load — critical for high-frequency applications where errors in estimation or control tuning tend to show up as instability or thermal problems only once the motor is under real load, not on the bench at idle. Manufacturing is carried out under our ISO 9001-certified quality process, with conformal coating and potting available where the application demands it. 

High-Frequency BLDC & PMSM Motor Controller Capability

Alongside our high-frequency asynchronous capability, we design and manufacture BLDC and PMSM controllers for high-speed applications — including sensorless designs proven in production at speeds well beyond 60000 rpm. High-frequency permanent magnet motor control brings a different set of challenges to standard-speed BLDC/PMSM design, and the controller architecture, current sensing, and commutation strategy all need to be engineered specifically for it rather than simply run faster.

Why High Frequency Changes the Problem

At low and moderate speeds, BLDC/PMSM control has comparatively generous timing margins — plenty of electrical cycle time to sense current, estimate rotor position, and update the control loop. As electrical frequency rises, that margin shrinks fast. A motor running at very high mechanical RPM, especially with more than one pole pair, can have an electrical frequency many times its mechanical speed — meaning the control loop has a fraction of the time per cycle that it would at lower speeds to do the same job, accurately, every time.

Commutation Strategy at High Speed

• Trapezoidal commutation remains the simplest strategy at high frequency due to their robustness and simplicity.
• Space Vector Modulation (SVM) and sinusoidal options are being developed at present but both have significantly higher MCU load and processing requirements so price does tend to be affected. 
• Control-loop tuning has to account for the reduced phase margin and stability headroom that comes with running a digital control loop close to its execution-rate limits, rather than assuming the same tuning that works at lower speed will simply scale up

Sensorless Rotor Position Estimation at High Speed

Sensorless control is the standard approach for high-frequency BLDC/PMSM applications — encoders and Hall sensors both become a liability at very high RPM, whether through mechanical wear, bandwidth limitations, or simply the added failure point of a spinning mechanical sensor. 

Back-EMF-based sensorless estimation is generally well-suited to high speed, since back-EMF amplitude increases with speed, making rotor position easier to detect accurately the faster the motor runs — the opposite problem to low-speed sensorless control, where signal is weak. The engineering challenge at high frequency shifts instead to sampling and processing that signal fast and accurately enough, without being compromised by switching noise, before the next commutation event is due.

Current Sensing at High Frequency

Three-shunt, low-side current sensing — our standard approach across current-generation platforms — has to be paired with sufficiently fast ADC sampling and signal conditioning to resolve accurate current values within a proportionally shorter electrical cycle. Sensing bandwidth and noise immunity become first-order design considerations at high frequency in a way they simply aren't at lower speeds.

Mechanical and Thermal Considerations

High-speed operation couples electrical and mechanical design more tightly than standard-speed applications:

• Switching and iron losses scale with electrical frequency, making drive-stage efficiency and thermal management critical rather than secondary — particularly relevant where the controller is physically integrated close to or inside the motor
• Torque ripple and current harmonics, if not well controlled, can excite mechanical resonances at these speeds far more readily than at lower RPM, so control-loop quality has a direct mechanical consequence, not just an electrical one
• Bearing and rotor dynamics need to be considered alongside the controller design, since electrical control quality and mechanical reliability become interdependent at sustained high speed

Proven Performance

This capability has been delivered in production for a European client requiring a combined sensorless BLDC and sensorless asynchronous motor controller, validated at speeds in excess of 60000 rpm, across a 36–48V operating range — demonstrating that the same platform architecture can be engineered to handle both motor types reliably at high frequency, not just one in isolation.

Manufacturing and Validation

As with all our controllers, high-frequency BLDC/PMSM designs are validated on our own test bench — dynamometer loading combined with precision power analysis — to characterise real efficiency, thermal performance, and stability under load at speed, not just at idle. Manufactured under our ISO 9001-certified quality process, with conformal coating and potting available where the application requires it. 

Custom Design Motor Controller / Spindle Inverter Capability

The high-frequency BLDC, PMSM, and asynchronous capability covered above didn't come from a standard product line — every one of these controllers was a custom motor controller design, engineered specifically for the motor, speed range, and application in question. High-frequency control in particular leaves very little room for a generic, off-the-shelf approach: the margin for error shrinks as electrical frequency rises, and a controller tuned for a 10,000 rpm application will not simply "run faster" at 100,000 rpm without a fundamentally different approach to current sensing, control-loop timing, and thermal management.

What "Custom" Means at High Frequency

• Control strategy matched to the motor and speed range — sinusoidal, FOC, or field-oriented induction control, sensored or sensorless, chosen based on what the application's speed and dynamic performance actually demand
• Control-loop execution rate engineered to the target electrical frequency, not simply the fastest the platform happens to support — ensuring sufficient timing margin is retained even at the top of the motor's operating range
• Current sensing and signal conditioning tuned for the frequency range in question, since sensing bandwidth and noise immunity requirements at 100,000+ rpm are not the same as at standard motor speeds
• Thermal and mechanical design considered alongside the electrical control strategy, given how tightly switching losses, torque ripple, and mechanical resonance couple together at sustained high speed
• Platform capability matched to single or multiple motor types, including combined controllers capable of driving both sensorless BLDC and sensorless asynchronous motors from one design, where a client's product range calls for it

Why Clients Choose Custom for High-Frequency Applications

Standard, off-the-shelf motor controllers are rarely designed with high-frequency operation as a first-class requirement — most are optimised for the far larger market of standard-speed applications. For clients working in CNC spindles, high-speed pumps, or other demanding high-frequency applications, that gap tends to show up exactly where it matters most: at the top of the speed range, where control accuracy and thermal margin are already under the most pressure. A custom design lets the entire platform — hardware, current sensing, control-loop timing, and firmware — be built around the actual operating envelope, rather than adapted from a design that wasn't built with high frequency in mind.

From Prototype to Volume

As with every custom controller we design, high-frequency platforms are built for manufacture from the outset — DFM-reviewed, produced under our ISO 9001-certified quality process, and validated on our own test bench under real load and speed conditions before they ever reach production. Whether the requirement is a handful of prototype units for evaluation or ongoing volume production, the same design and manufacturing process applies throughout.

Find out more about custom designed motor controllers today