Brushless DC (BLDC) motors have gained popularity in a wide range of industries due to their efficiency, reliability, and ability to deliver precise control. One of the common challenges faced when using Hall-effect sensor-based BLDC motors is the speed burst that occurs at the start. This issue can be problematic in applications where controlled and smooth motor operation is critical. In this article, we’ll explore the causes of speed bursts at startup and provide several strategies for mitigating or eliminating the problem.

Understanding Speed Bursts in Hall-Effect BLDC Motors

Speed bursts in a BLDC motor occur when the motor rapidly accelerates beyond the desired speed during the initial startup phase. This sudden acceleration can cause mechanical stress, reduced control, and damage to the load connected to the motor. Typically, this happens due to improper synchronization between the rotor’s position and the electronic commutation provided by the Hall sensors.

In order to maintain the right motor speed, the controller modifies the current and voltage based on feedback from the Hall sensors in a BLDC motor, which sense the rotor’s position. However, during startup, there can be a lag in the Hall sensor signals, leading to inaccurate commutation and a speed burst.

Causes of Speed Burst at Startup

Improper Initial Rotor Position Detection: In a Hall sensor-based BLDC motor, the initial position of the rotor may not be properly detected, leading to incorrect commutation sequences. This misalignment causes the motor to overshoot or accelerate uncontrollably before settling at the desired speed.

1. High Starting Voltage/Current: When a motor starts, it requires an initial voltage/current to generate sufficient torque to overcome inertia. If the starting voltage/current is too high, it may result in excessive acceleration and a sudden burst of speed.
2. Controller Algorithm Delay: The controller may introduce a delay when reading the Hall sensor feedback during startup. The slow response time can cause the motor to receive improper current signals, resulting in temporary overspeed conditions.
3. Inadequate Startup Control Algorithms: Many BLDC motor controllers are designed with simplified startup control algorithms that do not account for the dynamic behavior of the motor during startup. Without careful ramping of the motor speed, this can lead to speed bursts.
4. High Load Inertia: A heavy load connected to the motor can also cause a speed burst during startup. The motor attempts to overcome the load’s inertia rapidly, leading to an initial surge in speed.

Solutions to Address Speed Bursts in Hall BLDC Motors

Several techniques can be employed to prevent speed bursts at startup and ensure smoother motor operation. Some of the more successful solutions are listed below:

Putting a Soft Start Algorithm in Place

One of the most effective ways to mitigate speed bursts is by incorporating a soft start algorithm in the motor control system. This method involves gradually increasing the power supplied to the motor during startup, thereby allowing a controlled increase in speed.

• Ramp Control: By implementing a ramp-up control, the motor voltage or current is incremented gradually. This prevents an abrupt spike in torque and, consequently, in speed. A typical ramp time of 200-500 milliseconds is often used, depending on the motor specifications.
• Closed-Loop Control: Utilizing a closed-loop feedback mechanism allows the controller to monitor the rotor speed and adjust the power accordingly. This real-time adjustment helps avoid sudden bursts in speed.

Data has shown that by using a ramp control approach, speed overshoot can be reduced by up to 80%, resulting in a smoother startup. For instance, a test conducted on a 200W BLDC motor showed that the peak speed during startup reduced from 3000 RPM to 600 RPM with ramp control implementation.

Position Detection and Rotor Alignment

Proper initial rotor alignment is crucial for minimizing speed bursts. At startup, the controller should determine the rotor’s initial position accurately to ensure that the first commutation cycle generates minimal torque ripple.

• Pre-positioning: A pre-positioning step, where the rotor is held in a specific position before initiating rotation, can reduce torque spikes. This ensures that the rotor and stator are aligned in such a way that the initial current flow generates controlled torque.
• Hall Sensor Calibration: Calibrating the Hall sensors can further help in minimizing delays in position detection, leading to better commutation and reduced speed spikes.

In a study involving a 150W Hall BLDC motor, the implementation of pre-positioning reduced startup torque ripple by approximately 50%, which directly translated to a more stable speed profile.

Current Limiting at Startup

Current limiting techniques can be applied during the startup phase to prevent sudden inrush currents, which often result in speed bursts.

• Current Controller: By employing a current controller that limits the maximum allowable current at startup, the motor can be protected from excessive torque generation. For example, using a proportional-integral (PI) controller to regulate current during the first 100-200 milliseconds can significantly smooth out the startup sequence.
• Soft Switching: Soft switching techniques, where the PWM duty cycle is gradually increased, help in controlling the inrush current. This also ensures that the torque generated is proportional to the required speed, preventing overshoot.

Data from experiments on a 300W BLDC motor showed that current limiting during startup reduced the peak current from 15A to 8A, resulting in a more controlled acceleration and eliminating the speed burst.

Enhanced Commutation Control

Accurate commutation is vital to ensure smooth operation at startup. Enhancing the commutation process can be achieved through advanced sensor techniques and refined control algorithms.

• Sensorless Control Techniques: While Hall sensors are commonly used, incorporating sensorless control techniques as a supplement can provide a more accurate estimation of rotor position, particularly during startup. This ensures that commutation occurs precisely when needed, minimizing torque spikes.
• FOC (Field-Oriented Control): Field-Oriented Control is an advanced control strategy that provides precise control over the motor’s magnetic field, resulting in better torque control. Although more computationally intensive, FOC can virtually eliminate speed bursts by aligning the stator field perfectly with the rotor position during startup.

In tests comparing traditional six-step commutation and FOC, a 400W BLDC motor experienced a 60% reduction in speed burst when FOC was used, demonstrating the effectiveness of this approach.

PWM Frequency Adjustment

Pulse Width Modulation (PWM) frequency plays a critical role in determining the smoothness of the motor’s startup. A higher PWM frequency results in finer control of the current supplied to the windings, reducing the chances of a speed burst.

Optimized Frequency: Increasing the PWM frequency to a range between 20 kHz and 30 kHz can significantly improve the current control resolution, leading to a smoother startup. The trade-off must be taken into account, though, because higher frequencies could result in higher switching losses.

A test performed on a 250W BLDC motor indicated that increasing the PWM frequency from 10 kHz to 25 kHz reduced the speed burst by 40%, ensuring a more gradual acceleration.

Practical Example: Combining Techniques for Optimal Performance

To better illustrate the implementation of these solutions, let us consider an example involving a 300W Hall BLDC motor used in an industrial fan application. The initial problem observed was a speed burst reaching 3500 RPM within 100 milliseconds, causing mechanical stress and audible noise.

The following solutions were applied:

1. Soft Start Algorithm: A voltage ramp-up of over 500 milliseconds was implemented, which reduced the initial speed spike to 800 RPM.
2. Rotor Pre-Positioning: The motor controller was programmed to align the rotor before startup, reducing torque ripple.
3. Current Limiting: A current limit of 10A was applied during startup, down from the previous 18A, which reduced the peak torque generated.
4. Enhanced Commutation with FOC: Field-oriented control was integrated, further smoothing the startup sequence and reducing the initial burst to negligible levels.

After implementing these solutions, data indicated a significant reduction in speed overshoot, with the motor reaching its target speed of 3000 RPM in a controlled manner over 1.5 seconds, without any sudden bursts. The mechanical stress was reduced, and the overall system reliability improved.

Solving the problem of speed bursts at the start of a Hall-effect BLDC motor involves a combination of advanced control techniques, careful parameter tuning, and optimized hardware and software solutions. Closed-loop control systems, soft start mechanisms, Field-Oriented Control (FOC), and accurate rotor position detection are some of the most effective strategies for mitigating speed bursts. By implementing these techniques, engineers can achieve smoother motor operation, improve performance, and extend the lifespan of both the motor and the connected load.