Media Relations for a Connected World
Hall Effect Sensors for Brushless DC Motors
Josh Edberg, Senior Global Marketing Manager
Honeywell Sensing and Control
Hall effect sensors used in brushless DC motors (BLDCs) play a significant role in the reliability, performance and life cycle of many critical applications including robotics, portable medical equipment, HVAC fans and machine tools. These highly efficient motors use Hall effect sensors and control circuitry to detect the position of the rotor. Their role is to determine when to apply the current to the motor coils so the electronic controller can rotate the magnet at the right time and orientation. Accurate measurement of the DC motor’s position is important because this data is used to produce the maximum amount of torque on the motor shaft, which is a key element of motor efficiency.
Hall effect sensors can be mounted in one of three locations inside the motor, at the end of the motor’s shaft, and around the rotor shaft’s ring magnet to measure the motor’s position.
As energy efficiency, power consumption and cost savings become bigger design drivers, BLDC motor manufacturers need to adapt to these changing requirements to deliver more efficient motors. This means BLDC motor manufacturers need to carefully consider the bipolar latching Hall effect sensor used to commutate the motor. BLDC motors use electronic commutation to control power distribution to the motor, unlike brush DC motors that use mechanical commutation. They should evaluate these sensors based on several technical characteristics, including sensitivity, repeatability, stability over temperature and response time, to ensure that their motor design will operate most efficiently.
The fact is, Hall effect sensors for BLDCs should provide high repeatability and stability, and a fast response to changes in the magnetic field to deliver greater motor efficiency. Sensor manufacturers have achieved these goals through the use of chopper stabilization, which basically has become a defacto standard for Hall effect sensors. However, designers should consider new sensor technologies that don’t require this technique to achieve higher performance. Because of new technologies and processes, magnetic sensor manufacturers are not only achieving high sensitivity and magnetic stability without using chopper stabilization, they are also delivering better performance.
Before we take a look at some of those new technologies, let’s look at several key characteristics, sensitivity, repeatability, stability and response time, that designers need to evaluate when selecting a Hall effect sensor for a BLDC application.
BLDCs use Hall effect sensors and electronic
controllers to rotate the magnets at the right time and orientation.
High sensitivity sensors, typically rated at less than 60 Gauss, enable BLDC motor manufacturers to use smaller magnets or less expensive magnetic materials. This does two things: reduces cost and allows for the use of smaller BLDC motors, which is increasingly important for space-constrained designs and as the price of rare earth magnets continues to rise and supply availability becomes a greater issue.
High sensitivity also contributes to a wider air gap, which allows the sensor to be placed further away from the magnet and still be very reliable, giving the engineer greater design flexibility. If the designer does not require a wider air gap, the sensor will achieve higher sensitivity, delivering better reliability and repeatability, in the motor.
In this example, an eight-pole BLDC with a three-phase winding uses three bipolar latching Hall effect sensors to detect the position of the
Repeatability is important in these applications because a highly repeatable sensor changes state at the same angular position each time the magnet passes by the sensor. This means a highly repeatable sensor will maintain all of the angular measurements very close to the same value.
This is a critical factor because the timing between the current flow through the coil and the shaft’s position needs to be as accurate as possible to produce the maximum amount of torque on the shaft. A delay in a sensor’s response to changes in the magnetic field could lead to lower bandwidth and accuracy errors. Any error in the Hall effect sensor’s switching point reduces the motor’s torque, which impacts motor efficiency.
Stability is a critical characteristic of a Hall effect sensor because stability over temperature is required for precise position detection. The stability of a sensor measures how much the angular position changes over temperature or voltage. As an example, a highly stable Hall effect sensor requires the same Gauss (G) level to turn a part on whether it’s at 25°C or 125°C. Magnetic stability also improves jitter performance, which is an important characteristic in BLDC efficiency, resulting in less speed variation.
Response time measures the time it takes for a sensor to change state. A slow response time, can result in lower bandwidth and accuracy errors, which contributes to lower motor efficiency, as well as issues commutating a motor at high frequencies.
Miscommutation translates into lower effective torque constant (Kt) and higher torque ripple, causing additional electrical noise. This electrical noise can significantly impact efficiency and system performance. In contrast, a faster response time to a change in the magnetic field delivers higher efficiency in motor commutation.
Designing without Chopper Stabilization
In addition to evaluating Hall effect sensors for these key technical parameters to ensure a highly efficient BLDC, designers should consider new sensor technologies and designs that deliver a faster response time and higher repeatability to achieve the same goal.
One example is Honeywell’s SS360/460 family of bipolar latching Hall effect sensors. These non-chopper-stabilized devices provide high magnetic sensitivity of 30 G typical and (55 G maximum) over the entire temperature range of -40°C to 150°C without chopper stabilization of the Hall element, resulting in faster response times. With a faster latch response time and the capability to handle higher frequencies, these sensors can improve motor efficiency. Compared to chopper-stabilized sensors that traditionally use single or dual Hall elements, Honeywell’s parts use proprietary software and a quad Hall element to handle any drift in the switch point. This eliminates the need for chopper stabilization and results in a much more stable sensor that provides a fast response time, reduced sensitivity to packaging stresses, and less noise, resulting in improved motor efficiency. Initial customer feedback has shown up to a five percent increase in motor efficiency compared to a competitive chopper-stabilized part.
The Honeywell SS360ST, SS360NT and SS460S bipolar latching Hall effect sensors offer reliable switching points over its operating temperature range of -40°C to 150°C.
Typically, sensor manufacturers have used chopper stabilization to deal with the single and dual Hall element’s susceptibility to packaging stresses. These Hall elements have been traditionally used in Hall effect sensor designs. This averaging process on the hall die allows for greater stability over temperature and improves the device’s sensitivity by mitigating the inconsistencies from sensor to sensor due to packaging stresses.
It is true that chopper stabilization offers high sensitivity and high magnetic stability, making it suitable for latching sensors in BLDC motors, but they also have several drawbacks including slower response time, greater accuracy errors, due to sampling, and more electrical noise that all negatively effect motor efficiency.
Let’s look at response time as an example. Understanding that chopper stabilization is an averaging process, and since the averaging of the induced voltage across the Hall elements is continual in order to determine the output signal, it will always slows down how fast a sensor switches. This delay may range between 10 to 30 microseconds that can significantly effect motor efficiency; causing greater accuracy errors as the motor spins. It also means that these parts may not be suitable for use in very high frequency motors.
In contrast, a latching sensor without chopper stabilization delivers a faster response time, which means the motor is commutated at or closer to the correct time for higher accuracy and a more efficient motor.
Another tradeoff is additional electrical noise introduced into the circuit by chopper stabilization. This often results in additional design time, more filtering circuits and related costs. On the other hand, non-chopper-stabilized parts don’t cause additional electrical noise, simplifying the overall design.
In terms of stability and sensitivity, many designers think that chopper stabilization is the only way to achieve stability over temperature and high sensitivity in latching Hall effect sensors for BLDC motor commutation, contributing to a general misconception that non-chopper-stabilized parts are not stable. One of the most important factors that contribute to the stability of the sensor is the Hall element.
The issue is that many sensor manufacturers have traditionally used single and dual Hall effect elements, which was noted previously, are susceptible to packaging stresses, requiring chopper stabilization to provide more stable operation over temperature and voltage. By using a quad element, sensor manufacturers can produce higher stability parts without requiring this technique. The quad Hall element is less susceptible to stress-induced errors because the voltage is measured in four directions, cancelling the offsets in each element to deliver stable operation.
Honeywell conducted a low gauss latch competitive evaluation between the non-chopper-stabilized SS460S bipolar latching Hall-effect sensor and five high sensitivity chopper-stabilized competitor products. Tests included response time, repeatability, and sensitivity to air gap.
Test results for reliability and response time between the Honeywell SS460S bipolar latching Hall-effect sensor, using a quad Hall element together with proprietary software, and five competitor samples shows that the Honeywell part has a repeatable output with a response time that is between 10 μs to 20 μs faster than competitive chopper-stabilized products. All competitive samples showed a delay in response time between 10 μs and 30 μs. This delayed response time is due to the chopper stabilization process.
Testing also shows that chopper stabilization may cause repeatability issues due to variances in actuation. Although higher frequency of the chopper stabilization in some of the parts may have solved this problem, the chopper-stabilized parts still had a slower response time.
In summary, Hall effect sensors can significantly effect a motor’s efficiency. Motor designers should evaluate sensors based on several specifications including sensitivity, repeatability, stability, and response time to achieve a highly efficient motor. They also should consider Hall effect sensors that use new technologies and processes to deliver highly stable and sensitive parts without the drawbacks of increased electrical noise and slower response time often associated with chopper stabilization.
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