In the previous blog entry, I described two ways to control, or commutate, brushless DC (BLDC) motors by using information from internal Hall-effect sensors or back-EMF signals from the motor windings. So you might wonder if one or the other control technique better suits an application you have in mind.

To find out, I talked with Scott Eckles, application engineer and Mike Moore, director of engineering at the Hurst Manufacturing division of Emerson Motor Technologies. Hurst manufactures the BLDC motor (DMB0224B10002) used in the Renesas BLDC Motor Control Evaluation Kit I reviewed for evalkits.com.

I have distilled the key points Scott and Mike explained in our conversation.

Back-EMF commutation (sensorless):

1. Applications that require high-temperature operation might exceed the specified temperature range for Hall-effect sensors, whereas BEMF measurements do not need any added sensors.
2. Hall-effect sensors might not operate reliably in areas with high levels of ambient magnetic fields.
3. The BEMF technique requires fewer electrical connections with a motor and thus it can offer higher reliability.
4. Hall-effect sensors can add from about one to three dollars (US) to the cost of a motor.
5. Use the BEMF technique in light-load applications that don't require very accurate positioning.
6. Sensorless commutation might be better for applications in which you just have to maintain a speed.

Hall-effect-sensor commutation (sensored):

1. In an application that experiences a high-torque, or high-load, starting condition, say in a compressor, Hall-effect sensors provide rotor-position information right away so the controller can have the motor provide full torque right away.
2. BEMF commutation requires some movement of the rotor to generate a back EMF. This step introduces some jitter into the rotor's movement and a short a start-up delay.
3. When you have a motor application that requires accurate positioning, typically the sensored approach will provide better results than a sensorless approach. If you're must go to a specific location, you would likely use a sensored BLDC motor.

The comparison information in the lists above is difficult to find, so I appreciate Mike's and Scott's contributions. Any errors are entirely mine. I welcome your comments about BLDC motor control or applications.

--Jon Titus