BLDCs have many advantages and potential applications. But design engineers are still raising a plethora of questions about what they're actually capable of and how best to implement them. Here are some of the most frequently asked questions:
Compared to a brush-type motor, what kind of life can a brushless dc motor expect? There are no brushes to wear out, so the BLDC motor commonly has three to five times the life of a conventional, brush-type motor.
What's the maximum speed at which BLDCs can be used? For the outside-rotor, 58- and 98-mm OD BLDC motors, a maximum speed of 3000 rpm is recommended. Higher speeds can be achieved, but dynamic balancing would be required.
What's the minimum usable speed? For the outside-rotor, 58- and 98-mm OD BLDC motors, a minimum speed of 300 rpm is suggested. At lower speeds, cogging torque becomes an issue, and efficiency is affected.
What type of speed control is available? Pulse-width modulation is the most common, but 0- to 5-V, 0- to 20-mA, and variable-frequency controls also are available.
Can the motor produce a real-time tachometer signal? The brushless motor can generally produce a 36- to 100-line/revolution tachometer signal. Higher resolution would require a separate tachometer or encoder.
Can the BLDC motor pass EMI regulatory requirements, such as FCC or CISPR? Most BLDC motors meet these requirements—dBA levels of less than 20 are normal. And, levels of less than 10 dBA are achievable for motors with output-power requirements of less than 20 W. It's important to note that these devices are proliferating. As such, the need to meet EMI requirements is becoming more and more a demand in the industry. This is necessary to avoid interference with data-exchange systems involving microprocessors and similar devices.
In continuous-speed applications, the outside-rotor BLDC motor offers excellent instantaneous speed variation. This is due to its high rotor inertia, which mechanically integrates or dampens the effects of instantaneous load variation. Speed variations of less than 0.25% are typical, even under open-loop conditions. Closing the loop on the brushless dc motor has yielded speed variations of less than 0.1%.
Audible noise is a newer design criterion that designers now face. The use of a brushless dc motor can assist the designer in this regard. In office equipment, noise specifications of less than 35 to 60 dBA are not uncommon. This is the noise-level range found in an ordinary library or office. The 58-mm brushless motor's noise output is less than 35 dBA, while the 98-mm brushless frame contributes less than 55 dBA. The noise is measured with the motor under load and without any external cover.