How to Select a Brushless DC Motor
Going For Longevity With Brushless Technology
If an application requires high speed, quiet operation, low EMI and longevity, then brushless DC motors (BLDC) might be what you are looking for. There are many advantages to brushless motor technology and speed is one of them. Higher speeds are achievable because there are no mechanical limitations being imposed by the brushes and commutator. Another advantage is the elimination of the current arcing electro-erosion problem commonly experienced with brushed motors. BLDC motors also possess higher efficiency, and generate lower EMI (ElectroMagnetic Interference), which is excellent when used in RF (radiofrequency) applications. They also possess superior thermal characteristics over brushed motors since the windings are on the stator. The stator is connected to the case, thus the heat dissipation is much more efficient. As a result, the maintenance on a brushless motor is virtually non-existent.
Unfortunately, the higher cost of construction puts BLDC technology out of reach for many applications. You can easily spend twice as much on a brushless system and lose the simplicity of a brushed motor. Don't forget to save room for the control and drive electronics too. You'll need to mount it somewhere if it isn't integrated in the motor. Keep in mind, the motor can't be mounted too far away from the drive as long cable runs tend to introduce noise into the system. To compensate, the phase leads can be twisted and shielded from the sensitive feedback leads to reduce noise. As with brushed motors, brushless dc motors must overcome starting friction as well. Again, this is the sum of torque losses not depending from speed. Dynamic friction is dependent upon speed. In fact, dynamic torque friction is the only thing defining torque losses proportional to speed for BLDC. A function of speed (for example in metric units of mNm/rpm), dynamic friction is due to the viscous friction of the ball bearings, as well as to the eddy currents in the stator originated by the rotating magnetic field of the magnet.
Overall, you can expect the speed-torque curve to demonstrate excellent linearity for BLDC technology.
Driving Brushless Micro Motors
Unlike brushed (coreless) DC motors, brushless technology cannot be operated by connecting directly to a straight line DC voltage. Remember, brushless motors utilize electronic commutation and there are no brushes making physical contact with the commutator. The permanent magnet rotor initiates motion by chasing a revolving magnetic field induced by the current in the stator windings.
Creating this motion is done with electronics and is usually an on/off signal called Pulse Width Modulation (PWM). Normally supplied by a comparator, the PWM signal is a voltage generated as a result of a sinusoidal command signal and a saw tooth carrier or chopper frequency. The PWM signal is either on or off and delivered at a duty cycle governed by the chopping frequency. The PWM signal will be high when the command is greater than the carrier (chopper or switching frequency). The lower the chopping frequency, the more time the current has to gain amplitude. The motor will continue to accelerate and decelerate with an accompanying increase in current density. Such harsh changes in amplitude can result in more ripple in the output as well as shortened motor life, so it's important that the switching frequency is high enough. The discrete on/off states are controlled by 6 semiconductor switches which correspondingly send the amplified current through the correct phase. When the current is reversed by the semiconductor switches, the stator windings are utilized more efficiently because more than one winding will be energized.
In order to turn the phases on and off at just the right time, the drive requires feedback. This will help to keep the commutation angle around an ideal 90 degrees. Brushless motors are normally in a closed loop (servo) system to operate properly. In many cases, digital Hall effects are employed to provide the required feedback and commutate BLDC motors. For smoother operation, sometimes sinusoidal commutation (linear Hall effects) can be used.
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