Basics of a Brushless DC Motor Driver
A brushless dc motor driver is a device used to control the direction and amount of current that flows through a brushed DC motor. Motor drivers are typically characterized by their mounting configuration and features such as regeneration, programmability, and automatic restart after stalling.
Brushed DC motors rotate by switching the polarity of the voltage applied to them. Brushless motors, however, operate without brushes and a mechanical commutator. This requires more complex control electronics.
Voltage
The voltage of a brushless dc motor driver is an important factor that determines the maximum speed and torque at which the motor can operate. It must be high enough to prevent power loss, while also being low enough to ensure safe operation. Choosing an appropriate voltage depends on the motor type and application, load, ambient conditions, and cost.
Brushless DC (BLDC) motors sequentially energize an arrangement of electromagnetic coils on the motor’s stator that produce a force against permanent magnets arranged on the motor’s rotor, which rotates around the coils. This electronic commutation eliminates the need for carbon brushes and a commutator, and it operates more efficiently than brushed motors at the same speeds and loads.
A brushed DC motor can be driven using simple direct current (DC) voltage control, and this is common in applications where speed or torque don’t require precise control. The motor’s rotor is driven by an electric current through its coils, and the voltage generated is proportional to the number of revolutions.
For a motor to operate at its maximum speed, the coils and magnets must be properly matched. If the rotor is too small, it will not brushless dc motor driver generate the necessary voltage to reach its maximum speed, and if the rotor is too large, the moment of inertia will be high and the motor may overheat.
Current
To make an electric motor rotate, it must alternate the direction of current flow through its windings to generate a rotating magnetic field. This is accomplished with a mechanical linkage between its commutator and brushes. Brushes, made of a soft conducting material such as graphite, press against the commutator to establish sliding electrical contact with each segment in turn as the motor spins. This contact is interrupted only briefly when a reversing switch (typically a double pole switch) opens and closes. The continuous contact causes motor wear and abrasion that shortens the brush life and requires regular maintenance. It also creates an acoustic and electrical noise.
BLDC motors eliminate this friction by electronically commutating the rotor positions. Sensored and sensorless methods are used to determine the rotor position. Trapezoidal and sinusoidal commutation are the two basic approaches. Both rely on the detection of a voltage signal waveform at the motor’s phase windings, with trapezoidal commutation requiring that two out of three windings be energized simultaneously and sinusoidal commutation enabling a more gradual change in current flow between phases.
Regardless of the method, precise control is required to deliver a smooth, consistent motor current. A BLDC motor driver accomplishes this by delivering thousands of carefully-timed DC current pulses per second, with the net voltage that reaches the motor legs approximating a sinewave.
Pulse Width Modulation (PWM)
PWM is the quick switching of a signal with varying widths between two states, such as high and low. By adjusting the pulse-width-to-period ratio (also called the duty cycle) it is possible to control the average voltage or current delivered by a driver. This can result in different motor speeds, LED brightness levels or audio volume.
A typical PWM output waveform miniature bldc motor looks like the above. The period represents how long the signal is on and the width indicates when it switches between the two states. The frequency of the signal can also be adjusted to trade-off between speed and power efficiency.
More advanced PWM signals have more sophisticated waveforms which improve the harmonic quality of the output voltage. The most common method of doing this is called Space-Padded Pulse Width Modulation (SPWM). This approach filters out higher order harmonics and reduces the distortion factor DF to an acceptable level.
The selection of the PWM frequency is important because it can significantly impact both the current ripple and the noise generated by the driving circuit. A high frequency can increase the efficiency and power density of a design, while a low frequency can cause excessive switching losses which may result in heat dissipation, audible noise or electromagnetic interference (EMI). These issues can be addressed by careful component selection, proper layout and EMI compliance techniques.
Torque
A motor generates torque, which converts electrical energy into mechanical energy. The torque produced by the motor is dependent on many factors such as the shaft speed, gearing ratio and load. It is important to understand how these factors affect the torque.
Brushless DC motors utilize electronic commutation to control current, resulting in higher speeds and increased efficiency compared to their brushed counterparts. However, this electronic commutation introduces additional components and complexity to the motor. This increases the overall system cost and reduces reliability. In addition, there is an increase in noise resulting from the instantaneous opening and closing of switches in the commutator, especially at stall.
There are two common methods for electronic commutation in sensored and sensorless driver systems: trapezoidal and sinusoidal. The trapezoidal technique uses a zero-crossing of the back emf to determine the rotor position and energizes two windings at a time. The other windings remain unenergized. This stepping technique produces torque ripple.
The sinusoidal commutation technique requires more complex sensor circuits that analyze the phase current vs. logical state of the 3 hall sensors to determine the rotor flux direction and create a 6-step commutation table over 360 electrical degrees for a 1 pole pair motor. This reduces the torque ripple, and increases the speed range.
