Brushless DC (BLDC) Motor Control (Part 4): Current Control

The FOC approach also has two current loops. One is for the desired (Q) torque and the other for the undesirable (D) torque. The Q torque loop is driven with the commanded current output from the position-loop (or velocity-loop) controller. The D loop is driven with a command of zero to minimize the unwanted direct torque component.

What are the benefits of using the FOC approach instead of current control and commutation? The answer is higher top speed and improved efficiency at higher rotation rates. Note that for linear BLDC motors, which don’t generally move very fast in terms of electrical phases per second, there are few advantages to using FOC.

From the engineer’s perspective, using FOC is no more challenging than using standard PI current control. Implementing FOC in the controller is algorithmically complicated, but this complexity is typically hidden. To tune the FOC current-control gain settings, as before, K_p and K_i gain values need to be determined. This is typically done by an auto-tuner, which most motion-control vendors provide to customers.

FOC is a very important control technique due to its ability to increase efficiency and lower heat generation in BLDC motors at higher rotation speeds. While it once was a relatively high-end approach, new motor-control MCUs and DSPs have turned it into a relatively standard feature. It also tends to be the best approach when driving a rotary brushless motor in high-performance positioning or velocity-control applications.

Voltage-Mode Control of Brushless DC Motors

When it comes to the current-control portion of BLDC motor controllers, there’s also the possibility of not performing active current control at all. This is also called voltage-mode operation. Voltage-mode motor control is appealing first and foremost because it’s inexpensive, typically requiring only a switching bridge.

Are there safety concerns with voltage-mode control? Yes, particularly at startup or if the motor stalls. Without current control or at least current limiting, the danger is that excessive current will flow through the windings and damage the motor. This is especially true with motors designed for high-speed operation because they tend to have low motor-coil resistance.

Are there applications for voltage-mode operation of BLDC motors? Despite the limitations, the answer is definitely yes. Examples include cooling fans, pumps, compressors, high-speed surgical drills, shavers, and more. In all of these applications, explicit control of the motor velocity or torque isn’t a requirement because the motor back-EMF and/or the load that the motor drives against creates a natural bound on velocity.

Brushless Motor Amplifiers

The final component of a BLDC motor controller is the amplifier, which uses power switches to adjust the applied voltage to match the commanded current as closely as possible. While several different motor amplifier schemes exist, the most widely used architecture in high-performance position- and velocity-control applications is the triple half-bridge with leg-current sensing. A connection diagram for this digitally controlled switching amplifier is shown in Figure 5.