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Speed, torque and position: A functional understanding of brushless-DC motor systems

A brushless-DC motor is common in ceiling fan and pan-tilt-zoom (PTZ) camera designs. But you can’t use the same brushless-DC motor solution for both of these end products. A ceiling fan driver circuit, however excellent its performance, would be a horrible choice for a PTZ camera, and result in a product that just doesn’t work.

How do I know this? Intuitively, I recognize that the “function” of a ceiling fan motor and the “function” of a PTZ camera motor are very different, even though both are using a brushless-DC motor. So, there must be a property of motor functionality from an application perspective that can help establish a classification of motor types.

Figure 1 The function of a ceiling fan motor is different from the function of a PTZ camera motor. Source: Texas Instruments

So, what are these motors actually doing? The function of a ceiling fan is to spin at a constant speed to circulate air in a room. The function of a PTZ camera is to move the camera to a certain position and hold it there. I can add a third example to the mix: a cordless power drill. The user controls the drill’s torque by pressing the trigger, perhaps to drill a bit through a wall stud. These motors have very distinct implementations; thus, my answer to “what are the three motor types?” is an emphatic “Speed, torque and position.

Speed

A motor system falls under the speed category when its primary function is to operate at a constant or variable speed. These systems are called fans, blowers or pumps, since the primary load in the system is air, gases or a fluid. Some examples of applications in this category include the aforementioned ceiling fans, vacuum cleaner blowers (Figure 2), ventilator blowers and automotive pumps (fuel, oil and water).

Figure 2 A vacuum cleaner uses a speed-controlled blower motor. Source: Texas Instruments

Designers choose brushless-DC motors for speed control systems to achieve quiet performance compared to brushed-DC motors; high-power drive capability compared to brushed-DC and stepper motors; or high-speed operation compared to brushed-DC and stepper motors.

Speed control is the easiest of the three control schemes in relative terms, since controlling the voltage applied to the motor is equivalent to controlling its speed. For a brushless-DC motor controller, this simplicity means that it’s possible to integrate the motor-control algorithm into the motor driver for many applications.

There is a common buzzword for brushless-DC motors—sensorless—in many speed control systems because the motor will be generating a back-electromotive force (back-EMF) to estimate position. There is rarely a startup load too large for sensorless algorithms to overcome. The DRV10983-Q1 chip from Texas Instruments (TI) is an example of a sensorless sinusoidal fan driver with integrated speed control.

Torque

A torque type of motor’s primary purpose is to apply some rotational force. The load in this case is a solid. Some examples of torque control systems include cordless power drills and automotive body motors such as power windows and e-bikes.

Torque tends to be the most confusing category of the three, since many motors that may appear to address position or speed control end up addressing just torque control. A good example of this misperception is the power seat in an automobile (Figure 3). A passenger adjusts the position of the seat using switches to move the seat forward, backward, up or down.

Figure 3 Power seats in an automobile are torque control systems. Source: Texas Instruments

However, the motor system is not controlling the target position of the seat; the driver or passenger is. A power seat motor applies a certain amount of torque to ensure that the seat apparatus moves in the correct direction. The user decides when to stop the movement. The motor is, in effect, a “force on command” to move the seat.

In torque systems, the objective is to deliver as much mechanical torque as possible given the size constraint. Designers choose brushless-DC motors for torque control systems to achieve higher power output and higher power density when compared to brushed-DC motors. Stepper motors are rarely used in torque control systems because of their lower efficiency.

Torque systems are more difficult to implement than speed control systems. In some cases, you can get away with simple control integration into the driver using a sensored approach. If the motor does not stall and has enough gearing on the shaft, it is possible to implement sensorless control of torque systems. However, if your system requires significant torque at zero speed, sensorless is not going to work. TI’s 36-V, 1-kW, 99% efficient, 18-cm2 power stage reference design for three-phase BLDC motors is an example of a torque system implementing sensored trapezoidal control for a 36-V, 1-kW motor.

Position

A position motor moves an object to a certain position and holds it at that specific position. A position control loop must always be active, especially when the motor is still, to ensure the proper position. A servo drive is a common nickname for this sort of system. PTZ cameras, stage lighting equipment (Figure 4), industrial robots and gimbals all fall under this category.

Figure 4 Stage-lighting systems use position control to move the lighting fixture. Source: Texas Instruments

Designers choose brushless-DC motors for position control systems to achieve higher power output and higher efficiency when compared to brushed-DC motors and stepper motors.

Position control systems are the most difficult to implement from a control and sensing perspective. Since a servo drive must operate at zero speed to hold position, being sensorless is not an option. High-accuracy position control loops also require significant processing and configuration, leading to some intense computational requirements. TI’s 48-V, 500-W three-phase inverter with smart gate driver reference design for servo drives implements a position control system with encoder-based field-oriented control (FOC) for the 48-V, 500-W motor.

Motor classification by function

Some of you may be thinking, “What about systems that have two or three kinds of control going on at once?” A robotic arm (Figure 5) has to move to a specific position, but its movement will also be restricted to a certain speed and a certain torque. In such cases—and there are many—there is both a primary mechanism of control and additional limit controls. A robotic arm has to move to a certain position and stay there, but placing a speed limit and torque limit on top of the position control loop ensures proper operation.

Figure 5 Industrial robots such as cobots use servo motors. Source: Texas Instruments

If you skipped the entire article and are only looking for a summary, see Figure 6.

Figure 6 A summary view of how speed, torque and position operate in different motor control applications. Source: Texas Instruments

This sort of motor cataloging can help engineers look at motor problems in a different light. For example, quiet operation isn’t an issue for brushless-DC motors, but it is an issue for applications that use speed control. Speed, torque and position drive independent technology development in all three areas because the major concerns for each of the three motor types are fundamentally different.

This may also help drive integration in areas that have been classically very disintegrated. Speed control is over-represented with integrated solutions available while torque and position have relatively few. While the challenges are different, it presents an opportunity to create unique solutions that do not exist today.

What do you think? Does the concept of speed, torque and position resonate with you? Please let me know in the comments section.

Matt Hein, author of Signal Chain Basics blog # 163, helps develop and market TI’s next-generation motor-drivers, from the stages of roadmap definition to ecosystem creation.

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