Understanding brushless-DC motor systems, part 1

(This article was originally published on Planet Analog.)

Quick – what are the three motor types? The most common answer that I hear is “brushed-DC motors, stepper motors and brushless-DC motors,” which is basically a knee-jerk reaction to the question. Some of the more intrepid motor-drive experts I know might say “single-phase AC input, multiphase AC input or DC input,” categorizing each motor by its input mechanism rather than the specific motor type. I will admit, this is a pretty clever answer. There is certainly no shortage of different possible groupings in the taxonomy of motors, ranging from the use of position sensors to the supply voltage to the driver implementation; some engineers may be tempted to try to answer in these ways, even though the response doesn’t always fit into three distinct categories. In this technical article, I’ll address a new way to look at motor classification.

I have recently become fascinated with the idea that we should classify motors by their function. Many applications use different kinds of motors depending on the whim of the designer, but the same motor solution may not even be applicable in a different application that uses the same kind of motor. If that sentence gave you a headache (my communications colleague assured me that it had this effect on her), let me give you a better example: you can implement a pan-tilt-zoom (PTZ) camera axis with a stepper motor, a brushless-DC motor or even a brushed-DC motor. A brushless-DC motor is also the most common motor type in a ceiling fan (see Figure 1). But you can’t use the same brushless-DC motor solution for both of these end-equipment types. 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. Why do I know this? Intuitively, I recognize (as you probably do as well) 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. There must be a property of motor functionality from an application perspective that can help establish a different classification of motor types.

 

Figure 1: A PTZ camera versus a ceiling fan

 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 in 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.” Take a look at Figure 2 and Table 1 below.

Figure 2: Speed, torque and position

Type

Example

Load

Top priority

Sensorless capability

Speed

Ceiling fan

Air or fluid

High speed and low noise

Easy

Torque

Power drill

Solid

High power density

Very hard

Position

PTZ camera

Solid

High accuracy and efficiency

Extremely hard*

*I won’t say impossible, but this one is very close to impossible. Since a position control system operates primarily at zero speed, there is no back-electromotive force to sense, and therefore no sensorless signal to close the loop.

Table 1: Summary of speed, torque and position

Let’s talk more about speed, torque and position in the next installment of this two-part series. Until next time, let me know your thoughts in the comment section below!

Anonymous
  • Hello! I think that wort of motor is one which can operate in a stall condition without burning out. A more typical motor cannot operate in a full stall condition continuously without getting very hot. A torque motor is like a certain construction of brushed-DC or brushless-DC motor designed to operate with full voltage applied to it while being completely stalled without consequences.

  • Recently came across a valve actuator application that used a "Torque Motor".  You can check out what these are on wikipedia, https://en.wikipedia.org/wiki/Torque_motor.  Some variants of these motors are not capable of full rotations, so perhaps an odd-ball construction.  While this type of motor applies torque, maybe it doesn't fall so well into your torque category because I'm not sure the power density is there- power = torque RPM, and this type of motor might not even spin so power is 0!  Apparently a brushed-DC type of motor controller is sufficient for driving torque motors.  This particular application uses LVDT/RVDT feedback to sense position.