The broadest distinction between brushless DC motors is whether or not the motors are sensored or sensorless. This refers to the question as to whether or not the motor has in built positional sensors which are required to operate the motor with a sensored brushless DC motor controller or whether the motor has been built without these and is therefore requiring of a sensorless brushless DC motor controller.
The inbuilt sensors on a sensored BLDC motor are used to tell the motor controller the exact position of the rotor at any time. This allows the controller to operate the motor with a fine degree of control in closed loop mode. With a sensorless brushless DC motor, the controller does not know the exact position of the rotor, (which is why starting a sensorless brushless DC motor can sometimes be a problem). However, when a sensorless brushless DC motor is up and running, intelligent motor controllers such as the Zikodrive ZBDL15 are able to directly monitor the speed of the motor by reading the back-EMF signal.
One of the most common types of brushless DC motor uses the same NEMA frame sizes as standard stepper motors.
These motors are built in the same frame sizes as typical stepper motors such as NEMA 17 or NEMA 23 but use brushless DC designs rather than stepper. These motors are especially common because of their ease of installation and setup and offer a range of performance options.
As with stepper motors, a useful rule of thumb is the longer the ‘stack’ of the motor, the greater the torque. However this is only a rule of thumb and it is always important to check datasheets thoroughly as these factors can vary.
Gimbal motors are typically quite flat and have a central hole through the centre of the motor. The rotor in a gimbal motor is designed in a circular shape with permanent magnets mounted on the outer edge of the rotor. The electromagnets are then mounted on the external case with the inner circle rotating on bearings around the hollow centre.
Typically these motors include a large number of poles in comparison with other types of motors, especially smaller NEMA brushless DC motors. These extra poles have the advantage of helping control the speed of the motor more accurately as well as delivering a smoother, more consistent performance at lower speeds.
Single pole brushless DC motors are brushless motors which are based on a single pole pair. Whereas the gimbal motors looked at above have multiple pole pairs, enabling smooth transitions, enhanced performance at lower speeds and increased stability, a single pole motor cannot achieve this.
This fact points you in the direction of the most common applications for single pole motors. Broadly speaking, they are very poor at low speeds and are therefore much more suited to higher speed applications. As a result of the poor performance at lower speeds, the majority of single pole brushless DC motors are sensorless. One major advantage of these types of brushless DC motors however, is that they are typically able to reach the highest speeds of any type of brushless DC motor.
Typical applications for such motors include pumps and process control applications where faster speeds are important.
An outrunner brushless DC motor is very similar to a gimbal motor but rotates the outer casing of the motor around a fixed centre rather than the other way around. These motors are typically built on a higher number of poles and as such rotate relatively slower than other types of motor.
However, the increased number of poles and the external rotation offers a higher level of torque than many other types of brushless motor making it popular in applications where higher torque in a lower weight package is essential. Outside of industrial and commercial applications these motors are most commonly found in model applications such as model planes and similar.
One factor to consider when choosing a brushless DC motor is also the type of windings within it. With a delta type wiring the motor is set up with all of the phases wired to each other with power applied where the phases meet. However, in a Y (or star) winding all the phases are connected in the centre.
Broadly speaking the performance differences between the two windings are that the delta configuration enables a higher top speed at the expense of lower torque at low speeds. Conversely a Y winding offers a reduced top speed but greater control at the lower speed end.
It is generally accepted that the Y winding is more efficient although factors such as materials used and the quality of construction/assembly can also play a part. From the perspective of motor controls, the winding type makes no real difference to setup.