Brushless DC Motors Used in Industrial Applications
Brushless DC (BLDC) motors (also known as DC commutatorless motors, electronically commutated motors, AC synchronous motors or DC servomotors) are increasingly replacing brushed DC motors due to their “superior efficiency, long life, smooth torque delivery, and high speed operation.” 1 Yet, in the past, their application has been limited due to the additional cost of the complex motor controller necessary to operate these motors. However, controller costs have been trending downward in recent years such that application of brushless dc motors is on the rise and expected to grow. 2 While they have been successfully applied in the automotive, HVAC, electronic, computer, semiconductor and medical industries, BLDC motors have long been used in industrial applications such as actuators, feed drives for CNC machines, industrial robots, extruder drives, among others.
Despite the need of a complex motor controller, the simplicity of construction of BLDC motors offers several inherent advantages not provided with brushed DC motors in terms of low enertia, high torque and a very wide speed range. Constructed in an inside-out3 configuration with a rotor consisting an array of permanent magnets and a stationary armature that’s excited by an electronic commutation controller, BLDC motors exhibit better heat dissipation, improved efficiency and greater power density than brushed DC motors. Their compact size, reduced weight and high speed range is thanks to the lack of brushes and a mechanical commutator. To provide the position feedback required in industrial servo applications, BLDC motors have an encoder (optical or Hall Effect) that measures the rotor’s position. The feedback signal, generated by the encoder, is used by the motor controller to produce input signals to excite and electronically commutate the armature current such that the armature’s magnetic field rotates with the rotor following along in synchronism. 4 Additional advantages of BLDC motors include:
- Longer service life due to a lack of electrical and friction losses.
- Virtually maintenance-free due to a lack of brushes and mechanical commutators.
- Reduced EMI and noise because of the elimination of ionizing spikes from brushes.
- More suitable for hazardous environments (dirt, oil, grease and other foreign matter) since they can be completely sealed.
- Operational characteristics that include (1) high speed, short index moves, (2) heavy loads, high torque control, (3) short duty cycle moves and (4) high accel/decel capability. 5
For industrial applications, brushless DC motors are primarily used in servo, actuation, positioning, and variable speed applications where precise motion control and stable operation are critical for the satisfactory operation of the manufacturing or industrial process. They are commonly used as:
- Linear motors
- Actuators for industrial robots
- Extruder drive motors
- Feed drives for CNC machine tools
Linear motors produce linear motion6 without the need of a transmission system, such as a ball-and-lead screw, rack-and-pinion, cam, gears or belts, that would be necessary for rotary motors. Transmission systems are known to introduce less responsiveness and reduced accuracy. Direct-drive linear motors do not exhibit these shortcomings. In their simplest form, linear motors are essentially “unrolled rotary motors in which the poles of the stator have been laid in the direction of travel.” 7 There are many types of linear motors, ranging from stepper motors, dc brushed & brushless motors and AC synchronous motors. BLDC linear servomotors consist of a slotted stator with magnetic teeth and a moving actuator, which has permanent magnets and coil windings. To obtain linear motion, the motor controller excites the coil windings in the actuator causing an interaction of the magnetic fields thereby producing linear motion. As direct-drive linear motors, BLDC motors have the added advantages of maintenance-free operation with no mechanical connections, hysteresis or pitch cyclical error. 8
Servomotors9 are used for mechanical displacement, positioning or precision motion control based upon an input control and output feedback signal that establishes a tightly controlled, stable, closed loop operation. Servomotor drives are commonly used in machine tool servos, robotic actuator drives, among others. What sets servomotor applications apart from other types of motor control is their inherent high dynamic response, smooth torque production, high reliability and robust control even when there are wide variations in load inertia or motor parameters. 10 In the past DC stepper motors were used as servomotors; however, since they are operate with open loop control, they typically exhibit torque pulsations. 11 Brushless DC motors are more suitable as servomotors due to the feedback capability of the motor.
Actuators for industrial robots
Permanent magnet DC motors12 primarily function as the actuators to move the joints of industrial robots for pick-and-place or tool positioning in assembly, welding and painting operations. (It merits noting that when heavy payloads are involved, hydraulic motors are typically used.13) BLDC motors are preferred over brushed motors in robotic applications due to their compact size, power density, and maintenance-free characteristics. They also perform more reliably in less favorable or hazardous environments.
Extruder drive motors
The function of the extruder drive & motor is to provide energy to turn the screw that compresses the polymer. 14 DC drives are the most popular extruder drive due to their low cost and versatility. Since variations in screw speed can change the dimensions of the final extruded product, a precision motion control system is required to ensure product quality. Brushless DC drives have been frequently used in extruder drives15 because they offer full torque over the entire speed range with short-term speed errors as low as 0%. 16
Feed drives for CNC machine tools
There are two drives used in CNC machine tools: spindle and feed drives.17 Spindle drives provide the motion and power for drilling, milling or grinding operation while feed drives function as axis drive motors and essentially replace the “manual hand wheel controls used in conventional machine tools.” 18 While spindle drives use large DC shunt or AC squirrel cage induction motors, feed drives, on the other hand, typically use DC servomotors with an electronic controller. Brushless DC servomotors are used for their good heat dissipation, reduced rotor inertia and the advantage of maintenance free operation. 19
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