Brushless DC Motors vs. Stepper Motors

Brushless DC motor is a high performance motor, its biggest feature is that it has the external characteristics of DC motor but without the traditional contact commutators and brushes. It uses a permanent magnet rotor with no excitation losses. The heat generating armature windings are mounted on the stator outside, so heat dissipation is easy. Therefore, brushless DC motor has the advantages of no commutation sparks, no radio interference, long life, reliable operation and easy maintenance, etc.

Stepper motor is an open-loop control device that converts electrical pulse signals into angular or linear displacements. Under non-overload conditions, the motor's speed and stop position depend only on the frequency and number of pulses of the pulse signal, regardless of load changes.

Stepper motors can also be classified as brushless DC motors because stepper motors do not have commutation carbon brushes. A stepper motor is a specific term, while a brushless DC motor is a general term. A stepper motor may be a reluctance motor or a permanent magnet synchronous motor. Stepping is an open loop control method and the motor can be controlled in other ways.

Similarities between stepper motors and brushless DC motors

Stepper motors and brushless DC motors have some similarities. They are both driven by direct current. They both force the rotor to rotate by energizing the coils. They control forward and reverse rotation by changing the order of the excitation phases of the drive circuit. They both require electronic commutation and a drive to work. They both work by the interaction of the permanent magnets on the rotor and the magnetic field generated by the stator winding to produce the working torque. The motor has a high torque at low speeds and the torque decreases as the speed rises.

Differences in structure and construction between the two

As for the differences in motor structural components, brushless DC motors consist of permanent magnet rotors, multi-pole winding stators, position sensors and mechanical support parts such as rotor shafts, bushings and compression rings. Stepper motors generally are composed of front and rear end caps, bearings, center shaft, rotor core, stator core, stator components, corrugated washers, screws and other components.

As far as the differences in motor structure are concerned, brushless DC motors can be either outrunner or inrunner, but stepper motors can only be inrunner. The magnets of brushless DC motors are used in larger quantities, generally with ferrite magnets of lower magnetic density, while permanent magnet stepper motors generally use ferrite magnets, and hybrid stepper motors use neodymium magnets of higher magnetic density. The rotor shaft of Brushless DC motor can use ordinary steel, while the rotor shaft of stepper motor must use stainless steel or copper, a material that does not leak magnetism. In order to obtain higher torque, the gap between the rotor and stator of the stepper motor needs to be very small, generally around 0.5mm, while the gap between the rotor and stator of the brushless motor does not need to be so strict, so the stepper motor requires high machining accuracy, because the machining accuracy directly affects the vibration and noise level of the stepper motor.

Differences in the characteristics between the two

• 1. Stepper motors have a relatively large torque, especially at low speeds. Brushless DC motors have a large starting torque, and the torque is proportional to the armature current, which is relatively large at medium to high speeds.

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• 2. The rotation speed of stepper motors is in proportion to the input pulse frequency. Stepper motors have a high torque in the low speed range, and a reduced torque in the high speed range that results in a loss of step. Stepper motors are a kind of low speed and high torque motors, and the speed of most applications is in the range of 600~1500rpm. The rotation speed of brushless DC motors increases proportionally to the voltage supplied to the armature, while the speed decreases as the load torque increases. Brushless motors can produce stable torque from low speed range to high speed range. The maximum speed of brushless DC motors can reach several thousand to several tens of thousands of rpm.

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• 3. The rotational speed and position of stepper motors are controlled by open-loop command pulses, which do not require feedback circuit, but they suffer from out-of-step problems. Brushless DC motors require speed control for constant speed rotation, through feedback from speed sensors to control speed. Since the torque is proportional to the current, it is easy to control the torque of brushless DC motors.

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• 4. The working life of stepper motors is determined by the bearing life, which can reach tens of thousands of hours. The working life of brushless DC motors is also determined by the bearing life, but can reach tens of thousands to hundreds of thousands of hours.

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• 5. Stepper motors are mostly available in mounting sizes with industry standards, which are easier to obtain and have more varieties. However, brushless DC motors are mostly customized for specific applications in the industry and basically do not have industry standard sizes.

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• 6. Stepper motors have more noise and vibration compared to brushless DC motors, especially at low speeds.

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• 7. Stepper motors are less expensive than brushless DC motors because they usually do not require a feedback sensor. But brushless motors need a position sensor.

Summary and suggestions

When selecting motors such as stepper motors and brushless DC motors, a comparison of the characteristics and performance of these two types of motors can be used as a reference. However, the same category of motors includes a variety of specifications, so the comparison in this article is for reference only. The final decision needs to be confirmed by the detailed information in the technical specifications of each motor.