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There are three primary types of stepper motors available in the market: PM (permanent magnet), VR (variable reluctance), and hybrid. Each type has its own unique characteristics, applications, and benefits. Let’s explore the differences and determine which one might be the best fit for your needs.
Firstly, the PM type stepper motor is constructed with a rotor made of two permanent magnet rotors that are slightly offset from one another. These permanent magnets are axially magnetized, meaning the north and south poles alternate along the same axis as the motor shaft. When current is applied to the windings, the stator poles become magnetized and align with the opposite poles of the permanent magnet rotor. A commonly used PM type stepper motor is the 2-phase claw type.
This type of stepper motor is relatively straightforward in design and cost-effective to produce. The permanent magnets enable PM type stepper motors to generate higher torque compared to other types. However, their high-speed performance is limited due to significant losses during rotation. PM type stepper motors are typically used in simple positioning tasks, though higher resolution can be achieved through techniques like half-stepping or microstepping. Microstepping requires precise current control from the driver, impacting the motor's positional accuracy. Additionally, PM type stepper motors often use constant voltage drivers, which are more challenging to manage compared to constant current chopper drivers.
Next, the VR type stepper motor features teeth on both the rotor and stator, concentrating magnetic forces. Among the three types, this design is the simplest. A VR type stepper motor operates without permanent magnets, which means it lacks holding torque or detent torque when stationary. An advantage of the VR type is that it doesn’t require changing the polarity of the stator poles, simplifying its drive circuitry. However, its inability to finely control torque remains a drawback, as torque is proportional to the square of the current rather than the current itself. Despite this limitation, VR type stepper motors perform better at higher speeds compared to PM type stepper motors, thanks to their lack of permanent magnets.
The step angle of a VR type stepper motor is determined by the number of rotor teeth (Np) and the number of phases (Nphase). To achieve finer resolutions, increasing the number of rotor teeth is effective. However, increasing the number of stator slots complicates manufacturing processes. A VR type stepper motor has been around for a long time and is often chosen for mid to high-speed applications due to its reduced torque drop-off at higher speeds. However, it tends to produce more noise, limiting its application range. VR type stepper motors are less common than PM type stepper motors in the market.
Lastly, the hybrid type stepper motor combines the best features of both PM and VR types. It incorporates permanent magnets and toothed rotors and stators, allowing for superior magnetic flux focusing. This design achieves low losses by stacking magnetic steel laminations that efficiently conduct magnetic flux. Hybrid stepper motors are equipped with strong rare-earth permanent magnets and feature both north and south pole rotors, each with 50 or 100 teeth, depending on the model. The rotor cups are offset by half a pitch, ensuring that the south pole teeth from one rotor align between the north pole teeth of the other rotor.
When the windings are energized, the stator poles become electromagnets, pulling and pushing the permanent magnet rotor into fixed increments. Hybrid stepper motors offer high torque due to the rare-earth permanent magnets and maintain high resolution thanks to their small teeth. Their torque-current relationship is nearly proportional, making them easier to control than VR type stepper motors. Hybrid stepper motors can achieve high resolutions, such as 0.72° per full step or 500 steps per revolution, and even higher resolutions with microstepping.
In summary:
- **PM (Permanent Magnet) Type**: Uses permanent magnets, moderate torque, suitable for low to mid-speed applications.
- **VR (Variable Reluctance) Type**: Toothed rotor/stator, low torque, suitable for mid to high-speed applications.
- **Hybrid Type**: Combines permanent magnets and teeth, offering a balanced combination of torque and speed.
The hybrid type stepper motor is the most popular due to its excellent performance, despite being more expensive. Hybrid stepper motors are versatile and work well for most applications, whereas PM or VR type stepper motors are more suited to specific tasks. Modern stepper motor drivers are becoming increasingly intelligent, with advancements in functionality, distributed control, and ease of programming. Adding gearheads and closed-loop feedback systems can further expand the stepper motor's application range. Partnering with a manufacturer that offers comprehensive components and training can simplify the process significantly.
If you’re interested in learning more, check out our technical articles and white papers on stepper motor fundamentals. Please consider our high-torque hybrid stepper motors for your projects!
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