There are several factors to take into consideration when choosing a stepper motor for an application. Some of these factors are what type of stepper motor to use, the torque requirements of the system, the complexity of the controller, as well as the physical characteristics of the motor. The following contents focus on these considerations.
Variable Reluctance vs. Permanent Magnet or Hybrid
Variable Reluctance Motors (VRM) benefit from the simplicity of their design. These motors do not require complex permanent magnet rotors, so are generally more robust than permanent magnet motors. With all motors, torque falls with increased motor speed, but the drop in torque with speed is less pronounced with variable reluctance motors. With appropriate motor design, speeds in excess of 10,000 steps per second are feasible with variable reluctance motors, while few permanent magnet and hybrid motors offer useful torque at 5000 steps per second and most are confined to speeds below 1000 steps per second.
The low torque drop-off with speed of variable reluctance motors allows use of these motors, without gearboxes, in applications where other motors require gearing. For example, some newer washing machines use variable reluctance motors to drive the drum, thus allowing direct drive for both the slow oscillating wash cycle and the fast spin cycle.
Variable reluctance motors do have a drawback. With sinusoidal exciting currents, permanent magnet and hybrid stepper motors are very quiet. In contrast, variable reluctance motors are generally noisy, no matter what drive waveform is used. As a result, permanent magnet or hybrid motors are generally preferred where noise or vibration are issues.
Unlike variable reluctance motors, permanent magnet and hybrid motors cog when they are turned by hand while not powered. This is because the permanent magnets in these motors attract the stator poles even when there is no power.’ This magnetic detent or residual holding torque is desirable in some applications, but if smooth coasting is required, it can be a source of problems.
With appropriate control systems, both permanent magnet and hybrid motors can be microstepped, allowing positioning to a fraction of a step, and allowing smooth, jerk-free moves from one step to the next. Microstepping is not generally applicable to variable reluctance motors. These motors are typically run in full-step increments. Complex current limiting control is required to achieve high speeds with variable reluctance motors.
Permanent magnet and hybrid stepper motors are available with either unipolar, bipolar or bifilar windings; the latter can be used in either unipolar or bipolar configurations. The choice between using a unipolar or bipolar drive system rests on issues of drive simplicity and power to weight ratio.
Bipolar motors have approximately 30% more torque than an equivalent unipolar motor of the same volume. The reason for this is that only one half of a winding is energized at any given time in a unipolar motor. A bipolar stepping motor utilizes the whole of a winding when energized.
The higher torque generated by a bipolar motor does not come without a price. Bipolar motors require more complex control circuitry than unipolar motors (see “Basic Control Circuits“). This will have an impact on the cost of an application.
If in doubt, a unipolar motor or bipolar motor are good choices. These motors can be configured as a unipolar or bipolar motor and the application tested with the motors operating in either mode.
Hybrid vs. Permanent Magnet
In selecting between hybrid and permanent magnet stepper motors, the two primary issues are cost and resolution. The same drive electronics and wiring options generally apply to both motor types.
Permanent magnet motors are, without question, some of the least expensive motors made. They are sometimes described as can-stack motors because the stator is constructed as a stack of two windings enclosed in metal stampings that resemble tin cans and are almost as inexpensive to manufacture. In comparison, hybrid and variable reluctance motors are made using stacked laminations with motor windings that are significantly more difficult to wind.
Permanent magnet motors are generally made with step sizes from 30 degrees to 3.6 degrees. The challenge of magnetizing a permanent magnet rotor with more than 50 poles is such that smaller step sizes are rare! In contrast, it is easy to cut finely spaced teeth on the end caps of a permanent magnet motor rotor, so permanent magnet motors with step sizes of 1.8 degrees are very common, and smaller step sizes are widely available. It is noteworthy that, while most variable reluctance motors have fairly coarse step sizes, such motors can also be made with very small step sizes.
Hybrid stepper motors suffer some of the vibration problems of variable reluctance motors, but they are not as severe. They generally can step at rates higher than permanent magnet motors, although very few of them offer useful torque above 5000 steps per second.