The process of specifying electric motor technologies can present significant challenges for the appliance designer. Off-the-shelf motors tend to serve as starting points, because every particular application will involve varying design and performance criteria. For some applications, high motor speeds and long service life may be crucial, while other applications may call for reduced audible noise and vibration as an overriding requirement. Other operating parameters inherently come into play as well.

When selecting an electric motor for an appliance application, evaluating the differences among key competing motor technologies can help narrow the field. Insights into permanent magnet (PM) brush-commutated DC servo motors, brushless DC servo motors, and hybrid stepper motors can help with decision making.

As a practical matter, generalizations about motor characteristics should never be considered absolute—there will always be exceptions and lines may blur, especially since customized motor solutions often can be recommended and engineered to overcome perceived “deficiencies” relative to application need.

Motor Operating Dynamics

Brushless motors achieve commutation electronically via a permanent-magnet rotor, wound stator and rotor-position sensing scheme. Photo: Ametek Technical & Industrial Products Inc.

All PM brush-commutated and brushless DC servo motors operate by converting electrical energy into mechanical energy through the interaction of two magnetic fields. One field is produced by a permanent magnet assembly; the other field is produced by an electrical current flowing in the motor windings. The relationship between these two fields results in a torque that rotates the rotor. As the rotor turns, the current in the windings is commutated, or switched, to produce a continuous torque output.

Conventional brush-commutated DC motors use brushes (typically graphite with metal content) as part of the commutation process, while brushless motors achieve commutation electronically via a permanent-magnet rotor, wound stator and rotor-position sensing scheme.

Hybrid stepper motors combine the operating principles of two distinct step motor types: permanent magnet and variable reluctance. One or more pairs of laminated stacks featuring many sets of teeth along the outside diameter are positioned on the rotor shaft. A permanent magnet between each stack within a pair creates a north pole and south pole along the axis.

Like all stepper motors, hybrid steppers convert electrical pulses into mechanical movements. Whereas conventional brush or brushless DC motors rotate continuously, a stepper motor, when pulsed, rotates, or steps, in fixed angular increments. Step size, or step angle, is determined by the number of teeth, motor construction and type of drive scheme used for control. The most commonly specified step resolution is 1.8-deg., or 200 steps per revolution, resulting from 50 pole pairs generated by 50 teeth on each rotor lamination.

Selection Factors

Enlarge this picture This chart shows comparative data on motor properties and performance. Source: Ametek Technical & Industrial Products Inc.

All of these motor technologies invite comparisons. Initial factors usually considered by appliance designers include:

Type of Load. Brush and brushless DC servo motors can accommodate a wide range of loads, whether loads are constant, variable, high intermittent peak or unpredictable. Hybrid stepper motors are less tolerant of overload conditions. In fact, high peak loads can cause missed steps and stalls. Stepper types should be specified for applications only where near constant or predictable torque loads can be anticipated.

Physical Size and Power Density. High thermal impedance in PM brush DC motors due to windings located on the armature or rotor, creates a less efficient thermal path and diminished rate of heat dissipation. Therefore, a larger brush motor may be necessary when compared with a brushless motor to achieve a given continuous output torque. This larger motor size may be offset by the potential elimination of associated drive electronics, which can be a primary consideration when component space is limited.

Lower thermal impedance in PM brushless DC motors (windings located in the stator) delivers an efficient thermal path and high rate of heat dissipation—offering the potential for a smaller motor to achieve a given continuous output torque.

Even though hybrid steppers exhibit thermal paths similar to those of brushless DC motors, steppers must be sized to accommodate “worst-case” peak loading, which can require a relatively large package. Since steppers are good at producing high torque at low speed, they offer the potential to eliminate the need for a gearbox, yielding an overall smaller or more convenient package size.

Case Heating. For a given motor size and load point, brushless DC motors tend to run at lower internal temperatures than the other two types, due to the higher thermal impedance levels in brush types and the continuous drawing of full current for hybrid steppers. Steppers are typically designed to draw full current constantly.

Steady-state case temperature for PM brush-commutated DC motors is often significantly less than the winding temperature. Case temperature for brushless models often corresponds with the winding temperature and steppers will run hot relatively constantly.

Speed. Generally, PM brush-commutated DC motors should be operated in excess of 1,000 rpm to prevent brush particle accumulation in the slots between commutator segments that could result in shorting. Recommended operating speeds above 10,000 rpm are atypical, due to limitations inherent in brush-commutator systems, suggesting that where high speed must be realized, brush DC motors carry limitations. Speeds below 1,000 rpm can be realized with a gearbox.

High rotational speeds for brushless DC motors often will be limited only by the mechanical integrity of the rotor construction, speed-related internal losses and bearing selection. Speeds in excess of 10,000 rpm are possible with appropriate designs, and speeds below 1,000 rpm can be handled with a gearbox or directly, depending on drive capabilities.

Because of their high pole count, hybrid steppers typically are selected for low-speed operation below 1,000 rpm. Usable torque begins to drop off quickly in this range, although specifying a low impedance winding may slightly increase the maximum speed. Torque capability is quite high at low speeds and steppers can tolerate operation at multiple speeds through wide ranges.

Efficiency. Gearboxes and brush-and-contact resistance can lower the efficiency of brush DC motors. Improvements may be gained from ironless motor construction and precious metal brushes, although subsequent tradeoffs in power density and life can be expected.

Gearboxes and drive electronics will likewise decrease efficiency in brushless DC motors, although slotless construction techniques can help compensate by reducing cogging, hysteresis and viscous losses.

Hybrid steppers represent the least efficient among these motor types, because of continuous current draw.

Holding Torque Capability. Whether brush or brushless, an un-energized motor’s ability to hold a load will depend on its cogging and friction torque, which usually are low. Holding capacity can be increased using a gear reducer to amplify the holding torque reflected to the load. For superior holding capacity, brake installation is recommended.

An un-energized hybrid stepper’s ability to hold a load depends on its detent torque, which is similarly low. Since step motors are designed to operate continuously at full current, however, they can produce high holding torques indefinitely.

Audible Noise. Primary sources of audible noise in brush DC motors can include armature imbalance, bearings and brushes yielding moderate overall noise levels. Primary sources in brushless DC motors include rotor imbalance and bearings for lower overall noise levels. Noise in both motor types increases when a gearbox is introduced. Solutions such as rotor balancing and appropriate bearing and brush material selection can be employed to reduce noise.

In hybrid steppers, audible noise arises from vibration caused by resonances and bearings to create low to high overall noise levels. These levels can be improved by avoiding operating speeds that lead to resonances or instabilities and by introducing alternate drive schemes, such as microstepping.

Electrical Noise. For brushless DC and hybrid stepper motors less electrical noise is generated compared with brush DC motors, due to the nature of the mechanical commutation system in brush motors. Noise can be mitigated in brush types by adding suppression devices and filters and appropriately selecting brush materials.

Life Expectancy. In general, brushless and hybrid stepper motors will serve much longer than brush-commutated DC motor types. Life expectancy for brush-commutated DC motors is limited primarily by the life of the brushes, bearings and gearbox. Life expectancies in the range of 2,000 to 5,000 hours of operation are common, although actual service life can vary greatly depending on the motor design and operating current, voltage, speed and other conditions.

For brushless DC and hybrid stepper motors, life expectancies are in excess of 10,000 hours and will be limited essentially by bearing life and related radial and axial loads, temperature and environment.

Ease of Integration. Brush DC motors are relatively simple to engineer into an application, in part because commutation is performed mechanically without requiring additional components.

These motor types can be driven directly by a DC power supply, including a battery, and more advanced drive and control schemes can be integrated for functionality beyond basic operation. Applying brushless DC and hybrid stepper motors can be somewhat more complex.

Drive electronics for electronic commutation will be required for both types, although onboard solutions can help simplify the job for system designers. With all the parameters to consider and particular application requirements to satisfy, partnering with an experienced engineering resource can help point the way toward realizing a motor technology’s full potential.