Stepper Motors and Brush Less DC Motors


Stepper motors are used for accurate, repeatable positioning such as machine tool applications or for moving the head in an ink-jet printer. By using an electronic controller that applies electrical pulses to the motor windings, the motor shaft can be rotated in either direction in multiples of the step angle, which can range from 0.720 (500 steps per revolution) to 150 (24 steps per revolution). Stepper motors are available with rotational accuracies on the order of 3 percent of a step, which is noncumulative as the motor is stepped back and forth. By controlling the rate at which pulses are applied to the windings of the stepper motor, speed can be varied continuously from a standing stop to a maximum that depends on the motor and load. The three main types of stepper motors are:

1.Variable-reluctance stepper motor 2.Permanent-magnet stepper motor 3.Hybrid stepper motors

The permanent-magnet stepper motor, which has a cylindrical rotor (called a tin-can rotor) that is permanently magnetized with north and south poles alternating around its circumference. The stator of the permanent magnet motor is similar to that of the reluctance motor. As in the reluctance type, the rotor position is stepped by applying a sequence of pulses to the stator windings. Hybrid stepper motors that combine variable reluctance with permanent magnets are also available.

Conventional dc motors are particularly useful in applications that require high speeds and in those for which dc power is available, such as those in aircraft and automobiles. However, because they contain commutators and brushes, conventional dc motors have several disadvantages. These include relatively short service lives due to brush and commutator wear, particularly at very high speeds. Also, arcing as the brushes move between commutator segments can pose a hazard in explosive environments and can create severe radio interference. A relatively new development, the brushless dc motor, provides an excellent alternative when the disadvantages of conventional dc motors are prohibitive. Brushless dc motors are essentially permanent-magnet stepping motors equipped with position sensors (either Hall Effect or optical) and enhanced control units.

As in the stepper motor, power is applied to one stator winding at a time. When the position sensor indicates that the rotor has approached alignment with the stator field, the controller electronically switches power to the next stator winding so that smooth motion continues. By varying the amplitude and duration of the pulses applied to the stator windings, speed can be readily controlled. The result is a motor that can operate from a dc source with characteristics similar to those of a conventional shunt dc motor. Brushless dc motors are used primarily in low-power applications. Their advantages include relatively high efficiency, long service life with little maintenance, freedom from radio interference, ability to operate in explosive chemical environments, and capability for very high speeds (50,000 rpm or more).

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