0 Special-Purpose Electric Machines In addition to the types of machines we have studied so far, other types of special-purpose machines which operate on the same principle of previous machines (DC, synchronous, and induction machines) find widespread engineering applications in a variety of fields, including: –Robotics –Vehicle Propulsion –Aerospace –Automotive Control

1 Two-Pole Brushless DC Motor with Three-Phase Stator Winding This is in the reality a permanent-magnet synchronous machine. It is named DC not because of its structure but due to the fact that its operating characteristics resemble those of a shunt DC motor with constant field current. This characteristic can be obtained by providing the motor with a power supply whose electrical frequency is always identical to the mechanical frequency of rotation of the rotor. To generate a source of variable frequency, use is made of DC-to-AC converters (inverters), consisting of banks of transistors that are switched on and off at a frequency corresponding to the rotor speed. Accordingly, these motors require measurement of the position of the rotor to determine speed of rotation and to generate a supply current at the same frequency. The source of excitation is DC, although the current that actually flows through the motor windings is AC. Accordingly, this motor is a synchronous motor.

2 A Typical Bbrushless DC Servomotor Figure 18.1b

3 Conventional DC Motors Brushless DC Motors Advantages: Controllability over wide ranges of speed. Capability of rapid acceleration Convenient control of shaft speed and position. Disadvantage: Commutation (through brushes) causing wear, electrical noise, and sparking. Advantages: Controllability over wide ranges of speed. Capability of rapid acceleration Convenient control of shaft speed and position. No mechanical wear or sparking problem. Disadvantage: Need for more complex power electronics.

4 Stepping Motor Configurations Motors that convert digital information to mechanical motion: in printers; positioning the heads in magnetic disk drives in computer disk drives Characteristics: Angle of rotation of the motor is proportional to the number of input pulses; rapid response-starting-stopping-reversing command; self holding.

5 Single-Phase Induction Motor Applications: Refrigerator compressors; air conditioners; fans; electric tools; washer and dryer motors; and others.

6 Figure Split-Phase Motor Two separate stator windings, called main and auxiliary windings. The axes of the two windings are 90 o with respect to each other. The auxiliary winding current is designed to be out of phase with the main winding current, as a result of the different reactances of the two windings. Because of this the motor sees a two-phase (unbalanced) current that results in a rotating magnetic field, as in a 3-phase machines. Once the motor has started, a centrifugal switch is used to disconnect the auxiliary winding, since a single winding is sufficient to sustain the motion of the rotor.

7 Capacitor-Start Motor Another method for obtaining a phase difference between two currents that will give rise to a rotating magnetic field is by the addition of a capacitor. Explain the effect of the capacitor on the starting torque.

8 Requirements to be Considered on Motors for a specific Application Starting Characteristics Acceleration Characteristics Efficiency at Rated Load Overload Capability Electrical and Thermal Safety Cost!