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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

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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.

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A Typical Bbrushless DC Servomotor

Figure 18.1b

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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.

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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.

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Single-Phase Induction Motor

Applications: Refrigerator compressors; air conditioners; fans; electric tools; washer and dryer motors; and others.

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Figure 18.30

Split-Phase Motor

Two separate stator windings, called main and auxiliary windings. The axes of the two windings are 90o 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.

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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.

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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!