Special-Purpose Electric Machines [622707]

Special-Purpose Electric Machines
• The machines introduced in this lecture are used in many
applications requiring fractional horsepower, or the ability to accurately control position, velocity or torque. They include
– Brushless DC Motors – Stepping Motors – Single Phase Motors
• The Universal Motors • Single-Phase Induction Motors

Brushless DC Servomotor
A servo motor is a DC, AC, or brushless DC motor co mbined with a position
sensing device (digital decoder). Servos are extrem ely useful in robotics. The
motors are small and are extremely powerful for the size.
Brushless DC motor is not a DC motor but a permanen t-magnet synchronous
machine. The name is actually due to the fact that its operating characteristics
resemble those of shunt DC motor with constant fiel d current.
Figure 18.1b

Figure
18.3 Transistor and SCR Drives for a Brushless DC Motor

Figure
18.8 Stepping Motors
A special type of synchronous motor which is design ed to
rotate a specific number of degrees for every elect ric pulse
received by its control unit. Typical steps are 7. 5 or 15 o per
pulse. It is a motor that can rotate in both direct ions, move in
precise angular increments, sustain a holding torqu e at zero
speed, and be controlled with digital circuits. It moves in
accurate angular increments known as steps, in resp onse to the
application of digital pulses to the electric drive circuit.
Generally, such motors are manufactured with steps per
revolution. Step motors are either bipolar, requiri ng two
power sources or unipolar requiring only one power s ource.
e me m
ppω
ωθ
θ
22
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Figure
18.9 Variable Reactance (VR) Stepper Motor (a) Complete
Motor Assembly; (b) PM Rotor; (c) Stator Cross
Section; (d) Fully Assembled Stator; (e) Stator wit h
Windings.

Figure
18.21 Universal Motor
The universal motor is a rotating machine
similar to a DC motor but designed to
operate either from DC or single-phase AC.
The stator and rotor windings of the motor
are connected in series through the rotor
commutator. Therefore the universal motor
is also known as an AC series motor or an
AC commutator motor. The universal motor
can be controlled either as a phase-angle
drive or as a chopper drive.
The universal motor has a sharply drooping
torque-speed characteristics of a DC series
motor.
Typical applications in vacuum cleaners,
drills, and kitchen appliances.

Figure
18.25 Single-Phase Induction Motor
There are several types of single-phase induction m otors in use today. Basically they
are identical except for the means of starting. The y may be classified as: Split-phase
motors; and capacitor-start motors

Figure
18.30 Split-Phase Motor
They are constructed with two-separate stator windi ngs, called main and auxiliary
windings. The axes of the two windings are 90o with respect to each other. The
auxiliary winding current is out of phase with the main winding current, as a result of
the different reactances of the two windings. Once t he motor started, centrifugal switch
is used to disconnect the auxiliary winding. This m otor is used for fans, blowers,
centrifugal pumps (range 1/20 to ½ hp)

Figur
e
18.3
2Capacitor-Start Motor
Another method to obtain a phase difference between two current that will give rise to a
rotating magnetic field is by addition of a capacit or in series with the auxiliary winding.
The addition of the capacitor changes the reactance of the auxiliary current in order to
lead the main current. So the starting torque will be higher. Once the motor started,
centrifugal switch is used to disconnect the auxili ary winding.

Capacitor-Start Capacitor-Run Motor
By using two capacitors-one to obtain a permanent p hase split and the resulting
improvement in running characteristics, the other c apacitor (larger) will improve the
starting torque.
Figure 18.35

Motor Selection Criteria
• Available power (DC or AC) • Operating condition. • Starting characteristics (torque and current) • Operating speed. • Forward/reverse operation. • Acceleration characteristics (depending on load) • Efficiency at rated load. • Overload capability. • Electrical and thermal safety. • Life span and maintenance. • Mechanical aspects (size, weight, noise level, env ironment).
• EMC and EMI • Control complexity and Cost.