A maior rede de estudos do Brasil

Grátis
634 pág.
1 - principles of electrical machines and power electronics p_c_sen

Pré-visualização | Página 37 de 50

the converter. An inner current loop can be pro-
vided so that the motor current can be clamped to a specified value. 1 A block 
diagram of such a system is shown in Fig. 4.64. The output of the speed 
controller represents a torque command. Because torque is proportional to 
armature current, the output of the speed controller also represents the 
current command r::, which is then compared with the actual current I •. A 
limit on the output of the speed controller will therefore clamp the value of 
the motor current I •. 
The speed controller and current controller can have proportional (P) or 
proportional-integral (PI) control. 1 The selection depends on the require-
ment of drive performance. 
1 P. C. Sen, Thyristor DC Drives, Wiley-lnterscience, New York, 1981. 
N 
Speed 
controller 
Permanent Magnet DC (PMDC) Motors 19 7 
Current 
controller 
Power 
supply 
FIGURE 4.64 Closed-loop speed control with inner current loop. 
4.6 PERMANENT MAGNET DC (PMDC) MOTORS 
In permanent magnet de (PMDC) motors, the stator wound poles of a con-
ventional de motor are replaced by permanent magnets. The rotor has a 
conventional de armature, with commutator segments and brushes. Unlike 
the salient-pole field structure of the conventional de motor (Fig. 4. 7), perma-
nent magnet motors have a relatively smooth stator structure, as shown in 
Fig. 4.65. The outer shell is made of magnetic material and the permanent 
magnets, which are radially magnetized, are mounted on the inner periphery 
of the outer shell. 
PMDC motors have several advantages. Because of the absence of the 
field windings, copper loss is absent and this increases efficiency. No space 
is required for field windings; consequently, these motors are smaller than 
corresponding wound-pole motors and, in some cases, cheaper as well. 
Although permanent magnet materials tend to be more expensive, the reduc-
tion in the size of the motor reduces the cost of other materials, which 
Radially magnetized 
permanent magnets 
FIGURE 4.65 Cross-sectional view of a 
PMDC motor. 
("''' 
...... 
198 chapter 4 DC Machines 
compensates, at least in part, for the extra magnet cost. PMDC motors have 
a definite cost advantage in the smaller size range. 
The PMDC motors have some disadvantages. There is a risk of demagneti-
zation, which may be caused by excessive armature current. Demagnetiza-
tion may also be caused by excessive heating, which can occur if the motor 
is overloaded over a prolonged period. The magnetic field is always present 
even when the motor is not being used. The enclosure of the motor should 
be carefully designed so that no foreign matter is attracted that would be 
harmful to the motor. In addition, permanent magnets cannot normally 
produce the high flux density of a wound pole motor. However, with the 
development of high-flux-density rare earth magnets, PMDC motors are 
becoming viable alternatives to de shunt motors. 
The speed of a PMDC motor cannot be controlled by field flux and thus 
speed control must be achieved by changing the armature voltage. These 
motors are therefore used only where motor speeds below base speed are 
required. They do not offer the flexibility of operation beyond the base speed. 
PMDC motors are being produced in the millions each year and are being 
used in many applications, ranging from fractional horsepower to several 
horsepower. They are used extensively in automobiles-to operate wind-
shield wipers and washers, to raise and lower windows, to operate pumps, 
to drive blowers for heaters and air conditioners, and so on. They are often 
employed in equipment that is supplied from battery sources. PMDC motors 
up to 200 hp have been developed for industrial applications. 
The types of permanent magnet materials used for PMDC motors include 
alnicos, ferrites, and rare earth materials (samarium-cobalt and neodym-
ium-iron-boron). Alnicos are used in low-current, high-voltage applications 
because of low coercivity and high flux density. Ferrites are used in cost-
sensitive applications, such as in air conditioners, compressors, and refriger-
ators. For size-sensitive applications rare earth materials are used-for ex-
ample, in the automobile industry. Large industrial PMDC motors use rare 
earth materials. Servo industrial drives requiring high performance and 
high torque/inertia ratios use rare earth materials. Neodymium-iron-boron 
is cheaper than samarium-cobalt. However, the latter can stand higher tem-
peratures than the former. 
Most of the de motors likely to be used in the future are PMDC motors. 
4.7 PRINTED CIRCUIT BOARD (PCB) MOTORS 
The printed circuit board (or disk armature) motor, using permanent mag-
nets, has a configuration radically different from that of the conventional 
de motor. Figure 4.66 shows the construction of such a motor. The rotor 
has no iron and is formed of a disk of nonconducting, nonmagnetic material. 
The entire armature winding and the commutator are printed in copper on 
both sides of the disk. The brushes are placed around its inner periphery. The 
disk armature is placed between two sets of permanent magnets mounted on 
Printed Circuit Board (PCB) Motors 199 
PCB Disk (rotor) 
FIGURE 4.66 PCB motor assembly. (Courtesy of PMI Motion Technologies.) 
ferromagnetic end plates. This configuration provides axial flux through the 
armature. The radial current flowing through the disk armature interacts 
with the axial flux to produce torque that rotates the rotor, as in any de motor. 
This type of motor has several advantages: 
Because of its low rotor inertia, it has a high torque/inertia ratio and thus 
can provide rapid acceleration and deceleration. The motor can accelerate 
from 0 to 4000 rpm in 10 milliseconds. 
The armature inductance is low because there is no iron in the rotor. 
Because of the low inductance, there is little arcing, which leads to longer 
brush life and high-speed capability. Low armature inductance makes the 
armature time constant low. Consequently the armature current can build 
up very quickly (in less than 1 millisecond), which implies that full torque 
is available almost instantly, a key to quick motion response and accu-
rate tracking. 
The motor has no cogging torque because the rotor is nonmagnetic. 
These motors are particularly suitable for applications requiring high 
performance characteristics. Examples are high-speed tape readers, X-Y 
recorders, point-to-point tool positioners, robots, and other servo drives. 
Typical sizes of these motors are in the fractional horsepower ranges. How-
ever, integral horsepower sizes are also available. 
I"'" 
1, •.. •• 
., . 
.. ,, ,, 
',.,• 
200 chapter 4 DC Machines 
PROBLEMS 
4.1 Two de machines of the following rating are required: 
DC machine 1: 120 V, 1500 rpm, four poles 
DC machine 2: 240 V, 1500 rpm, four poles 
Coils are available which are rated at 4 volts and 5 amperes. For the same 
number of coils to be used for both machines, determine the 
(a) Type of armature winding for each machine. 
(b) Number of coils required for each machine. 
(c) kW rating of each machine. 
4.2 A four-pole de machine has a wave winding of 300 turns. The flux per pole 
is 0.025 Wb. The de machine rotates at 1000 rpm. 
(a) Determine the generated voltage. 
(b) Determine the kW rating if the rated current through the turn is 25 A. 
4.3 A de machine (6 kW, 120 V, 1200 rpm) has the following magnetization 
characteristics at 1200 rpm. 
11 (A) 0.0 0.1 
Ea (V) 5 20 
0.2 
40 
0.3 
60 
0.4 
79 
0.5 
93 
0.6 
102 
0.8 
114 
1.0 
120 
1.2 
125 
The machine parameters are Ra = 0.2 !1, Rfw = 100 D. The machine is driven 
at 1200 rpm and is separately excited. The field current is adjusted at lr = 
0.8 A. A load resistance RL = 2 n is connected to the armature terminals. 
Neglect armature reaction effect. 
(a) Determine the quantity Ka<l> for the machine. 
(b) Determine Ea and la. 
(c) Determine torque T and load power PL. 
4.4 Repeat Problem 4.3 if