Wound-rotor AC Motors: Squirrel-cage
ac motors are relatively inflexible with regard to speed and torque
characteristics, but a special wound-rotor ac motor has controllable
speed and torque. Application of wound-rotor ac motors is markedly
different from squirrel-cage ac motors because of the accessibility
of the rotor circuit. AC motor performance characteristics are obtained
by inserting different values of resistance in the rotor circuit.
Wound-rotor ac motors are generally started with secondary resistance
in the rotor circuit. The ac motor resistance is sequentially reduced
to permit the motor to come up to speed. Thus, ac motors can develop
substantial torque while limiting locked-rotor current. This secondary
ac motor resistance can be designed for continuous service to dissipate
heat produced by continuous operation at reduced speed, frequent
acceleration, or acceleration with a large inertia load. External
resistance gives ac motors a characteristic that results in a large
drop in rpm for a fairly small change in load. Reduced ac motor
speed is provided down to about 50% rated speed, but efficiency
Multispeed AC Motors: Consequent-pole
ac motors are designed for one speed. By physically reconnecting
the leads, a 2:1 speed ratio can be obtained. Typical synchronous
speeds for 60-Hz ac motors are: 3,600/1,800 rpm (2/4 pole), 1,800/900
rpm (4/8 pole), and 1,200/600 rpm (6/12 pole).
Two-winding ac motors have two separate windings that can be wound
for any number of poles so that other speed ratios can be obtained.
However, ratios greater than 4:1 are impractical because of ac motor size and weight. Single-phase multispeed ac motors are usually
variable-torque design, but constant-torque and constant-horsepower
ac motors are available.
Power output of multispeed ac motors can be proportioned to each
different speed. These ac motors are designed with output horsepower
capacity in accordance with one of the following load characteristics.
AC Motors: Basics of AC Motor Design Engineering