Quantcast Figure 1-21.-Torque-motor brake and ball-jack assembly.

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into the coil. The armatures are mechanically linked to
the levers. The levers pivot on the pins. When the
magnetic pull overcomes the pressure of the coil
springs, the pressure of the brake shoes on the drum is
removed, allowing the drum to turn. The drum is
mechanically coupled to the motor shaft or the shaft of
the device driven by the motor. The coil is connected to
the voltage supply lines. The method of connecting the
coil (series or parallel) is determined by the coil design.
The magnetic brakes are applied when the coil is not
energized. A spring or weight holds the band, disk, or
shoes against the wheel or drum. When the coil is
energized, the armature or solenoid plunger overcomes
the spring tension and releases the brake.
The ac solenoid brake frame and solenoid are of
laminated construction to reduce eddy currents, which
are characteristic of ac systems. Because the magnetic
flux passes through zero twice each cycle, the magnetic
pull is not constant. To overcome this, shading coils are
used to provide pull during the change of direction of
the main flux. The principal disadvantage of an ac
solenoid is that it draws a heavy current when the voltage
is first applied.
Figure 1-21.-Torque-motor brake and ball-jack assembly.
against the brake drum. This action stops and holds the
The torque-motor brake uses a specially wound
windlass drive shaft.
polyphase, squirrel-cage motor in place of a brake-
The torque-motor brake can be released manually
release solenoid. The motor may be stalled without
by raising the lever(1). However, the lever must be held
injury to the winding and without drawing heavy cur-
rents. Figure 1-21, view A, shows the complete mechan-
manually in the UP position; otherwise, the brake will
ical arrangement of the torque-motor brake assembly,
be applied.
and figure 1-21, view B, is an enlarged view of the ball-
Dc Dynamic Brake
jack assembly. his assembly is used with an anchor
Dynamic braking is similar to the slowing down of
The mechanical connection between the torque-
a moving truck by the compression developed in its
motor shaft and the brake operating lever (1) is through
engine. A dc motor also slows down when being driven
a device called a ball-jack assembly, which converts the
by its load if its field remains excited. In this case, the
rotary motion of the torque-motor shaft to a straight line
motor acts as a generator and returns power to the
supply, thereby holding the load. In an actual braking
system, however, the dc motor is disconnected from the
When power is applied to the torque motor, the shaft
line. Its armature and field are connected in series with
turns in a clockwise direction, resulting in an upward
a resistor to form a loop. The field connections to the
movement of the jack screw (2). The thrust element (3)
armature are reversed so the armature countervoltage
in the jack screw pushes upward against the operating
maintains the field with its original polarity.
lever (1) to release the brake. As soon as the brake is
fully released, the torque motor stalls across the line and
Figure 1-22 shows the connections in the dynamic
holds pressure against the spring (4), keeping the brake
braking system of a series-wound dc motor. The field
switching is carried out by switches S1, S2, and S3,
which are parts of a triple-pole double-throw (TPDT)
When the voltage supply to the torque motor is
assembly. These switches are magnetically operated
interrupted, the torque spring forces the brake shoes

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