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

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.

AC TORQUE-MOTOR BRAKE

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

windlass.

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

motion.

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

released.

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

1-16