Emergency stop switches at each level served.
ELECTRIC (ELECTROMECHANICAL)
ELEVATORS
These switches allow operators at any level to
stop the elevator should a malfunction occur.
The platform on electric elevators is raised and
Overtravel switches. These switches stop the
lowered by groups of cables that pass over sheaves and
elevator if it should fail to stop at the uppermost
then to the hoisting machinery drums. The hoisting
level.
drums, coupled together, are driven through a reduction
Overload protection.
gear unit by an electric motor.
This feature prevents
damage to the system from an overload
The motor is of the two-speed type. The control
condition.
arrangements are such that the elevator starts and runs
on the high-speed connection. The low speed is used
for deceleration as the elevator approaches the upper or
Elevator controllers are designed with a
lower limit of travel.
double-break feature that prevents improper operation
The two-speed electric motor is controlled through
if any one contactor, relay, or switch should fail to
a system of contractors, relays, limit switches, and
function properly. Pushbuttons are interlocked to
selector switches. Automatic operation is obtained by
prevent operation of the elevator unless the platform is
selecting the levels between which the platform is to run.
at the same level as the pushbutton. Some elevators are
The start pushbutton can then be used to close contractors
equipped with hatchway door mechanical interlocks to
through safety switches to operate the elevator at high
prevent opening the door unless the platform is at the
speed. Just before reaching the desired level, the control
same level.
transfers the motor to the low-speed winding through
the action of cam-operated limit switches. On reaching
A governor-actuated safety device is provided
the desired level, the control circuit is disconnected by
under the platform to grip the guide rails and stop the
a cam-operated stop switch, releasing the contractors and
platform if there is an overspeed in the DOWN
setting the brake to stop the platform.
direction. Also, spring bumpers are provided at the
bottom of the hatchway to prevent mechanical damage
to the hull or platform due to overtravel in the DOWN
direction.
For safety in operation, all doors at each
level are interlocked to prevent operation
The operation of the elevator depends on the
unless they are closed. Also, all hatch covers
position of the selector switch. The selector switch
are interlocked to prevent elevator operation
determines which decks the elevator will run
unless they are fully opened.
between. This switch also makes all master switches
inoperative, except those pertaining to the selected
levels.
The following protective features are incorporated
in the control:
Suppose the selector switch is set in the second
Slack-cable switches. These switches prevent
platform to the third deck position (fig. 5-23). Refer to
figure 5-23 as you read the sequence of events which
operation of the elevator if any cable should
become slack.
follow:
5-39
