Building Design and Construction Handbook - Merritt - Ventech!

VERTICAL CIRCULATION 16.29
and lowering the car and for other purposes, a driving machine, sheaves for controlling
rope motion, control equipment for governing car movements, a counterweight,
and safety devices (Fig. 16.13).
16.9.1 Driving Machines
Components of an electric driving machine include an electric motor, a brake, a
drive shaft turned by the motor, a driving sheave or a winding drum, and gears, if
used, between the drive shaft and the sheave or drum. The brake operates through
friction on the drive shaft to slow or halt car movement. Hoisting-rope movement
is controlled by the driving sheave or the winding drum around which the ropes
are wound.
Traction machines are generally used for electric elevators. These machines
have a motor directly connected mechanically to a driving sheave, with or without
intermediate gears, and maintain and control motion of the car through friction
between the hoisting ropes and the driving sheave. Also called a traction sheave,
this wheel has grooves in its metal rim for gripping the ropes.
Geared-traction machines, used for slow- and medium-speed elevators, have
gears interposed between the motor and the driving sheave. The gearing permits
use of a high-speed ac or dc motor with low car speeds, for economical operation.
Recently, helical gear machines have been employed effectively for variablevoltage,
variable-frequency (VVVF) control ac elevators. Whereas conventional
worm-geared machines limit car speeds to 450 or 500 ft/min, the dual efficiency
of the helical gearbox coupled with an ac motor produces car speeds of up to 800
ft/min. Progress in solid-state design has virtually eliminated the classic singleand
two-speed ac-drive systems.
Gearless traction machines, in contrast, are used with ac or dc motors for elevators
that operate at speeds of 500 ft/min or more. This type of elevator machine
is essentially a large motor with a traction sheave and brake mounted on a common
shaft. Gearless dc and ac motored (VVVF control) machines are effectively used
for car speeds of 500 ft/min or more. Since the gearless traction machine consists
of a custom-built motor, traction sheave, and brake on a custom motor frame, these
machines are the most expensive elevator drive systems.
A winding-drum machine gear-drives a grooved drum to which the hoisting
ropes are attached and on which they wind and unwind. For contemporary elevators,
the winding-drum drive system is applied only to dumbwaiters and light-duty residential
units.
16.9.2 Elevator Control
The system governing starting, stopping, direction of motion, speed, and acceleration
and deceleration of the car is called control. Multivoltage control (also known
as variable-voltage control) or rheostatic control has been commonly used for electric
elevators, due largely to the relative simplicity of controlling the dc motor. The
advent of larger power transistors has resulted in control systems known as VVVF
control, that can be applied to ac motors to produce smooth starting and stopping
equal to the classic dc elevator control system.
Multivoltage control usually is used with driving machines with dc motors. For
elevator control, the voltage applied to the armature of the motor is varied. Because