An electric-elevator installation requires, in addition to the car described in Art. 16.8 and the hoistway components described in Art. 16.7, wire ropes for raising 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).
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.
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 buildings usually are supplied with ac power, the variable voltage generally is obtained from a motor-generator set that converts ac to dc. This type of control commonly is used for passenger elevators because it combines smooth, accurate speed regulation with efficient motor operation. It also permits rapid acceleration and deceleration and accurate car stops, with low power consumption and little maintenance.
But multivoltage control costs more initially than rheostatic control.
Variable-voltage, variable-frequency control is a means used to produce smooth acceleration, deceleration, and stopping of common ac motors at nonsynchronous speeds. VVVF control offers much higher efficiency than that realized through dc motors and is gradually replacing the various means used to control dc motors, along with the dc motor.
Car Leveling at Landings
Elevator installations should incorporate equipment capable of stopping elevator cars level with landings within a tolerance of 1â„2 in under normal loading and unloading conditions. Because changing car loads vary the stretch of the hoisting ropes, provision should be made to compensate for this variation and keep the car platform level with the landing. Most elevators employ automatic leveling.
Automatic leveling controls the driving motor to level the car. Elevators typically employ a two-way, automatic leveling device to correct the car level on both overrun and underrun at a landing and hold the car level with the landing during car loading and unloading.
Terminal Stopping Devices
For safety, provisions should be made to control car movement as it approaches a terminal landing and to keep it from passing the terminal. For the purpose, special speed-limiting and stopping devices are needed.
An emergency terminal speed-limiting device is required to reduce car speed automatically as the car approaches the terminal landing. This should be done independently of the functioning of the operating device, which actuates the elevator control, and of the normal terminal stopping device if it should fail to slow the car down as intended.
The normal terminal stopping device slows down and stops the car at or near a terminal landing independently of the functioning of the operating device. It should continue to function until the final terminal stopping device operates.
The final terminal stopping device is required to interrupt automatically the electric power to the driving-machine motor and brake after the car has passed a terminal landing. But this device should not operate when the car has been stopped by the normal terminal stopping device. When the final terminal stopping device has been actuated, normal car operating devices should be rendered incapable of moving the car.
Car and Counterweight Safeties
A safety is a mechanically operated device that is capable of stopping and supporting the weight of an elevator car and its load when the device is actuated by a car-speed governor. The safety should be actuated when the car travels at more than 15% above its rated speed.
Car safeties are generally mounted on the safety plank, or bottom member of the car frame. When tripped, springs on the safeties push shoes against the guide rails hard enough to make the car slide to a stop. (When a hoistway is located above an accessible space, safeties, such as those used for cars, should also be provided on the counterweight frame.) The safeties are typically released by upward motion of the car.
The governor may be conveniently located in the machine space, where the device will not be struck by the car or the counterweight if either should overtravel.
The governor may measure car speed from the rotation of a sheave around which is wound a wire rope connected to the car and held under tension. When the car goes too fast, the governor trips jaws that grip a wire rope connected through linkages to a safety and release a spring to actuate the safety. Also, electrical switches on the governor and the safety are opened to remove power from the driving machine and apply a friction brake to the drive shaft.