All electric elevators are of the traction type, incorporating multiple-hoist ropes with counterweights. Hoisting machinery should preferably be placed at the top of the hoistway. Under these circumstances, the weight of this machinery, plus the elevator lodgings, must be carried by the building structure.
Where required cars open on two opposite ends, usual height of door opening is 225 cm and minimum floor-to-floor height is 1.5 times door opening plus 15 cm.
For low rise, slow-speed applications, the hoisting machinery can be placed in the basement adjacent to the hoistway if top mounting is impractical. Consideration should be given to sound isolation of this machinery, particularly hotels.
(a) Gearless traction elevators – Are utilized for speed above 150 m/min.
(b) Geared traction elevators – Are used for slower speed passenger elevators and for all freight services. For low speed applications where elevators usage infrequent or where minimum investment is desired, the alternating-current rheostat type of control is suitable, in which the hoisting motor utilized alternating current.
Leveling, or alignment between car floor and building floor, is not precise, variable-voltage control utilized a direct-current hoisting motor. A wide range of car speed is possible, together with smooth stops and rapid acceleration and declaration. Automatic leveling is always provided with these types of control.
A motor generator set is required for each hoisting motor. This set does not need to be in the machinery penthouse. Actually, a basement space is preferable, Variable voltage control is, of course more expensive than alternating current rheostat control.
(I) Hydraulic Elevator System:
A low-cost and low-maintenance-cost elevator is the hydraulic (plunger) elevator. The car moves at a relatively low speed and the elevator system is limited to low-rise buildings, generally about six serviced levels.
The elevator system does not have cables, counter weights, brake drums, MG sets, or costly control equipment and has fewer safety devices than the cable type elevator system-hence the low initial system cost and lower maintenance requirements.
The car is moved by a hydraulic hoist or plunger attached to the car floor structure. The elevator machine is an oil pumping system. Oil is pumped from a reservoir into the base of the plunger system and the car is lifted; removing oil from the plunger base with the same pump lowers the car.
The shaft has guide rails for car movement. The shaft pit has a buffer spring. There are limit switches. If there is an oil leak, the car will slowly descend. It is a very safe elevator system.
Elevator system maintenance is simple but continuous. The major maintenance problem is floor leveling. The car may stop slightly above or below the floor level, and if passengers are not watchful, there may be accidents when entering or leaving the car. A slight change in reservoir oil level could result in floor leveling problems.
Whereas the variable voltage traction elevator machine is probably the best system for floor leveling, the hydraulic elevator is classified by some users as the forest for floor leveling. Floor leveling generally can be controlled within plus or minus 25 mm. with a hydraulic elevator, very similar to a rheostat traction elevator.
Floor leveling with a variable voltage traction elevator machine generally can be controlled to within plus or minus 6 mm. The maintenance of those components that are similar to the cable elevator is identical to the cable elevator.
The final factor for this elevator is its energy operating cost. As the elevator does not have counterweights, the energy requirements are relatively large-almost twice as great when compared to cable elevators. These elevators are frequently used as service elevators in low-rise buildings.
Superficial comparisons of power costs should be avoided. Electric traction elevators of are counterweighted, so only approximately 50% of the load in the both direction is imposed upon the hoisting motors. The hydraulic motor carries full load on the apron and no load on the down run.
Thus, for a given load, the horsepower required is greater for hydraulic than for electric traction. However, power consumption is approximately equal for both types of equipments. Initial cost is great for electric hydraulic than that of equivalent traction elevator but maintenance cost can be lower.