Cranes – Design, Practice, and Maintenance phần 3 potx

50 Cranes – Design, Practice, and Maintenance

Fig. 3.1.2 Fluid coupling

fluid coupling is an excellent type of drive as it gives smooth accelera￾tion of the complete belt system.

The slipring motor

The slipring motor is a drive which is now little used but it is still worth

mentioning. The alternating current slipring motor is speed-controlled

by resistances. These resistance-steps can be switched on or off by the

controller. If torque is required: the more resistance, the lower the

speed. ‘No resistance’ gives the speed curve of the normal squirrel cage

motor. The brushes of the motor need regular maintenance; the resist￾ances can burn out and rust. Therefore resistances made of stainless

steel have preference.

Fig. 3.1.3 Slipring motor: resistance controlled

Drives; Calculating Motor Powers 51

The Ward–Leonard drive

The Ward–Leonard (WL) drive can be considered as a ‘better DC dri￾ve’. (The DC drive with resistance control is not further described.) The

more complicated WL drive has great advantages compared to drives

with slipring motors or DC motors with resistance control.

The main motor, which is a squirrel cage motor, runs at a constant

speed during the workshift on the crane. It drives a Ward–Leonard

generator for each mechanism. The generator is directly coupled to the

main motor and gives a regulated voltage and current to the respective

motor which forms the drive-element of the crane mechanism. The

speed control of this drive-element can be stepless.

With a three-field generator like the Ward–Leonard–Kra¨mer the

maximum torque can be fixed exactly at the desired level. This gives

excellent drives for the hoisting mechanisms of grabbing cranes which

dredge under water and for the drives of cutter-dredgers and similar

devices. Cosphi compensation is not necessary. The Ward–Leonard–

Fig. 3.1.4 Ward–Leonard–Kra¨mer (hoist motion)

52 Cranes – Design, Practice, and Maintenance

Kra¨mer drive has advantages when the current-supply delivery net is

weak or when the main drive element is a diesel engine. A factor, which

must be carefully monitored, is the average accelerating torque. Knowl￾edge of how to design and manufacture these powerful Ward–Leonard

drives has unfortunately been largely lost.

Direct current full-thyristor systems

In the last twenty years the direct current full-thyristor drive has become

the successor to the resistance-controlled AC drives and DC drives and

the Ward–Leonard drives.

The stepless controlled full-thyristor direct current motor is available

for all mechanisms and all capacities. It can be regarded as fool proof.

Regular maintenance is needed to attend to the brushes, and collectors

in the motors. Dust caused by wear and tear of the brushes has to be

removed from time-to-time and the brushes have to be adjusted,

checked, and replaced to prevent breakdown and loss of efficiency.

These motors can be totally enclosed or drip-watertight, self-ventilated

or ventilated by an external, continously running ventilator (force-venti￾lated). Field weakening can occur, normally to a level of approximately

1500 to 2000 rev
min depending on the power range and field compen￾sation. The normal voltage is 400 V or 500 V. Cosphi compensation is

needed to achieve a cosphi of approximately 0,9.

Alternating current drives with frequency control

To reduce maintenance on the motors as much as possible, the manu￾facturers of electrical systems have developed and now use AC motors

with frequency control. Since 1995 a good working system has been

achieved. AC frequency control is also available for hoisting mechan￾isms using large amounts of power.

The motors are of a simple design. However these are special squirrel

cage motors. The electrical control is somewhat more complicated than

that of the full-thyristor systems, and forced ventilation is not normally

required. Control of these motors is always stepless. Field weakening,

up to 2000 to 2200 rev
min – based on a four-pole motor, is possible by

increasing the frequency. Torque–speed curves can be adjusted within a

limited range.

It is safe to assume that the research and development of the design

of motors will continue and that further advances will be made. How￾ever, this drive offers the most appropriate and suitable answer for the

next ten years. Cosphi compensation may be necessary to achieve a

cosphi level of approximately 0,9 depending on the type of the drive.