A BORDAJE I NSTITUCIONAL
2. A LGUNOS CONCEPTOS PARA PENSAR LA EVALUACIÓN DE LA DOCENCIA UNIVERSITARIA
Electricity is the normal power used for driving cranes, so this is the main system considered here. The types of drives used for the crane mechanisms are given below.
The squirrel cage motor with fluid coupling
In simple cases, it can be worthwhile to consider the alternating current squirrel cage motor, with a fluid coupling as the driving element for the horizontal movements. However, there is then no possibility for real speed regulation. This can only be executed by ‘inching’ or small and repeated movements, commanded by the push button knob or the toggle-switch. For driving a belt conveyor, the squirrel cage motor with
Fig. 3.1.1 Squirrel cage motor with fluid coupling
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 resistresist-ances made of stainless steel have preference.
Fig. 3.1.3 Slipring motor: resistance controlled
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)
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兾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兾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.
Fig. 3.1.5 DC full thyristor
In low speed crane-travelling mechanisms, the option of using one drive for all the motors under the two sill-beams of the cranes is poss-ible. Because all the motors will receive the same frequency, synchroniz-ation between the motors is not absolutely necessary providing that the wheel loads and the wind loads on each sill-beam of the crane do not differ significantly. However, it is preferable to use one drive for each sill-beam and also to make ‘cross-over’ connections between the motors on the two sill-beams. This ensures exact synchronization.
Warning Especially with AC frequency control, but often also with DC-Full-Thyristor Control the Electromagnetic Compat-ability (EMC ) due to the Higher Harmonics plays an important role.
To prevent disturbances by this Electro Magnetic Inter-ference (EMI) special double-shielded cables must be used.
These screens or shields consist of a copper foil wrapping and optimized copper wire braiding.
On both ends of the cable special EMC glands must be used. These must be well-earthed and connected to steel boxes.
In the bigger motors insulated bearings should also be used.
Fig. 3.1.6 AC frequency control: torque–speed diagram for hoisting/lowering
Fig. 3.1.7 2B800 kW Holec AC frequency control motors in the hoisting winch of a grab-unloader
Hydraulic drives
We now concentrate on the Ha¨gglunds hydraulic drive, which consists of a control system; an electric motor; an oil tank; a pump; and a hydraulic motor. The pump is driven by an electric motor, which runs with a fixed speed. The oil flow from the pump is controlled by either a Squashplate or a tilting cylinder block, the angle of which can be changed by a signal from the control system. The motor pumps the oil which flows into the motor cylinders and presses the pistons radially out towards the camring. The speed of the motor is stepless variable.
This system has a low moment of inertia and a high starting torque (200 to 300 percent of a nominal rated torque). A brake system can also be provided on these drives.
Fig. 3.1.8 Winches with Ha¨gglunds hydraulic drives