What is the purpose of an electric crane drive?
The primary purpose of crane electric drives is to regulate the velocity of elevating and movement mechanisms within a definite range. First of all, the actual type of an electric drive depends on the physical principles regulating velocity of particular types of motors.
The following are the most frequently used types of motors:
- direct current motor
- asynchronous wound rotor electric motor
- asynchronous short-circuit rotor motor
Direct current motors became widely used in heavy metallurgical production, including casting cranes, due to their simple construction and an easy-to-use steering mechanism, practically linear mechanic and control characteristics and, what is most important, a long starting moment. The drawbacks of this type of motors are high manufacturing price, direct current dangers for human operators, wear-out, and the need to run preventive maintenance of the commutator-and-brush assembly.
To provide direct current supply for the crane, it is required to install a three-phase rectifier.
Currently direct current motor cranes are out of production.
Asynchronous wound rotor electric motors have been widely used on cranes in the Soviet Union times and are also popular nowadays. Motors of this type are used for reconstruction of older cranes, as well as in the newly produced ones. Wound rotor motors are characterized by a significant starting torque, almost constant velocity under various loads and a lower start current as compared to short-circuit electric motors. Its drawbacks include large dimensions, high production price, as well as (the same as direct current motors) quick wear-out and the need to run preventive maintenance of the commutator-and-brush assemblies.
Ways to regulate rotation velocity of wound rotor electric drives
There is a number of control systems used for wound rotor electric motors. The change of torque and velocity of such motors is achieved through inclusion of additional electric devices into the rotor circuit. Depending on the required range of regulation, the following systems can be used:
- rheostatic systems(in other words: resistible, with active resistance)
- throttle systems with no velocity regulation (cushion start, in other words: with reactive resistance)
- throttle systems with thyristor regulators
- other systems.
The switching facilities within stator and rotor circuits can be as follows:
- power contactors, actuators, reversers (and other mechanic switching facilities)
- thyristors (thyristor keys)
- hybrid systems
Comparative analysis of control systems for wound rotor electric drives
While comparing various control systems, it is important to correctly define the qualitative and quantitative factors and the evaluation criteria. When selecting crane equipment, one should take into account the following system factors:
- the costs of equipment, installation and operation commissioning – the initial implementation costs;
- energy efficiency of the equipment – the factor that allows evaluating the actual funds spent combined with the initial costs, i. e. recoupment of the system as compared to the competing solutions during operation;
- declared and real allowable operating conditions (temperature, dust content, dust protection, frequency of maintenance works, etc.);
- declared crane control characteristics and their actual attainability, etc.
Apart from comparing the technical characteristics of equipment from different manufacturers, it is also important to pay attention to the operational cyclogram of the upcoming project. Each of the velocity control methods can show the maximum economic effectiveness under particular conditions and must be evaluated separately based on its economic effectiveness and the quality of control procedures. The right choice of velocity control methods and the appropriate equipment used during the initial stage determine both the initial project cost and its total value during the entire period of crane operation.
We develop and produce contemporary control systems for wound rotor motors under the TEDF trade mark.
All of the basic features and technical characteristics are described in the Catalog of thyristor panels.
Asynchronous short-circuit rotor electric engines were rarely used on cranes in the Soviet period due to the technical complexity of velocity control. Their use was limited to small telpher elevators, overhead cranes and other types of one- or double-speed slow-going mechanisms. In other cases direct current and wound rotor motors could not be replaced. This situation changed only after frequency converters became wide spread and cheaper.
The primary advantages of asynchronous short-circuit rotor electric motors are the absence of commutator-and-brush assemblies, small weight and easy production, high performance coefficient and cos φ as compared to wound rotor motors. The major drawbacks are high starting current (up to 5-7 In) and the complex structure of rotation velocity control equipment.
Ways to control rotation velocity of short-circuit rotor electric motors
Lowering prices for semi-conductor devices during the late 90-ies made crane producers to start using contemporary control systems. Starting from 2000, the systems based on frequency converters have been getting more popular in the CIS countries. These devices facilitate smooth change of voltage and frequency on the motor connection terminals, thus enabling control of rotation velocity, while preserving the nominal torque throughout the entire regulated range. The major advantages of frequency converter systems are considered to be as follows:
- the allowed amount and softness of velocity control (up to 1:100)
- energy efficiency as compared to older control systems
- possibility of operation within the weakness zone of a motor field
- easy modification of crane mechanism characteristics through changing drive assembly parameters
- cheapening of motors through use of short-circuit rotors
- reliability and easy replacement of drive units, etc.
Also, the systems using frequency converters feature a number of additional limitations that must be taken into account:
- small range of operational temperatures
- high prices for high power figures
- low dust protection
- insignificant overload capability
- high qualification requirements for the service engineers and operational personnel
- a laptop often required to run testing operations
- long productions terms (which is particularly important in case of malfunctions)
Production of frequency converter systems always involves custom solutions and creation of specific projects for every single crane.
The Dvesta company specializes in production of crane control systems based on frequency converters:
Our company has already implemented over 100 different projects for newly produced cranes and also reconstruction works with the initial capacity of 800 kW.
Please find detailed information on our experience and competence in Technical solutions.