SHIFTING THE CRANE TO A FREQUENCY DRIVE
SHIFTING THE CRANE TO A FREQUENCY DRIVE – is it about saving or performance? This is an improvement of the lifting mechanism, which allows to increase both these indicators.
The operation of a crane on a conventional relay-contactor circuit is applicable in cases where the following factors do not affect production figures:
- jerks when turning on the mechanism of crane movement;
- lack of control over the accuracy of the load positioning;
- motors overload at start and stop;
- turning on the reverse of mechanisms only after full stop;
- inability to change the speed of the crane movement mechanism.
Frequency converters are introduced on cranes in cases when it is necessary to ensure a smooth start, acceleration, stopping of the mechanisms, to ensure the absence of “counter-currents”.
The use of a variable frequency drive (VFD) as part of overhead, gantry, portal or other types of electric cranes provides the following indisputable advantages as compared to the relay-contactor control circuit of electric crane drives:
- ABSOLUTE CONTROL OVER LOAD AND CRANE MOVEMENT.
As a rule, obsolete relay-contactor control systems for electric drives of a crane provide a speed control range of not more than 1:3. The speed of movement at each position of command controller is unstable and depends on the resistance moment on the motor shaft. Work in such conditions requires considerable experience and skill, from the crane operator especially when it is necessary to quickly and accurately position the load. After all, runout of the movement mechanisms is ensured mainly by the drum brake and depends on the inertia of the mechanism, therefore it cannot always be predictable. Using the VFD allows to control the speed of the labor movements up to a complete stop. This fact greatly increases the comfort of the crane operator, labor productivity and safety of work (no uncontrolled runout). When using the VFD, it becomes possible to use methods that ensure the elimination of the load swing during acceleration or deceleration of movement mechanisms. It also saves the crane operator from having to independently combat the swing.
- ENERGY EFFICIENCY.
As a rule, start-control resistors are present as part of relay-contactor drive control systems. Their task is the formation of artificial mechanical characteristics of an electric motor in dynamic modes of operation (acceleration / deceleration). When the electric drive operates on resistors, a significant amount of heat is released, which makes the drive operation completely inefficient from the point of view of energy consumption. The introduction of the VFD allows to significantly reduce this type of loss, since the acceleration and stable operation of the motor occurs without the participation of resistors, i.e. with the highest possible coefficient of efficiency. However, when the VFD is in braking mode, the kinetic energy of the moving mechanism is released. Most often, it is dissipated in the form of heat on the brake resistor of the frequency converter.
With a further increase in energy efficiency, it is possible to get rid of such losses. To do this, a frequency converter with an electric power recuperation module to the grid should be used. In this case, all the kinetic braking energy of the mechanism is converted into electrical energy and returns to the supply grid. However, the presence of a recuperation module significantly affects the cost of the frequency converter, therefore the selection of this type of inverter must be confirmed by appropriate calculations.
There is another budget way to increase the energy efficiency of a VFD, which is implemented in some applications. It consist of connection of circuits of the intermediate DC link of frequency converters included in the VFD crane into a common bus. In this case, the energy released during braking of some mechanisms is used to power the drives of other mechanisms operating in the acceleration mode or steady speed.
- REDUCED MAINTENANCE AND REPAIR COSTS
This fact is one of the main advantages of the VFD implementation.
Electric drives built on a relay-contactor control system are characterized by such phenomena as current and torque surges at the start of the motors or switching the speed of the command controller. Naturally, this adversely affects the power grid, switching equipment and mechanical part of the drive. There is a burning of contacts, the wear of gear wheel of reducers, etc., which requires periodic maintenance, and this in turn leads to financial and time losses. The influence of these factors is minimized when the crane is shifted to VFD. The frequency converter provides a smooth change in output values when applying commands to change the speed, therefore the consumed and output current of the frequency converter, as well as the torque developed by the motor, as a rule, do not exceed their nominal values. This ensures the absence of current surges and shocks in the dynamic modes of the drive operation. Also, a pleasant addition is virtually no wear of the brake pads of the drum brakes of the mechanisms, since the braking is made by the motor itself, and the brake is applied to the virtually stationary drum. Accordingly, there is no need for frequent adjustment of the brakes.
It is possible to switch the crane to the frequency drive regardless of its type: overhead, gantry and portal crane. The term of works execution and the cost are influenced by such factors as: number of mechanisms that require the implementation of FC;
- power of motors,
- operating mode;
- selection of the manufacturer and model of the frequency converter.
Frequency converter selection
As a rule, selection of a frequency converter is guided by such basic criteria.
– Rated current is one of the main parameters when choosing a frequency converter, it should be not less than the rated current of the motor, and even better – exceed it by 1 nominal.
– Type of control: vector or scalar. Frequency converters that support vector control are somewhat more expensive than scalar frequency converters of similar power. For the crane lifting mechanism drives, it is necessary to use a frequency converter with vector control, which provides more accurate control of the motor torque. Movement mechanisms allow the use of scalar control of the frequency converter. In addition, scalar control is used with a multi-motor system of the VFD (several motors operating from one frequency converter).
– Overload capacity characterizes the ability of the frequency converter to withstand short-term overcurrent of a motor. As a rule, the majority of modern frequency control drives allow current overload at the level of up to 150% for 1 min.
In addition to the basic, there are many secondary criteria for choosing a frequency converter. These may include the presence of the required number of discrete inputs / outputs that provide remote control of the drive, the presence of a built-in programmable logic controller that provides simple control algorithms, the presence of an interface of communication with the lower level of automation, if any, climatic performance, degree of protection, the presence of application functions (for example, mechanical brake control), etc.
Switching of overhead crane or other type of crane to radio control includes the following works:
- Expert inspection of the crane for the possibility of upgrade.
- Development and approval of Technical Conditions (TC) for the crane upgrade
- Manufacture of the interface panel based on frequency converters.
- Installation and commissioning of the crane.
- Making a record in the data sheet about the nature of upgrade.
- Conducting an unscheduled full technical certification of the crane (UFTC).
INSTALLCRANESERVICE LLC may perform all of these activities both selectively or turnkey. To calculate the cost, it is necessary to provide an electrical diagram of the crane, power of the mechanism motors, as well as data on the temperature mode of the crane operation (eg. switching of a gantry crane to a frequency drive, or a casting, portal, overhead crane that operates outdoors).
To obtain information on the cost and timing of switching of the crane to the frequency drive, please contact our specialists by phone numbers listed in the CONTACTS section.
The cranes, manufactured by INSTALLCRANESERVICE LLC with the use of frequency converters: