Many people still identify the energy efficiency of an industrial crane by how much electricity it consumes, but the issue is more complex than that. How does the motor start, how smooth is the movement, how much load is put on the drive, the brake, the rope or the chain? How much time is lost and energy wasted in a work process due to inaccurate settings, due to a piece of equipment that is difficult to operate? A modern, energy-efficient crane operation is therefore not based on one big spectacular solution, but on several smaller components. In our series of articles, we will take a look at these. Let's start with the control system!
Two main ingredients
A key component of a modern crane control system is inverter, or VFD. It is intended to adjusts the speed of the electric drive to the needs of the task.
Instead of suddenly revving to full speed, the engine accelerates gradually, decelerates in a controlled manner and maintains the desired speed more accurately. This is particularly important with cranes, because lifting, trolley movement and crane travel all involve different types of load.
Moving an empty hook, setting a delicate workpiece and lifting a heavy load do not require the same motor control.
Another equally important component is the soft start. It has a narrower remit than the VFD.
Its purpose is to reduce the starting current consumption and the sudden torque peak. This in itself makes a big difference: it means less electrical stress on the grid, it saves on the engine and reduces the mechanical stresses that run through the structure every time it starts.
An important difference, however, is that the soft starter basically controls the starting and stopping phases, while the frequency converter can control speed and torque during the entire movement.
Priority
Proper control is of paramount importance because energy efficiency in cranes is not always the same as in a fan, for example, where reducing the speed of the fan can result in spectacular energy savings. A lifting device needs a lot of energy to move several tonnes, even if it does so more slowly. Therefore, energy savings must be assessed in other terms. Examples include: the amount of work the drive does for a given movement; or the amount of unnecessary acceleration, which makes the handling more refined.
These features are as important as power consumption. Sudden starts and braking can cause a sway, which slows down positioning. The operator then corrects, waits, restarts and stops again. In a single cycle, this may seem like a few seconds, but with hundreds of movements a day, it can be a serious waste of time.
Controlled acceleration and deceleration helps the load to move more smoothly, the adjustment to be more precise and the rhythm of the workflow to be smoother.
The finer movement also has an effect on the structure. Less load is placed on the wheel, the rail, the drive, the rope drum, the for chain, the brake and the support structure. This does not mean that the maintenance can be omitted and a frequency converter or soft starter will prevent any failure.
But I do know that well sized and correctly adjusted controls can reduce unnecessary stress, so it can have a positive impact on the amount of energy used, as well as on service life and operational safety.
It is also worth paying special attention to peak loads. In many cases, it is not the average consumption that is the biggest problem, but the simultaneous start-up of many appliances and the resulting sudden power consumption.
Soft starting and variable frequency control can help to make starts more smooth, which can also be better for the electrical grid and internal power distribution.
The lesson is simple: energy efficient crane operation is not just a matter of electricity consumption. The drive system can also have a big impact on this characteristic of a hoist. In the next post, we will look at how optimum cycles, braking strategy and everyday operating habits can further improve the long-term economics of cranes.
