They usually work behind the scenes: Pumps, fans and compressors are of great importance in many areas of our everyday lives. They have to work reliably day after day. Without the tasks they perform, such as ventilating and supplying buildings or supplying and disposing of drinking water, our lives would be much more difficult. All of these applications require perfectly functioning pumps, fans and compressors.

Cascade control with position controller, speed controller, current controller, actuator, distance (inductance and resistance of the machine), disturbance variable load torque, equation of motion and integration of speed to position. Copyright: AHaumer – Own work

As different as the requirements and operating conditions may be for the various applications, the main task always remains the same: pumps, fans and compressors must be driven as efficiently as possible. This requires not only good products, but also appropriate concepts that take lifecycle costs into account.

In the past, the appliances were connected directly to the mains and the power output was regulated via throttle valves and slide valve systems, as is still the case today in some cases. Nevertheless, today there is an increasing demand for speed control that allows the drives to be continuously adjusted to the power required. The energy balance can be greatly improved in this way, particularly for turbomachines with a quadratic characteristic curve, which in practice leads to considerable savings in energy costs.

In addition, maintenance and service costs are also reduced, as the soft start and the generally reduced number of starts and stops drastically reduce wear on these machines.

But how should a system be structured if, for various reasons, it cannot manage with just one machine? In these cases, the user then needs a solution for multi-pump operation that still offers the advantages described above.

There are many reasons for using multi-pump systems. The most important reason for this is the optimum setting of the operating point depending on the power required in the system. This leads to optimum efficiency of the overall system. In comparison, with a large control range using only one pump designed for the worst-case scenario, this can mean an efficiency of only 10 to 20 percent, for example. In comparison, the multi-pump system is usually over 70 percent. Overall, you can then assume a much better energy balance.

In addition, in many cases a multi-pump system can be controlled better and more precisely than just one pump. Especially when using a master-slave system (see below), the system runs optimized. In addition, a multi-pump system offers greater redundancy and therefore higher availability of the overall system than a single-pump system.

Cascade control, which can be implemented with the VLT® AQUA Drive frequency inverter from Danfoss, is ideal for controlling such a system with several flow machines. In principle, a pump, a fan or a compressor, which is then connected to the frequency inverter, takes over the control and, depending on requirements, the inverter switches on other machines to provide the total power required in the system. Three basic types of such cascade circuits can be distinguished: The standard cascade controller, in which only one flow machine is frequency-controlled, the master-slave circuit, in which all machines are speed-controlled, and a mixture of these. The special thing about such a solution is that it can now be implemented using a frequency inverter without the need for an additional, more expensive and higher-level control level.

With standard cascade control, the guide pump is speed-controlled via a frequency converter to enable infinitely variable control. It also controls the downstream pumps, which are connected directly to the supply network via a relay, depending on a setpoint specification or measured value recording. The VLT® AQUA Drive can control a total of three pumps as standard, and even up to 5 pumps or fans with the cascade extension.

The principle is relatively simple: At the beginning, the frequency inverter first ramps up the speed-controlled pump to the required output. When this pump reaches the 100 percent mark, the first downstream pump, which must have the same output as the controlled pump, takes over operation. The speed-controlled pump provides any additional power required. This game is repeated in each stage of the cascade. However, this also results in an optimum performance ratio of the pumps: 1:1:2:4:8 … where the first pump uses the speed-controlled motor. If the required output falls below a corresponding threshold value so that the frequency-controlled pump can no longer compensate, the fixed-speed pump that is still delivering the highest flow rate at this time is disconnected from the mains. Table 1 shows typical, valid power combinations, always in relation to the speed-controlled pump as a reference value with 100 percent. The advantage of this circuit is that there is always a constant pressure in the system, without any pressure surges, which reduces the load on the system and ensures quieter operation.

Table 1:

Rev. PumpFixed speed 1Fixed speed 2Fixed speed 3Fixed speed 4Fixed speed 5
100 %100 %200 %
100 %100 %200 %200 %
100 %100 %100 %300 %
100 %100 %200 %400 %
100 %100 %100 %300 %300 %
Principle of cascade control