Especially for pump drives that are typical used in water supply and water treatment plants, or blower compressors for stabilizing ponds, variable-speed operation using a drive converter saves a considerable amount of energy, makes processes more flexible and therefore reduces the stressing on the complete plant or system. Thanks to these cost savings, the pay-back time for a frequency converter drive is often just a few months.
The cost-saving potential and pay-back time of such an investment can be quickly and simply calculated using our SinaSave energy-saving program. Using plant-specific data, such as flow rate, delivery head, pumping profile over a day and over a year, working shifts and working days etc., SinaSave determines the energy demand of variable-speed systems compared to alternative concepts. These alternative concepts include various types of closed-loop throttle controls, bypass systems or pole-changing motors. The program calculates the pay-back time from the purchase price of the appropriate drive converter, the energy saving based on the actual cost of energy and other additional cost-reducing factors associated with variable-speed drives. These factors include, for example, the elimination of mechanical controls, lower stressing on the overall system and a higher overall efficiency.

With traditional control techniques, the motor always runs with the speed designed for the maximum pumped quantity and the excess quantity is throttled using mechanical actuator elements. When all is said and done this means that the energy is simply wasted. On the other hand, with converter-based drive systems, the speed is adapted to the actual operational requirements and therefore also the amount of energy drawn. In other words – the motor only draws the power that is precisely necessary at that particular operating point. In this case, power factor and efficiency remain almost constant over a wide range. By changing the speed, the pump characteristic is shifted to an optimum for the specific process. The overall result is a significantly lower energy demand than for fixed-speed drives with the same power rating that use mechanical control principles in the partial load range. The energy-saving effect is especially high for pumps, fans, compressors and blowers. The reason for this is that in this case the power drawn is proportional to the speed to the power of three. Even slight speed changes result in a considerable energy saving effect. Depending on the plant or system characteristic, the energy saving as a whole can be up to 50% - whereby for steep system characteristics, the energy saving potential is especially high.

Low stress operation reduces service & maintenance costs

When current peaks are eliminated this also eliminates the associated torque surges – that can be quite significant. The soft starting and stopping reduces the stressing on the complete mechanical transmission line as well as the piping system and increases its lifetime.
In addition to the soft starting and stopping that reduces the stress on the system, variable-speed operation avoids unfavorable operating conditions such as cavitation, gas being pumped along with the medium, compression surges and vibration that can also damage the plant. Partial load eddy effects and the associated eddy losses are eliminated.
Additional advantages of frequency converter operation include the fact that mechanical controls are eliminated, the power factor is improved as well as the closed-loop process control that is optimized when compared to mechanical principles.

The pumped quantity is controlled faster and more precisely

When using variable-speed drives, the flow rate can be controlled far more accurately with short response times than when applying mechanical control concepts such as throttles. Even for brief fluctuations in the water demand or the amount of wastewater, the pump system can more quickly and more precisely adapt the pumped quantity to the actual requirement. This results in a higher degree of reliability and quality in water supply and wastewater treatment plants.

The matching drive converters completely utilize the cost-saving potential

Siemens drive converters, such as the SINAMICS G150 and MICROMASTER units and SIMOVERT MV for medium voltage, have a concept that is designed to very effectively utilize the energy-saving potential. These drive converters have been specifically designed for drive applications without regenerative feedback into the line supply – especially for pumps, fans and blowers.