Aki Maenpaa, Business Unit Manager, Drives and Motors, ABB Southern Gulf and Pakistan spoke to H20 on the innovations in low voltage AC drives and their role in improving the energy-efficiency of water pumping systems.
Aki Maenpaa
Meeting today’s energy challenges requires us to address the correlation between economic growth and energy use. One of the ways to do that is through energy efficiency. In pumping systems, where a variable output is often required, energy consumption costs can be dramatically cut by using low voltage AC drives, also called Variable Frequency Drives (VFD) or Variable Speed Drives (VSD), to control the motor speed, rather than operating it at full speed or throttling the output. “In water and wastewater treatment, pumping systems are used to regulate water flow. But you don’t always want to run the pumps at full speed as the flow keeps varying. You can achieve significant savings on the energy front by using VFDs or VSDs,” said Aki Maenpaa who handles the drives and motors business unit of ABB Southern Gulf and Pakistan.
ABB has designed and developed a series of low-voltage AC drive modules, called the ACQ810, to meet the needs of squared torque pump control in water and wastewater applications. “This drive’s specific features ensure accurate control of water flow throughout the processing cycle, from raw water, through utilisation to wastewater treatment,” said Maenpaa.
Elaborating on how such low-voltage AC drives work, Maenpaa said that if the AC motor is directly connected to the grid, it will always run at its fixed or nominal speed. Throttling can be used to regulate the water flow, but this achieves only a marginal reduction in the motor’s energy consumption. On the other hand, a frequency converter regulates the flow and the speed of the motor directly from the motor, which also helps control the amount of energy consumed. “Rather than having an electric motor running continuously at full speed, an AC drive allows the user to variably control the PRODUCT FOCUS: VARIABLE FREQUENCY DRIVES motor speed, depending on demand,” said Maenpaa.
To drive home the point, he pulled out a Pump Control Method slide (see below) and continued, “If you squeeze the water flow, you will have some reduction in energy consumption but not much. But if you use a VFD or VSD, your energy savings will be relatively huge. If we assume a 60% flow, with throttling you will use 72% of the energy, but with VSD, you will only use 27-28% of energy.”
When customers select pumps, they tend to maintain wide safety margins. “If the safety margin is high, the potential savings too, tend to be high, because the variable drives will help run the pumping systems with less energy,” said Maenpaa.
If customers incorporate VSDs into the pumping system right at the beginning, when it is being set up, would it be possible to avoid the tendency to overspecify, and the costs associated with it? Maenpaa certainly thinks so. He said, “If we work with proper pump curves at the initial stage, we can make proper dimensioning and select the right motor and frequency converter.”
Modern VSDs come equipped with advanced features, like soft pipe filling and multi-pump control. The former provides a pump with soft-start, enabling a smooth build-up of flow in pipes while increasing the life time of the pipe work and pumping system. “If you start a pump with full power, the resulting pressureimpact on the pipeline will reduce its life span,” said Maenpaa. Multi-pump control is used in scenarios where several parallel pumps are operated together and the required flow rate is variable. Maenpaa said, “In a booster pumping station, with parallel pumps, it is typical to start the pumps at their nominal speeds, one by one, till the required flow is achieved.
However, for most pumps, their most efficient speed is less than the nominal speed. So ideally, you should start the first pump to its more efficient speed, and if you still need more flow, start the second pump to the most efficient speed and so on. The multi-pump control makes this possible, providing the most energyefficient way to operate parallel pumps. In fact, the nominal speed should be used only during emergency situations, for instance, when the level control alarm is triggered.”
The ACQ810 drive modules are equipped with the aforementioned features, and some more, like flow calculation, sleep & boost and pump cleaning. Flow calculation provides the drive with a flow meter routine, which accurately determines the flow rate within a process. “This function avoids the need for costly external flow meters and is suitable for applications where the flow data is not needed for invoicing purposes,” said Maenpaa. The Sleep & Boost function detects slow rotation when water consumption is typically low, like in the night time for example, and runs the pump to boost pressure prior to shutdown. The pressure is continuously monitored and pumping restarts when the pressure falls below the minimum level. “Sleep & Boost extends the pump’s sleep time through saving the equipment and helping to reduce energy consumption. It is suitable for potable water pumping systems,” added Maenpaa.
On the other hand, pump cleaning, which prevents pump and pipes from clogging by initiating a sequence of forward and reverse runs of the pump to clean the impeller, is beneficial for wastewater pumping stations.
Pump Auto Change, Redundancy and Level control are the other important features. Maenpaa continued, “If you have multiple pumps, you have to ensure that no pump remains idle for too long. The Pump Auto Change function balances the operating time of all the pumps in the parallel pumping system over the long term, which helps increase the mean time between repairs (MTBR).” Redundancy ensures system redundancy, so should one or more pumps fail or require maintenance, the remaining pumps continue operating. Level control is used to control the filling or emptying of wastewater storage tanks. “This function prevents sediment build-up on the tank walls by randomly varying the surface level within a range preset by the user,” he said.
Additionally, for systems where the consumption rate varies based on demand, the Pump Priority feature can boost energy-efficiency by operating pumps closer to their best efficiency point. Pump-specific protections help to maintain disturbance-free operating time in a process.
“Although designed with built-in intelligent pump features, the ACQ810 drive modules can be used for other water and wastewater applications such as fans, conveyors and compressors to obtain significant energy savings,” said Maenpaa. All the energy and CO2 savings generated by the drive module are monitored using a built-in energy counter, which displays energy savings in kWh, local currencies or volume of CO2 emissions. Energy consumption is compared to previous time frames defined by the user and if there is an energy consumption increase, an alarm is generated.
An additional energy-saving function called the energy optimiser, has been designed into the drive. The energy optimiser is a software-based algorithm generated by a highly advanced motor model developed by ABB. Maenpaa explained, “With the energy optimiser, when a pump system operates at partial load, the motor magnetising current is reduced, thus lowering the losses of the motor and drive. This means that the total efficiency of the drive system is maximised. Improved efficiency has a direct impact on the energy costs.” Additionally, the drive’s internal fan has an automatic on/off control which provides further energy savings by switching the cooling fan off when the drive is not active.
The ACQ810 comes with preprogrammed application macros that can be used in both single pump and multipump systems to configure the drive to control typical pumping tasks. Maenpaa said, “The user is helped by an intuitive user interface with several assistant screens. The start-up assistant guides the user through essential settings; the maintenance assistant indicates preventive maintenance needs of a drive, motor or pump, while the diagnostic assistant helps locate failures or reasons for performance changes and suggests remedies.”
Maenpaa noted that less than 10% of motors worldwide are equipped with a VSD. “One high efficiency 75MW motor (three per cent higher than ‘standard’ efficiency as per NEMA) can save 23,000 kWh, corresponding to 11.5 tonnes of CO2, in one year of continuous operation. Energy efficiency is thus a key source of potential CO2 emissions reductions,” he said.
