Getting smarter all the time
Variable speed drives are getting smarter every day - but are operators taking advantage of this, asks Craig Rapson of Danfoss Drives
In 1968 Danfoss released the world’s first massed-produced variable speed drive (VSD) – the VLT5. This was an oil-filled unit with no display and looked very much like a transformer. Back then a three-phase supply would be connected, the unit given a speed reference and told to start. Now, over 40 years later, the VLT Aqua drive has been designed specifically for water and wastewater applications with great intelligence and many dedicated software features – and guess what? The majority of the time the product is still utilised as a ‘dumb’ speed controller as it was in 1968.
In the past 10 years or so, although we have seen the cost of VSDs drop by around 3%, year on year, the intelligence and efficiency of this technology has exceeded all expectations. VSDs now have the ability to do more than just run a motor at different speeds. Onboard microprocessors have the power and memory to run more application specific tasks.
This power can be used for various features that would have previously been carried out by additional hardware connected to the control systems. The result is a lower installed capital cost to control the process, while achieving the lowest possible running costs.
Three features that have been designed for the water industry to improve reliability, simplify installation, while reducing the initial and lifecycle costs associated with the investment: Low Flow Detection, Cascade Control – Master Follower and Impeller Deragging.
Low Flow Detection
Low Flow Detection is primarily used to protect the pump in a situation where we have a loss of flow, for whatever reason. In a traditional system this can mean investing in a low flow transmitter and the associated controller.
The pipe would be drilled, the transmitter fitted then the pipe welded. Installation also includes running the cable back to the MCC in the switch room to the detection controller and the commissioning of the PLC. Typically in a medium to large water treatment plant there could be around 40-50 of these types of installations.
If this is compared to what is available in the new VLT AQUA Drive, every piece of hardware used in the traditional installation can be eliminated. No external transmitter is needed, no drilling of the pipe, no transmitter cabling at all, no additional PLC programming and it gives a very high accuracy level of protection. This integrated feature in the standard drive is totally non-intrusive.
During commissioning of the drive, operators are asked to close the outlet valve to dead head the pump. The low flow detection feature in the drive is then selected. The drive will then request permission to proceed. After selecting yes, the drive will run to pump up to the default value of 85% speed at which point it records the power consumption. It then slows the pump down to 50% speed, all the time recording the power consumption to give us a power curve.
This whole sequence takes around 10-20 seconds to complete. After the commissioning is complete and the pump is started, the drive will continuously monitor the pump power and if the power for a certain speed drops below the curve, the pump can be tripped or a warning given. A slight delay can occur if required to stop nuisance tripping. For a site with 40-50 low flow systems, by utilising this technology, the savings could be in excess of £20,000 inclusive of labour and hardware.
Cascade Control – Master Follower
The Master Follower function means pumps can be automatically staged and destaged to meet the demand requirements of the system. For example – Pump 1 will start and run up to a speed where the drive system decides that is now more efficient to run with two pumps in parallel. At the point a second drive/pump is staged in and both pumps run at the same speed following a common speed reference between all drives. Again the demand may rise and once again a third pump is staged in for best efficiency.
The same also applies when the demand drops and we automatically destage a pump. Control of up to 13 parallel pumps is possible.
If required the system can be set to runtime balancing and this means the system looks at the running hours of all pumps and alternates the duty & assist and standby pumps for evenwear across the pumps. In some circumstances it may be a preference to have the running hours unbalanced as there can be an argument it is better to have a different service schedule for each pump rather than all at once. The balancing feature is simply disabled by the user if this is a specified.
All of this is done without the need for additional external sequencers and associated wiring. There has always been the question of when is it best to run with one pump, or two pumps in parallel and I will explain how we can overcome this.
As the first pump is ramping up, just prior to when it stages in a second pump, it records the power consumption pre and post staging. If the recording after staging shows the power consumption has increased, it means the setting was incorrect and it was better to run longer with one pump.
The drive system records this and automatically adjusts it stage setting so the next time it stages it does so at the most efficient point. This is constantly monitored and automatically adjusted every time a staging of pump takes place. This means that not only will the system offer the highest efficiency, but as the pumps begin to wear across their lifetime and the best efficiency point (BEP) changes, the drive system will automatically react to ensure we maintain the highest efficiency possible.
Ragging of wastewater pump impellers is a serious ongoing issue. Traditionally it reduces pump efficiency and can cause pump blockage resulting in major downtime to lift the pump out of service to
carry out the required maintenance.
New technology now has dedicated de-ragging software features within the drive. This optimises the efficiency of the pump by constantly monitoring the motor shaft power consumption, relative to flow, and reacts with a reverse spinning cleaning cycle when a deviation from the power consumption of the system design is recorded. This intelligent system is fully flexible and can be optimised to meet the individual needs of applications. During the commissioning sequence, the user can determine when the system should react – at start command, stop command, during running, by digital input or time based. A combination of any of these events is possible to ensure the highest pump efficiency is maintained and ragging of the impeller is minimised. The user can also define how many times the drive should spin the impeller in reverse, at what speed and for how long. The LCP display of the drive shows a clear indication of the status of the deragging function.
Please be aware that not every pump can be run in reverse and the operator should always check with the pump manufacturer before specifying this type of control. Modern day drives technology not only offers the possibility to reduce lifecycle costs, but now, with dedicated software features, can also reduce the level of initial costs by eliminating the need for costly external devices traditionally used to give the required control or protection in a pumping system.
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