Understanding how best to use VSDs with pumps

Retrofitting pumps with variable speed drives (VSDs) is now a well-established practice, and likely to become more so with steep rises in energy costs. Recent estimates that over 75 per cent of pump systems are oversized - many by more than 20 per cent - reveals the potential for energy saving as a result of employing VSDs to more accurately match pump systems to actual system requirements. However, despite the apparent benefits, retrofitting is not without potential drawbacks, both in terms of the installed motors to which the VSD are fitted, and the pumping systems themselves.

When considering adding a VSD to an existing motor, care should be taken to match the electrical characteristics of the motor and frequency converter; otherwise the risk of premature failure is introduced into the system. For example: the high rate of switching of the inverter PWM waveform can lead to problems, especially with regard to older motors, whose insulation systems may deteriorate more rapidly due to the rapid rate of voltage change; the adoption of filters in the output of the inverter can eliminate this problem. In addition, voltages can be induced in the shafts of larger motors, potentially leading to circulating currents that can damage bearings. This can be addressed by using insulated non-drive-end bearings and, for higher powers (over 100kW) common mode filters may also be required.

Additional electrical problems that can arise as a result of retrofitting VSD are that the rate of the PWM wavefront rise can cause electromagnetic disturbances, requiring adequate electrical screening (screened output cables). Furthermore, long cable runs can result in 'transmission line' effects that cause damaging raised voltages at the motor terminals.

Location, location, location

Finally, there is the question of the where the VSD is to be located: it may require installation in a less arduous environment than the control gear it replaces. Ventilation requirements are particularly relevant, as the life expectancy of the inverter is generally directly related to the temperature of its internal components. In addition, if corrosive or damp conditions are prevailing, then the VSD will need to be installed in suitably IP rated enclosure. Moreover, if the environment is potentially explosive, then the enclosure will have to be mounted in a safe area, remote from the pump motor.

The above electrical problems related to VSD retrofitting are relatively well known; the same is not true, however, as regards the actual system problems that can result, with resonance being one such example. Pumps, their support structure, and the piping are subject to a variety of potential structural vibration problems (resonance conditions). Fixed-speed applications often miss these potential resonance situations because the common excitation harmonics, due to factors such as running speed, vane passing frequency and plunger frequency, do not coincide with the structural natural frequencies.

However, with VSD applications, the excitation frequencies are variable and the likelihood of encountering a resonance condition within the continuous operating speed range is greatly increased. The most common excitation mechanism for resonance conditions is pressure pulsations: these may be amplified further by acoustic resonance within the pump or adjacent pipework. Happily, a number of 'tried and tested' methods are available to predict and avoid resonance conditions; these range from generally accepted hydraulic resonance calculations to passing frequency analysis and structural resonance examinations using finite element analysis (FEA).

Lateral critical speed

In addition to resonance, the risk of the pump rotor encountering a lateral critical speed increases with the application of a VSD. Lateral critical speeds occur when running speed excitation coincides with one of the rotor's lateral natural frequencies. The resulting rotor vibration may be acceptable or excessive, depending on the damping mode adopted. Variable speed vertical pumps are more likely than horizontal machines to suffer from the excessive vibration condition. This is because the lower natural frequencies of these pumps are more likely to coincide with running speed. Small, vertical close-coupled and multi-stage pumps do not generally suffer from this type of problem.

Having explored some of the potential pitfalls, it is worth underlining that when installed correctly by a team that is familiar with the above issues, a variable speed drive can also solve problems in addition to saving energy. The average power factor in a building for example is only 90 per cent; an inverter has a unity power factor of 99 per cent, thereby ensuring the maximum performance for a pump is attained.

Frequent changes to a production line as new products are brought on line or as a shift changes can also be accommodated far more easily with a VSD-driven pump, accurately matching the volume of fluids, gases or powders to demand can improve both production efficiency and consistency. Replacing inefficient mechanical throttling and choking allows operators to trim production to a better outflow, not just saving energy in one place; a whole line can work at its most efficient speed and system production can be fine-tuned to suit immediate needs. Mechanical stresses on pumps and other powertrain components are also reduced, extending the life of the equipment and reducing maintenance costs.