By Kody Cook, Journalist, Pump Industry Magazine
Variable speed drives (VSDs) are used across a vast range of industries to control the speed of pumps and motors and maintain optimal operational efficiency. Here, we dive further into how they work, the differences between the types of drives and the purposes they serve.
VSDs are used in a variety of operations to control the speed and torque of an AC motor by converting fixed frequency and voltage input to a variable frequency and voltage output. This is useful for increasing system performance and efficiency by controlling the speed to precisely match the load.
Greater system optimisation brings long-term benefits of reduced wear and tear on motors and pumps, as well as reduced energy consumption. Where process output requirements vary by 30 per cent or more, matching the load with a VSD can reduce energy use significantly.
These improvements lead to significant savings, not just due to reduced energy costs but also because reduced strain on equipment means less maintenance costs and less downtime, boosting overall productivity.
Motor systems fitted with VSDs can bring other benefits, including:
- Accurate control of pressure, flow and temperature
- Improved safety and amenity, through reduced heat and noise levels
- Integration of VSD control with building management systems
In some pumping systems, valve-throttling flow-control is used to prevent pressure build-up. This is not as efficient as using a VSD because energy to the pump is not reduced. A VSD enables precise flow control without the energy losses of throttling, ensuring the system isn’t running at full-speed when not necessary.
Some modern electronic VSDs are also known as variable frequency drives (VFDs) as they work by varying the AC electrical input frequency to control drive speed.
The difference between VSDs and VFDs
While VSDs and VFDs serve a similar purpose – controlling the speed of motors and pumps in order to optimise the load to speed ratio and increase overall efficiency – the means by which they achieve this is different, and they are therefore suited to different systems and conditions.
VFDs vary the speed of a motor by varying the frequency to the motor and, as such, they can only be used with alternating current (AC) motors. VSDs adjust speed by varying the voltage to the motor, allowing them to be used for both AC and direct current (DC) systems.
VFDs may be of little benefit where precise motor speed control does not assist the production process or where hours of reduced demand are few. VFDs are also not recommended for applications where slowing down the machine causes operating problems, such as insufficient torque or poor cooling.
VFD technology has widespread uptake with AC induction motors, with VFDs favoured due to their accurate speed variability from zero rpm to over 100 per cent of the rated speed. VFDs also enable motor control in either direction.
VSDs in pump systems
VSD systems are often a viable option for pumps that experience highly variable demand situations, for example in irrigation systems. When delivering water sequentially to irrigation units of variable size and elevation, subtle variations, such as changes in river or bore height and more frequent operations such as filter backflushing, can all require variable flow or pressure. Each of these situations has unique pressure and flow requirements.
Conventional methods for flow control in irrigation include throttling valves, impeller trimming or pump speed adjustments. These adjustment methods are reasonably permanent and unable to easily cater for the variable load requirements of pressurised irrigation. When situations demand flow adjustments from a pump, VSDs offer convenience and great potential for energy cost savings.
Potential drawbacks
While VSDs and VFDs can provide significant increases in efficiency and life-span of equipment, there are potential risks and drawbacks associated with them that can be avoided with
appropriate design and operation of systems.
Structural resonance
Structural resonance refers to excessive vibrations of non-rotating components or supporting structures. The high vibration levels are potentially harmful to equipment and surrounds.
Fixed speed applications often miss these potential resonance situations because the common vibrations of continuous operating conditions rarely coincide with the structural natural frequencies.
For VSD applications, the excitation frequencies become variable and the likelihood of encountering a resonance condition within the continuous operating speed range is greatly increased. Pump vibration problems typically occur with bearing housings and the support structure.
Pressure pulsations are the common excitation mechanism. These pressure pulsations may be further amplified by acoustic resonance within the pump or the adjacent piping. There are a number of methods for predicting and avoiding potential resonance situations. These methods include:
- Simple hydraulic resonance calculations
- Passing frequency analysis
- Model testing of the machine
Model testing can supplement the regular vibration test. Often, a pump intended for variable speed operation will only be tested at one single speed.
Rotor dynamics
VSDs can also increase the risk of the rotating element of a machine encountering a lateral critical speed. 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 modal damping associated with the corresponding mode. Additionally, drive-induced torque harmonics may cause resonance conditions with torsional rotor dynamic modes, however, such conditions are usually correctable or preventable.
Variable speed vertical pumps are more likely than horizontal machines to exhibit operational zones of excessive vibration. This is because such pumps’ lower natural frequencies are more likely to coincide with running speed.
VSDs and VFDs are highly useful devices that are used throughout the industry to maximise efficiency of systems. While using them presents new risks and challenges, operators who do their due diligence and properly plan and test their systems will find that these devices can deliver significant cost savings.
