VARIABLE SPEED DRIVES AND THEIR USE IN RESOURCE EFFICIENCY AND CLEANER PRODUCTION

Forced draft fans in dry cooling devices
like the one pictured can be fitted with
VSD's  to allow them to  vary speed in
response to climatic conditions or
tower performance

The speed of an induction motor is a function of its number of poles and the frequency of the power supply. Variable speed drives (VSD’s) – also called variable frequency drives (VFD’s) – are used to vary motor speed by changing the frequency of the alternating current and voltage applied. They have many uses in industry, which is to be expected given the wide range of potential applications for continuous speed control within industrial processes.

VSD’s may be controlled through the intervention of a plant operator, using an interface such as a simple button/knob which can be used to increase and decrease speed, or through a SCADA system, which can be used to manually changed the frequency output. Generally however, they are used within the context of a control loop, which varies speed automatically in response to an output from some other measured variable. For example, pump speed could be varied in response to the output from a flow meter in the pipeline carrying the fluid being pumped. Alternatively, frequency setpoints could be written into control software based on the status of the process being controlled. For example, agitator speed could be set to a minimum value while a vessel is being filled, with the speed increased after filling has been completed. There are an infinite number of ways that VSD’s can be employed in control schemes such as this. Just a few examples of common applications are:

• Changing the speed of agitation in a mixing vessel
• Changing conveyor speed on a production line
• Changing the speed of a pump (and hence the flow) in fluid handling
• Changing the speed of a compressor
• Changing the speed at which racks are lifted from baths in electroplating
• Changing the pressure inside a boiler (on the fireside) by changing the speed of FD and ID fans.

So far I have discussed how VSD’s give us more control over industrial processes, but what then is the contribution of VSD’s to industrial sustainability? For one, improved control means reduced rework, which translates into reduced consumption of energy, water and materials. VSD’s are however probably more recognised for their growing use in industrial applications on the basis of the energy savings they provide. Simple examples include their use to control poorly loaded screw compressors (minimising “off-load” losses), and their use in controlling the speed of fans (where flow is directly proportional to speed, but power varies with the cube of speed). Controlling fluid flow by manipulating pressure drop (e.g. the use of a control valve or damper in conjunction with a pump or fan running at a fixed speed) consumes far more energy than that required were a VSD used to control pump or fan speed instead. VSD’s also provide exceptional “soft start” capabilities (while using less energy than traditional soft starters) and can also be used to control torque characteristics and to boost torque (e.g. at start-up).  While there are losses incurred in using a VSD (of the order of 1 – 3%) the efficiency gains at the system level arising from their use generally far exceed these. As far as energy efficiency projects involving induction motors go, I tend to find many more viable opportunities through VSD applications than I find in areas such as motor replacement.

VSD’s are however far more versatile from a sustainability point of view than simply being an electrical energy efficiency tool. Their use in material usage reduction can yield spectacular results where correctly applied. Consider the following drivers of material usage in the industries in the table below, and how VSD’s can be applied to improve material usage performance:

INDUSTRY	PROCESS	DRIVER OF MATERIAL USAGE	HOW TO EMPLOY A VSD
Drum manufacturing/reconditioning	Spray painting	The speed at which drums are rotated during spray painting is an important driver of paint thickness.	Install a VSD and allow the operator to manipulate drum speed to control paint thickness, along with other variables such as solvent ratio, air pressure and nozzle design.
Powder coating	Curing	Curing time and temperature affects curing quality and rework rates	Install a VSD on the conveyance system to permit variations in residence time in the curing ovens for items of different dimensions. To be used in conjunction with oven temperature profile.
Waste management	Oil recovery in a plate separator	Residence time in the plate separator impacts on efficiency of oil recovery	Use a VSD on the supply pump and vary the flow based on the composition of the incoming effluent, thereby changing the residence time. A turbidity meter in the separator outlet could be used for automatic feedback control.
Batch chemical manufacturing	Material dosing	Accuracy with which individual chemicals are added to a batch	Use VSD’s on dosing pumps (liquids) and screw conveyors (solids)  to vary dosing rate, slowing down the flow as the target value is reached and eliminating overshoots

How else could VSD’s be employed? The answer is really that the opportunities are limited by your creativity. There are few manufacturing processes in which speed is not an important variable, and the first thing to understand is: “what is the impact of speed on the process being analysed?”. Armed with this understanding, the next question would be: “does a VSD, with its ability to provide speed control over a continuous range, provide leverage in terms of reducing resource consumption and minimising pollution?” In my experience, the answer is very often a resounding “yes”. Your final consideration would be whether this leverage/benefit justifies the investment in the VSD. In real terms, VSD’s have become cheaper over time, despite gains in the features they offer. Combine this fact with the significant benefits they can provide, and they are often easy to justify from a financial and environmental perspective.