Variable speed drives (VSD’s) are considered state-of-the-art in controlling driven processes, but it is important to realize they may not be the answer to every process control problem. End users need to understand their application requirements and mechanical and electrical system constraints in order to ensure that a VSD is the right solution, and to specify it correctly. In the first part of this series, we’ll summarize some of the main process and load characteristics which should be taken into account before specifying a VSD:
- The need for speed (…control, that is)
Many processes can benefit from the ability to periodically or continuously reduce output by reducing the speed of driven equipment. For example, piping systems are often “over-designed” to accommodate future expansion or simply provide some operating headroom. If driving the motor at full speed results in output that must be “turned down”, via control valves for instance, there is a potential to increase overall system efficiency and reduce energy consumption using a VSD. Bearing in mind that capital costs tend to be higher for VSD use, it is important to determine the percentage of time and reduction in flow needed and use that information to estimate the reduction in motor output that could be tolerated. For example, in centrifugal applications the motor horsepower varies by the cube of the speed. As a result, a reduction in motor speed of just 15% can reduce horsepower required (and thus kW consumed) by almost 40% (.853 = .61).
On the other hand, some systems are designed such that speed reduction provides no operational benefit. For example, in tightly designed piping systems with plenty of static head available, system flow tends to be more efficiently controlled with valves, especially when capital equipment costs are factored in. It is often only when systems are dominated by friction losses and these can be significantly decreased by decreasing flow rate that VSD advantages become clear.
Also, many processes could benefit from reduced stresses during starting even if speed control is not required. In such cases, a reduced voltage (i.e. “soft”) starter can be used. The soft-starter will typically reduce inrush current, which in turn reduces mechanical and electrical stresses on driven equipment. While a VSD also can provide this “ramp up/ramp down” function, it may well be over-kill (adding unnecessary cost and complexity) if variable process speed is not needed.
- Load torque
VSD’s are sized directly by continuous amperage output, not horsepower. While the latter can be used to approximate VSD sizing, it is the amount of current required in both steady state and over-load conditions which determines the correct drive to specify. The torque needed to operate the driven load affects the amperage required. Loads with constant torque or constant horsepower profiles require a higher amount of torque during start-up, which places greater amperage demands on the drive at ramp-up. It is therefore important to understand load torque profiles to know which drive amperage rating and overload capacity to select.
- Variable torque: this profile is characteristic of centrifugal pumps and fans. The torque increases as load speed increases, resulting in lighter demands on the drive at start-up. The torque increases with the square of the increase in speed.
- Constant torque: typical of conveyors, positive displacement pumps/compressors, and screw feeders, this profile is characteristic of a load requiring effectively the same torque at any speed within the operating range.
- Constant horsepower: in this profile, typical of machine tool applications, winders, and some load-driven conveyors, the torque required varies inversely with speed (decreases as speed increases).
VSD’s rated for variable torque applications have higher continuous amp ratings because they are not being taxed as much at start-up or when needing to respond to changing loads. For example, a drive sized for a 20 HP variable torque application, capable of supplying a continuous 31 amps at 480 volts, would only be rated for a 15 HP constant torque application, supplying a continuous 23 amps. This is to prevent the drive electronics from over-loading when faced with the need to maintain torque under heavy duty applications.
Next week, in Part II we’ll discuss electrical system considerations in VSD applications. In the meantime, please contact me with any questions, comments or application needs you may have at firstname.lastname@example.org. Thanks for reading, and we’ll see you next week!