Living sustainably

Energy efficiency

System optimisation

  1. Introduction
  2. Optimising pump and fan applications
  3. Control technologies

Variable speed drives: typical questions and answers

What factors should I consider to ensure successful implementation of variable speed drives?

  1. Work environment - what atmosphere will the drive and motor be subject to (volatile/non-volatile gases, moisture, dust etc.)? What is the ambient temperature?
  2. Speed range - what speed range will the process require? What is the average operating speed? What is the allowable speed error?
  3. Multimotor - will the drive be controlling one or more motors? If several, will they start simultaneously or sequentially?
  4. Required acceleration time - what is the maximum and minimum time for acceleration of the total drive inertia?
  5. Process duty cycle - what percentage of the total operating time will the equipment operate at each speed? 
  6. Potential overheating - will overheating be a problem, especially for constant torque-reduced speed applications?
  7. Protection features - what drive and equipment protection is required to maintain process continuity? Some drives trip instantly on an over-current condition, while others will maintain a constant torque to the motor and reduce the speed of the motor to maintain the current required (for several seconds up to several minutes depending on the drive backup supply).
  8. Load torque requirements - what are the torque requirements of the process?
  9. Extent of diagnostics - how critical is the down time of the load? If you want an extensive fault analysis, a digital drive provides the most accurate and precise fault indications, as it can store process and motor variables prior to the fault. Analog drives can normally indicate bus over-voltage, input under-voltage, drive over-temperature, output ground and drive over-current.
  10. Drive system considerations - coordinated drive systems require accurate speed control between drives, and some digital drives can perform in a 'master-slave' manner with direct communication that eliminates any additional inter-drive controller. You can have networking drives and process logic controllers (PLC) in a complete process monitoring and control system, if the drive has the required communication hardware and software. If process changes require different limits and responses, the limits and drive operating parameters can be downloaded from a computer or PLC while the process is running.

What information do I need to evaluate energy savings in a variable speed application?

  1. Method of flow control to which adjustable speed is compared:
    • output throttling (pump) or dampers (fan)
    • recirculation (pump) or unrestrained flow (fan)
    • adjustable-speed coupling (eddy current coupling)
    • inlet guide vanes or inlet dampers (fan only)
    • two-speed motor.
  2. Pump or fan data:
    • head v's flow curve for every different type of liquid (pump) or gas (fan) that is handled
    • Pump efficiency curves.
  3. Process information:
    • specific gravity (for pumps) or specific density of products (for fans)
    • system resistance head/flow line or per cent static head and rated head and flow requirements
    • equipment duty cycle, i.e. flow levels and time duration.  
  4. Efficiency information on all relevant electrical system apparatus:
    • motors, constant and variable speed
    • variable speed drives
    • gears
    • transformers.

If you do not have precise information for all of the above, you can make reasonable assumptions for points 2 and 4. However, if the information in points 1 and 3 is unknown, you will not be able to conduct a valid energy saving evaluation.

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