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Motors and Drives

Large numbers of motors are over-sized for duty.  Many are fixed speed when they serve a fluctuating load.  Just think about a lift for example with 1 person in it or 10 people in it.  The same motor elevates the occupants but the demands on the motor change with the number of folk in the lift – sounds simple but this basic “issue” is often not addressed and results in waste of motive power.

The good news is that it doesn’t have to stay that way.  There are controls available that can be retro-fitted to a motor to allow the power drawn to more closely match the demand.  We call these Variable Speed Drives or VSDs and they are super bits of kit that reduce energy consumption of fans, pumps, compressors, conveyors and so on.

Let’s look at a typical example:

  • One of the studios at a Cinema complex
  • the ventilation system is fitted with an 11kW Supply Fan and a 11kW Extract Fan
  • The fans operate 14 hours / 7 days / 52 weeks
  • 12p/kWh electricity cost
  • 22kW x 14hr x 7 day  52 weeks = 112,112 kWh a year costing £13,353

Think about the occupancy of this space.  Numbers vary throughout the day but the ventilation system chugs away at the same speed and power draw constantly.

So, we install air quality monitors to detect and quantify CO2 in the extract ducts and these are wired back to a micro-processor in a VSD set to maintain  a target of, lets say, 1000ppm CO2 creating a fully automated system that will adapt both Supply & Extract to maintain air quality and adjust performance to match the number of occupants – the more people watching a film the faster the fans turn and they are further controlled by a real-time clock and calendar to ensure there is no over-run.  

How much energy / money can this save? 

There is a physical phenomena known the Fan Affinity Law which states that the power drawn by a motor is directly proportional to its’ speed cubed (a square-law curve if you are interested).

What that means in practice is that slowing down a motor can achieve much larger savings than you might otherwise expect.

For example, if you slow a motor down by 10%, that is to 90% of full speed, power drawn becomes 0.9 x 0.9 x 0.9 = 0.729 or 73%

So a 10% speed drop results in a 23% energy saving – nice.  A 20% reduction results in almost 50% energy saving.  So for the cinema studio that would be 56,056kWh worth £6,727/year.

Payback on 2 suitably sized, properly installed and commissioned VSDs will be well inside 1 year.