The Energy Chronicles

Variable Speed Drive (VSD), Another View

For many a variable speed drive still holds some mystery. I found it’s best understood if we take something we do nearly every day and relate it to the four common methods used to control the speed of fans, pumps, motors, and air compressors. Those four methods are Variable Speed Drive, Load/UnLoad, Active Rotor Length (air compressors), and Modulation.

Car Analogy:

Imagine driving on the interstate at 70 mph, coming up to a 55mph construction zone and you need to slow down. You could slow down in one of 4 ways:

  1. Variable Speed Drive: Just raise your foot off the throttle to slow the engine. Then to speed up the throttle is put down again. Your foot manages the engine speed at the direction of your brain.
  2. Load/No Load: Throttle is all the way to the floor, but the clutch is pushed to slow down. The engine continues at maximum speed, but when not connected to power the wheels, the car slows. Let off the clutch pedal, the wheels engage, and you speed up. This uses a lot of energy and will prematurely wear out your clutch. We do not recommend you try it.
  3. Active Rotor Length: Throttle is all the way to the floor, but the vehicle is shifted to a lower gear. The engine continues at maximum speed, but the wheels will turn slower. Again, energy is wasted from running the engine at maximum speed.
  4. Modulation: Throttle is all the way to the floor, but the brake is applied. This slows the car down, but puts a lot of damaging heat into your brakes….for a while. Obviously this is also not recommended.

In summary,

Variable Speed Drive is what we’ve learned to do on the road and is intuitive. It is the most economical as it reduces losses in efficiency throughout the drivetrain rather than one section.

Load/No Load reduces the load on the engine, but it is still using a significant amount of energy.

Active Rotor Length would probably be slightly less economical than Load/No Load, because the drivetrain losses are engaged all the time rather than switched on/off.

Modulation would be the worst as a high load would be put on the engine all the time.

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