Vacuum Cleaner Speed

Discussion in 'General Science' started by AlbertHall, Feb 9, 2017.

  1. AlbertHall

    Thread Starter AAC Fanatic!

    Jun 4, 2014
    I have noticed that when the hose of a standard domestic vacuum cleaner is blocked it sounds like the motor speeds up.
    Does it really speed up?
    And why isn't it running more slowly as it has more work to do?
  2. MaxHeadRoom


    Jul 18, 2013
    Sounds like another recent thread!;)
    A vacuum motor has a impeller type fan and when the inlet or oulet is blocked on this kind of mechanism it reduces the load, in the case of the vacuum, being mainly Universal motors, which operate in a runaway condition the only thing that limits rpm is windage, load, friction.
    Any reduction in current also causes the series field to reduce and a speed increase occurs.
    In the case of an induction motor, for e.g. blocking the outlet of say a fluid pump with a impeller causes a drop in current and off loads the motor.
    A different kind of pump the result is opposite, say in a gear pump, where blocking the outflow will cause Increase in current.
    Last edited: Feb 9, 2017
  3. MrChips


    Oct 2, 2009
    One may think that when the hose is blocked the motor has to work harder.
    Not so. When the hose is blocked the air pressure that the impeller is working against decreases. There are fewer air molecules that the impeller has to bat around.

    I work with high vacuum systems. It took me awhile to wrap my head around how turbo pumps work. They literally have to strike a molecule out of the end-zone, so to speak, (or over the boundary. It's simple cricket, shall we say).
  4. crutschow


    Mar 14, 2008
    The vacuum cleaner motor does work to move the air through the hoses and the impeller pump.
    When the hose is blocked, it's moving no air through the hoses, creating a maximum vacuum in the pump, so the thinner air just circulates round and round in the impeller chamber, which requires less motor torque and it speeds up.
  5. AlbertHall

    Thread Starter AAC Fanatic!

    Jun 4, 2014
    OK, I get it thanks.
    It's been puzzling me for quite a while so I'm glad I asked :)
  6. Glenn Holland

    Active Member

    Dec 26, 2014
    Interesting that tornadoes have the same behavior as a vacuum cleaner fan where the rotating funnel cloud (and the core of the thunderstorm) move air by centrifugal force.

    If a funnel cloud has not made contact with the ground, the surrounding air can flow into the end of the funnel and upward into the rotating core of the parent thunderstorm. This requires that energy be taken from the rotation of the funnel to accelerate the air mass. However, if the funnel makes contact with the ground and this blocks the inward air flow, the rotation (and the angular wind speed) will speed up.

    Therefore, tornadoes that have been on the ground for a long time will inherently have more destructive power than one that has just touched down.
    RichardO likes this.