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555 timer to make 120HZ motor drive?

Discussion in 'Electronic Basics' started by Eric R Snow, Jan 24, 2006.

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  1. Eric R Snow

    Eric R Snow Guest

    This probably won't work. I don't know enough about motors and
    electronics to know. I was looking at some zero crossing solid state
    relays. Thinking that one could be used to switch on a 1 HP single
    phase motor. The type with a starting winding and no caps. Then, after
    wiring up a VFD (Variable Frequency Drive) to a three phase motor, I
    got to thinking that maybe the aforementioned single phase motor could
    be started on 60HZ and then switched to 120 HZ. From what I've read
    about VFDs the voltage rises with the frequency so that at 120HZ a 115
    volt motor would be supplied with 230 volts. If this is so, could a
    230 volt DC supply be switched by a zero crossing SSR at 120HZ
    controlled by a 555 timer? What effect would the induction from the
    motor windings have on the SSR? Obviously the complex VFD electronics
    aren't just there for fun. But would the described scheme work? Well
    enough to get useful work from the motor?
    Eric R Snow

  2. The speed of real three phase motors can be controlled by what you call a
    VFD. Especially lower speeds can be obtained without degrading the power.
    But driving a motor at two time its nominal speed will destroy it. The
    bearings can not handle the forces neither can the coils in the stator. A
    one phase motor will almost sure have other mechanical limitations. For
    instance, when the speed is too low, the start coil will not be switched off
    and will be fried.

    SSR's that switch inductive loads like motors need a so called snubber
    network. Google for it if you want to know more.

    I like your aim for experimenting but strongly advise to start with less
    complicated and less dangerous projects.

    pertrus bitbyter
  3. Eric R Snow

    Eric R Snow Guest

    Thanks for the reply Petrus. You should know that most 3 phase
    induction motors made in the USA can be run at twice nameplate RPM.
    Especially if they are 1725 RPM motors. The biggest problem with
    running non-VFD rated motors is when run below rated RPM. The fans may
    not keep the motor cool. The solution is to install another fan. The
    info about motor ratings was obtained from bothe motor makers and VFD
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