Induction motor soft start

Discussion in 'The Projects Forum' started by zebedee, Feb 12, 2011.

  1. zebedee

    Thread Starter New Member

    Jun 23, 2009
    I want to build a soft start circuit for a single phase induction motor.
    The motor rating is 1 HP so a DOL start is far from ideal.
    The no load current is 3 amps (measured) and the full load current is given as 5.7 amps on the rating plate.
    Thoughts so far,
    1). Series resistor shorted out by an SSR after a short time delay but where do I get a resistor that will tolerate 2 KW or so for 1/2 sec?
    2). A simple automatic phase control circuit.
    3). NTC series resistor but is there one that will survive the surges?

    Phase control seems favourite.
    Suggestions please for the way forward.
  2. beenthere

    Retired Moderator

    Apr 20, 2004
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    Induction motors rotate at the frequency of the AC input.

    To start one slowly, a variable frequency drive would be needed if you want full torque during startup.

    A "chopper" circuit could be used to allow less overall power into the motor, but torque would suffer.

    For the power you are looking at, either solution would be rather expensive.
  4. GetDeviceInfo

    AAC Fanatic!

    Jun 7, 2009
    multi phase motor soft starts typically use a graduated voltage drop. At the run end, or close to it, you may have a hard contact pull in to reduce any further heating on the solid state drivers. Thermistors mounted on the drivers or on thier heatsinks in close proximity, can provide for programmed (memory) overload protection.

    Single phase may be a different animal in regards to torque requirements, as mentioned.

    Another approach can be phasing. In multiphase, Y start, Delta run is a start strategy. For single phase, a start cap (phase shift) switched to a run cap is common, along with several other configurations including a resistive split phase. Much would depend on the type of motor you have, more specifically the types of windings present.
    Last edited: Feb 12, 2011
  5. zebedee

    Thread Starter New Member

    Jun 23, 2009
    Sorry about the hijacking.

    My application is a simple one and doesn't really warrant a sophisticated or expensive solution.

    Last week I bought a pillar drill with a 3 Phase motor on it. I have changed the motor for a single phase 1 HP one and need to soft start it to prevent high current surges on the 240 Volt mains input.

    I feel that if I could connect it at a zero crossing point and increase the phase angle to 180 degrees over the next 50 cycles or so, that should work....but I don't know how to do it.
    Last edited: Feb 12, 2011