AC+DC motor control with Triac Bridge

Discussion in 'General Electronics Chat' started by radbrad, Jan 17, 2011.

  1. radbrad

    Thread Starter New Member

    Jan 17, 2011
    Hi everyone,

    Long time listener, first time caller. Hoping to get more involved with the forums!

    I want to be able to connect either an AC or DC motor to the same connections on my current project (not simultaneously, but for motor replacement/upgrade reasons). The AC motor runs at 120VAC @1.4A and has starting windings and main windings (can't remember the name of that style). The DC motor is rated for 90VDC@2A and needs speed control.

    Both motors need direction control.

    I've decided that a Triac bridge should do the job well. I'm not sure whether or not the design I've created will work or not, though. Here are my main questions:

    1) Will the Triacs shut off correctly? I'm concerned about the current lagging due to the inductive load, but not sure how to correct it/design for it.
    2) Will I be able to rectify the signal correctly for the DC motor?
    3) I'm also not sure about my snubber circuits - I don't want any transient noise issues, so I'm trying to go heavy on the protection. Is it too much/little?

    The thought is using my MCU with a zero-cross input I can pass both a full AC waveform for the AC motor but also do phase control (chop down the AC sine wave) for the DC motor direction and speed.

    When I do a SPICE simulation using a resistive load and triacs, everything comes out exactly as expected for the DC chopping. I have no idea how to correctly model the motors, but adding any inductance makes everything go haywire (I think it's a model problem, plus I'm just simulating a triac optoisolator).

    I have only recently become aware of triacs, so please go easy if I'm totally off my rocker. If everything's groovy then hopefully this at least helps the next guy with similarly crazy ideas - I searched and found a lot on motors and triacs, but nothing pertaining particularly to this setup.

    Attached are schematics and simulation results for phase control showing how it would toggle the Triac gates to control the DC motor in forward and reverse. One note on the schematic - the triac opto symbol incorrectly shows a zero crossing detector. The MOC3052 is a standard triac optoisolator.