Newbie needs a motor control

Discussion in 'General Electronics Chat' started by walthor, May 1, 2013.

  1. walthor

    Thread Starter New Member

    May 1, 2013
    Well gentlemen I’ll be quite honest, I joined this forum for one reason only, to pick your brains. When it comes to electricity and electronics I’m a complete clutz.
    My hobby is radio controlled model powerboats. I love to build and occasionally see the fruits of my labors take a trophy home from a race.
    I was recently remodeling the bathroom and needed to do some cutting. I have a small table saw that measures 10” X 10”. Not heavy duty by any means.
    Well it didn’t take long before I fried the motor cutting ½” ply.
    I searched all around but couldn’t find a small 110 DC motor the right size. What I did find was a larger ElectroCraft DC servo motor. A friend tested it for me and found that it could take over 100V at 0.7A no load so I modified the saw to accept the larger motor.
    Then I found the motor needed to be run up gradually or it would fry the rectifier. I proceeded buying a thermistor. Very interesting event. The thermistor got so hot after about 20 seconds that it melted the plastic case and the connecting leads de-soldered themselves from the disc.
    Next a 3 position switch. 0 is off, 1 goes through the thermistor and 2 goes to the motor (both go to the rectifier first). Turned out 120V is too much for the motor. Next came the
    Powerstat type 10C Variable transformer in front of the rectifier. Works great but lots of weight there.
    I wanted to eliminate the variac using a resistor and then someone told me I should put a diode in the circuit rather than a resistor to slow down the motor. Something with the rectifier and the diode working together to reduce the output.
    Can anyone enlighten me to what value I would need or tell me what I need to do to find that value?
    Here is the current setup (pun not intended);
    1,) 120 power in to a switch
    2) from switch to variac
    3)from variac to RS604 rectifier
    4)from rectifier to the motor