DC Motor speed control

Discussion in 'General Electronics Chat' started by Tomsmachine, Oct 5, 2017.

  1. Tomsmachine

    Thread Starter New Member

    Jun 17, 2017
    8
    0
    I have a Dart DC motor controller, with a POT. speed control.The pot has a travel of about 300 degrees, but uses about 60 degrees, from minimum to maximum speed. The pot is what is supplied with the control from the manufacturer. Specs on the pot are 5K ohms, 2W. if measured from the high side of the pot Ohms are 5k min speed, 3.5k maximum speed. So an idea to expand the range of travel, is to take a multi contact switch, (the old speed control) and add a string of resistors, first being 3.5k, next .5k, then .5, the last .5K in total 5k.
    Leave the wire wound pot in place and just leave the wiper at the 0 point. The multi switch would be in series of the wiper, can I just use 1/8w resistors to do this.

    Tom
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    18,211
    5,592
    Normally the pot goes into an OPamp or other high impedance input, so 1/8th would work.
    Max.
     
  3. dendad

    Distinguished Member

    Feb 20, 2016
    2,752
    739
    You need to look at the input circuit of the speed control. Is the pot connected with 2 wires or 3?
    If just 2 are used, your idea will probably work, but for 3 wires it could be different as the pot is acting as a voltage divider.
    And if you have the room, use at least 1/2 watt resistors not 1/8 watt. They probably used a 2 watt pot for a reason.

    As for the speed, is it a PWM out, and does the PWM go from 0% to 100% over just part of the pot range?

    Is it one of these?
    http://www.dartcontrols.com/catalogs/250GSeries.pdf

    and if so, have you looked to see if the internal Min and Max trim pots are set correctly? That may be you the range is reduced.

    Another thought I had, with a switch, the speed control will go open circuit momentarily as it is switched, but that I hope will not cause trouble.

    Have you considered a 2K pot with a 3K3 resistor in series so the controller sees 5K3 across it, but the variance is 5K3 to 3K3?
     
    Last edited: Oct 5, 2017
  4. Tomsmachine

    Thread Starter New Member

    Jun 17, 2017
    8
    0
    It is a 3 wire pot., and a model 253, covered by the manual you noted. Yes, the trim pots are set as I need them. The max rpm is at its limit, which is the side with all the deadband. Motor is rated at 230V, both field and armature, field limit with this controller is 210V, and armature is 215V as the high limit boost a bit over the 180V rating. The motor is only 1/4hp, while the controller max is 2hp. So it is not overloaded electrically.

    This project has been the first that ever really kickede my butt in so many ways. This issue is about .01% of the problems the last month with this machine.
    Thanks for your responses.
    Tom
     
  5. jayanthd

    Active Member

    Jul 4, 2015
    863
    72
    If it is a microcontroller based device then maybe they are controlling the phase angle of the AC fed to the motor using TRIAC. Por is used to control the phase angle.
     
  6. Tomsmachine

    Thread Starter New Member

    Jun 17, 2017
    8
    0
    This is a DC motor, shunt wound field, That gets a steady 200V, and voltage is varied to the armature.for speed control.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
    18,211
    5,592
    These Dart controllers are usually simple SCR bridge controllers, similar to the lower end KB line.
    The pot does Not control the SCR's directly but goes through a couple of Op-amp stages first
    Max.
     
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