Phase control Shaded pole motor

Discussion in 'General Electronics Chat' started by Vindhyachal Takniki, Mar 13, 2015.

  1. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    348
    6
    1. I have to control the speed of shaded pole motor. For this i have selected phase control method using TRIAC.
    I have a connected a 10K pot at adc input of MCU, as pot is rotated MCU sense the reading & accordingly control the phase. (voltage = 220vac)

    2. Now if I connect a 100W bulb at the output & then rotate the pot, I am successfully able to control the intensity of bulb like in gradually goes dimmer or brighter.
    Also I have connected a ac voltmeter at load, I can see gradual increase/decrease in voltage of bulb as phase control is done.

    3. Problem arises when I connect motor;

    Case1: When there is no parallel capacitor at motor connected (ceramic 450V):
    Motor start to show on/off behaviour. When volateg of pot at MCU adc pin goes high, motor gets fully on & below that it goes fully off.
    However if I connect bulb instead of motor, it works fine.

    Case2: parallel capacitor at motor connected (ceramic 450V):
    Motor shows random behaviour. Sometimes it picks up the load & starts roating & sometimes it do not even run. At some point full speed is on, at some point only by half speed


    What could be problem?
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    that will not give you much torque, for a shaded pole motor you should use a vfd. the phase control wworks for a light dimmer and will change the speed of the motor, but will give you less torque when you slow down the motor.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,509
    2,369
    Shaded pole motors are not a good choice for speed control, in fact most 1ph induction motors are a poor choice.
    Very small shaded pole fan motors can be ran down to a certain speed in some cases.
    Have you tried it with a direct hard wired Triac circuit bypassing the MCU control?
    Max.
     
  4. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,034
    1,626
    The speed of shaded pole motors are easily controlled by phase angle control but you need to modify your triac circuit to work with inductive loads not just resistive.

    You can cheat a bit on your motor inductance by adding more capacitance across the motor to make it appear as a more resistive load but you will likely still get some random speed changing.
     
  5. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,226
    If you add a small load to the motor shaft it will help with the speed control.
    You don't want to add a capacitor.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,509
    2,369
    Can you show the schematic and how you have the interface to the MCU, means of isolation etc?
    Max.
     
  7. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    348
    6
    I will give schematic. Meanwhile as my circuit, I have a optocupler between MCU * gate pin of triac.
    According to user input, I keep control of phase of ac signal.
     
  8. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    348
    6
    attached is schemtic. I am using 220vac.
     
    • sch.pdf
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  9. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,509
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    That shows a capacitive split phase motor rather than shaded pole?
    Cap split phase motor do not phase control well as they tend to drop out of run on low rpm and a load.
    Max.
     
  10. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    348
    6
    That only schematic to view. It actually contians a shaded pole motor
     
  11. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    348
    6
    move up
     
  12. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    348
    6
    1. I have tried & recorded some results.
    2. Snubber circuit is added.
    3. I have attached a pdf to show some results:
    4. Page 1 shows how circuit is setup i.e how drop across sense resistor is measured by CRO. (circuit says current probe but its where drop is measnured across 5.4ohm resistor.)
    5. Page 2 shows voltage & current relationship.
    6. Page 3 shows a circuit where I have placed a capacitor in parallel to motor.
    7. Page 4 shows its relationship voltage & current relationship.
    8. Page 5 shows the capacitor which I had used. I don't know what its exact value. But by checking it at LCR meter, value shows around 2.2uF.
    9. Page 6 shows sense resistor (5 resistors in parallel of 27 each) , & how drop across it is measured by CRO. I placed ground wire close to sense tip of probe, so as to have minimum distance.


    10. On doing phase control as soon as I try to do it suddenly entire voltage across the motor goes down.
    11. What I am planning to do now, is to place a zero crossing detector across sense resistor & will do phase control whenever current goes zero. Will see what happens then.
    12. Should I proceed this way or there is any other method?
     
  13. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    348
    6
    1. Attach is the ckt diagram showing snubber circuit & triac connections. Ckt again don't work.

    2. I had bought a low cost ceiling fan regulator & strangely it works. I had checked its circuit, the additional feature is it has diac to control firing angle rather than MCU pin for controlling it via opto-triac.
    They are using 4q triac.s

    3. I think my snubber circuit values are wrong?
     
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