Transformer for DC motor

Discussion in 'General Electronics Chat' started by raychar, Mar 2, 2013.

  1. raychar

    Thread Starter Member

    Nov 8, 2011
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    Hello,

    I have a DC motor: when powered with Ni-Cd 7.2V power pack, running with attached gearbox. Starting current drawn is 13A and then 6A after one second and is steady at this current drawn.

    Now, I want to drive it by transformer. I got a 6Vx2 of 60VA one. With two 6A diodes to have full-wave rectification, 6800uF capacitor attached. The measured open output voltage is 8.4VDC. When connecting it to the DC motor, the measured voltage goes down to 5.5V and current drawn is 5.6A.

    By calculation, only 5.5Vx5.6A=31W was used on the transformer and 5.5v is too low for the motor to run.

    Did I get the wrong rating transformer? or what improvement can I make in order to get 7V in running motor?

    Thanks in advance
     
  2. t06afre

    AAC Fanatic!

    May 11, 2009
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  3. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Also, the power supply must be capable of supplying the starting current of 13 amps, not just the running current.
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
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    In this case I think a cheap swithmode power supply will be much better and cheaper
     
  5. #12

    Expert

    Nov 30, 2010
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    There is also the idea that a center tapped transformer with 2 diodes only uses half the windings on each pulse. A 4 diode rectifier uses all the secondary winding during each power pulse. Point is, proper utilization of the whole transformer to reduce size and price.
     
  6. raychar

    Thread Starter Member

    Nov 8, 2011
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    Hello,

    From the formula in attached information of "5c007"; V (Avg) D.C. = 0.45 X Sec. VA.C. it is center-tapped mode.

    If I need 7.2VDC, I need to have 7.2/0.45=16VAC or 8Vx2 transformer. I think I have to buy a new one...and don't know if there is such rating??

    As the starting high current time is short, I think the transformer can withstand it??? and can provide this short high current pulse to make motor rotate.

    Yes, SMPS is my first opinion, however, I don't want make it on my own and the it seems there is no such voltage range in market. I only find the common one; 5v, 12v,....

    Regards

     
  7. crutschow

    Expert

    Mar 14, 2008
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    You might try some high current (12A or larger) Schottky diodes and a larger filter capacitor. For 1V ripple at 6A the capacitor needs to be about 48,000μF.
     
  8. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Interesting. Would the large capacitor provide some extra energy to handle the motor starting current ?
     
  9. crutschow

    Expert

    Mar 14, 2008
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    Not enough to be a factor.
     
  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Probably the opposite, the larger cap will have a much slower voltage rise time at turnon, reducing chance of good starting. The larger cap also gives much lower DC peak voltage when running.

    Have you tried running the motor with no cap, just on the rectified pulses?
     
  11. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    The bridge configuration with 4 Diodes has double the diode voltage drop losses.

    It also doubles the voltage. You would likely end up with over 16Volts that drops to about 9 Volts under load.
     
  12. #12

    Expert

    Nov 30, 2010
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    I'm having difficulty seeing your point of view. Could you re-label these drawings or make a drawing that shows me what you were thinking about?
     
  13. crutschow

    Expert

    Mar 14, 2008
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    Don't understand that statement. The peak voltage may be some smaller but the average DC voltage will be higher which would seem more important than the peak voltage. :confused:
     
  14. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Hi Crutschow, DC motors will run from an average DC voltage, but they start based on peak voltage as that produces a higher instantaneous current.

    Assuming a very large cap his startup voltage will rise slowly to the average voltage of 5.5v which causes too little current for the motor to start turning.

    With no cap the voltage peaks might be 9v or more, which can cause enough short term current to get the motor past its startup friction.

    Similar effects are seen with DC motor traction drives using low freq PWM instead of high freq PWM, as high freq PWM acts like a very low average voltage and the motors won't start turning. Low freq PWM enables higher peak currents through the motor and improves low speed running torque and startup torque.
     
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