Motordriver problem - works with one motor, but not another

Discussion in 'General Electronics Chat' started by Scalpel78, Sep 18, 2014.

  1. Scalpel78

    Thread Starter Member

    Aug 11, 2013
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    Hi, I've made a motor driver PCB which uses the Allegro A4950 motor driver. I supply 7,4V to the driver, and when I connect a Micro Metal Gearmotor it works perfectly.

    However, when I connect a motor that is inside a small RC car I bought from ebay nothing happens. If I bypass the motordriver, and put the 7,4V directly to the poles of the RC motor it works.

    When I measure the voltage out from the motordriver I get the expected 7,4V, so I don't see why there is a difference between the power delivered directly from the LiPo battery, and the output of the motor driver for that particular motor.

    Here is the A4950 breakoutboard, and the micromotor that works;
    [​IMG]

    And here is the other motor which only works when I connect it directly to the battery, but not to the output of the motor driver:
    [​IMG]
     
    Last edited: Sep 18, 2014
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    It might be the difference between milli amps & amps. Motor driver may have an over current shut down feature.
     
  3. Scalpel78

    Thread Starter Member

    Aug 11, 2013
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    On the PCB you can see I've added a little jumper wire. Originally there was an SSA33L schottky diode there, which has a maximum rating of 3A. The A4950 motor driver has a max rating of 3.5A, and up to 6A transient output current. In my mind the schottky will limit the power to maximum 3A (without stopping the current flow), but I'm not sure if I'm correct about that. Anyway, replacing the schottky with a jumper wire didn't make any difference.
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Are you sure the Schottky wasn't there to protect against reverse-voltage power supply?
     
  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    I s one motor a brushed motor and the other a brushless DC motor. The former runs in plain DC; the latter is more like a 3-phase AC motor.

    John
     
  6. faley

    Member

    Aug 30, 2014
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    The drive is PWM. The Micro Metal Gear-motor is brushed. As jpanhalt pointed out, therein lies the problem.
     
  7. john*michael

    Member

    Sep 18, 2014
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    Both motors are brush motors if you can run them with a DC supply. I'm guessing that R1 is a sense resistor for sensing the current to the motor. All of the motor current goes through this resistor. If the value is incorrect for your motor, your circuit won't work. Try shorting this resistor
     
  8. ronv

    AAC Fanatic!

    Nov 12, 2008
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    My money is on the car motor drawing a lot of current. Can you measure the resistance of the 2 motors?

    PS. Put the diode back in.:eek:
     
  9. #12

    Expert

    Nov 30, 2010
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    The diode is not a current limiter. It has a surge rating far in excess of 3 amps. Put it back in. If it smokes, replace it with a 5 amp Schottky diode.
     
  10. Scalpel78

    Thread Starter Member

    Aug 11, 2013
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    Thanks for the comments.

    When I designed the PCB I put the schottky there as a reverse polarity protection. I wasn't sure what the 3A 30V rating actually indicated.

    R1 is, correctly a current sensing resistor. According to the datasheet (http://www.farnell.com/datasheets/1481251.pdf) a reasonable value is 0.25ohms. I've tried two of those, and also tried replacing with a 0.1ohm resistor.

    I don't know what kind of motor the one in the ebay car is. All I know is that it works when I connect the LiPo battery directly to it, but not through the motor driver.

    Do you think I should replace R1 with a 0 ohm resistor?
     
  11. john*michael

    Member

    Sep 18, 2014
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    Thanks for the data sheet. You will notice that the resistor is used to limit current - set by the voltage on Vref and the resistor value. Certainly a zero ohm resistor will not hurt anything so long as you don't exceed the maximum current the device can handle, which is 3.5 amps. However, the device will current limit if the peak voltage on the resistor exceeds one-tenth of the voltage you place on Vref. So if you use a 0.1 ohm resistor, 2 amps would produce 0.2 volts on the LSS pin attached to the resistor (0.1 x 2). The data sheet specifies that you should not exceed 0.5 volts on LSS.

    The voltage on the LSS pin is compared to Vref / 10. If you placed 3 volts on Vref, Vref / 10 = 0.3 volts, and the current limit would kick in if the voltage on LSS exceeds 0.3 volts. Since you have a 0.1 ohm resistor, 3 amps are required to produce 0.3 volts at LSS, so you would current limit at 3 amps.

    When the current limit is exceeded, the PWM kicks in. The part applies motor power until it hits the current limit, then shuts off for a fixed time until turning on again, "chopping" the applied power. By adjusting Vref, you adjust motor current which is proportional to torque.

    You can adjust the current limit by changing the voltage on Vref, but Vref must be between 0 and a maximum of 5 volts or you will damage the device.

    If you place a zero-ohm resistor in the circuit, and some positive voltage on Vref, the motor should run because no PWM occurs. Of course, you will have no speed control but at least you can see if everything is working. The danger is that you could exceed the maximum continuous current so don't leave it on too long.
     
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I agree with the suggestion of high current, RC motor are traditionally very high current, the RC outrunners can typically use well over 20amps for a very short period.
    Max.
     
  13. john*michael

    Member

    Sep 18, 2014
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    1 afterthought - don't forget that the traces can have resistances in the 0.1 ohm range, so you may have to play around to get the value right when you are using low values.
     
  14. #12

    Expert

    Nov 30, 2010
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    Excellent post by a newcomer. Welcome aboard, john*michael.
     
  15. john*michael

    Member

    Sep 18, 2014
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    Thanks MaxHeadRoom - I am a motor guy, not an RC guy. If the motor draws more than 3.5 amps, a different controller would be required.
     
  16. Scalpel78

    Thread Starter Member

    Aug 11, 2013
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    Thanks for that good writeup john*michael!

    I'll try with a zero ohm resistor, and also reduce Vref from 5V to 3.3V.

    I can also try to measure the current drawn by the motor when I connect it directly to the battery. If it draws more than 3.5A, what are my options for reducing the current drawn?
     
  17. john*michael

    Member

    Sep 18, 2014
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    For DC motors, torque is directly proportional to current, and speed is proportional to voltage. Not much hope for reducing current. Even if the voltage is reduced, the current remains the same for a given torque output.
     
  18. #12

    Expert

    Nov 30, 2010
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    7.4 volts, right? Motor measures almost zero ohms when not connected to anything, right? Try 2 ohms in series with the motor. It might not start, but this is a discovery process.
     
  19. Scalpel78

    Thread Starter Member

    Aug 11, 2013
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    So, I've reduced Vref to 3.3V, replaced R1 with a zero ohm resistor, and added a 2.2 ohm resistor in series with the motor, but there is no difference.

    I measured the current when connecting the motor directly to my 7.4V lipo, and it comes out at about 1A. Not sure if it spikes higher during startup.
     
  20. MaxHeadRoom

    Expert

    Jul 18, 2013
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    A long shot, but per Johns post #5, a thought occurred to me that if the RC motor ran uncontrolled in its original application, it could possibly be a BLDC motor, these motors only have two leads and look like a brushed motor but have internal 3 winding commutation circuit with a P.M. rotor.
    Just another possibility?
    Max.
     
    Last edited: Sep 20, 2014
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