Brushed DC Motor Questions

Discussion in 'General Electronics Chat' started by Konduction2, Feb 23, 2014.

  1. Konduction2

    Thread Starter Member

    Feb 12, 2014
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    Although all these questions are interrelated, please pretend they are not and answer each of them separately for the sake of addressing all of my questions. Thank you all so much!

    1) I've heard that brushed DC motors overheat when given insufficient voltage.
    If that is not entirely true, can you explain why people say that?

    If that is true, please critique what I think the explanation is:

    "low voltage causes low speed, which causes low back-EMF, which causes very low resistance, which causes very high current draw, which causes heat"

    Is that explanation correct?
    If yes, am I thus correct in inferring that:
    -If you supply insufficient voltage but also manually spin the shaft quickly in the same direction, then it will not overheat?
    -If you supply sufficient voltage but also hold the shaft still, then it will overheat?

    If that explanation is not correct, could you please explain?

    2) I've also heard that brushed DC motors act like inductors and consequently cause a near short/current spike at the instant they are powered (assuming they had previously been unpowered for a long time).
    Is this true?
    If yes, then:
    -Why doesn't powering it with PWM cause it to have many current spikes and thus overheat?
    -Would the current spike still happen if an external force kept the shaft spinning until the instant the motor is powered?

    3) Do brushless DC motors overheat when given insufficient voltage?

    4) I have a brushed DC motor being powered by an LM386, which is amplifying music signal. While my motor never feels hot, the LM386 op-amp gets so hot that it slightly melts my breadboard. Besides converting my LM386 output into a PWM or using a heatsink, is there a way to stop my LM386 from overheating? If you suggest using a current regulator, please recommend a specific model that will work for me (the LM386 only outputs up to 5V)
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    1/Untrue, If so how would you control the rpm of a DC motor?
    Excess torque (load current) causes overheat.
    Low back EMF is accompanied with low applied voltage so the net effect is the same.

    2/ If full voltage is applied on a stationary motor the current is initially limited only by the very low resistance of the armature.

    3/NO, unless load conditions are taken into respect.

    4/ PWM is a way to decrease the power dissipation 0f the switching device.
    Max.
     
    Last edited: Feb 23, 2014
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  4. Konduction2

    Thread Starter Member

    Feb 12, 2014
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    Ah sorry, #1 is a repost - I went into it thinking I'd make it a lot more specific than last time, but then I deleted those more-specific parts afterwards for simplicity. Surprisingly, you addressed my deleted questions, lol.

    1) When you say "excess torque (load current)", does that imply that the motor will output more torque if there is more load? Does this also mean that the motor outputs the most torque when the motor is stalled? Does this also apply to AC motors?

    2,3) Okay, thanks!!

    4) Thanks! Why does PWM use less power?
     
  5. pilko

    Active Member

    Dec 8, 2008
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    If a DC motor (or any motor) is fan cooled, then it may run hotter when slower due to the reduced cooling.
    The motor temperature will vary with current, air temperature, air flow and heat transfer efficiency. Some motors are ribbed to give better heat transfer.
     
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  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    1/ A typical DC motor has the maximum torque at zero rpm, the torque curve is fairly flat up to the max rated rpm.
    Current = Torque, Voltage = RPM.
    When voltage is applied and the motor starts to rotate, the BEMF tends to counter the applied DC until the max rpm is reached for this particular voltage, at this point the BEMF is almost equal to the applied V, so very little current flows.
    If a load is placed on the motor, this tends to reduce the rpm and therefore the difference between the Applied and BEMF is greater, allowing a higher current to flow.
    The greater the load, the greater the current and the higher the torque.
    If a motor is allowed to operate into the peak torque range for any length of time, overheating occurs and can result in burn out of the motor if this current value is not monitored and reduced by the drive.

    4/ PWM does not necessarily use less power, the supply voltage is usually at least 10% higher than the motor rated voltage, the switching device issues a pulse width that is sufficient to equal the same mean current required for the load, if the control device operates in the linear zone heat is produced in the device, with PWM the voltage is either Maximum or zero, so the switching device does not operate in the linear zone.
    If your device overheats when used in a PWM fashion, it could be insufficient heat sink or underrated device?
    Max.
     
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  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Pwm uses less power because you can control the motor speed by turning the power on and off at some duty cycle. So either the input power is used to do work (turning the motor) or it is off. For any variable power supply or rheostat, some of the input power is lost to heat.

    The one exception is if a switching variable power supply is used, but in this case, the switching variable power supply uses Pwm - it is just at the variable switching voltage regulator onstead of at the motor speed controller.
     
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  8. Konduction2

    Thread Starter Member

    Feb 12, 2014
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    Ahhh okay I see. Thanks guys!!!
     
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