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Very silly question regarding DC motor

  1. Jan 18, 2014 #1
    If in the DC motor specs doc says that it can handle + - 16 Volts maximum and 3A of peak current but the nominal voltage is 6 V and the continuous current is 1A....

    it means that it is safe if you feed it with 3A during the time of start up?

    Supposedly it is only meant to be at the start time, in order to obtain maximum acceleration, but then immediately after that you have to limit the current to 1A.

    Just have so many doubts about the values which I could send to the motor...
     
  2. jcsd
  3. Jan 18, 2014 #2

    uart

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    If you power the motor from something like a battery, then typically the start up current will be significantly larger than the normal running current. As long as you don't overload it however (as in impose a mechanical load which the motor cannot overcome sufficiently to reach nominal operating speed) then the current will drop of it's own accord as it reaches operating speed. This phenomenon is caused by the "back EMF" of the motor.
     
  4. Jan 18, 2014 #3
    I am asking this because this DC motor is the actuator in an Inverted Pendulum control system. The Pendulum rotates freely over a pivot which is mounted over a cart, which moves along a rack as well.

    So the manner to control the movement of the cart is to supply a voltage signal to the motor whose rotation is going to be transformed into linear movement of the cart via the pinion rack gear. So we have a controller in which the control law is going to be implemented, who supplies a voltage signal into a power amplifier (who supplies the necessary electrical power) and then into the DC motor.

    The experiment is first to swing up the pendulum from the downward position to its upright position, and then balance it. So my main concern is due to the maximum levels of voltage and current allowed by the motor, if I could reach them during the swing up phase in order to get the maximum acceleration of the cart and do the swing up in less time, or if that is going to be a mess since there is going to overload the motor.

    Due to the fact that it is a low inductance motor, in the mathematical model one could depreciate L, and thus the current at star up is only limited by the armature resistance, let´ s call it R.

    The idea is to implement a current saturation into the control law in order to not overload and damage the motor during the experiment. So the amount of linear acceleration of the cart that you can get from the DC motor is bounded by the maximum current you are able to supply to the motor.
     
  5. Jan 19, 2014 #4

    uart

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    In that case you should be fine, as the control law is taking care of the current. Actually you'll find that electric motors are quite robust in terms of short duration over currents.
     
  6. Jan 19, 2014 #5
    Thank you very much, Uart. Your help is very appreciated. Actually the control law is in terms of Voltage, and as far as I am reading more about Dc motors, I am leaning towards to limit the current up to 1A (and not to 3A), just for being preventive.
    The controller is going to take longer to swing up the pendulum, and I hope it will be able to do it. I have to do more math and simulations already.
     
  7. Jan 19, 2014 #6
    From my experience, it is neither the voltage or current that breaks the machine, it is long exposure to high temperatures. That is, the insulation degrades due to the high temperatures. Current are of course the central factor in the temperature distribution in the machine, but thanks to the "heat capacity" of the different materials in the machine (materials store energy/heat), a short overloading (increased current) may not raise the temperature of the machine above its rating. Think of it as a RC circuit, with a time constant. And if it overshoot the temp rating, its only for a short time so that the insulation degradation is low.

    Thats why manufacturers makes machines with different "duty rating", S1,…,S10.

    As for voltage, a over-voltage can make the insulation break due to breakdown/flashover, or partial discharges can occur inside pockets/holes in the insulation gradually degrading the insulation until it flashes over. This is usually only a problem in high voltage machines, above 1000V.
     
  8. Jan 19, 2014 #7
    Thanks fellows,

    my major concern is that in my application the motor is not going to rotate at a set regime, but it is going to do a sequence of starts-stops-changes of directions, whatever it needs to balance the pendulum, or at the swing up.. the time it takes to get it up.

    So I am not able to estimate now at which speed it is going to work and thus know the effect of the EMF will have on the net voltage.

    The control law is a Voltage signal feed to the motor so I must set the limit accord to that.
     
  9. Jan 20, 2014 #8
    Hi,

    I am only want to raise a last thought. It is clear that the maximum current is going to be at the start up of the motor. In my application I am going to put the motor into a sequence of successive starts and stops, changes of direction ...etc.

    If I permit 3A to be the current limit, (that is setting the Voltage saturation limit up to 7.5V approximately, as R=2.6 Ohms) besides of that the fact that it is going to drop immediately, I really do not know how much time it is going to work over the nominal limit and thus if it is going to be capable of refrigerate and the thermal limit is going to be reached.

    But if I set the limit up to 1A, I know that the motor it is going to work under safety limits all of the experiment, but I do not know if I am going to have enough energy available to reject disturbances... swing up properly...etc.
     
  10. Jan 20, 2014 #9

    uart

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    One option would be to try including a crude thermal model alongside your controller (power dissipation and thermal resistance etc).

    Or alternatively (and easier) you could probably mount a small thermal cutout switch directly onto the body of the motor.

    Not overly limiting the peak current will definitely allow your controller to function better with a faster response time.
     
  11. Jan 20, 2014 #10
    I would try it out. Set the voltage limit "low" at first, measure the current and maybe the temperature of the motor (simple PT100 element or similar, its also very fun an interesting to just experience it). Measure the current with a ammeter or oscilloscope (if available. Current rise/peak may be to fast to see on a normal multimeter)

    In a DC motor current is directly proportional to torque, so a high torque means high current. High torque also relates to acceleration (remember Newton!?). So as a rough guess, the highest "load torque" is at startup when the pendulum is horizontal. At vertical the load torque is zero, and hence current is zero. At a small deflection the load torque varies sinusoidal with the angle, the motor must overcome this torque and a bit more to raise it up vertical again.

    Is it a school or private project?
     
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