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Capacitor with a motor?

  1. Dec 23, 2008 #1
    What does a capacitor serve when it is connected in parallel to a motor............
    as we see in most of the good remote controlled toy cars??

    Also are there any motors that would rotate in only one direction and resist the motion in the other direction(keeping the direction of the current same)?
  2. jcsd
  3. Dec 23, 2008 #2


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    This must be the case of a DC motor otherwise the capacitor would act as a partial short circuit, (never plug a capacitor into an electrical outlet!).

    The typical small DC motor consists of coils and brushes and permanent magnets. (Called a PM DC motor.) The current through the coils pushes therm from one magnet to the other and this turns the commutator which switches the current around to continue this process in a cycle. Since you are switching the coils on and off the current drain on the battery jumps up and down. When it is high it will waste energy heating the battery due to internal resistance.

    The capacitor acts as a reservoir and buffer so that when the motor draws little current the cap recharges and when the motor needs high current the capacitor discharges through the motor.

    Most PM DC motors are bidirectional i.e. reverse the current you reverse the motion.
    You could add circuitry which will achieve what you desire. Put a diode in series with the motor and current will only flow in one direction. Also if you put another diode (high current rated) an low value resistor in parallel to the motor what will happen is that when it tries to reverse motion it will act as a generator putting a load on the resistor. This will not stop reverse motion but will dampen it. This is in theory. I have no practical experience as to how well this will work or what values to use. I've attached a rough circuit diagram to show you what I mean.

    If the parallel diode has a high enough rating then you can dispense with the resistor all together. This is very similar to the circuit you would use to "debounce" a relay.

    Attached Files:

  4. Dec 23, 2008 #3


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    I would expect the motor in the car is a DC motor. Note that the capacitor is an open circuit to a DC current.

    In parallel, the capacitor is a shunt, such that it takes current (stores or release charge) when there is a time varying voltage. This can prevent a current surge into a DC motor, and basically smooths the response of the motor with respect to rapid voltage/current changes.
  5. Dec 24, 2008 #4
    Well now the answer to the first question is quite clear to me.............but I have some doubt regarding the second question.....

    I think that you didn't understand my problem.............

    I am trying to design a car(a toy model) controlled by a remote.
    But I am not willing to put a ball bearing in place of the front wheel and want it to be a 4 wheel drive.
    Now the problem is that..........in ordinary motors, when the motion of one motor starts the other wheel(connected to the other motor)also starts moving with it without any resistance to motion. The problem arises when moving on curves.....I try to stop one motor(which must stop the wheel also)....but the wheel continues to move. So the curve cannot be made successfully.
    There is no steering mechanism in the front wheels and so some circuitry has to be made.

    I am not able to visualize that circuitry................

    Can anyone help..................
  6. Dec 25, 2008 #5
    This would have been better posted in the electrical engineering section.

    Describe the appearance of the capacitor.

    The small disc capacitors (usually brownish orange in color) soldered direct to the motor leads at the motor chassis are noise suppression capacitors. The motor's inductive load is electrically noisy. The capacitors prevent electrical noise propagating back to the control electronics.

    Brushed DC motors are very noisy in this regard.

    Electronic speed controlled (ESC) motors develop noise spikes when they switch phase. This can propagate back to the control ESC logic itself without the suppression capacitors.

    Use a small mica disc capacitor of about 0.001 to 0.01 uF. Select a voltage rating at least 5 times--even 10 times--the battery voltage.

    As far as the two motor steering... Assuming you're using brushed DC motors with field magnets, shorting one motor will allow it to act as a brake. Partial shorting--that is, shorting though a load resistor--will soften the braking force. You'll want to short the motor without shorting the batteries, of course. You can test this action by shorting a motor through various resistors and spinning the rotor.
    Last edited: Dec 25, 2008
  7. Dec 25, 2008 #6
    Capacitors are placed in parallel to inductive loads (Such as an AC motor) for power factor correction.
    Last edited: Dec 25, 2008
  8. Dec 26, 2008 #7


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    Ahhh I see. Yes I misunderstood your problem. That's a little more involved. The simplest thing I can think of is to introduce a relay which applies battery current when the motor is "on" but will short the motor when there is no current applied. The shorted motor will resist moving. But there's a lot of tweaking you'll need to do to keep from shorting while applying voltage and burning up your battery.

    Some of the store bought RC car's I've seen have a free pivot in the front wheels but no active steering. The pull of the front wheel motors will turn the axle to effect steering and even with one freely rotating the other under power should suffice to turn it. You'll need a centering spring to keep it aligned when both are under power and tweak it to get the best maneuverability.

    Beyond that I suggest you google around for some hobbyist RC websites.
  9. Dec 27, 2008 #8
    There are two solutions to this.

    1) The Electronic Solution
    You can use a SPDT relay (Single Pole, Double Throw or "two-way switch" relay) to short the motor by having the relay switch between the +ve rail and the other contact of the motor. I've attached a schematic of the circuit.

    2) The Mechanical Solution
    Connect a worm gear to the motor, then run a cog of that to drive the wheel. The wheel will move more slowly, but will have a lot more torque. The wheel also cannot be turned by hand, and will stop dead when the motor stops turning. For optimum turning I'd suggest a tri-state system:
    Moving forward = both motors full on.
    Turning left = right motor on full, left motor 25% voltage.
    Turning right = left motor on full, right motor 25% voltage.
    Stopped = both motors off.
    This allows for a smoother movement, puts less strain on the mechanical parts and saves battery power (motors have a lot less friction to contend with).

    To be honest I'd stick with the electrical solution, but it's up to you.

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