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12V switch in series with two 12V DC motors & 24V power supply. Is this ok?

Discussion in 'Power Electronics' started by mjosbesh, May 9, 2016.

  1. mjosbesh

    mjosbesh

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    Feb 27, 2016
    I have two identical 12V DC motors working together on either end to move an object. I'm considering wiring them in series so if there's an issue both motors will stop.

    I have a 24V power supply and a 12V 2 channel switch to control the motors. So my question is if I wire the 12 V switch in series with the Two 12 V DC motors will the switch only have 12V running through it or will it be 24V running through it?
     
  2. Bluejets

    Bluejets

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    698
    Oct 5, 2014
    Don't think much of your series motors idea, too many possible problems. Voltage does not run through the switch. When the switch is off, 24v appears across it and when it is on, zero volts across it.
     
  3. Gryd3

    Gryd3

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    Jun 25, 2014
    I second that...
    Motors are not a simple 12V device... They are dynamic. If one motor is seeing a greater load than the other, the voltage will no longer be split evenly. There is a high probability of one of the motors seeing a higher voltage than the other. I can't say "don't do it", but be mindful of how you are loading the motors.

    Now... let's discuss voltage and current a little...
    Voltage does not run through anything... it's present 'across' something... in that from one terminal to another, the voltage is present. Current run's through things... and it's the relationship between the voltage across it, and the resistance it presents that determines the current through it.
    What are your motors rated for? Everything in a series circuit passes the same current... so if both motors draw 500mA then 500mA *must* also pass through the switch when it is running. When the switch is open, there is no current flow, and the *entire* source voltage will be present across the switch.
    So, please size the switch to a higher voltage, and check the motor ratings to make sure the switch can handle the current.

    Running too high a voltage on anything could allow sparks to form while the component is turning on/off, or could allow an arc from one component to another! (Worst case)
    Running too high a current through anything can cause it to heat up. Excessive heat leads to damage, and/or smoke & fire.
    !! It's important to note, that increasing voltage also increases current !!
    A switch should only ever be on or off though... so the current through a switch will almost always be decided by what else is in series with it...
     
    mjosbesh likes this.
  4. mjosbesh

    mjosbesh

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    Feb 27, 2016
    Thanks. I did some testing. I currently have a 12V30A power supply and 48V7.5A power supply here so I used them to test the motors. I have the two motors (connected to linear actuators) installed in a testing rig which applies 2500N of resistance force independently to each actuator.

    So here's what happened... (no switch in place. just hard wired)
    Test 1: 12V30A > 2 motors wired parallel = It worked without failure... but slowly
    Test 2: 48V7.5A > 2 motors wired in series = Extremely Fast! Too Fast and too much force. One of the motors broke bc a retaining ring holding an internal bearing was pushed out of place.

    Test 1 operated as I expected but Test 2 did not. I don't want the motors to spin faster, I just want them to be stronger... Here's whats confusing. The manufacturer tells me to increase my power supply from 12V to 24V to increase the actuator force but they claim on their specs that 24V and 12V will = the same speed and say that the 12V version and the 24V version are exactly the same motor. Can this be true? Or does increased DC Voltage always mean increased speed of a DC motor?
     
  5. Gryd3

    Gryd3

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    Jun 25, 2014
    48V in series will most certainly cause it to spin much faster!
    You have gone from providing 12V to each motor, to providing (in theory) 24V to each motor... but it may not have been an equal 50-50.

    A 12V motor and a 24V motor are far from equal...
    If they are selling them as two different descriptions or items, they are fudging numbers! Very bad...

    An increase in voltage will equal an increase in speed of the motor.
    An increase in current will equal an increase in force of the motor...
    Tricky thing though... If the motor does not have the force required at the specified voltage, the only way to increase the current it to buy a different motor, rewind the coils in the motor, or increase the voltage...
    Voltage and current are related.
    So.. to solve you dilemma, if the motors are 24V safe, then you can use anything 24V and below. You may need a different power supply for this. Look into PWM motor controllers as well. You can use them to vary the power to the motor.
    There may not be a happy medium though... a motor without load will always go faster than one with a load... so you may have to settle for a weaker motor that runs a little slower if they will see a varyation in load
     
    mjosbesh likes this.
  6. mjosbesh

    mjosbesh

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    Feb 27, 2016
    Thank you @Gryd3 . I'd like to expand on my question.

    Assume the outside dimensions of the motor are identical regardless of voltage.

    1) Does a 24V motor always spin faster than a 12V motor or can a 24V motor be manufactured so it spins at the same rpm as the 12V

    2) Is a 24V motor more powerful than a 12V motor of the same size?

    3) Pretty much all actuator manufacturers i've come across offer the same model actuator with 12V and 24V options and claim the 12V & 24V versions have identical push/pull force, and identical speed (in regard to the actuator shaft travel speed loaded and not loaded, not motor rpm).
    Am I correct to think that the travel SPEED could only be the same if 1) the gear ratio is different or 2) the motors are manufactured such that the 12V & 24V motor spins at the same rpm?

    Also, I assume it's important to know that in all cases the dimensions of the 12V and 24V motors are identical.
     
  7. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    1) This depends entirely on the motor design. The motor should indicate it's designed RPM, *or* it's RPM per volt. 2) Again, depends entirely on the motor design. The 24V motor may be designed to draw very little current. You should look for the motor's torque rating or *power* handling capabilities.
    3) You are correct in thinking they gear it differently, or use different motors.
     
    mjosbesh likes this.
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