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A motor I have ranges from 12 to 30 volts, but it says the current has to be at 0.5 amps (500ma) I am powering it from a 12.6v 20000mah battery. Basically I would to use that motor with 24 volts. There are step up voltage converters available, but the lowest current I could find with one steps it up to 3 amps, which is what I don't need. I've heard people say that the motor will only draw what it needs, but what is the limit before current starts being forced into it? I only need 500ma, not 3A. Will this damage my motor?

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  • \$\begingroup\$ You need to post the spec of that motor and tell us what kind of motor it is. Motors are complicated beasts. \$\endgroup\$ – Trevor_G Apr 23 '17 at 14:03
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You can only force one of current or voltage. The load, which in this case is your motor, will determine the other.

For example, you can use a constant-current supply that drives 500 mA thru the motor. You then have no control over the voltage. When the motor is just starting, the voltage will be low. As the motor speeds up, the voltage will go higher.

In your case it seems you want a supply that limits its output to 30 V or 500 mA, whichever is lower. That is certainly doable. It is only "forcing" one of current or voltage at a time. When it's controlling the current to 500 mA, the voltage could be anywhere in the 0-30 V range. When it's controlling the voltage to 30 V, the current could be anywhere in the 0-500 mA range.

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  • \$\begingroup\$ So how would I be able to stabilize the voltage at 24v, current is not an issue for me, I just do not want to burn the motor. \$\endgroup\$ – ToastHouse Apr 23 '17 at 12:27
  • \$\begingroup\$ @Toast: That's all about how the power supply is controlled. \$\endgroup\$ – Olin Lathrop Apr 23 '17 at 12:31
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Your specifications seem a little off.

Motors are usually defined with a tight voltage and a max current, which makes me wonder about the style of this motor.

A traditional DC motor will consume all the current it can take through it's stopped coil resistance. The latter will be a low number so start up currents can be large. Once up to speed that current will drop to a much lower value depending on the mechanical load on the motor.

How FAST the motor can go for any given load is defined by the applied voltage.

So that leaves me wondering what exactly this motor is, and how fast you want it to run.

If you do not care about the speed, simply apply your voltage source and add a current limiter to stop the current from exceeding the 500mA value.

If you need a constant speed, you need to adjust the current anywhere up to 500mA, possibly using pulse width modulation (PWM), until that motor speed is reached and continue to control it to maintain that speed.

ADDITION:

When you do not have a proper motor specifications, you really need to test it with a bench power supply that lets you vary and view the voltage and current that the motor requires under stalled and full load conditions.

When stalled, whatever voltage generates the 500mA current value is the max voltage you should apply to the motor. Make sure the motor does not get overly warm during that testing. Also verify that the motor functions as expected at that voltage.

Then design your driver not to exceed the numbers you measured.

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  • \$\begingroup\$ I'm using this motor for a film camera, the motor was made in 1958 if that helps. Reversing the polarity does not reverse the motor direction. The voltages represent FPS, which is why I am trying to get to 24v. \$\endgroup\$ – ToastHouse Apr 23 '17 at 14:59
  • \$\begingroup\$ @ToastHouse.. even more curious.... \$\endgroup\$ – Trevor_G Apr 23 '17 at 15:15
  • \$\begingroup\$ @ToastHouse, see my addition to my answer. \$\endgroup\$ – Trevor_G Apr 23 '17 at 16:01
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Voltage is a measure of the ElectroMotive Force. This is the potential which causes a current to flow.

Current is never forced. It is the voltage which, by definition, is the forcing effect. This is true for all circuits.

If you restrict the current in a circuit, a lower voltage is the result. If you have a fixed voltage, the current depends on the load. In this case, the more work the motor does, the higher the current.

The spec does not say that the current has to be 500 mA, it says the current will be 500 mA (as a result of the voltage). This is clearly inaccurate - maybe it applies to full rated voltage at stall-load - maybe not.

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  • \$\begingroup\$ So if I were to apply 24 volts at 3 amps, the motor would still work properly? I am trying to avoid burnouts. \$\endgroup\$ – ToastHouse Apr 23 '17 at 12:20
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    \$\begingroup\$ Your first sentence is incorrect. Current can be "forced", just like voltage can. That's what constant-current supplies are. The issue is that you can't force both at the same time. \$\endgroup\$ – Olin Lathrop Apr 23 '17 at 12:25
  • \$\begingroup\$ Current can't be forced (co-erced) through something without applying a higher voltage. Current can be restricted by a supply - I think we have a linguistic disagreement here. \$\endgroup\$ – Sean Houlihane Apr 23 '17 at 13:30
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    \$\begingroup\$ Current can be the controlled variable though @Sean, and if you have the voltage overhead to do so, that can be translated as "current driven" i.e. forced. \$\endgroup\$ – Trevor_G Apr 23 '17 at 14:05
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    \$\begingroup\$ Forcing current requiring a high voltage is no different than forcing voltage requiring a high current. Eventually both types of supplies (constant voltage, constant current) have a limit. However, the both are possible and both exist. \$\endgroup\$ – Olin Lathrop Apr 23 '17 at 14:34

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