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Heat motor voltage vs amps

Discussion in 'Motor actuators and drivers' started by evolaco, Jul 23, 2018.

  1. evolaco

    evolaco Member

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    Hi, on a 24v and 350w motor that I want to use to make a FFB, would it be better to use it at 12V and a ratio of 5/1 and all the amperage you need or is it better to use 24V and 10/1 and use half the amperage?
  2. evolaco

    evolaco Member

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    I do not know if I have not explained myself well, what affects more to warm a brushed motor, more voltage or more amperage?
  3. yobuddy

    yobuddy Well-Known Member Staff Member Moderator SimTools Developer SimAxe Beta Tester Gold Contributor SimTools 2.0 Beta Tester

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    Higher voltage will always win I believe.
    Its simply more efficient.
    But it can be harder to come up with a power supply for a 24v system, so a lot of people use 12v.
    Hope that helps!
    Take care,
    yobuddy
  4. Alexey

    Alexey Active Member

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    If you give your motor less voltage it will use less amps and thus produce less heat.
    What heats the motor is Watts. When you buy a motor it will have a defined voltage and wattage.
    The stated wattage of the motor will indicate how much power the motor can handle before overheating.
    Motors can go over the stated wattage when being used outside of their design specification. Like how your FFB wheel will have the motor in a "stalled"
    state for most of its life.

    What you need to find is the motors performance graph which will indicate how much current the motor will draw at stall vs voltage input. That will tell you the watts.
    Most of the time this wattage will be well above the design specification.

    What you need to do is take the motor apart and drill holes in the face and back plate and mount a fan to draw air through the core of the motor. The outside body of the motor will heat up but the core of the motor is the source of the heat so that is what you need to focus on cooling.
  5. evolaco

    evolaco Member

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    @Alexey But can the maximum torque (nm) be obtained with less voltage than the maximum of the motor? that is, if I use a 48v and 20a motor, could I give 12v and the maximum amps of the 40a or 60a source to have a lot of torque but less speed? the use of a my1020 of 1000w and 48v direct seems good idea and with 12v it would turn faster than what is needed for a steering wheel
  6. Alexey

    Alexey Active Member

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    Unfortunately that's not how motors work, you cannot force current through a motor. You can force voltage through it but not current.
    The simple thing to do is buy a motor that suits your needs to begin with. Sometimes it is hard to do so but that's life.
    Forget about fast or slow RPM, the wheel does not turn for you. You turn the wheel so motor speed is irrelevant in most cases. (except for when you gear a motor down and use some crazy reduction like 1/1000).

    If you are going for a direct drive FFB wheel then forget about the stated rpm of the motor because when you use the motor the rpm is effectively zero.

    1000W would be hard to power, I'm sure if you used 24V on that 1000W motor it would still provide sufficient torque for a Direct drive FFB wheel.
  7. evolaco

    evolaco Member

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    with 12v can not get the maximum torque of that engine? @danove_b uses a 36v motor and feeds it with 11-12v and according to the amps it gets the torque indicated by the motor, if it is 10a 1nm with 30a it gets 3nm, regardless of the voltage being only 12v.
  8. Alexey

    Alexey Active Member

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    images.png
    This graph is a good explanation of a motors response to different voltage level inputs. I think you may have missinterpreted something with regards to danove. He is using less voltage with the specific intention to reduce current draw.
  9. evolaco

    evolaco Member

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    but danove is getting the necessary NM with the amps that the motor indicates, if 30A gives 3nm it does not need to give more voltage, no more watts are needed, if not more torque and that according to danove is related exclusively to l amps of current, the voltage only gives the motor speed
  10. Alexey

    Alexey Active Member

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    Yes at a certain voltage his motor gives an acceptable torque that he is after. A higher voltage will give him even more torque but also a higher current draw.
    Again, the voltage applied is directly related to the amount of torque that is available. Also again, disregard rpm unless you are attaching a gearbox.

    You control the maximum torque by selecting the corresponding voltage as depicted in the graph.
  11. BlazinH

    BlazinH Well-Known Member SimTools 2.0 Beta Tester

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    For practical purposes only amps create heat though. If it were watts then a 600 watt motor for example at 12v or 24v would generate the same amount of heat given the same load. But in reality the 24 volt motor will generate a lot less heat since it uses 1/2 the amperage the 12 volt motor does to do the same amount of work.
    Last edited: Aug 1, 2018
  12. Alexey

    Alexey Active Member

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    You are partly correct, yes higher current will cause more heat in a closed system but current is directly related to voltage. Say a 12V system where a 10 ohm resistor is used. This resistor will draw 1.2 amps from the supply (I=V/R). This sytem is drawing 14.4 Watts of energy. However this resistor is a 20W rated resistor. This means that the resistor can handle 20W of energy before overheating (the ability to dissipate heat faster than gaining heat energy). Now lets bump up the voltage to 24 volts. The current is now 2.4 amps (I=V/R), so the resistor is now trying to drain 57.6 watts of power (P=VxI) when it is only rated at 20W. This is precisely why when you go to buy a heat sink, it will state its heat dissipation in Watts.

    Now we cant use this same terminology for motors because the systems are physically different with the internals.

    A 12V 600W motor has a resistance of around 0.24ohms with 50 amps of current.
    A 24V 600W motor has a resistance of 0.96 ohms with 25 amps of current.

    You see how it is the internal resistance of the system that is changing rather than the input voltage changing the wattage of the system?

    So if you were to apply 24 volts to that 12V 600W motor your initial current draw would be (24x0.24) = 100 amps!!
    Lets do the opposite, we will give the 24V motor 12V so the current is now down to 12.5A.

    When a motor states a wattage, this is telling you what power the motor can handle safely.
    Now when you state there is a 600W rated motor at either 12 or 24 volts, both motors will heat up to the same temperature (as long as they are physically the same size for heat dissipation).

    What causes heat is resistance to current flow. If you had a resistance in a conductor of absolute zero then there would be no heatup of that conductor.
    Heat in an electrical system is essentially wasted energy, If a DC motor was 100% efficient then it wouldn't get hot at all. Unfortunately that is not the case.
    During stall (where our motors tend to stay at for extended periods of time) the motors efficiency is near zero.

    Now there are other variables that will dictate the performance of that motor such as the design of the armature, magnets and winding type.
    As you increase voltage on a motor its magnetic field strength increases which in turn increases max RPM and maximum torque levels.
    There is a limit however, at a certain voltage the motor will reach magnetic field saturation at which point more voltage will not mean more performance (one could call it diminishing returns).

    That was fun.
    • Agree Agree x 1
  13. Jake

    Jake New Member

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    That was very helpful. Maybe you could answer me this question? What if you take a 120 watt 12 volt motor and use it at 120 volts, but only 1/10th the time. Will it work the same, with the same amount of heat as running it continuous at 12 volts? I'm thinking like powering it 100 of 1000 cycles per second.
  14. SilentChill

    SilentChill Problem Maker SimTools 2.0 Beta Tester

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    You would melt it, 12v motor at 120v lots of smoke, I would guess
  15. Jake

    Jake New Member

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    Ok, I might have phrased this wrong. What if you run the 12 volt motor with 120 volts at a number of cycles that produces the same RPM that's produced at 12 volts?
  16. SilentChill

    SilentChill Problem Maker SimTools 2.0 Beta Tester

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    You've completely lost me now lol
  17. Jake

    Jake New Member

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    With electronics you can switch the voltage at 120 volts off and on. If you switch it on for 1/1000th sec then off for 9/1000th sec, or whatever ratio is needed to produce the same RPM on the motor, will it still overheat the motor?
  18. Alexey

    Alexey Active Member

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    We already do PWM the motor as it is to control torque. Speed is of no importance when it comes to a FFB wheel as the wheel is mainly being turned by the user.
    In a motion sim however this is different, where speed and power are a critical factor to how well the simulator performs.

    Adding too much voltage to the motor puts it into a state of saturation where adding any more voltage/current to the motor will not increase the magnetisation of the motor and thus just wasting power as heat instead. Large motors have much larger thresholds for magnetic saturation than smaller motors, therefore you could get away with a very large 12V motor running at 100V. However the efficiency is still very poor and so I would hazard a guess that half of the power that you would be putting in at 100V would be turned into heat instead of work. The motor would probably last a lap or two before melting away the insualtion from the windings of the motor. The melting of insualtion can happen easily at the rated voltage of a motor, let alone a voltage 100x its rating.

    There is no such thing as a free breakfast with these motors. Even if you did find a way to efficiently cool the motor and the windings, make a 100V H-Bridge controller, make a 200 amp 100v power supply to power the thing (and trip every circuit breaker in the house when you turn it on). It would be cheaper and better in every way to just buy a 1000W motor in the first place. The difference between a 1000W motor and a 200W motor is about $50.

    Its good to think outside the box so certainly don't stop doing that but unfortunately DC motors are very limited in what we can do with them. As it is we are pushing past the design purpose of these motors.
    DC motors want to be running at the rated RPM with a light load, not constantly at stall with maximum loads.
    • Informative Informative x 1