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Question Motor and power supply question?

Discussion in 'DIY peripherals' started by WatchDoge, Jan 29, 2020.

  1. WatchDoge

    WatchDoge New Member

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    I'm building an FFB wheel and I'm looking at a motor with stall current at 80A , but my PSU has 30A at 12v , so I'm confused if the motor will run at all or will it run at full speed?
  2. Gadget999

    Gadget999 Well-Known Member

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    My Motion Simulator:
    2DOF, DC motor, Arduino, 6DOF
    use a server power supply - they can give 70 amps

    or use a battery between the power supply and the h-bridge
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  3. Sieben

    Sieben Active Member

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    You should look at the motor rated current, not the stall.
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  4. WatchDoge

    WatchDoge New Member

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    So you mean if a motor has Stall current at 80A but Rated at 25A i should be fine using it for Force Feedback with 30A psu ?
  5. Sieben

    Sieben Active Member

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    Yes. May be that should be called a nominal current, but simply just follow the specs it has. e.x. 12v14a/ 24v8a etc.

    Stall current of a motor is another thing.

    WatchDoge
    What motor is that?
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  6. WatchDoge

    WatchDoge New Member

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    For example lets take Mabuchi rs-550-vc-7525 , Or Johnson rs-775
    Johnson has rated current lower than my PSU , im still confused about Stall current , in my mind if force feedback is working the motor is not spinning , trying to move , so that makes me think it s producing maximum torque and current , running on stall current , so im still little confused about all that Rated and Stall current thing

    Attached Files:

  7. Sieben

    Sieben Active Member

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    Stall current is another motor spec. It does not mean that motor shall have that amperage to be powered with. At all.
    More over, excess amperes in PSU, only will heat that quick and no more in stall mode, with no power gain. So for much powerful motors like MY1025 with 24v13.5a the 12v33.3a PSU only heats up that quicker and nothing more. So better give them there nominal or even less, in this applications. I power mine MY1025, with 12v12,5a PSU with 1:3 gear, and that's more then just enough. ) I powered rc540 with all the same in direct mode PSU, and that smoked less then in 1 min . )
    If reckon right, the g25 wheel has 24v/1,75APSU. The motor is like rs550 too.
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    Last edited: Jan 30, 2020
  8. WatchDoge

    WatchDoge New Member

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    Wow thanks , i really appreciate that , now I understand .
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  9. Zed

    Zed VR Simming w/Reverb Gold Contributor

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    My Motion Simulator:
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    Motor rated and stall currents are just what the motor will pull when running under some specified load or stalled as long as the power source can supply it. Motors used in force feedback are generally in stall or even being driven in reverse so the currents can go up to what wiring, windings, and brush resistances allow.

    The reason for the difference between running and stall currents is because of back EMF when the motor is running and no back EMF (electromotive force - voltage) when stalled. When motors spin they also act like generators at the same time and that voltage opposes the voltage that’s applied to the motor. The faster it spins, the more back EMF. That’s why currents drop off as motors spin up.

    What happens when power supplies aren’t able to deliver full rated motor current is motors don’t have their rated torque and spin up slower. They may not reach full rpm and so back EMF is less and currents are higher, leading to more heat. Heat is I*I*R so current has a bigger impact on heating than resistance does. (But obviously current and resistance are interrelated.) Under stall, it’s just a resistance heater and current is set by Ohm’s law and what the power supply can deliver.

    Excess current availability does not cause heating. Power supply current ratings don’t mean the power supply will supply that current. It just means it can. What matters is the voltage in the circuits we are using. If the power supply can’t supply the current a motor can use, the voltage will sag. The only exception to that are constant current power supplies that will increase or decrease voltage to regulate the current at some level. These power supplies aren’t those. We use power supplies that regulate voltage.

    Ideally you would choose a power supply for force feedback motors that exceeds the stall current rating of the motor plus a safety factor so the power supply doesn’t always run at full rated current unless the supply is rated for 100% duty cycle at rated current. Running a power supply where voltage is sagging because the power supply is maxed out can overheat and damage components in the power supply and even lead to fires. (This is qualified because the force feedback isn’t always running at 100%. In practice, using a supply rated at the stall current may be fine but using one rated below still means pushing the supply beyond rated limits.)

    If people are having to use underrated power supplies to cut down on available torque, the motor is just too big or has too much leverage If the motor can take it, no big deal, and it may even be more capable of standing the heat developed in it, but the maxxed out power supply will possibly be being overstressed.
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  10. Sieben

    Sieben Active Member

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    Excess current availability does not cause heating

    Zed, practically it isn't true concerning our application of the motors and psu used. I've already described the condition on it. And thus, turned back to 12amps. practical, it does heat up the motor quicker and severe. With all the same voltage but different current.
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  11. Zed

    Zed VR Simming w/Reverb Gold Contributor

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    My Motion Simulator:
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    But with different current is the key. If a power supply can deliver 10A at 12V but a motor only draws 5A, the extra ability of the power supply to deliver 10A doesn’t cause additional heating. A motor drawing 5A will reach identical temperatures on a 5A supply as a 10A supply. On a supply that can only supply 4A at 12V temperatures should be lower because there is less current available. This is neglecting cooling from rotation and other things.

    Maybe I’m not understanding what you are saying? I’ve certainly tried to answer the wrong question before.
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  12. Sieben

    Sieben Active Member

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    Zed,
    DC Motro MY1025, rated 24v/13.5A, should have ~40A Stall with 12v, and ~85A stall with24V. So when powering him with 12v/33A PSU, all got heated up quick, including flybacks etc. Even got loss of FFB for a couple of seconds, even though the flybacks should be capable of holding 60 amps. And returned to 12.5A Power. Because of that, i kind of and giving this note, that it could be allright, to have not that extra amps in PSU. It will get heated up, and without them too.
    (In my power config also have 4F capacitor, bunch of caps and diode flybacks)
  13. Sieben

    Sieben Active Member

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    Also a good reading.
    https://www.ecmweb.com/design/article/20901278/the-highs-and-lows-of-motor-voltage

    I bet this it.

    An increase in current is a danger to the motor only if that current exceeds the motor's nameplate current rating. When amps go above the nameplate rating, heat begins to build up in the motor. Without a timely correction, this heat will damage the motor. The more heat and the longer the exposure to it, the more damage to the motor.
  14. Zed

    Zed VR Simming w/Reverb Gold Contributor

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    My Motion Simulator:
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    That’s what I’m saying, though. You are undersizing your power supply to keep from overheating the motor because stall currents are too high and cause too much heating.

    It works for now and may continue, but running the power supply like that beyond its rating stresses the supply and can lead to a blown supply and whatever comes with that.

    If you need to limit current, a wire wound resistor designed to take the load will lower the voltage on the motor due to the voltage drop over the resistor and keep the power supply in rating.
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  15. Sieben

    Sieben Active Member

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    Checked MY1025 motor rated 24v14A, with 12v33.3A PSU the stall current draw with MMOS with 100 Gain. In a game or with Spring Desktop Effect 100%, no matter, it draws only 10-12 AMPS. Only doubles and jumps to 20-24 AMPS with rapid rotation changes. And that's OK and exactly how it works with this jumps. But. So have a doubt now that all those DD MY1020 rated 36-48V, 26.7 AMPS, do really draws the stall current as it stated to be, with MMOS, and with all that only 12 volts (i.m. MY1025 should have ~ 40A? Stall under 12 Volts. ) or compensate for the lack of rated voltage with extra current draw then, as were stated by the DC DD builders.

    What's more interesting here, it seem that 100% gain MMOS is just like a EMC with Gain 100% but with zero Constant force. If Constant 100% it kinda, out powering the MMOS. Motor get that excessive heat etc. So EMC apparently must be really drawing more from the PSU then. Indirect features judgment but still.

    The 12v12a psu gives ~7A motor draw, but get all the same full PSU 12A draw if connected and measured directly without BTS7960 and MMOS control.

    Haven't measured directly with EMC, gotta make wiring lay out requirements for EMC now etc, but it is apparently clear that the motor got heated up quickly and more in a much shorter time. Also can judge by other means like glowing and burned wires in 10amp relay driver diy when put the EMC constant force slider 100% with 12v33a psu.

    Bay the way, and that's may be why people report the EMC be more sharp powerful too than MMOS etc. And I test the EMC Lite before too and that's crazy. Have to put the force slider in a game (Iracing) way down below the mmos rates.

    Thus the MY1025 is kind of tuned already for operation under keeping it in it's current nameplate ratings limits, so it can hold the heat etc as planned for it.