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Am I getting motors correct? I'm building a third party psu for a slot car race track. Obviously easy bit is transforming 240Vac down to 12Vdc. But I am unsure how thiswill react with my motor. THe motor is rated as 12Vdc at 0.42A. Am I correct in thinking that if the motor is corrected in series with rheostat (as thus to adjust the Volts suppled to the motor) it will slow down? Or will it for some reason decide to take even more than 0.42A enter image description here


CHeeers Motor in question http://www.pendleslotracing.co.uk/scaleauto-short-can-motor-30000rpm-sc-0005.html


Also, the main issue. I've got two tracks which essentially act as uninsulated wires. On each loop the respective Rheostat will of course cause a Volt drop and therefore less p=iv into the M1. But conservation of enerhgy,that v drop will now of course go into M2. How can I make it so that the motors are independentanly speed controlled? I've heard of pwm or something but that is out of bounds.

Please help oh mighty ones

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  • \$\begingroup\$ PWM is basically turning on and off really fast, but make it so the on to off ratio changes according to speed. Like ON for 0.5 milliseconds and OFF for 0.5 milliseconds and repeat this makes it go half speed. ON for 0.75 milliseconds and OFF for 0.25 milliseconds makes it go 3/4 speed. Although that is recommended, how about replacing the potentiometer with a MOSFET and connect the gate to a potentiometer acting as a voltage divider? \$\endgroup\$ – Bradman175 May 3 '16 at 23:57
  • \$\begingroup\$ Motors will not like PWM directly since they are intensely inductive - i have the welded-shut 30A relays (on a 1A motor field) to prove it. You will want to add some snubbing to that PWM output so it becomes effectively a variable voltage DC-DC converter. \$\endgroup\$ – Harper - Reinstate Monica May 4 '16 at 0:06
  • \$\begingroup\$ Thanks guys, we haven't learnt of pwm yet, (Only know it from pc heatsink fank) Is there a way I could split the DC output into 2 different circuits? such that R wil control their own M independently? \$\endgroup\$ – Iain May 4 '16 at 0:46
  • \$\begingroup\$ @Harper relays won't work with PWM. You'd use a MOSFET, transistor or an optocoupler connected to a TRIAC. \$\endgroup\$ – Bradman175 May 4 '16 at 3:21
  • \$\begingroup\$ I mentioned relays only as a cautionary tale as to how much inductive kick a motor has. \$\endgroup\$ – Harper - Reinstate Monica May 4 '16 at 3:24
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If you have a 12V supply and each motor has it's own rheostat, they won't affect each other. Cranking up the rheostat on one car will increase the total resistance in only that leg of the circuit, more resistance means less current and a slower motor. Now conservation of energy applies, but not as you've described above, the drop across the rheostat creates heat energy (a rheostat is really a glorified heating element), that energy will not be fed into the other car because the power supply is not trying to put out a fixed amount of energy, the power supply will supply only as much energy as is needed at a fixed voltage, the current is free to be whatever the cars need. When you have loads in parallel on a good power supply, they won't have any effect on each other.

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  • \$\begingroup\$ Cheers, helped alot. So to check, if I were to replace the DC supply with the end of a rectification circuit from the wall (wall adapator transformer). The motors would always draw (0.42A) regardless of the volts supplied. And then by changing the voltage supplied (P=iv) would cause faster motor? \$\endgroup\$ – Iain May 4 '16 at 21:21
  • \$\begingroup\$ The power draw of the motors will go up if you crank up the voltage, they will spin faster. It doesn't matter how you change the voltage, whether you get a higher voltage supply (more speed), or burn some off with a rheostat (less speed), either way, you're changing the voltage at the motor, the motor has no idea how you changed the voltage, all it knows is "I have more voltage now, I'm going to spin faster) \$\endgroup\$ – Sam May 5 '16 at 0:01
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The only thing that will cause one motor to interfere with the other is the internal resistance of the power supply. When a motor starts or when the motor overloaded or stalled by too much load, it will draw more than the rated current. If that makes the supply voltage drop, the other motor will slow down. That can happen with PWM or any other type of control, but electronic control can limit overload current thus minimizing the effect. You should use a power supply that can put out at least two or three times sum of the rated currents of two motors.

Without knowing the internal resistance of the motor, it is difficult to estimate the resistance value that you will need for the rheostat. Probably 25 ohms would be a reasonable starting point.

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