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I am trying to make an electric DC motor, with brushes. For that I am using copper enameled wire. Wires are available by max. current they can carry.

How can I calculate the current the motor will draw.

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2 Answers 2

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The maximum current the motor will draw is the stall current. And if you want your motor to be robust, you will rate it so that it can survive being stalled, so you will use a wire that can carry the stall current.

The stall current is simple to calculate : it's V/R where V is the supply voltage, and R is the resistance of the winding (ignoring brush resistance. You can include that if you know it)

To calculate R you need to know the length of each turn * the number of turns, and the resistance per metre (or per km) of your chosen wire, which should be somewhere in the same datasheet that tells you its rated current.

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  • \$\begingroup\$ Thanks for thr guidance. Can you please tell me the source(if available online) as I would like to understand more about it. \$\endgroup\$
    – TontyTon
    Commented May 27, 2018 at 5:25
  • \$\begingroup\$ If I take wire with greater rating, its resistance would decrease and stall current would increase. How can stall current depend on wire the wire itself. Better the wire higher the stall current!!! \$\endgroup\$
    – TontyTon
    Commented May 27, 2018 at 6:45
  • \$\begingroup\$ Yes it can be an interesting problem. \$\endgroup\$
    – user16324
    Commented May 27, 2018 at 11:17
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The initial or stall current will be determined by the brush and coil resistance.

The running current will be reduced due to the back-EMF of your motor. This will depend on the efficiency of your design and construction both mechanically and electrically. A well designed, small DC motor would have a no-load current in the range of 10% to 25% of the stall current.

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  • \$\begingroup\$ Is it not possible to predict the current by weighting the load and adding the weight of rotor(taking diameter as variable) at a particular voltage. \$\endgroup\$
    – TontyTon
    Commented May 26, 2018 at 11:32
  • \$\begingroup\$ No. Current will depend on rotational friction. Weight is not the correct property to be looking at. For acceleration and deceleration the moment of inertia is what matters. When running at constant speed the moment of inertia doesn't matter but the friction does. \$\endgroup\$
    – Transistor
    Commented May 26, 2018 at 11:45

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