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I have system I want to move with a DC motor that requires about 1 kW of power when taking into account rolling resistance, drag resistance and when moving on an inclined plane with a specific velocity. I had trouble finding small enough motors to provide that power so I asked a friend for help and he told me to use a gear system with low power motors.

So I ended having a question in my mind: Does the motor power, say a 1 kW motor, need to match the 1 kW requirement of the system or is it the torque provided by this motor that needs to match the torque of the system I want to move, in this case ((9.55 * 1000 kW)/ 600 rpm) = 15.91 Nm? If it's the torque then that means I can use a 200 W with known torque and a gear system to multiply the torque and match that 15.91 Nm requirement? If it's the power then is my only option using enough motors to match that 1 kW value, say 5 200 W motors? I'm sorry if the question is not totally clear, I'll do my best answer any questions.

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  • $\begingroup$ Rule of thumb - about 2% out: Watts = RPM x kg.m_torque. $\endgroup$ – Russell McMahon Oct 3 '20 at 9:48
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What you'll find is that most electric motor (not all) exhibit a curve like the following

enter image description here

So at very low rpms they tend to have a very stable torque and low power. As rpm increase at some point Power tends to flat out and torque start to drop, because just like @jko said : $$Power = M_t \cdot \omega=M_t \cdot 2\cdot\pi \frac{n}{60}$$

So there is always a tradeoff, that's the important thing you need to keep in mind.

I don't know the application (although I suspect it is some sort of vehicle) and/or how you arrived to the number of 1kW, however if you are certain about that value, there is no getting around the power of the motor. So you'd need at least a 1 kW motor.

Having said that, if you are not concerned about velocity (or how fast you get things done), I think you don't need to worry about the power, and you can focus on the Torque. In that case the tradeoff is time. You will be able to do something big with a small motor given enough time.

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The motor power needs to match the power of your system. Power = speed x torque.

Gears can provide different speeds and torques, but power is constant (minus any losses from friction). Try to use only one motor if possible.

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  • $\begingroup$ So there's no way to make a motor with rated power of 100 W move a system that requires 200 W? Even with gears? I feel like I'm gettinng confused about the power of the motor and the power to move a system. $\endgroup$ – Lucas S. Oct 1 '20 at 14:59
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    $\begingroup$ Gears will probably make the 100W motor move the system, but at a slower speed, Power = force x velocity, and presumably most of the "force" required to raise the system up the slope comes from the weight of the system, which is constant. $\endgroup$ – alephzero Oct 1 '20 at 15:34
  • $\begingroup$ Typically the system requirement is in terms of torque, not power. So you can use an undersized motor to move the load but it will be slower than desired. $\endgroup$ – jko Oct 1 '20 at 15:59
  • $\begingroup$ Does that mean that, using the example values I provided above, if I somehow can get a motor with 500 W rated power that provides 16 Nm of torque at 1000 RPM, theoretical values, it will be enough? Because it looks like I'm definitely confusinng the meaning of power in this case. 500 W is just the energy consumption of the motor and not the power it can output? $\endgroup$ – Lucas S. Oct 1 '20 at 16:03
  • $\begingroup$ You're just confusing electrical and mechanical power, which use the same units but manifest themselves differently. If you have a "500 watt" motor, that's usually the mechanical output rating. The electrical power needed to supply that is higher due to inefficiencies. A 500 watt motor can rotate a 16 Nm load at only 300 rpm max. $\endgroup$ – jko Oct 1 '20 at 17:35

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