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I'm currently making a radio controlled fighting robot and have pulled apart some drills which have a brushed RS550S DC motor (0.5Nm 18V 60A so 1080W) and a gearbox attached. Right now I'm looking at replacing the brushed DC motors on the gearbox with a brushless motor because I can't find any ESC's that would work with a brushed motor at 18V 60A.

So my question is how can I find an equivalent brushless motor to replace the brushed motor. I have seen people say brushless is more powerful etc but I have no proof of that. Can a 14.8V 45A 1400KV brushless motor potentially have the same torque as a 18V 60A brushed DC motor? I can't find much info on torque for brushless motors which is why I'm asking here and I don't want the motor blowing up my gearbox particularly quickly. Is it as simple as if the brushed motor is 1080W I need a 1080W brushless?

Thanks If you need any further details I'll be happy to provide.

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closed as off-topic by Chris Stratton, Voltage Spike, PeterJ, Finbarr, Dmitry Grigoryev Apr 17 '18 at 8:12

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  • \$\begingroup\$ This is likely physically impractical as the mounting styles are quite different, following from the fundamentally different construction of the two types of motors. \$\endgroup\$ – Chris Stratton Apr 10 '18 at 19:57
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You have calculated the brushed motor's input power using Power(W) = Voltage(V) X Current(A). You can calculate the output power using Power(W) = T(Nm) X RPM X 2Pi/60. Output power is equal to input power minus losses. For estimation, you can assume output power equals input power or assume losses are 15% or so for a better estimate.

If you are comparing two motors for use with the same gearbox, you need the motors to have about about the same torque for full speed and rated power. Just matching the power ratings is not enough.

I have seen people say brushless is more powerful etc...

There may be some truth to that, but it doesn't mean much without numbers. If the output ratings are the same, one is not more powerful than the other. However you may find that a brushless motor is more efficient, smaller and lighter than a brushed motor with the same speed and torque rating.

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Can a 14.8V 45A 1400KV brushless motor potentially have the same torque as a 18V 60A brushed DC motor?

Perhaps, depending on how the motors are rated.

Torque is proportional to current, and the torque constant (Kt) is the inverse of the velocity constant (Kv), so if both ratings are under the same conditions then a 1400Kv 60A motor should have more torque than a 1400Kv 45A motor. However if the 45A is at continuous rated power while the 60A is at stall then a direct comparison is invalid.

Brushless motors generally have more power because they use Neodym magnets which are more powerful than Ferrite magnets. A stronger magnetic field requires fewer turns to generate the same voltage - resulting in lower winding resistance, lower 'copper' loss, higher efficiency, higher stall current and higher maximum torque.

The graph below shows the inverse relationship between speed and torque. Torque is directly proportional to current (minus a small no-load current). The lines are dotted close to stall because current may need to be limited (by lowering voltage) to permit continuous operation here. Sensorless brushless motors have a minimum synchronous speed so they cannot normally operate in this region.

enter image description here

I don't want the motor blowing up my gearbox particularly quickly. Is it as simple as if the brushed motor is 1080W I need a 1080W brushless?

Your controller needs have an active current limit which can be adjusted to limit torque, and then you won't have to worry about blowing anything up. The power rating itself doesn't tell you anything about torque, but if two motors have similar power and Kv (or Kt) then they should be roughly equivalent.

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