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I'm designing a Full Bridge for a Brushed DC motor control. My circuit will have a current measurement, so I'll can control the output current, sinked by the motor.

Whit this current control, I think that I could safely overvoltage my motor, using a battery with a higher voltage that the specified to the motor, since I'll never allow the current to overcome the max. current of the motor. With this, I could achieve bigger velocities.

My doubt is if you see another problens, rather the overcurrent, when I do a overvoltage? I could see the insulation of the motor and also the mechanical problems, like the gear, shafts, bearing and etc, these could be overstressed too.

But, eletrically speaking, the limiting factor, since I can limit the current, is the motor insulation, right?

Someone can see other problems?

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  • \$\begingroup\$ If you're limiting the current, that will prevent you from achieving bigger velocities. \$\endgroup\$ – pjc50 Mar 23 '15 at 16:05
  • \$\begingroup\$ If I limit the max. current, I'll just limit the max. motor torque. So, it will accelerate slowly, but I'll achieve the max. velocity the same way... The final velocity is determined by the applied voltage. What I'll do, its to limit the PWM in the accelaration, and increase its as long as my motor will getting more velocity... \$\endgroup\$ – Luis Possatti Mar 23 '15 at 16:11
  • \$\begingroup\$ Well, the obvious problem is that you'll exceed the design speed of the motor, which could result in physical failure of the rotor, its bearings, and/or whatever mechanical load is connected to the motor. You can't assume there's a lot of design margin on these things, especially in low-cost motors. \$\endgroup\$ – Dave Tweed Mar 23 '15 at 16:24
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In addition to the obvious mechanical problems with bearings, if it's a brushed motor you will quickly wear out the brushes. If the brushes spark, you can mitigate this by rotating the brush assembly (on a motor that allows this, most small motors don't) to minimise sparking at a specific speed - provided you don't need to run the motor in both directions.

However, such sparking can quickly erode brushes and commutators at which point the motor runs inefficiently and accelerates its failure. On big motors, commutators can be machined to restore smooth operation (until they are completely worn away!) Carbon brushes will take some abuse and are usually replaceable, metal brushes may simply melt in the arc...

If you're OK with shortening life to a few hours, a moderate overvoltage - depending on the motor, maybe up to twice its rating - is survivable. Perhaps acceptable for a toy, definitely not for a vehicle.

This is the main reason why high performance applications have moved to brushless motors - if the drive electronics permit, the bearings become the main limiting factor in speed.

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  • \$\begingroup\$ Thank you for your reply! I think that I'm okay about the problems you have said. I'm design this motor control for robotic wars, so I'm more interested in the power of the motor than it durability... In addition, could you explain me more detailed the part that you said about the mitigation of the brush sparks? I use in my motor Carbon Brushes, and it sparks so much! Thank you! \$\endgroup\$ – Luis Possatti Mar 23 '15 at 17:08
  • \$\begingroup\$ A couple of Wiki links help explain it: en.wikipedia.org/wiki/Commutator_(electric)#Brush_construction and the reason behind sparking variation with speed: en.wikipedia.org/wiki/… To minimise sparking you want to commutate at the "zero crossing" point of the (distorted) field. I don't expect your motor has a handle to make this easy! IMO it's worth sacrificing a cheap (smaller!) carbon brush motor to learn what it can do... \$\endgroup\$ – Brian Drummond Mar 23 '15 at 19:47

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