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In a robotics application I have to install a 12V (30A) DC motor actuator in a vehicle. The typical setup indicated by the manufacturer is to directly connect it to the battery, because of inrush current surges from the DC motor.

However, the vehicle in question has 24V batteries. Our intention is to use a 24-12V DC converter and add a battery in parallel to handle the reverse current surges.

Are there any safety caveats with that solution? What points should be taken in account when specifying the converter? What kind of batteries (lead, LIPO, LIFE, etc) are more suitable for the operation? Has anyone done a similar setup before?

Arthur Nunes
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Our intention is to use a 24-12V DC converter and add a battery in parallel to handle the reverse current surges.

You can do a lot better. A motor is a DC converter already, so to run a motor from a DC converter is probably more expensive and less efficient. All you need to do is find a motor driver that can run on 24V, then configure it to have a current limit such that your motor doesn't overheat. It's excessive power and the consequent heat, not excessive voltage, that is the usual limit for motors. If you get enough voltage to cause arcing in the windings that can be a problem too, but 24V isn't anywhere near that limit.

I'd also consider more robust overvoltage protection. A battery's ability to absorb energy from the motor is limited by its internal resistance, the inductance of its leads, and its current level of discharge.

Do some analysis of the mechanical energy your system can have at a maximum, and then work backwards to see if your battery could absorb that in a very short time without driving the supply voltage high enough to break stuff. Also consider what happens if your vehicle is driving down a hill: in this case your battery must absorb the gravitational potential of your vehicle faster than friction can slow it down.

Then, add something like a crowbar circuit to handle pathological cases or to save yourself the cost of a new motor controller if your calculations were off.

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Phil Frost
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  • it is a DC motor actuator, not just the DC motor. It comprises a DC motor + motor driver + a digital controller which accepts position commands through a digital interface. The operating voltage is fixed in a range 16-9V. – Arthur Nunes Jun 06 '14 at 19:53
  • @ArthurNunes it would still be better to get such a thing that could operate on 24V. If it's doing any sort of PWM control on the motor, then it's *already* a buck/boost converter: it's just the wrong one for your application. You *could* feed it with a DC converter, but you are just doubling your costs and losses. – Phil Frost Jun 06 '14 at 19:58
  • In addition, the actuator is used to steer the vehicle, not to propel it. – Arthur Nunes Jun 06 '14 at 19:58
  • @ArthurNunes steering the vehicle is just propelling the wheels. You still need to consider what happens to excess mechanical energy. For example, what happens when the vehicle is driving along, then bumps into an obstacle which jolts the wheels? A steering motor might be smaller in scale, but that doesn't make the problem go away. – Phil Frost Jun 06 '14 at 20:01
  • I'm tied in that matter because the manufacturer does not provide units with other operating voltages, and its a specific actuator. What I am considering right now is: should I invest in a DC converter that can handle the inrush current surges (that will probably be more expensive) or the setup with a simpler converter and the battery can handle it? – Arthur Nunes Jun 06 '14 at 20:01
  • @ArthurNunes A DC converter can't handle what I think you mean by "inrush current surges". When a mechanical force turns the motor, the motor converts that mechanical power into electrical power (usually it's called a generator in this application). A DC converter converts energy, but it still has to *go* somewhere. Something needs to get hot, or you need to store it chemically in a battery, or in the electric field of a capacitor, or *something*. – Phil Frost Jun 06 '14 at 20:03
  • The idea would be a converter that could forward the reverse energy back to the 24V battery bank of vehicle. Or one with a protection like an active crowbar circuit already implemented that could handle the power coming in. – Arthur Nunes Jun 06 '14 at 20:09
  • The actuator also already has current limit built in, so it does not returns more current thant it would handle normally. – Arthur Nunes Jun 06 '14 at 20:13
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I don't know what you mean by security caveats with that solution as there's nothing wireless and if the person got to the device, they would be able to dismantle it one way or another.

For a high wattage motor, efficiency is key in a converter as well as current sourcing capability. I would recommend a buck converter for its simplicity, efficiency and lack of more requirements (such as isolation, variable voltage, etc.). If you're only using the battery for inrush and reverse current coverage, I'm wondering why you don't go towards a supercap setup which would be lighter and probably cheaper. Your battery of choice depends on weight and cost requirements.

horta
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