Any engineers out there? Seeking help on dumping excess energy

General discussion of the MCP motion controller product line
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Any engineers out there? Seeking help on dumping excess energy

Post by dbulota » Fri Apr 12, 2019 7:47 am


I have project that uses 2 small dc motors, a Roboclaw 2x15, 2 x 12v LiPo 3s batteries (2200 mah each) connected in series (so 24v), and a raspberry pi (See attached illustration). The motors are set in motion through an external force which is a "clothes line" that is constantly moving thanks to another independent power source. One of the two DC motors turns clockwise and the other counterclockwise. The motors cannot be disengaged from the "clothes line" so are constantly turning unless one motor is put into a stall.

The entire unit (my vehicle) can have 3 states : 1- Stationary, 2- Forward (variable speed), 3- Reverse (variable speed). An external mechanism/brake can hold the unit stationary in which case the motors turn at the same RPM (one CW and the other CCW) and generate electricity. When movement is required, i wish to apply regenerative braking through the Roboclaw on one of the motors (M1), and let the other turn freely all while it's RPM increases at equal rate as the motor that is braking. So if M1 brakes and reduces rotation by 1000 RPM then M2 will increase RPM by 1000. The external brake is disengaged of course when there is movement.

Another option would be to draw power from the batteries to rotate M2 faster that the speed of the clothes line that is making it rotate. So M2 turns at 2000 RPM CW when stationary and I make it turn at 3000 RPM CW, M1 with reduce its RPM to 1000.

Over a period of time, my unit produces more energy than it consumes as it's often stationary and batteries reach full capacity. I set a voltage limit to protect the batteries on the Roboclaw. At this point I am at a loss and have many questions since I've "melted" about 6-7 motors so far that have over-heated and don't understand if it's a configuration issue with the Roboclaw or a component I need to add such as a mosfet/heat-pad to dump excess energy or something else.

A few basic questions as I have little experience with the Roboclaw:

1- Since the motors continually rotate (generate current), where does the energy go once batteries are full ? Is this maybe why my motors overheat ? A simpler question : does a DC motor over heat if rotated by an external force below the max RPM specifications?
2- If I added a component like a mosfet/heatpad to dissipate excess energy, would that solve my problem ?

Any help or guidance is appreciated. Thanks,
Project diagram
Diagram.jpg (218.77 KiB) Viewed 165 times

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Re: Any engineers out there? Seeking help on dumping excess energy

Post by Basicmicro Support » Mon Apr 15, 2019 10:12 am

You can use a voltage clamp as is recommended for Power Supply powered Roboclaws. However the power resistor will need to be big enough and heatsink well enough to disipate all the energy you are pulling out of the rotating close line. That could be a considerable amount.

You would set the max voltage limit to just below the batteries full charged state(so you dont ov er charge the batter). Then set on eof the auxiliary pins(S3,S4 or S5) as a voltage clamp output. This will then drive a power mosfet which will connect a power resistor from the main power + lead through the resistor to the mosfet and then to ground.

Keep in mind the voltage clamp activates at 1v ABOVE the maximum main battery voltage settings and disables again at the maximum main battery voltage limit.

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