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What are downsides of wiring 2 brushed DC motor drivers in parallel, so that they share the same control pins?

I have two identical brushed DC motors that each have a stall current of 2.5A. I have two identical brushed DC motor drivers that can drive a continuous 3A.

For my application, the motors have to spin at identical speeds and direction. Since a single driver can't handle the current should both motors stall, I'm going to use two drivers to handle each motor separately, but wire the control pins together.

Is there any reason this wouldn't work?

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  • \$\begingroup\$ Are motor mechanical loads entirely independent? If not, I can well imagine speed differentials escalating toward failure. Do your drivers employ feedback from the motors (ie: rpm sensors)? \$\endgroup\$ – glen_geek Sep 26 '16 at 22:30
  • \$\begingroup\$ You won't get identical speeds of course, without some sort of closed loop control for each individual driver, but it'll sort of work. \$\endgroup\$ – Brian Drummond Sep 26 '16 at 22:30
  • \$\begingroup\$ @glen_geek, No, the motors will be driving the same load, and no, there's no feedback. \$\endgroup\$ – Cerin Sep 27 '16 at 5:50
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Two brushed DC motors, fed from the same voltage, or from different controllers fed with the same control, each turning their own load that requires the same torque, will turn at roughly the same speed, not exactly the same speed. If the load torque varies on one motor, its speed will further vary.

When you say the 'motors will be driving the same load', that is ambiguous. Depending on how it's interpretted, it may mean it's fine and will work, it may mean it cannot work.

If the two motors are coupled together via the load so that a small angular difference between them will generate all the torque needed to keep them at the same angle, then all will be fine. Say both motors are coupled by a gear or belt drive to a common gear on the load. Or one is used each end of a rigid lead-screw. If the motors are nominally the same, and have the same control, they will be able to share the torque sufficiently well to be rigidly paralleled like this.

If the arrangement will not generate the torque needed to correct the position of the motors, say each motor drives its own leadscrew, to drive a table sideways with a nut on each side, then the motors will get out of sync and twist the table rather than driving it straight. This application needs position feedback on each motor, and each motor driving from its own controller, to control the motor angles to be equal moment by moment. People often use stepper motors in this configuration because, when driven from the same controller, they do match angles.

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As an alternative to trying to precisely speed match the two motors, I typically run one motor in "Master" mode, with feedback, and control its speed. The second motor is run in as a torque slave to the master, with no feedback.

This requires that you be able to get to the inner torque loop of the master to use it as a direct reference to the torque loop of the slave, but it also eliminates the possibility of the two motors fighting each other, which they will in speed mode.

A second method of this type of control, is to run the slave in speed, but with a 100% speed reference, and then control its current limit from the torque command of the master. With some controller/drive combinations, that don't allow for direct torque control, this works well.

Caveat for pure torque control: If the load can ever be uncoupled from the slave, it will accelerate to potentially destructive speeds when in torque mode.

Caveat for current limit control: If the load can ever be uncouple from the slave, it will accelerate to 100% speed. This can be limited, by setting its speed reference to something just over (say 5 or 10%) the reference to the master.

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