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I am considering two 50:1 metal gearmotor 37Dx70L mm 12 V with 64 CPR encoder (helical pinion) from Polulu for an autonomous mobile robot.

I am looking for a motor driver to control the speed using an Arduino.

Is a dual-channel motor driver which has a maximum current for each channel of 3 A OK for this motor, or should I choose a motor driver that has a maximum current greater than the 5.4 A stall current?

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  • \$\begingroup\$ What do you think the driver would do if you continuously took 5.4 amps from it? \$\endgroup\$
    – Andy aka
    Commented Feb 23, 2022 at 18:12

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Most likely you need a driver rated much higher than the stall current, unless you choose a fault-proof intelligent acceleration and motion sensing control that prevent over-temperature faults or have some other unknown compromise.

Thermal and current sensing are excellent secondary feedback for damage protection and servo control in addition to position feedback.

On the other hand, if you don't use the feedback and controls properly, you can damage the gears in the motor with excess torque from excess current.

So the engineered solution estimates the load current from acceleration and load demand, and inertial power from braking and uses controlled acceleration with current confined to limits with feedback sensing, measured velocity error and then position error for a perfect servo control with smart recovery for unexpected disturbances that are anticipated by design.

The "Stall Current" is simply the rated voltage applied across the coil resistance , thus 12 V / 5.5 A ~ 2.18 Ω. This generates 12 * 5.5 = 66 W of heat, yet the rated power = 10 W for 50:1 gear yet foot note indicates that is "constrained by gearbox load limit". Thus at 10W rated max input power the motor draws I=P/V = 10 W / 12 V = 0.833 A and the coil loss then is \$P=I^2R= 0.833A^2*2.18 Ω = 1.512 W = 15\$% of 10W

One typical driver rated for 5 A stall current is the Toshiba TB9051FTG (Ron(Pch+Nch))<0.45 Ω (Max @Tj = 150°C, VBAT = 8 V.)

Comparing the driver/motor electrical resistance will tell you relative power loss but you also need thermal resistance to compute max temperature rise. i.e. <0.45 Ω vs 2.18 Ω will reduce the output power and thermal resistance will indicate the temp rise, depending on the board heatsink design.

If you can tolerate the voltage drop of 0.833 A * 0.45 Ω = 0.375 V and implied additional power loss , OK. The drivers will be operating at maximum temperature in this state, which is never a good thing long term, so choosing to derate power or max current will improve reliability some TBD years and reduce losses a few percent with added torque.

It all depends on your design specs for the system on what compromises you are willing to tolerate. Performance, Cost, or Reliability

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  • \$\begingroup\$ What should be the range of much higher? From the datasheet, the stall current is 5.4A. So, is a motor driver which can supply 10A continuous and 30A peak is ok? or is 20A continuous and 60A peak is needed or more? \$\endgroup\$
    – Msr09
    Commented Feb 23, 2022 at 18:23
  • \$\begingroup\$ Thank you very much for your detailed reply . . . \$\endgroup\$
    – Msr09
    Commented Feb 23, 2022 at 23:25

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