Hi,
removed.

 

Mechanical issue?

 

Mechanical issue?

yes. the two motors have different RPM with 10%.

 

Depending on the manufactured quality, new motors may have somewhat different torque to voltage curves. Likewise, driver circuits built from 10% tolerance parts will often be a bit different. IN this case I suggest exchanging te driver circuits to see if it is the driver circuit or the motor. If the motors are not both new, then the condition of the commutators may be different, or there may be a mechanical difference.
Now a question: Is this bias apparent at all speeds? Or only at some low speed? That could indicate a difference in threshold of some active device.

 

It might also be a mechanical difference between the two motors. AND, if they are rotating in opposite directions because of being on opposite sides of the vehicle, it is very possible that the brushes are positioned to favor one direction. That is often done with motors for some applications. So if the brush positions re adjustable that is one more thing to check.

 

Depending on the manufactured quality, new motors may have somewhat different torque to voltage curves. Likewise, driver circuits built from 10% tolerance parts will often be a bit different. IN this case I suggest exchanging te driver circuits to see if it is the driver circuit or the motor. If the motors are not both new, then the condition of the commutators may be different, or there may be a mechanical difference.
Now a question: Is this bias apparent at all speeds? Or only at some low speed? That could indicate a difference in threshold of some active device.

Thanks.
I ordered some more motors, and see if I can pick the same RPM pairs.

 

It might also be a mechanical difference between the two motors. AND, if they are rotating in opposite directions because of being on opposite sides of the vehicle, it is very possible that the brushes are positioned to favor one direction. That is often done with motors for some applications. So if the brush positions re adjustable that is one more thing to check.

Thanks.
Good point.

 

If you want the two motors to precisely match speed, then you either need control loops with speed feedback from each motor, or use a stepper type motor.

 

If these are the cheap eBay motors with wheels (usually yellow) then don't expect anything you build with them to go in a straight line, the variation is well over 20%. I 3D-printed some simple 3cm slotted discs with 20 slots which were a push fit onto the rear motor shaft and used a cheap interrupter board off eBay. Though the motors are spec'd as 3 - 6v they don't have any useful torque below about 5v so we ran them on 11.1v LiPo pack and used PWM to control the speed. Which worked OK though above 10v for any length of time they tend to liquefy internally!!! They have a large L/R time constant so to get any starting torque at low speed you need higher volts to drive enough current through the winding with a current limiter to stop the windings burning out and then reduce the duty cycle (and average volts) once the motor starts turning to get the desired speed.