# What are the reasons two DC motors with same specifications and running with same power supply run at different speeds?

I am using two DC motors in my circuit.

They are running on same single supply and also using same motor driver but one of them is running slower than the other one.

There is 1.54 volt difference between the output of both motor drivers to the motors.

If I swap motors then I can clearly see that speed of the motors change, the slower one earlier now runs faster and vice-versa.

• Speed is proportional to voltage so if you have a 1.54 V difference then your problem may be explained. Start debugging: (1) Why is there a 1.54 V difference? (2) What happens if you swap motors? Put all your findings in the question, not buried in the comments. Jan 12, 2021 at 16:19
• If you're trying to control their speed open loop, you'll be running around in circles forever. Jan 12, 2021 at 16:38
• It sounds like one driver is running higher voltage than the other, and the motors are just responding to that higher voltage. If that's not the case -- please clarify your description of the problem. Jan 12, 2021 at 17:25
• Which motor driver, and how much current do the motors draw? Show us a photo of your setup. Jan 12, 2021 at 20:55
• What type of DC motors? Shunt. What are the specifications? Jan 12, 2021 at 22:33

They can have same printed specs but they are different - not the same friction, not as strong magnets, different windings or something else. The brushes also can have different placements. If they run as unloaded generators differently when both have the same r.p.m the motors are different, no matter what's printed on them or papers.

Board routing or wiring can also be an issue from resistance of conductors which is most likely going on here (so make sure you calculate resistance of conductors to the motors). If you want any control over speed then closed loop control is a good way to go.

One way to find out is to measure the voltage drop of the wire from the supply to the motor.

There is 1.54 volt difference between the output of both motor drivers to the motors.

and

If I swap motors then I can clearly see that speed of the motors change, the slower one earlier now runs faster and vice-versa.

Then the problem lies with the motor drivers, not the motors.

Are the motor drivers the same make and model? Are they adjustable at all?