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motors in parallel or in series?

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        So if you have 2 motors that are mechanically connected (ie.driving the same chain), you should connect them in series? By your definitions above that would allow them to in essense self balance the current..

        On a related topic, what would be the issues/benefits with using 2 motors this way to drive a larger load? Or would it be best to find larger motors? From an efficency standpoint..

I second the parallel suggestion.

Many Rover 5 bots have four motors. The two motors driving a single tread run in parallel. All of the reasons OddBot gave to use motors in parallel still apply.

2 motors in series under equal load would only have half the voltage which would likely defeat the purpose of having 2 motors in the first place. I'd go in parallel

A brushed DC motor has a very low resistance but as it runs back emf is created which further reduces the current flow. If you measure the resistance of a motor, calculate how much current that motor should draw based on it's resistance and then measure the current drawn with the motor running (no load) you will find the motor draws far less current than expected because of the back emf.

SERIES:

If you wire two or more motors in series then as one motor comes under more load and it's back emf decreases, the voltage drop across the motor decreases. This causes more voltage across the motor(s) with a lighter load and therefor more power goes to the unloaded motor(s). When one motor stalls, it is almost a short circuit and all the power goes to the other motor(s).

A simple test to demonstrate this is to wire two small dc motors in series with a wheel on each motor. As you slow one motor down by touching the wheel, the other motor speeds up. It is an electronic version of a differential.

With my 6 wheel chassis, if the motors were connected in series then when one wheel came off the ground or hit loose sand, it would get more power feed to it while the motors with traction would loose power. The vehicle would get bogged andthe wheel in the air or loose sand would spin furiously.

Parallel: 

By having them in parallel, they all try to maintain the same speed. When one motor gets more load than the others, it draws more power to compensate without affecting the others.

A simple test is to run a motor off of a 7805 voltage regulator and load it up. The motor will try and maintain the same speed by drawing more current and will only fail to maintain speed if the regulator fails to maintain the voltage. (put a heatsink on the regulator before trying this one).