Small DC Motors – Should They Be Connected in Series or Parallel?

batteriesdc motormotor

I have beginner motors kits, and I'm trying to build a car. All wheels are working and moving, but when they touch the floor, they stop. Seems like they don't have enough energy/power.

I tried with 2 AA batteries and with 4 AA batteries, and both options don't work.

Will it work better if I connect each motor directly to four AA batteries (AKA parallel)?

Or maybe add some more batteries?

What is the problem? Why is it not moving when it's on the floor?

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UPDATE:

After reading all the answers (And I learned a lot) I tried to play with it before taking it apart. In the end, I found that
the issue was that not all the wheels exactly pointed the same direction (As you see in the picture above). As you can see, a little bit of touch and it's working.

enter image description here

Best Answer

You pretty much always want to connect motors in parallel. Imagine if the more force you applied with one leg while walking, the faster the other leg would move. That's motor in series.

Your problem is the motors probably produce enough power, but all that power in speed instead of torque. This is usually the case with electric motors. So if the wheels did spin as fast as the motor wants them to (like when how it spins when the wheels do not touch the ground), the car would move too fast, but they cannot spin at all because they don't have enough torque.

So what you need to do is to reduce the RPM and turn all that excess RPM into torque in the process. One way to do that is a gearbox, which you do not have. But you have wheels, and wheels play into it too.

What moves the car is the force at the edge of the wheel. What happens is that when the motor tries to turn the wheel, it produces a force on the edge of the wheel which is applied to the ground. Due to Newton, the ground applies an equal and opposite reaction by means of rolling friction. Rolling friction is a type of static friction which means that it will produce an equal and opposite force against whatever force is applied, but only up to a limit. If the wheel cannot produce enough force at the edge of the wheel to overcome this limit, then the force of rolling friction is equal and opposite to the wheel edge force produced by the motor and nothing moves.

But if the motor can produce enough force at the edge of the wheel to exceed the maximum amount of rolling friction the ground can apply, then there is excess force leftover which is then used to accelerate the car and the wheel turns.

But a motor does not produce force. It produces torque and needs a lever to convert the torque into a force. A wheel is a round lever, or a lever that has been duplicated an infinite number of times and arranged all around a shaft.

A big wheel means a longer lever which means the force applied by the ground at the edge of the wheel produces a much larger torque at the center of the wheel which the motor must overcome. So use smaller wheels. Much smaller wheels in this case.

And just like a lever where it can turn lots of movement with low force into small amounts of movement with high force, adjusting wheel size does the same thing. A larger wheel will move the robot farther per rotation (and thus travel faster at the same RPM) but require more torque, whereas a smaller wheel will move the robot less per rotation (and thus travel slower at the same RPM) but requires less torque.

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