I am trying to build a circuit that has wheels rotating that are connected to a motor. This motor will then generate 5 Volts to a battery which then charges lets say a cell phone. I have two wheels that have motors on both, that are connected in series which is then hooked up to a portable charger and a multimeter. I got an open circuit voltage of 5 Volts but when i connect it to the load (the portable charger), i am only getting about 1 Volt. How many motors, or of what kind, do I need to get the output to be 5 Volts under load?
DC motors in series to get 5 Volts
currentvoltage
Best Answer
What you need to understand first is
How DC motors work
Most likely you have a brushed DC motor. It will have an equivalent circuit like this:
simulate this circuit – Schematic created using CircuitLab
When you use a motor as a generator,\$V_{emf}\$ will increase with the speed. However, V_mot that you can measure on motor's terminals is \$V_{emf} - R_{mot}*I\$. When you measured voltage on an open circuit, there was (almost) no current flowing, so you have measured pure \$V_{emf}\$. When you connect the load, there is some current, that causes voltage drop on internal motor resistance, and measured voltage is smaller.
How to find out what motor you need?
Assuming you want to use just a single motor:
EDIT:
Series vs. parallel
If you want to have two motors, first make sure that they are both same type. That will not guarantee their specs will be exactly the same, but we want them to be as closely matched as possible.
Series connection will give you higher voltage with the same current, but if you have 5V under the full load, when the current consumed by your charger/battery will go down, the output voltage will rise to 2 times the voltage of a single motor under no load. Make sure that your device can withstand this.
Parallel connection will allow for more current to be supplied. As I have already mentioned, it's the current flowing through a motor, that causes the voltage drop from higher voltage under no load to smaller with full load. Parallel connection of two motors will reduce the current flowing through a single motor by two times, thus reducing also the voltage drop. I would add a diode in series with each motor, to prevent current flowing in reverse through a motor if for some reason the voltages they generate differ.
Additional thoughts on your device