Motors? what do you know about them? A shaft that has a field winding (the winding responsible for magnetism in any motor) and the armature winding, it rotates when current passes through armature winding. Not only Current 'I' but the potential difference 'V' is also important for a motor to rotate. This implies it's power that is important. For any electronic Device to work Power is very important. Toy Motors are some cheap DC motors, which you can find in almost any toy car, etc. You would have played with those. In this instructable, we are gonna discuss some knowledgeable things about Toy Motors, the current across them, etc.
Step 1: Toy Motors and the Battery Mystery
If you are going to a shop asking for a motor having the voltage reading 9 Volts and current rating 1 Ampere, that can't happen as such a motor can't exist. Always keep in mind Current is like Juice, it depends on our capacity how much can we drink it without getting our stomach burst, similarly its CIRCUIT'S CAPACITY how much current can it bear without getting circuit's components burst due to heat. It is our circuit and its components which demand the current from source. That's why we have a maximum or stall current reading and minimum current rating on the motor specifications. One more thing, the source that you are using has ratings in mAh or Ah, i.e., milli-Ampere hour or Ampere hour. For example: if we are using a 9 V, 1000 mAh battery; it will provide 9 Volts all the way but current again depends upon the circuit. If our circuit. is demanding 1A or 1000mA, Battery will last for an hour, and if it is demanding 500mA the battery will last for 2 hours. thus when you are going to buy a motor of rating 9 volt and 1A or a battery for 9V and 1A, open Instructables again.
Step 2: Starter Current Concept
If the motor is asking for current the battery will supply it obviously, but how will motor demand a current value that's important to understand. DC motors need a high starting torque to continuously move. practically, most motors you will be using for making robots will be Geared DC motors, they will require a much high torque to start i.e., due to larger load, thus they will need a higher current to start. Theoretically, this current is required to overcome current generated by back EMF and that is why we need a starter for a DC motor. We can put a potentiometer at the start to limit the current and increase the value slowly. But just in case you are having 8-10 motors fixed on an automatic robot where you can't get you hand, and thus we have to think of other starter methods. The textbooks don't provide a simple and practical method. What we can do here is little playing with the graph of DC. It can be done by using circuit reactive elements such as resistors, capacitors or inductors. Resistors will limit the voltage but will do it on the whole time period of a turn on. L's or inductors will resist the change in current for a very short time (dI/dt) thus it will act as a short between wire much before the back EMF problem is solved. The cap's store charge when the voltage is turned on and will get charged and thus by connecting it across the terminals or in parallel with the motor, it will increase the voltage slowly to the maximum value, as you can see in the graph.
Step 3: RPM, Load and Current
Sometimes one must have wondered, Geared DC motors have different RPMs for the same potential difference. To control speed or RPM of a DC motor we can limit the input power to the motor. But as the current depends on the circuit, we will harshly be able to control it without disturbing the circuit. Thus the only option we are left with is Voltage control and for constant voltage we can only limit the voltage by various voltage regulators like if we want to run a motor at 5 volts we can use IC LM7805 for limiting the input voltage(works up to 35 volts), or we can reduce the RPM by introducing a gear box at the shaft. This method particularly increases the load at the shaft as a result of which motor asks for a larger current value. For an example, I was performing an experiment on DC motors and their load capacity and I got following data: Current for a toy DC motor was 50mA, for 100 RPM motor it was 190mA, for 600 RPM it was 600mA, etc. Thus, choosing an incorrect pair of motor- battery set can limit your Bot's performance to some minutes maybe.