How to the Common Wires of a Stepper Motor?

The best thing to do is to look up the make and model of the motor and the manufacturers data sheet will tell you the answer. Most stepper motors use the same color coding so any six wire motor data sheet can supply the right answer. But otherwise you can read the coils of the motor with an ohm meter and draw a schematic from your readings.

1. DC motor vs Stepper motor vs Continuous servo - Which one should I use? [closed]

A DC motor (suitably geared) can be much more powerful and faster than a stepper motor, but needs a feedback mechanism to obtain precise positioning.You intend to operate your 'robot' arm by remote control, but "do not need feedback". Without feedback you cannot tell the arm to move to a precise position. However if you are controlling it with an R/C transmitter, and the motor is geared down to a low speed so you can stop the arm when it reaches the position you want, then you will be providing the feedback. In that case it is not really a robot, but just a remote controlled mechanism. An R/C servo can be made to rotate continuously by disconnecting the feedback pot from the output shaft, then it will rotate continuously when commanded off center. This effectively would owngrades' the servo to a simple geared motor with H-bridge speed control. This could be used for traction motors in an R/C vehicle, but is not good for steering.The average R/C car uses a servo (geared DC motor with feedback) for steering because the operator needs it to turn in proportion to moving the stick or wheel on the transmitter. Without that automatic tracking, the operator would have to rely on visual feedback and quick reflexes - and driving fast would be impossible. Cheap toy R/C cars often use a simpler steering mechanism which just has a motor that rotates until it hits a stop at full lock, and a spring to center it when the motor is turned off. These are harder to steer because you only have 3 positions (left/center/right) but even that is better than allowing it to rotate 360 with no defined positions.

2. How to move a Stepper Motor to a desired angle with raspberry pi?

You said,I came to know about rotary encoders but these wont be any good for my robotic arm.But why wo not they be any good for your robotic arm?There are several approaches to position control. From best to worst, your options are generally: The difference between (2) and (3) above is that you must always assume the arm traveled the correct number of steps if you do not have any encoder. If the torque applied to the arm exceeds the running or cogging torque of the stepper motor, then the arm will not be in the position you think it's in. If you use a relative encoder, then you will at least be able to detect that something moved the arm. A stepper motor is completely open loop and you have no means of determining if you are in the correct position unless (if) you happen to pass the soft stop/limit switch.

3. How can I get a stepper motor to engage with a freely spinning wheel?

Stepper motors have significant cogging. If that is getting in the way, then a stepper is probably not appropriate here.Take a look at using a DC motor with a shaft encoder, or some way of knowing position. Then it's a matter of a servo control loop to drive the DC motor to get the desired position. If you specify a low-cogging motor, the mechanical resistance when left open circuit can be quite small

4. How does stepper motor changes its poles?

A stepper motor works on steps that is its rotor cut stator flux in steps. The circuit which helps to rotate this machine is called chopper circuit.Consider a motor with 3 pair of stator windings. Each winding is done such that winding starts from one slot and that winding goes to opposite slot and wounds it too and a terminal comes out. So is done for rest two. So each winding covers two slots and has two terminal coming out. So total 6 terminals come out from 6 slots(2 slot per winding). This is nothing different from stator construction of induction motor.(Except that induction motor have distributed winding while stepper motor has concentrated winding per phase)Now, out of the 6 terminals, 3 end terminals of the each is shorted (star connected). We are thus left with 3 terminals. These 3 terminals are given supply in a certain sequence. As there are 6 slots so there are 6 poles. Let the 3 terminals of the stator be named as a, b and c. So, if 'a' is given supply, current flows through it and goes to winding say 1 in slot on stator and comes out from opposite slot say 1. As a result, slot 1 bears N pole and slot 1, S pole. The N pole attracts the nearer S pole on rotor and the opposite S pole attracts rotor nearer N pole. Thus, the rotor moves by certain angle and stays until supply is cut off. Now if 'b' terminal is supplied cutting off 'a' then the rotor will rotate further by some angle. Thus is how stepper motor works. If this supply is given in a sequence, the motor rotates completely 360 degree.Here, there are 6 poles totally on stator. Thus (360/6)=12 sequential supply is needed for stator to make rotor rotate one complete rotation. This sequence is given by a microprocessor and thyristor bridge(chopper) circuit.How does stepper motor changes its poles?.