1. Longer Life which the the only thing to affect the life of the brushless dc motor is the bearing’s life.
2. No maintenance because there are no brushes and physical commutator.
3. More efficient.
4. High reliable.
5. High speed which can operate at speeds above 10,000 rpm in both loaded and unloaded conditions
6. Less noise because its internal parts are completely enclosed.

Sensored motor as a sensor to help the sensored speed control know the orientation of the motor shaft.

Sensor brushless motor systems always know the position of the rotor, which is especially critical at low speed as well as during the start condition when there is no rotor movement. Sensor brushless motors are often used in applications where starting torque varies greatly or where a high initial torque is required.  For the sensorless brushless motor, in contrast, a sensorless speed control does not know the position of the rotor until it is spinning at a certain speed and generating enough back-EMF to calculate its approximate position. 

This method of stepping the motor energizes both phases constantly to achieve full rated torque at all positions of the motor. If a stepper motor has 200 steps, one pulse equals one step. So, 200 pulses from the NC computer results in 360 degrees of motor shaft rotation. A unipolar stepper motor driver operating in full step mode energizes a single phase. A bipolar stepper motor driver energizes both coils to make a full step. See the images below. The first image is single coil full step operation while the second is dual core full step mode. 

Half Step

The Half step mode energizes a single coil then two coils then one again. Alternating between energizing a single phase and both phases together gives the motor its higher resolution. A 200 step stepper motor operating in half step mode would have 400 positions, twice the normal resolution. However, the torque will vary depending on the step position because at times a single phase will be energizes while at other times both phases will be energized. Higher end drivers compensate by increasing the current through the single coil when a single coil is energized. This makes up for the loss in torque, making the half step mode very stable. See the image below. 
Micro-stepping 
The micro-stepping mode is the most complex of all the stepping modes. That is why some stepper drivers only offer full and half step modes. Micro-stepping is when the current applied to each winding is proportional to a mathematical function, providing a fraction of a full step. The most common divisions are 1/4th, 1/8th, 1/10th, etc. However, there are some drivers that provide up to 1/256th of a full step. Micro-stepping provides greater resolution and smoother motor operation. This is very advantageous as it reduces the need for mechanical gearing when trying to achieve high resolution. However, micro-stepping can affect the repeatability of the motor.

Detent torque:
A stepper motor’s detent torque is the amount of torque the motor produces when the windings are not energized. The effect of detent torque can be felt when moving the motor shaft by hand, in the form of torque pulsations or cogging.
Because detent torque has to be overcome in order for the motor to move, it reduces the ideal torque that the motor can produce when it’s running. Overcoming the detent torque requires more power from the motor, and the amount of power needed is proportional to speed. So the faster the motor turns, the greater the effect that detent torque will have on the motor’s actual torque output.
On the other hand, detent torque can be beneficial when stopping the motor. The momentum of the moving rotor is countered by the detent torque and the friction in the rotating components. Therefore, a higher detent torque will help the motor to stop more quickly. The detent torque typically ranges from 5 to 20% of the holding torque.

4 Wires: Bipolar only
5 Wires: Unipolar only
6 Wires: Universal
8 Wires: Universal

Unipolar and bipolar is the type of connection each motor.The only difference is that bipolar can not be connected as unipolar (because it has only 4 wires) ,but unipolar can be connected ae bipolar.

Bipolar stepper motors
Bipolar motors have a single winding per phase. The current in a winding needs to be reversed in order to reverse a magnetic pole, so the driving circuit must be more complicated, typically with an H-bridge arrangement (however there are several off the shelf driver chips available to make this a simple affair). There are two leads per phase, none are common.
Because windings are better utilized, they are more powerful than a unipolar motor of the same weight. This is due to the physical space occupied by the windings.

Unipolar stepper motors
A unipolar stepper motor has two windings per phase and of course two phases, one winding for each direction of magnetic field. Since in this arrangement a magnetic pole can be reversed without switching the direction of current, the commutation circuit can be made very simple (eg. a single transistor) for each winding.

1. Low cost for control achieved
2. High torque at startup and low speeds
3. Ruggedness
4. Simplicity of construction
5. Can operate in an open loop control system
6. Low maintenance
7. Less likely to stall or slip
8. Will work in any environment
9. Can be used in robotics in a wide scale.
10. High reliability
11. The rotation angle of the motor is proportional to the input pulse.
12. The motor has full torque at standstill (if the windings are energized)
13. Precise positioning and repeatability of movement since good stepper motors have an accuracy of 3 – 5% of a step and this error is non-cumulative from one step to the next.
14. Excellent response to starting/stopping/reversing.
15. Very reliable since there are no contact brushes in the motor. Therefore, the life of the motor is simply dependent on the life of the bearing.
16. The motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control.
17. It is possible to achieve very low-speed synchronous rotation with a load that is directly coupled to the shaft.
18. A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input pulses.

Disadvantages

1. Require a dedicated control circuit
2. Use more current than D.C. motors
3. Torque reduces at higher speeds
4. Resonances can occur if not properly controlled.
5. Not easy to operate at extremely high speeds.

The first difference you notice is that they have no brushes or commutator (the parts of a DC motor that reverse the electrical current and keep the rotor—the rotating part of a motor—constantly turning in the same direction). In other words, stepper motors are examples of what we call brushless motors. (You’ll also find brushless motors in many electric vehicles, hidden away in the wheel hubs; used in that way, they’re called hub motors.)

The second major difference is in what rotates. Remember that in a basic DC motor, there is an outer permanent magnet or magnets that stays static, known as the stator, and an inner coil or coils of wire that rotates inside it, which is the rotor. In a brushless hub-motor, the coils of wire are static in the center and the permanent magnets spin around them on the outside. A stepper motor is different again. This time, the permanent magnets are on the inside and rotate (making up the rotor), while the coils are on the outside and stay static (making up the stator).

The third big difference between an ordinary DC motor and a stepper motor is in the design of the stator and the rotor. Instead of one large magnet on the outside (the stator) and one large coil rotating inside it (the rotor), a stepper motor has an inner magnet effectively divided up into many separate sections, which look like teeth on a gear wheel. The outer coils have corresponding teeth that provide magnetic impulses, attracting, repelling, and making the teeth of the inner wheel rotate by small steps. This will become clear in a moment when we look at some pictures.

The final difference is that a stepper motor can stay still, in a certain position, once it’s rotated through a particular angle. That’s obviously crucially important if you want a motor to power something like a robot arm, which might have to rotate a certain amount and then remain in precisely that spot while another part of the robot does something else. This feature is sometimes called holding torque (torque is the rotary force something has, so “holding torque” simply means a stepping motor’s ability to stay still).

These applications can choose from brush dc motors, brushless dc (BLDC) motors, or a combination of both. Most motors operate in accordance with Faraday’s law of induction(see here). Still, there are key differences between these motors and in the employment opportunities that await them.

Brush DC Motors
Around since the late 1800s, dc brush motors are one of the simplest types of motors. Sans the dc supply or battery required for operation, a typical brush dc motor consists of an armature (a.k.a., rotor), a commutator, brushes, an axle, and a field magnet (Fig. 1)(see “Brushed DC Motor Fundamentals”).

1. Simple in construction, a general-purpose dc brush motor includes an armature or rotor, a commutator, brushes, an axle, and a field magnet. Naturally, a battery or power supply is required.

The motor’s properties are determined by the material it’s made of, the number of coils wound around it, and the density of the coils. The armature or rotor is an electromagnet, and the field magnet is a permanent magnet. The commutator is a split-ring device wrapped around the axel that physically contacts the brushes, which are connected to opposite poles of the power source (Fig. 2).

2. A split ring wrapping around the axle, the commutator makes physical contact with the brushes, which connect to opposite poles of a power source to deliver positive and negative charges to the commutator.

The brushes charge the commutator inversely in polarity to the permanent magnet, in turn causing the armature to rotate. The rotation’s direction, clockwise and/or counterclockwise, can be reversed easily by reversing the polarity of the brushes, i.e., reversing the leads on the battery.

Brushless DC Motors
In terms of differences, the name is a dead giveaway. BLDC motors lack brushes. But their design differences are bit more sophisticated (see “Brushless DC (BLDC) Motor Fundamentals”). A BLDC motor mounts its permanent magnets, usually four or more, around the perimeter of the rotor in a cross pattern (Fig. 3).

3. Viewed from the top, this brushless dc (BLDC) motor employs four permanent magnets mounted to the top of its rotor, eliminating the need for connections, a commutator, and brushes.

Efficiency is a primary selling feature for BLDC motors. Because the rotor is the sole bearer of the magnets, it requires no power, i.e., no connections, no commutator, and no brushes. In place of these, the motor employs control circuitry. To detect where the rotor is at certain times, BLDC motors employ, along with controllers, rotary encoders or a Hall sensor (see “Brushless DC Motor Control Made Easy”).

BLDC motors are synchronous motors, which means their rotors and stators turn at the same frequency. They come in single-, dual-, and tri-phase configurations (see “Brushless DC (BLDC) Motor” ).

To Brush 
When it comes to a loosely defined range of basic applications, one could use either a brush or brushless motor. And like any comparable and competing technologies, brush and brushless motors have their pros and cons.

On the pro side, brush motors are generally inexpensive and reliable. They also offer simple two-wire control and require fairly simple control or no control at all in fixed-speed designs. If the brushes are replaceable, these motors also boast a somewhat extended operational life. And because they need few external components or no external components at all, brush motors tend to handle rough environments reliably.

For the downside, brush motors require periodic maintenance as brushes must be cleaned and replaced for continued operation, ruling them out for critical medical designs. Also, if high torque is required, brush motors fall a bit flat. As speed increases, brush friction increases and viable torque decreases.

However, torque may not be an issue in some applications and could actually be desirable. For example, electric toothbrushes require higher speeds with decreasing torque, which is good for the brush and your teeth and gums.

Other disadvantages of brush dc motors include inadequate heat dissipation caused by the rotor limitations, high rotor inertia, low speed range due to limitations imposed by the brushes, and electromagnetic interference (EMI) generated by brush arcing.

Or Not To Brush
BLDC motors have a number of advantages over their brush brothers. For one, they’re more accurate in positioning apps, relying on Hall effect position sensors for commutation. They also require less and sometimes no maintenance due to the lack of brushes.

They beat brush motors in the speed/torque tradeoff with their ability to maintain or increase torque at various speeds. Importantly, there’s no power loss across brushes, making the components significantly more efficient. Other BLDC pros include high output power, small size, better heat dissipation, higher speed ranges, and low-noise (mechanical and electrical) operation.

Nothing is perfect, though. BLDC motors have a higher cost of construction. They also require control strategies that can be both complex and expensive. And, they require a controller that can cost almost as much as if not more than the BLDC motor it governs.

The Choice Lies In Our Apps
The bottom lines for making a choice between components of any type are the type of application and the cost cutoff for the end product. For instance, a toy robot targeting the six- to eight-year-old market may require four to nine motors. They can all be brush or brushless dc components or a mixture of both.

If this robot only performs basic movements or is part of an introductory kit, there’s no need to go with long-life BLDCs that cost more than brushed counterparts. The toy or kit will probably end up in the recycling bin well before the brush motors have burned out.

Typical brushed dc motor applications include motorized toys, appliances, and computer peripherals. Auto makers enlist them for power windows, seats, and other in-cabin designs because of their low cost and simple design.

BLDC motors are more versatile, mainly because of their savvy in the speed and torque departments. They also come in compact packages, making them viable for a variety of compact designs. Typical apps include computer hard drives, mechanical-based media players, electronic-component cooling fans, cordless power tools, HVAC and refrigeration, industrial and manufacturing systems, and direct-drive turntables.

The automotive industry also puts higher-power BLDC motors to work in electric and hybrid vehicles. These motors are essentially ac synchronous motors with permanent magnet rotors. Other unique uses include electric bicycles where motors fit in the wheels or hubcaps, industrial positioning and actuation, assembly robots, and linear actuators for valve control.

*Article from: http://electronicdesign.com/electromechanical/what-s-difference-between-brush-dc-and-brushless-dc-motors