What is an air gap?

A simple way to define air gap is to say it is the air in the space between the stator and the rotor of the motor.

More specifically, we can have similar definitions for alternating current or AC motors and direct current or direct current motors.

In an AC motor, the air gap is the air space between the rotor core and the stator. On the other hand, it is said to be the space between the poles and the armature of a DC motor.

The flux gap is the distance between the high permeability material in the stator (stator "iron") and the corresponding high permeability material in the rotor (rotor "back iron"). This material is usually made from thin laminations of Fe-Si steel.

As we all know, in general, the flux gap should be as small as possible. 

Note that the flux gap includes magnets. Rare earth magnets (magnetized or unmagnetized) have essentially the same permeability as air. Therefore, from the stator point of view, the magnets are indistinguishable from air and should be included as part of the flux gap.

How does the air gap work?

To understand how air gaps work, first remember that electric motors and generators are rotating electrical machines.

This means that electric motors and generators work very similarly. The difference is that electric motors convert the supplied electrical energy into mechanical energy. Instead, generators take specific mechanical energy and convert it into electrical energy.

In both cases, the energy conversion process occurs when the stator and rotor work together to generate magnetic flux through their own copper windings. This is where the air gap comes into play.

A magnetic field is formed in the air gap, and one of the above-mentioned windings is responsible for generating the magnetic flux, which has to pass through the air gap twice for each pole of each phase of the motor.

Some of the key factors related to air gap operation are:

Magnetic force is inversely proportional to the square of the distance. As the air gap size increases, the magnetic attraction decreases and becomes more difficult to control.

Increasing the size of the air gap increases the magnetizing current, which represents the amount of current required to drive magnetic flux through the air gap.

The more poles a motor or generator has, the more times the flux must cross the air gap per revolution.

All these factors lead us to conclude that the smaller the air gap, the better. However, a smaller air gap means less separation between the moving parts of the rotor and the stator. This is why monitoring the air gap in motors and generators is critical, as the slightest variation in the alignment of the air gap with these characteristics can create operational problems in the machine.

The magnetic circuit consists of a soft iron ring, a copper winding (2A, 250 turns) represented by a green rectangle, and an air gap in the ring. Flux density (units of Tesla) is represented by the distance between the flux lines and the color, with red being the highest density and blue being the lowest. The flux density is clearly greatest in rings with small flux gaps. This ring also has the least amount of flux "leaking" into the surrounding air.

The reason for this difference is that the air gap increases the reluctance of the circuit. Reluctance is flux in a magnetic circuit, just like resistance is current in a circuit. Therefore, the magnetic flux in a circuit depends on the total reluctance and the applied magnetomotive force (number of turns times the current), just as the current depends on the total resistance in the circuit and the applied voltage.

Let's take a closer look at how the flux varies with the flux gap itself. We can do this by drawing a line across the flux gap and measuring the flux density at each point on the line.

Doing this for flux gaps of 1 mm and 4 mm it is clear that the flux in the middle of each gap remains the same. It can also be seen that the flux in the 4 mm flux gap is four times smaller than the flux in the 1 mm flux gap. So to produce the same flux density in a 4 mm gap we need to quadruple the windings at the same current, or keep the same number of windings and quadruple the current. This concept can also be applied to electric motors and explains why engineers usually go to great lengths to keep the flux gap as small as possible.

Influence of Flux Gap Size on Torque of Simple Motor

The torque produced by the motor depends on the flux density in the air gap, and as the size of the flux gap increases, the torque decreases gradually

From the above discussion it is clear that in general we want the flux gap to be physically as small as possible to increase the motor torque output, hence it is the motor constant. However, in addition to manufacturing tolerances, we also need to consider the thickness of the magnet. In general, if you make the rotor magnets longer, the flux density at their poles also increases. This will increase the torque output of the motor.

If you don't know much about the internal clearance of the brushless motor, you can contact our salesman, and they will give you a professional answer.

The movement of the robot itself requires the use of motors. There are a variety of motors available for robotic applications. Each type of motor serves a different purpose. Motors aid in the movement of the robot and act as actuators in the mechanical design of the robot. Robotic applications may involve the following types of motion:

1) Vertical movement - usually by shoulder rotation to move part of the robot up and down

2) Radial movement - moving part of the robot in and out

3) Rotational movement - Rotation clockwise or counterclockwise around a vertical or horizontal axis or around a plane in a 3D frame

4) Pitching motion - up and down motion while rotating motion

5) Rolling motion - rotation of a part of the robot on a parallel axis relative to the rest of the robot's body

6) Yaw motion - right or left rotational motion of a part of the robot

7) Motion - the movement of the robot on a surface or medium

All these types of movements are achieved with the help of various motors or pumps assembled with the drive train and end effectors. In this tutorial, the use of motors to provide the main motion for the robot itself or its components will be discussed. This tutorial will examine different types of motors, their applications, motor selection, and robotic car design.

Motor type

There are many types of motors available in industry. For robotic applications, some type of motor is usually used. Motors commonly used in robotics applications can be divided into the following categories:

• AC motor

• Brushed DC motor

• Brushless DC motor

• Geared DC Motor

• servo motor

• Stepper motors

Today we will introduce three types of motors (AC motors, brushed DC motors, brushless DC motors)

AC motors are driven by AC current. They are typically used in heavy duty applications requiring high torque (high load or load capacity). That's why these motors are used in robotic assembly lines deployed in manufacturing cells. Mobile robots are usually powered by a DC power source (battery or series of batteries), which is why AC motors are rarely used for such robots.

Brushed DC Motor:

Brushed DC motors use brushes to conduct current between the power supply and the armature. There are several variants of brushed DC motors, but in robotics, permanent magnet DC motors are used. These motors are known for their high torque-to-inertia ratio. Brushed DC motors are capable of delivering three to four times more torque than their rated torque. A brushed DC motor consists of six different components: shaft, commutator, armature, stator, magnets, and brushes.

Brushed DC motors have two terminals. When voltage is applied to both terminals, proportional speed is output to the shaft of the brushed DC motor. A brushed DC motor consists of two parts: the stator including the housing, permanent magnets and brushes, and the rotor consisting of the output shaft, windings and commutator. Its stator remains stationary while the rotor rotates relative to the stator. The stator produces a stationary magnetic field around the rotor.

The rotor, also called the armature, consists of one or more windings. When these windings are energized, they generate a magnetic field. The poles of this rotor field are attracted by the opposite poles created by the stator, causing the rotor to rotate. As the motor turns, the windings are continuously energized in different sequences so that the poles produced by the rotor do not exceed those produced by the stator. This switching of the magnetic field in the rotor windings is called commutation.

Brushless DC Motor:

Brushless DC motors are similar in structure to brushed DC motors, but they are driven by a closed-loop controller and require an inverter or SMPS for power. These motors have permanent magnets that rotatably fix the armature. Compared to brushed DC motors, they have a closed-loop electronic controller in place of the commutator assembly. These motors are typically used in industrial robots that require precise control of motion and positioning. However, these motors are very expensive and involve complex structures and electronics.

To choose motors for the robot:

To choose the right motor, many different parameters must be considered, such as the load a particular motor can handle, the torque required to move the robot without overloading, the number of revolutions per minute the motor makes while under load, etc.

Since there are many types of motors, one should be selected based on the application. For example, to run a robotic arm, servos are often used. Wheeled robots are simple in design and use electric wheels to navigate the ground. Wheels are also easier to design and manufacture than tracks or outriggers. There are some drawbacks to using wheels, such as navigating obstacles or low friction areas that are not easy to use wheels.

The most commonly used motors in such robots are DC motors. DC motors provide high torque with high efficiency. By applying torque in response to a load, a DC motor can be characterized by a speed and torque curve. DC motors used in hobby robots are usually preferred with voltage ratings of 3, 6, 12 and 24 volts. If the voltage applied by the motor is lower than what is given in the datasheet, the torque will not be able to overcome the internal friction - mainly from the brushes. Also, if you apply a higher voltage to the motor than it supports, it can get hot and damaged.

If you are still worried about finding a motor, please contact the salesman of BG Motor, they will give you a very professional motor answer

Neodymium magnets can be called NdFeB magnets, which is a general term for strong magnets. The chemical formula is Nd2Fe14B, and it is one of the artificial permanent magnet materials with the strongest magnetic force so far. The material grade of NdFeB magnets is N35-N52, which can be processed into different shapes according to specific requirements, such as round, square, punching, magnetic tile, magnetic rod, convex or trapezoidal, etc. Therefore, how to distinguish between bonded NdFeB and sintered NdFeB?

In fact, both magnets are NdFeB. These two magnets are distinguished according to their production process. Bonded NdFeB magnets are formed by injection molding, and sintered NdFeB magnets are formed by high temperature heating

1. Production process of bonded NdFeB magnets

Bonded NdFeB magnets are formed by injection molding. The density is lower than that of sintered NdFeB because it contains a resin matrix, and the density varies according to the content of the resin matrix. However, sintered NdFeB magnets are heated at high temperature through a complicated process and do not contain non-magnetic resin components. Therefore, the performance of sintered NdFeB is higher than that of bonded NdFeB.

2. Production process of sintered NdFeB magnets

Sintered NdFeB magnets are anisotropic magnets produced by powder sintering. Generally, only blanks can be produced by sintering, and magnets of various shapes need to be mechanically processed (such as wire cutting, slicing, grinding, etc.). Sintered NdFeB is a hard and brittle material that is difficult to process, so it has large losses during processing, high cost, poor dimensional accuracy, poor corrosion resistance, and the surface needs electroplating, but the advantage is high performance, which has reached more than 50M

To sum up, the magnetic properties of bonded NdFeB magnets are relatively low, isotropic bonded magnets are usually below 10M, while anisotropic bonded magnets can reach about 16M. However, injection-molded NdFeB magnets also have their irreplaceable advantages. For example, the effective utilization rate is relatively high; the performance can be adjusted according to the needs of different products; the injection-molded magnets have high resistivity, which can effectively reduce eddy current loss and motor heating when used in the field of high-speed rotors; the sintering process is not suitable for molding complex shapes and structures products, and injection molding can easily solve this problem.

Of course, the different materials of NdFeB magnets are recommended according to the different scenarios of customers. If you are not very clear about the magnetic materials, you can contact the customer service of our official website at any time, and they will contact and recommend to you professionally.