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Many engineers have not figured out the difference between brushless motors and brushed motors for a long time? What is the difference between a synchronous motor and an asynchronous motor? Are servo motors all AC motors? Are servo motors all synchronous motors? Is a stepper motor a DC motor or an AC motor? Whether the steering gear is a servo motor, etc...
Do you have a lot of doubts about this too? This article will try to explain the systematic knowledge in plain language. I hope that after reading it, you will have a clear understanding of the classification of motors.DC Motors - Brushed MotorsAnyone who has studied physics in middle school knows that in order to study the fact that a current-carrying conductor is subjected to force in a magnetic field, we have practiced a severed palm of our left hand, which is also the principle of a DC motor.All motors are composed of a stator and a rotor. In a DC motor, in order to make the rotor turn, the direction of the current needs to be changed continuously, otherwise the rotor can only turn half a turn, which is like a bicycle pedal.So DC motors need commutators.DC motors in a broad sense include brushed motors and brushless motors.Brushed motor is also known as DC motor or carbon brush motor. DC motor is often referred to as brushed DC motor. It adopts mechanical commutation, the external magnetic pole does not move and the internal coil (armature) moves, and the commutator and rotor coil rotate together. , the brushes and the magnets do not move, so the commutator and the brushes are rubbed and rubbed to complete the switching of the current direction.Disadvantages of brushed motors:1. The sparks generated by the mechanical commutation cause friction between the commutator and the brush, electromagnetic interference, high noise and short life.2. Poor reliability and many failures, requiring frequent maintenance.3. Due to the existence of the commutator, the inertia of the rotor is limited, the maximum speed is limited, and the dynamic performance is affected.Since it has so many shortcomings, why is it still widely used, because it has high torque, simple structure, easy maintenance (ie, carbon brush replacement), and cheap.DC Motor - Brushless Motor
Brushless motor is also called DC variable frequency motor (BLDC) in some fields. It adopts electronic commutation (Hall sensor), and the coil (armature) does not move the magnetic pole. At this time, the permanent magnet can be outside the coil or inside the coil. , so there is a distinction between an outer rotor brushless motor and an inner rotor brushless motor.
The brushless motor construction is the same as the permanent magnet synchronous motor.However, a single brushless motor is not a complete power system, and the brushless basically must be controlled by a brushless controller, that is, an ESC to achieve continuous operation.What really determines its performance is the brushless electronic governor (that is, the ESC).Generally, there are two kinds of driving currents for brushless motors, one is square wave and the other is sine wave.Sometimes the former is called a DC brushless motor, and the latter is called an AC servo motor, which is exactly a type of AC servo motor. (video portal)Brushless motors operate in different ways, and can be divided into inner rotor brushless motors and outer rotor brushless motors.The inner rotors are all three-phase and are more expensive.The outer rotor is usually single-phase, the price is affordable, and the mass production is close to the carbon brush motor, so it has been widely used in recent years.The price of the three-phase outer rotor is close to the price of the inner rotor. Well, as you can guess, the disadvantage of a brushed motor is the advantage of a brushless motor.It has the advantages of high efficiency, low energy consumption, low noise, long life, high reliability, servo control, stepless frequency conversion speed regulation (up to high speed), etc. It is much smaller than the brushed DC motor. The control is simpler than the asynchronous AC motor, and the starting torque is large and the overload capacity is strong.DC motor - speed regulation principleSpeed regulation of DC motor: the so-called speed regulation, that is, to obtain the required torque by adjusting the motor speed.The DC (brush) motor can adjust the speed by adjusting the voltage, connecting the resistance in series, and changing the excitation, but it is actually the most convenient and most commonly used to adjust the voltage. At present, the main use of PWM speed regulation, PWM is actually through high-speed switching to achieve DC Voltage regulation, in one cycle, the longer the ON time is, the higher the average voltage is, and the longer the OFF time is, the lower the average voltage is. It is very convenient to adjust. As long as the switching speed is fast enough, the harmonics of the power grid will be less, and the current will be more continuous. .
However, the brushes and commutators are worn for a long time, and at the same time, there is a huge current change during commutation, which is very easy to generate sparks. The commutator and brushes limit the capacity and speed of the DC motor, which makes the speed regulation of the DC motor encountered. bottleneck.
For the brushless DC motor, only the input voltage is ostensibly controlled during speed regulation, but the self-controlled frequency conversion speed regulation system of the motor (the brushless DC motor itself has a rotor position signal acquisition device such as a rotor position detector, and the rotor of this device is used The position signal to control the commutation time of the variable voltage variable frequency speed control device) automatically controls the frequency according to the variable voltage, which is almost the same as the DC (brush) motor, which is very convenient.Since the rotor adopts permanent magnets, no special excitation winding is required. Under the same capacity, the motor is smaller in size, lighter in weight, higher in efficiency, more compact in structure, more reliable in operation and better in dynamic performance. Drive and other aspects have been widely used.Three-phase AC motor - asynchronous motorAC motors are divided into synchronous motors and asynchronous motors. Synchronous motors are mostly used for generators, and asynchronous motors are mostly used for motors.The outer casing of the motor is the stator, and there are three-phase symmetrical AC windings on the stator. Due to the sequential changes of the three-phase electricity, a rotating synthetic magnetic field is formed, and the rotation speed of the magnetic field is the synchronous speed.The synchronous speed is n=60f/p, f is the frequency, and p is the number of pole pairs. For example, for a 2-pole motor connected to the national grid at 50Hz (that is, the number of pole pairs is 1), then the speed n=60*50/1=3000r /min.Similarly, the synchronous speed of 4-pole, 6-pole and 8-pole motors is 1500, 1000, 750.The asynchronous motor mechanism is simple, and the rotor is a closed coil, such as a squirrel cage type.The rotor coil will cut the rotating magnetic field, generate an induced electromotive force, then generate an induced current, and finally generate a rotating magnetic field, so that the rotor becomes an electromagnet and will follow the stator magnetic field to rotate, so the rotating speed of the rotor must be < the rotating magnetic field of the stator, so that the Cut the magnetic field lines.That is, the asynchronous speed of the rotor is less than the synchronous speed, and there is a speed difference between the rotor and the stator magnetic field, so it is called an asynchronous motor.The rated speed of asynchronous motors produced by different manufacturers is slightly different. The 2-pole motor is about 2800+r/min, and the 4-pole, 6-pole, and 8-pole asynchronous motors are about 1400+, 950+, 700+.The speed of the asynchronous motor is high when it is no-load, and the speed is reduced when there is a load.The asynchronous motor has simple structure, convenient maintenance, reliable operation and low price, and is widely used.Three-Phase AC Motor - Synchronous MotorIf the rotor speed = the rotation speed of the stator magnetic field, it becomes a synchronous motor. At this time, the stator needs to be turned into an electromagnet or permanent magnet, that is, the stator is energized. At this time, it can rotate without cutting the magnetic field lines. The speed is the same as the rotation speed of the magnetic field, that is, a synchronous motor is formed. The rotor structure of the synchronous motor is more complicated than that of the asynchronous motor, and the price is higher. It is not as widely used in production and life as the asynchronous motor.Three-phase AC motors - speed regulation of asynchronous motors
Speed regulation of asynchronous motor: In theory, the asynchronous motor can control the frequency and voltage of the AC power, or the resistance of the rotor and the distribution of the magnetic poles of the motor.Because the range of voltage regulation and speed regulation is not large, it can only be used in occasions where the speed regulation requirements are not high, and the application is not extensive. Frequency conversion speed regulation: When it comes to frequency conversion, everyone may have heard of it.The full name of variable frequency speed regulation is variable voltage variable frequency speed regulation (VVVF), which means that the voltage is changed when the frequency is changed, so that the speed regulation range of the asynchronous motor is large enough.Inverters can be divided into two categories: AC-AC frequency conversion and AC-DC-AC frequency conversion.AC-AC frequency conversion: The AC power is directly converted into another frequency AC power through the power electronic device. The maximum output frequency cannot exceed half of the input frequency, so it is generally only used in low-speed, large-capacity systems, which can save huge gears Gearbox.The AC-DC-AC inverter rectifies the alternating current into direct current, and then converts it into alternating current with controllable frequency and voltage through the inverter. With PWM technology, this type of inverter can realize a wide range of voltage and frequency conversion.For electric vehicles, asynchronous motors are durable, have strong overload capacity, and are so mature in control algorithms that they can be used.Three-phase AC motors - speed regulation of synchronous motorsSpeed regulation of synchronous motor: Synchronous motor has no slip rate. Under the condition of certain structure, the control voltage cannot change the speed, so before the appearance of frequency converter, the speed of synchronous motor cannot be adjusted at all.The appearance of the frequency converter has given the AC synchronous motor a huge speed regulation range. Because the rotor also has independent excitation (permanent magnet or electric excitation), its speed regulation range is wider than that of the asynchronous motor, and the synchronous motor is full of new vitality. The synchronous motor variable voltage variable frequency speed regulation system can be divided into other-controlled variable frequency speed regulation and self-controlled variable frequency speed regulation.For other-controlled variable frequency speed regulation, similar to the variable frequency speed regulation of asynchronous motors, it can also be controlled by SVPWM and other control methods according to its mathematical model, and its performance is even better than that of ordinary AC asynchronous motors.The self-controlled variable frequency synchronous motor used to have a variety of names in the development process, such as a commutatorless motor; when a permanent magnet is used and a three-phase sine wave is input, it can be called a sine wave permanent magnet synchronous motor; and if a square wave is input, then It can be called a trapezoidal wave permanent magnet synchronous motor. Yes, this is similar to the brushless DC machine (BLDM) mentioned before. Do you feel that you have left a big circle and have turned back, but you are now very concerned about the variable frequency speed change The understanding must be deeper, so the brushless DC motor uses DC input, but uses the frequency conversion technology of the synchronous motor (the structure is the same as the permanent magnet synchronous motor), and the DC brushless motor is used on the Model3.Single-phase AC asynchronous motor - single-phase AC series excitation motor (with brush)Single-phase AC series motor, commonly known as series motor or universal motor (UniversalMotor is called abroad, named after AC and DC universal), the armature winding and the excitation winding work together in series.Single-phase series-excited motors are also called AC-DC dual-purpose series-excited motors, which can work with either AC or DC power.The advantages of the single-phase series motor are that it has high speed, large starting torque, small size, light weight, not easy to stall, and a wide range of applicable voltages. It can be adjusted by voltage regulation, which is simple and easy to implement.Therefore, it is widely used in power tools, such as angle grinders, hand drills, etc.The structure of the single-phase series motor is very similar to that of the DC series motor. The main difference is that the stator core of the single-phase series motor must be laminated with silicon steel sheets, while the DC magnetic poles can be laminated or laminated. make a whole structure.The speed regulation of the single-phase series motor, most of which use the method of regulating the voltage, is to change the electromotive force.The controllable phase-shift voltage regulation adopted by the voltage speed regulation method of the single-phase series excitation motor utilizes the trigger voltage of the thyristor to lag behind the input voltage to realize the phase-shift trigger of the input voltage.There are hardware and software methods in terms of implementation methods.
The voltage regulation method and the thyristor speed regulation technology are adopted, which have the characteristics of simple circuit and small size of components. It is a simple and effective method of thyristor.
(a) AC current change curve;(b) When the current is a positive half-wave, the rotation direction of the rotor (c) When the current is negative half-wave, the rotation direction of the rotorSingle-phase AC asynchronous motor - single-phase AC squirrel cage motor (brushless)When the single-phase current passes through the armature winding, the pulsed magnetic field is generated instead of the rotating magnetic field, so the single-phase asynchronous motor cannot start automatically.In order to solve the problem of starting, the asynchronous motor with single-phase AC power supply is usually made into two-phase.The main winding is directly powered by a single-phase power supply; the auxiliary winding is 90° spatially different from the main winding (electrical angle, equal to the mechanical angle divided by the number of pole pairs of the motor).The secondary winding is connected in series with a capacitor or resistance and then connected to a single-phase AC power supply, so that the current passing through it has a certain phase difference with the current in the main winding.Make the resultant magnetic field an elliptical rotating magnetic field, possibly even close to a circular rotating magnetic field.The electric motor thus obtains a starting torque.The motor using the resistance phase separation method is cheap. For example, the secondary winding can be wound with a thinner wire, but the phase separation effect is poor, and the resistance consumes energy.After this kind of motor starts and reaches a certain speed, the secondary winding is usually automatically cut off by a centrifugal switch mounted on the motor shaft to reduce the loss on the resistance and improve the operation efficiency.Generally used in occasions with low starting torque requirements, such as small lathes, small refrigerators, etc., the disadvantage is that the speed cannot be adjusted.The use of capacitor phase separation has a better effect. It is possible to make the synthetic magnetic field of the motor close to the circular rotating magnetic field at a certain operating point of the motor, so as to obtain better working characteristics.In order to make the split-phase asynchronous motor obtain better starting performance or better operating characteristics or both, the required capacitance (value) is different and can be divided into three types.Stepper Motors - Open Loop Stepper Motors(Open-loop) Stepper motors are open-loop control motors that convert electrical pulse signals into angular displacements, and are widely used.In the case of non-overload, the speed and stop position of the motor only depend on the frequency and number of pulses of the pulse signal, and are not affected by the load change. When the stepper driver receives a pulse signal, it drives the stepper motor to rotate. A fixed angle, called "step angle", the rotation of which runs step by step at a fixed angle.The angular displacement can be controlled by controlling the number of pulses, so as to achieve the purpose of accurate positioning; at the same time, the speed and acceleration of the motor rotation can be controlled by controlling the pulse frequency, so as to achieve the purpose of speed regulation. (video portal)Stepper motor is a kind of induction motor. Its working principle is to use electronic circuit, that is, driver, to convert direct current into multi-phase sequential control current of time-sharing power supply.Although a stepper motor is powered by DC current, it cannot be understood as a DC motor. A DC motor is a power motor that converts DC electrical energy into mechanical energy, while a stepper motor is an open-loop control motor that converts electrical pulse signals into angular displacement.Stepper Motor - Stepper Servo ComparisonNote that the stepper motor is used in low-speed occasions--the speed per minute does not exceed 1000r/min, and the best working range is 150~500r/min, (closed-loop step can reach 1500).The 2-phase stepping motor is prone to low-speed resonance at 60~70r/min, resulting in vibration and noise, which needs to be avoided by changing the reduction ratio, increasing the number of subdivisions, and adding magnetic dampers.Precautions for subdivision accuracy, when the subdivision level is greater than 4, the accuracy of the step angle cannot be guaranteed, and the accuracy requirements are high. It is best to use a stepper motor with more phases (ie, a smaller step angle) or a closed-loop stepping ,servo motor.(Open loop) Stepper motors are 7 different from servo motors:A control accuracy - the control accuracy of the servo motor can be set according to the encoder, and the accuracy is higher;B low frequency characteristics - stepper motors are easy to vibrate at low frequencies, but servo motors do not;C Torque frequency characteristics - the torque of the stepping motor decreases as the speed increases, so its maximum working speed is generally less than 1000r/min, and the servo motor can output the rated torque within the rated speed (generally 3000r/min), and the rated speed is within the rated speed. The above is constant power output, and the maximum speed can reach 5000 r/min;D overload capacity - the stepper motor cannot be overloaded, and the maximum torque of the servo motor can be overloaded by 3 times;E running performance - the stepper motor is open-loop control, and the servo motor is closed-loop control;F speed response - the starting time of the stepper motor is 0.15~0.5s, the servo motor is 0.05~0.1, and the fastest can reach the rated 3000r/min in 0.01s;G efficiency index - the efficiency of stepper motor is about 60%, and the efficiency of servo motor is about 80%;In actual use, it will be found that servo motors are expensive and much more expensive, so synchronous motors are more widely used, especially in synchronous belt drives, flat belt conveyors and other occasions where positioning accuracy is not very high.Stepper Motor - Closed Loop Stepper MotorClosed-loop stepper motor: In addition to the open-loop stepper motor, there is also a stepper motor that adds an encoder at the end of the motor to achieve closed-loop control.The closed-loop control of the stepper motor uses position feedback and (or) speed feedback to determine the phase transition suitable for the rotor position, which can greatly improve the performance of the stepper motor.Servo system with no out-of-step phenomenon. Advantages of closed-loop stepper motors:1. High-speed response. Compared with the servo motor, the closed-loop stepping has a very strong follow-up to the positioning command, so the positioning time is very short. In applications with frequent starts and stops, the positioning time can be significantly shortened.2. Produces more torque than ordinary servo. Make up for the lack of step-out and low-speed vibration of ordinary stepping systems.3. High torque can also be generated under 100% load, no out-of-step operation, and there is no need to consider torque loss and other issues like ordinary stepping systems.4. Using closed-loop drive, the efficiency can be increased to 7.8 times, the output power can be increased to 3.3 times, and the speed can be increased to 3.6 times.Higher running speed, more stable and smoother speed can be obtained than open-loop control.5. When the stepping motor stops, it will be completely still, and there is no micro-vibration phenomenon of ordinary servo.
Where low-cost, high-precision positioning is required, it can replace the application of general-purpose servo systems.
Stepper Motor - Comparison of Stepper Closed Loop ServoThe closed-loop stepper motor automatically adjusts the winding current according to the load size. The heat generation and vibration are smaller than that of the open-loop stepper. The encoder feedback is provided, so the accuracy is higher than that of the ordinary stepper motor. The motor response is slower than the open-loop stepper and faster than the servo motor. There is a position error in the command, and the error will gradually decrease a few milliseconds after the command stops.The high-speed torque is larger than the open-loop step, and it is commonly used in 0-1500rpm occasions.Summary: The closed-loop stepping motor has the characteristics of low cost, high efficiency, no jitter, no stop micro-vibration, high rigidity, no tuning, high speed, high dynamic response, etc. It is a cost-effective alternative to high-cost servo systems and low-end open-loop stepping systems. highest solutionServo motor - ordinary servo motorServo motor (servo motor) is also called executive motor, which can control the speed and position accuracy very accurately, and can convert the voltage signal into torque and speed to drive the control object.Different from the principle structure of the stepper motor, the servo motor puts the control circuit outside the motor, and the motor part inside is a standard DC motor or an AC induction motor.The servo motor is positioned by pulses. When the servo motor receives 1 pulse, it will rotate the angle corresponding to 1 pulse.Each time the motor rotates by an angle, the encoder will send out a corresponding number of feedback pulses, and the feedback pulses and the pulses received by the servo driver form a closed-loop control, so that the servo driver can precisely control the rotation of the motor to achieve precise positioning.Servo motor control: Generally, industrial servo motors are controlled by three loops, namely current loop, speed loop and position loop, which can feedback the angular acceleration, angular velocity and rotational position of the motor respectively.The chip controls the drive current of each phase of the motor through the feedback of the three, so that the speed and position of the motor can be accurately run as scheduled. AC servo has the characteristics of constant torque under rated speed. Common 200W, 400W low and medium inertia AC servo rated speed is 3000rpm, the maximum speed is 5000rpm, and the speed is high.The torque is proportional to the current, and it can work in torque mode, such as locking screws, crimping terminals and other occasions that require constant torque.The AC servo has very little noise and vibration and low heat generation.The motor inertia rotor inertia is small under the same volume, and the 400W servo inertia is only equivalent to the rotor inertia of the 57 base 2NM stepper motor.The servo has a short-time overload capability, and the motor overload multiple during acceleration and deceleration should be considered when selecting a model.The servo adopts closed-loop control, and there is a position tracking error like closed-loop stepping.Servo needs debugging before it can be used.When the original torque of stepper and servo motors is not enough, it is often necessary to work with a reducer, and a reduction gear set or a planetary reducer can be used.Sero motor - steering gearServo, commonly known by the Chinese, is a type of DC servo motor, which was first used for small model aircraft, and is now used for small robot joints.From the structural analysis, the steering gear includes a small DC motor, plus sensors, control chips, and reduction gear sets, which are installed in an integrated housing.The rotation angle can be controlled by an input signal (usually a PWM signal, but also a digital signal).Because it is a simplified version, the original three-loop control of the servo motor is simplified into one loop, that is, only the position loop is detected.The cheap solution is a potentiometer, which is detected by resistance, and the advanced solution is to use a hall sensor, or a grating encoder.Generally, the price of the steering gear is low and the structure is compact, but the precision is very low, and the position stabilization ability is poor, which can meet many low-end needs.With the boom of consumer-grade small robots in the past two years, small and lightweight servos have suddenly become the most suitable joint components.However, the performance requirements of robot joints are much higher than that of rudders, and as a commercial product, the quality requirements of rudders are much higher than those of DIY players.
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