Brushless, synchronous and asynchronous, servo stepper, this article summarizes the types/advantages/disadvantages/applications of various motors

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Brushless, synchronous and asynchronous, servo stepper, this article summarizes the types/advantages/disadvantages/applications of various motors [Copy link]

 

What is the difference between a brushless motor and a brushed motor? What is the difference between a synchronous motor and an asynchronous motor? Are all servo motors AC motors? Are all servo motors synchronous motors? Is a stepper motor a DC motor or an AC motor? Is a steering gear a servo motor? And so on...

Many experienced drivers may not be able to fully explain these issues. For these issues, you can refer to the following article to learn about the classification, usage, advantages and disadvantages of motors.

DC Motor - Brushed Motor

Anyone who has studied high school physics knows that in order to study the forces on a current-carrying conductor in a magnetic field, we have trained our left hands to have a broken palm, which is exactly 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 rotate, the direction of the current needs to be constantly changed, otherwise the rotor can only rotate half a circle, which is like a bicycle pedal. Therefore, a DC motor needs a commutator.

In a broad sense, DC motors include brushed motors and brushless motors.

A brushed motor is also called a DC motor or a carbon brush motor. The DC motor we often refer to is a brushed DC motor. It uses mechanical commutation. The external magnetic poles are stationary, while the internal coil (armature) moves. The commutator and the rotor coil rotate together, and the brushes and magnets are not stationary, so the commutator and the brushes rub against each other to complete the switching of the current direction.

Disadvantages of brushed motors:

1. The sparks generated by mechanical commutation cause friction between the commutator and the brush, electromagnetic interference, loud noise and short life.

2. Poor reliability, frequent failures, and frequent maintenance required.

3. Due to the existence of the commutator, the rotor inertia is limited, the maximum speed is limited, and the dynamic performance is affected.

Given that it has so many disadvantages, why is it still widely used? Because it has high torque, simple structure, easy maintenance (i.e. replacing carbon brushes), and is cheap.

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The original poster explained very clearly "What is the difference between a brushless motor and a brushed motor? What is the difference between a synchronous motor and an asynchronous motor? Are all servo motors AC motors? Are all servo motors synchronous motors? Is a stepper motor a DC motor or an AC motor? Is a steering gear a servo motor, etc."

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DC Motor - Brushless Motor

Brushless motors are also called DC variable frequency motors (BLDC) in some fields. They use electronic commutation (Hall sensor), the coil (armature) is stationary and the magnetic pole is moving. At this time, the permanent magnet can be outside the coil or inside the coil, so there are outer rotor brushless motors and inner rotor brushless motors.

The construction of brushless motors is the same as that of permanent magnet synchronous motors.

However, a single brushless motor is not a complete power system. The brushless motor must be controlled by a brushless controller, also known as an electronic regulator, to achieve continuous operation.

What really determines its performance is the brushless electronic speed controller (also known as ESC).

Generally, there are two types of driving currents for brushless motors, one is square wave and the other is sine wave.

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The following content comes from Zhihu author Pan Yunzhe's understanding of brushless DC and permanent magnet synchronous motors. Lao Yuge thinks it's good, so I share it with you:

I have seen too many misunderstandings about brushless DC and permanent magnet synchronous motors. Today, I will put the final nail in the coffin of this issue in the Chinese world. If anyone argues about what BLDC and PMSM are and what the difference is, please follow my advice. It is guaranteed to be accurate.

Brushless DC motor, called BLDC motor (brushless direct current motor) in English

Permanent magnet synchronous motor, called PMSM in English

The origin of the term brushless DC motor

First, let's talk about brushless DC motors. Why is it called brushless DC? We have to start with brushless motors. Historically, and even now, some motors use brushes to commutate, which is called brushless DC motors.

For the motor to rotate, the motor winding must pass through an alternating current, and the stator's magnetic field must always be one step ahead of the rotor's magnetic field. If it is not ahead, the stator and rotor magnetic fields are aligned, the rotor cannot rotate, and no torque is generated. So if you want to save trouble on power supply and use direct current directly, you have to have a mechanical commutator. ( Why direct current? Because battery power is direct current, so brushed motors were once widely used in power tools. More importantly, remember that before the invention of alternating current, the power grid transmitted electricity through direct current? At that time, in the early 19th century, most people used this kind of brushed motor to generate electricity or start ) This brush is a mechanical commutator, which can ensure that the current passing through the motor winding is always alternating.

But brushes have many disgusting problems. First of all, modern brushes are generally made of graphite, which is a bit like pencils. Small dust will appear when grinding, and they may even be completely worn away and become unusable. This dust not only limits the application scenarios of the motor to be dust-resistant, but also severely limits the life of the motor. Moreover, if something is rubbing against something, it will inevitably generate a lot of heat at a high speed, so the speed cannot be high. It can be seen that the existence of brushed motors is mainly to solve the problem of direct current power supply. If you have AC power supply, who would bother to use this brushed motor that is complicated to manufacture and has a short lifespan? (Of course, there are some specific scenarios where brushes are used not only because of DC, but this is off-topic. Such scenarios are rare and can be ignored.)

Later, everyone knew that due to the development of electronic components, the price of electronic components became very low, and with the popularization of super-strong permanent magnets (that is, neodymium iron boron rare earth permanent magnets), DC brushless motors came into being and became popular. Why do we emphasize DC and brushless? Because its purpose is to replace (DC) brushed motors. My power supply is also DC, but I can achieve motor commutation with electronic components, and electronic components are getting cheaper and cheaper, so why should I use brushes? Therefore, brushless DC motors gradually replaced brushed motors. Even though brushed motors are becoming less and less, the name of brushless DC, which once emphasized its identity, has been passed down.

The origin of the name of permanent magnet synchronous motor

When talking about permanent magnet synchronous motors, we cannot talk about permanent magnet synchronous motors first. We must first talk about induction motors, also known as asynchronous motors.

Induction motor, also known as asynchronous motor, was invented by Nicholas Tesla in 1887. In fact, the working principle of induction motor is very complicated. I believe that students who have studied motor courses in electrical engineering can understand it, so I will not explain the induction motor in detail. There are two important properties of induction motor. First, induction motor has no excitation part, that is: the magnetic field of its rotor is not generated by connecting to an external power supply or permanent magnet, but by the induced current of the stator magnetic field; second, asynchronous motor, as the name implies, is asynchronous, that is, the rotation speed of the stator magnetic field is inconsistent with the rotation speed of the rotor, and there is a speed difference. The greater the speed difference, the greater the torque of the induction motor.

Why is the induction motor widely used in industry? Because it is so convenient to use. In a normal factory, the power grid will have three-phase electricity. Once the three live wires of the power grid are connected to the induction motor, the motor can rotate, and the speed can be automatically adjusted according to the load size. No electrical control circuit is required. In addition , the induction motor is tough, stable, and durable. As long as the bearings are not damaged and the insulation is not a problem, it is easy to use the induction motor for tens of thousands of hours. For most occasions where it is enough to rotate, it is perfect. But on the other hand, because it can be used as long as it is connected to electricity, the speed and torque are completely dependent on the load, and the controllability is very poor. If you want the motor to rotate to a fixed position or follow a certain trajectory, it is even more impossible. (Of course, I mean when it is only connected to industrial frequency AC power. If the input of electricity is controlled by yourself, it is actually still possible, such as the induction motor in Tesla electric cars)

What should we do for some occasions with high requirements? Specifically, I hope to control the speed, torque and position of the motor. What should I do? Therefore, the synchronous motor appeared. Why is it called a synchronous motor? Because the rotor has an excitation source, the magnetic field of the rotor and the position of the rotor are fixed. As long as the stator magnetic field is ahead of the rotor magnetic field a little, the rotor will rotate. The stator magnetic field is always ahead of the rotor magnetic field, no more and no less, just a little bit. Their relative positions remain unchanged, and the magnetic fields of the rotor and stator run synchronously in the same cycle, so it is called a synchronous motor. In this way, as long as we control the magnetic field of the stator, we can control it however we want.

As I just said, the excitation of the rotor magnetic field, that is, the source of the rotor magnetic field, can be achieved by powering on like an electromagnet, or it can be achieved by permanent magnets. Then the one achieved by permanent magnets is called a permanent magnet synchronous motor. Why use permanent magnets? The biggest reason is of course because of convenience. Think about it, if something has to rotate and provide a magnetic field that is constant relative to itself (that is, relative to the rotor itself), what other methods are there besides permanent magnets? Moreover, permanent magnets represented by neodymium iron boron have a high energy density and can achieve higher torque. If it is to be achieved by powering on, even if it can be achieved, the low energy density and the large amount of wasted Joule heat generated by powering on are also a problem.

Therefore, the key point of the permanent magnet synchronous motor is the use of permanent magnets, and the difference from the induction motor (that is, the asynchronous motor) is to emphasize synchronization. Why do we emphasize synchronization? Because asynchronous motors are popular earlier than synchronous motors. Asynchronous motors can be used by plugging in AC power (where does the AC power come from? That's right, it depends on the reverse output of the asynchronous motor!), while synchronous motors still rely on chips and other electronic components to achieve synchronization. Where did people get chips in 18xx?

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What is the difference between a brushless DC motor and a permanent magnet synchronous motor?

In fact, from the most basic definition, it is difficult to discuss brushless DC motors and permanent magnet synchronous motors together. Brushless DC emphasizes the power supply and electronic commutation of the motor, while permanent magnet synchronous emphasizes synchronization and permanent magnet excitation. One talks about the power supply, and the other talks about the inherent properties of the motor itself.

Just like you ask, what is the difference between a black car and an SUV? Isn't it a bit like discussing whether a white horse is a horse?

Really?

Reality is often more complicated than imagined. In our actual industrial world, there are some unwritten rules.

We just said that the brushed DC motor uses brushes for phase commutation, and there are only two possibilities for one contact and brush of the commutator on the motor rotor, that is, contact or non-contact, so there is only a difference between 0 and 1. Then the two ends of the motor winding will be in an on-off square wave state.

The brushless DC motor is designed to replace the brushed motor, so naturally, this feature is also retained. The controller of the brushless DC motor is simply the difference between 0 and 1, just turn it on and off. In order to maximize the characteristics of the square wave controller, the motor rotor will be designed with a square wave magnetic field, so naturally magnets with equal radial distances will be used. That is, magnets of equal thickness. Then the motor's back electromotive force will tend to be a trapezoidal wave. (Of course, a perfect square wave magnetic field and trapezoidal wave back electromotive force are impossible. In fact, the magnetic field will be distorted to a certain extent. Moreover, having magnets of equal thickness does not necessarily mean a square wave magnetic field. The shape of the magnetic field is very complex and is also affected by other design factors.)

Similarly, permanent magnet synchronous motors are generally distinguished from asynchronous motors. Asynchronous motors are driven by three-phase alternating current, which is a relatively perfect sine wave. So we usually drive permanent magnet synchronous motors with sine waves. In order to maximize the utilization efficiency of sine waves, the motor magnetic field will also be designed as a sine wave magnetic field, and the back electromotive force will also be sinusoidal. Therefore, you will see that some PMSM magnets use petal shapes of unequal thickness, the purpose of which is to produce a good sinusoidal magnetic field.

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What is the difference between motors with square wave and sinusoidal magnetic fields?

Motors with sinusoidal magnetic fields are more suitable for servo control. The sine function is so wonderful, it is a panacea in differential equations, and analysis and processing are simple. When those who work on control see the sine wave signal, they will wake up with a smile in their sleep.

Correspondingly, square wave motors are not as sinusoidal as motors for servo control, because there are many harmonic components in the Fourier transform of square waves, and harmonics are unwanted in many scenarios. Of course, square waves are not incapable of servo control, and they can be used in scenarios with low requirements (and most application scenarios do not have high requirements).

Sine wave motors can really achieve a high degree of sinusoidality, with a total harmonic distortion (THD) of less than 1%. Signal analysis may think that 1% THD is nothing, but for something as elusive as a magnetic field, the accuracy is already very high. However, it is difficult for square wave motors to achieve true square waves, because magnets will have magnetic leakage. Once the magnetic leakage occurs, the square wave will disappear, and arcs will form at the two sharp corners of the square wave, like the shape of toast.

Of course, square wave motors are not without advantages. With the same current, square wave motors will theoretically have greater torque . After all, those harmonic magnetic fields can also participate in generating torque.

Of course, the so-called BLDC is a square wave and PMSM is a sine wave, which is just an unwritten rule. Whether to comply or not depends entirely on the manufacturer's mood. In the academic community, if these two words are used, they are basically defined literally. After all, if I insist on holding a square wave permanent magnet motor and say it is a PMSM, you can't say I'm wrong, because it is indeed a synchronous motor and it does have permanent magnets. And if I hold a sine wave motor and say it is a BLDC, you can't say I'm wrong, as long as I use a DC power supply + inverter drive motor.

The above content comes from Zhihu author Pan Yunzhe's understanding of DC brushless and permanent magnet synchronous motors

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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, affordable, and mass-produced, it is close to the carbon brush motor, so it has been widely used in recent years.

The price of the outer rotor three-phase is already close to that of the inner rotor.

Well, you can guess that the disadvantages of brushed motors are the advantages of brushless motors.

It has the advantages of high efficiency, low energy consumption, low noise, extra long life, high reliability, servo control, stepless frequency conversion speed regulation (can reach very high speed), etc. It is much smaller than brushed DC motors, simpler to control than asynchronous AC motors, has large starting torque and strong overload capacity. As for its disadvantages... it is more expensive and difficult to maintain than brushed motors.

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DC motor - speed regulation principle

Speed regulation of DC motor: The so-called speed regulation means obtaining the required torque by adjusting the motor speed.

The speed of a DC (brushless) motor can be adjusted by adjusting the voltage, connecting a resistor in series, or changing the excitation. However, adjusting the voltage is the most convenient and commonly used method. Currently, PWM speed regulation is mainly used. PWM actually achieves DC voltage regulation through high-speed switching. Within one cycle, the longer the on time, the higher the average voltage, and the longer the off time, the lower the average voltage. It is very convenient to adjust. As long as the switching speed is fast enough, the harmonics in the power grid will be less and the current will be more continuous.

However, the brushes and commutator wear out over a long period of time. At the same time, there is a huge current change during commutation, which easily generates sparks. The commutator and brushes limit the capacity and speed of the DC motor, causing the speed regulation of the DC motor to encounter a bottleneck.

For brushless DC motors, only the input voltage is controlled during speed regulation. However, the motor's self-controlled variable frequency speed regulation system (the brushless DC motor itself has a rotor position detector and other rotor position signal acquisition devices, and the rotor position signal of this device is used to control the switching timing of the variable voltage variable frequency speed regulation device) automatically controls the frequency according to the voltage conversion. It is almost the same as a DC (brushless) motor in use, which is very convenient.

Since the rotor uses permanent magnets, no special excitation winding is required. Under the condition of the same capacity, the motor is smaller in size, lighter in weight, more efficient, more compact in structure, more reliable in operation, and has better dynamic performance. It has been widely used in the driving of electric vehicles and other aspects.

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Three-phase AC motor - asynchronous motor

AC motors are divided into synchronous motors and asynchronous motors. Synchronous motors are mostly used as generators, while asynchronous motors are mostly used as motors.

The outer shell of the motor is the stator, which has three-phase symmetrical AC windings. As the three-phase electricity changes in sequence, a rotating synthetic magnetic field is formed, and the rotation speed of the magnetic field is the synchronous speed.

The synchronous speed n=60f/p, f is the frequency, p is the number of pole pairs. For example, for a 2-pole motor (i.e., the number of pole pairs is 1 pair) connected to the national power grid at 50Hz, the speed n=60*50/1=3000r/min.

Similarly, the synchronous speeds of 4-pole, 6-pole, and 8-pole motors are 1500, 1000, and 750.

The asynchronous motor has a simple structure and the rotor is a closed coil, such as a squirrel cage type.

The rotor coil will cut the rotating magnetic field, generate induced electromotive force, and then generate induced current, and finally generate a rotating magnetic field. In this way, the rotor becomes an electromagnet and will rotate following the stator magnetic field. Therefore, the rotor speed must be less than the rotating magnetic field of the stator, so that it can cut the magnetic lines of force.

That is, the asynchronous speed of the rotor is less than the synchronous speed, and there is a speed difference between the rotor and stator magnetic fields, 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+, and 700+.

The speed of an asynchronous motor is high when it is unloaded, and decreases when it is loaded.

Asynchronous motors have a simple structure, are easy to maintain, operate reliably and are cheap, so they are widely used.

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Three-phase AC motor - synchronous motor

Synchronous motor:

If the rotor speed is equal to the stator magnetic field rotation speed, 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, there is no need to cut the magnetic lines of force to rotate, and the rotation speed is the same as the magnetic field rotation speed, forming a synchronous motor.

The rotor structure of synchronous motor is more complicated than that of asynchronous motor, and its price is higher. It is not as widely used as asynchronous motor in production and life. It is mainly used as generator. Now thermal power plants, hydropower plants, steam turbines and water turbines are basically synchronous motors.

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Three-phase AC motor - speed regulation of asynchronous motor

Speed regulation of asynchronous motors: In theory, the speed of asynchronous motors can be regulated by controlling the frequency and voltage of the alternating current, or the resistance of the rotor and the distribution of the magnetic poles of the motor, but in practice, stepless speed regulation is achieved by adjusting the frequency and voltage.

Since the voltage and speed regulation range is not large, it can generally only be used in situations where the speed regulation requirements are not high, and is not widely used.

Variable frequency speed regulation: When it comes to variable frequency, everyone may have heard of it.

The full name of variable frequency speed regulation is variable voltage variable frequency speed regulation (VVVF), which means changing the voltage when changing the frequency, so that the speed regulation range of the asynchronous motor is large enough.

Frequency converters can be divided into two major categories: AC-AC frequency conversion and AC-DC-AC frequency conversion.

AC-AC frequency conversion: converts AC power directly into AC power of another frequency through power electronic devices. The maximum output frequency cannot exceed half of the input frequency, so it is generally only used in low-speed, large-capacity systems, and can eliminate the need for large gear reducers.

The AC-DC-AC inverter first rectifies AC power into DC, and then converts it into AC with controllable frequency and voltage through an inverter. Combined with PWM technology, this inverter can achieve a wide range of voltage and frequency conversion.

For electric vehicles, asynchronous motors are durable, have strong overload capacity, and the control algorithms are so mature that they can be used.

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Three-phase AC motor - Speed regulation of synchronous motor

Speed regulation of synchronous motors: Synchronous motors have no slip rate. When the structure is determined, the control voltage cannot change the speed. Therefore, before the advent of frequency converters, synchronous motors were completely unable to regulate speed.

The emergence of frequency converters has given AC synchronous motors a huge speed regulation range. Because their rotors also have independent excitation (permanent magnets or electric excitation), their speed regulation range is wider than that of asynchronous motors, giving synchronous motors new life.

The synchronous motor variable voltage and frequency speed regulation system can be divided into externally controlled variable frequency speed regulation and automatic controlled variable frequency speed regulation.

For the externally controlled variable frequency speed regulation, similar to the variable frequency speed regulation of asynchronous motors, the control can also be achieved by using control methods such as SVPWM 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 has had many names during its development, such as commutatorless motor; when permanent magnets are used and three-phase sinusoidal waves are input, it can be called a sinusoidal wave permanent magnet synchronous motor; and if square waves are input, it can be called a trapezoidal wave permanent magnet synchronous motor. Yes, this is similar to the brushless DC machine (BLDM) mentioned earlier. Do you feel like you have gone around in a big circle and have come back? But now you must have a deeper understanding of variable frequency and speed change. Therefore, the brushless DC motor uses DC input, but uses the frequency conversion technology of the synchronous motor (the structure is the same as that of the permanent magnet synchronous motor). The DC brushless motor is used in Model 3.

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Single-phase AC asynchronous motor - single-phase AC series motor (brush)

Single-phase AC series-excited motor, commonly known as series-excited motor or universal motor (Universal Motor is called abroad because it is universal for AC and DC), works with the armature winding and the excitation winding connected in series.

A single-phase series-excited motor is also called an AC/DC dual-purpose series-excited motor. It can work using both AC and DC power supplies.

The advantages of a single-phase series-excited motor are that it has a high speed, a large starting torque, a small size, a light weight, is not prone to stalling, has a wide applicable voltage range, and can be adjusted in speed by voltage regulation, which is simple and easy to implement.

Therefore, it is widely used in power tools, such as angle grinders, electric drills, etc.

The structure of a single-phase series-excited motor is very similar to that of a DC series-excited motor. The main difference is that the stator core of a single-phase series-excited motor must be made of stacked silicon steel sheets, while the magnetic poles of a DC motor can be made of either stacked or integral structures.

The speed regulation of single-phase series-excited motors is mostly done by adjusting the voltage, that is, changing the electromotive force.

The voltage speed regulation method of the single-phase series-excited motor adopts controllable phase-shift voltage regulation, which uses the trigger voltage of the thyristor to lag behind the input voltage to achieve phase-shift triggering of the input voltage.

There are two implementation methods: hardware and software.

It adopts the voltage adjustment method and the thyristor speed regulation technology, which has the characteristics of simple circuit and small component size. It is a simple and effective method of thyristor.

(a) Alternating current variation curve;

(b) When the current is a positive half-wave, the direction of rotation of the rotor

(c) When the current is in the negative half-wave, the direction of rotation of the rotor

Single-phase AC asynchronous motor - Single-phase AC squirrel cage motor (brushless)

When single-phase current passes through the armature winding, a pulsating magnetic field is generated instead of a rotating magnetic field, so the single-phase asynchronous motor cannot start itself.

In order to solve the starting problem, asynchronous motors powered by single-phase AC are often actually made into two-phase ones.

The main winding is powered directly by a single-phase power supply; the auxiliary winding is 90° (electrical angle, equal to the mechanical angle divided by the number of motor pole pairs) spatially different from the main winding.

The secondary winding is connected in series with a capacitor or resistor 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.

The synthetic magnetic field is made into an elliptical rotating magnetic field, which may even be close to a circular rotating magnetic field.

The motor thus obtains starting torque.

Electric motors that use the resistor phase-splitting method are inexpensive. For example, the secondary winding can be wound with a thinner wire, but the phase-splitting effect is poor and energy is consumed in the resistor.

After this type of motor starts and reaches a certain speed, the auxiliary winding is usually automatically cut off by a centrifugal switch installed on the motor shaft to reduce resistance losses and improve operating efficiency.

It is generally used in situations where starting torque requirements are not high, such as small lathes, small refrigerators, etc. The disadvantage is that the speed cannot be adjusted.

The effect of using capacitor phase separation is better. It is possible to make the motor's synthetic magnetic field close to a circular rotating magnetic field at a certain working point of the motor, thereby obtaining better working characteristics.

In order to make the split-phase asynchronous motor obtain better starting performance or better running characteristics or both, the required capacitance (value) is different and can be divided into three types:

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Stepper Motor - Open Loop Stepper Motor

(Open-loop) stepper motors are open-loop controlled motors that convert electrical pulse signals into angular displacement and are widely used.

In the case of non-overload, the motor speed and stop position depend only on the frequency and number of pulses of the pulse signal, and are not affected by load changes. When the stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle, called the "step angle". Its rotation is run step by step at a fixed angle.

The angular displacement can be controlled by controlling the number of pulses, thereby achieving 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, thereby achieving the purpose of speed regulation. (Video Portal)

A stepper motor is an induction motor that uses an electronic circuit, the driver, to convert direct current into a multi-phase sequence-controlled current that is supplied in a time-sharing manner.

Although the stepper motor is powered by direct current, it cannot be understood as a DC motor. A DC motor is a power motor that converts direct current electrical energy into mechanical energy, while a stepper motor is an open-loop control motor that converts electrical pulse signals into angular displacement.

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Stepper Motor - Stepper Servo Comparison

Note that stepper motors are used in low-speed applications - the speed does not exceed 1000r/min, and the best working range is 150~500r/min (closed-loop stepping can reach 1500).

Two-phase stepper motors are prone to low-speed resonance at 60~70r/min, causing vibration and noise, which needs to be avoided by changing the reduction ratio, increasing the number of subdivisions, adding magnetic dampers, etc.

Note on subdivision accuracy: when the subdivision level is greater than 4, the accuracy of the step angle cannot be guaranteed and high accuracy is required. It is best to use a stepper motor with more phases (i.e., smaller step angle) or a closed-loop stepper or servo motor.

7 differences between (open loop) stepper motors and servo motors:

A Control accuracy - the servo motor control accuracy can be set according to the encoder, with higher accuracy;

B Low frequency characteristics - stepper motors are prone to vibration at low frequencies, but servo motors are not;

C Torque-frequency characteristics - the torque of a stepper motor decreases as the speed increases, so its maximum operating speed is generally less than 1000r/min. A servo motor can output rated torque within the rated speed (generally 3000r/min), and has a constant power output above the rated speed, with a maximum speed of up to 5000 r/min.

D Overload capacity - stepper motors cannot be overloaded, and servo motors can be overloaded 3 times their maximum torque;

E. Operation performance: Stepper motor is open-loop controlled, servo motor is closed-loop controlled;

F Speed response: The starting time of stepper motor is 0.15~0.5s, and that of servo motor is 0.05~0.1s. The fastest speed can reach the rated 3000r/min in 0.01s.

G efficiency index - the efficiency of stepper motor is about 60%, and that of servo motor is about 80%;

In actual use, you will find that servo motors are expensive, much more expensive, so synchronous motors are more widely used, especially in synchronous belt drives and flat belt conveyors where positioning accuracy requirements are not very high, stepper motors are often used.

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Stepper Motor - Closed Loop Stepper Motor

Closed-loop stepper motor: In addition to open-loop stepper motors, there are also stepper motors with encoders added to the tail of the motor to achieve closed-loop control.

The closed-loop control of a stepper motor uses position feedback and/or speed feedback to determine the phase shift that corresponds to the rotor position, which can greatly improve the performance of the stepper motor.

A servo system without step-out phenomenon.

Advantages of closed-loop stepper motors:

1. High-speed response: Compared with servo motors, closed-loop stepping has a very strong followability to positioning instructions, so the positioning time is very short. In applications with frequent start and stop, the positioning time can be significantly shortened.

2. It produces greater torque than ordinary servos, making up for the lack of step loss and low-speed vibration in ordinary stepping systems.

3. It can generate high torque even at 100% load without losing steps. There is no need to consider torque loss like ordinary stepping systems.

4. By applying 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.

It can achieve higher operating speed, more stable and smoother rotation speed than open-loop control.

5. When the stepper motor stops, it will be completely still, without the micro-vibration phenomenon of ordinary servo.

It can replace general servo systems in applications where low-cost, high-precision positioning is required.

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Stepper Motor - Stepper Closed-Loop Servo Comparison

The closed-loop stepper motor automatically adjusts the winding current according to the load size. The heat and vibration are less than the open-loop stepper. There is encoder feedback so the accuracy is higher than the ordinary stepper motor. The motor response is slower than the open-loop stepper but faster than the servo motor. There is a position error during operation, and the error will gradually decrease a few milliseconds after the command stops.

The high-speed torque is greater than that of the open-loop stepper and is commonly used in 0-1500rpm applications.

Summary: Closed-loop stepper motors have the characteristics of low cost, high efficiency, no jitter, no stop micro-vibration, high rigidity, no tuning, high speed, and high dynamic response. They are the most cost-effective solution to replace high-cost servo systems and low-end open-loop stepper systems.

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Servo motor - ordinary servo motor

Servo motor, also called actuator motor, can control speed and position with high accuracy, and can convert voltage signals into torque and speed to drive the controlled object.

Different from the principle structure of stepper motors, servo motors place the control circuit outside the motor, so the motor part inside is a standard DC motor or AC induction motor.

The servo motor relies on pulses for positioning. When the servo motor receives one pulse, it will rotate the angle corresponding to the pulse.

Every time the motor rotates an angle, the encoder will send a corresponding number of feedback pulses. The feedback pulses and the pulses received by the servo driver form a closed-loop control, so that the servo driver can accurately control the rotation of the motor and achieve precise positioning.

Servo motor control: Generally, servo motors used in industry are controlled by three loops, namely current loop, speed loop and position loop, which can respectively feedback the angular acceleration, angular velocity and rotational position of the motor.

The chip controls the driving current of each phase of the motor through feedback from the three, ensuring that the speed and position of the motor run accurately as planned.

AC servo has the characteristic of constant torque at rated speed. Common 200W and 400W low and medium inertia AC servos have a rated speed of 3000rpm and a maximum speed of 5000rpm, which is high.

The torque is proportional to the current and can work in torque mode, such as locking screws, crimping terminals, etc. where constant torque is required.

The AC servo has very little operating noise and vibration, and generates little heat.

The motor inertia and rotor inertia are small under the same volume. The 400W servo inertia is only equivalent to the rotor inertia of a 57 base 2NM stepper motor.

The servo has a short-term overload capacity, and the motor overload multiple during acceleration and deceleration needs to be considered when selecting the servo.

The servo adopts closed-loop control, and there is position tracking error just like closed-loop stepping.

The servo needs to be debugged before it can be used.

When the original torque of stepper and servo motors is not enough, they often need to work with a reducer. A reduction gear set or a planetary reducer can be used.

电机知多少

Servo motor - steering gear

Servo is a common name given by Chinese people. It is a type of DC servo motor. It was first used in small model airplanes and is now used in small robot joints.

From a structural perspective, the servo consists of a small DC motor, a sensor, a control chip, and a reduction gear set, which are installed in an integrated housing.

The rotation angle can be controlled by an input signal (usually a PWM signal, but sometimes a digital signal).

Since it is a simplified version, the original three-loop control of the servo motor has been simplified to one loop, that is, only the position loop is detected.

A cheap solution is a potentiometer that detects through resistance, while an advanced solution uses a Hall sensor or a grating encoder.

Generally, servos are cheap and compact, but have low precision and poor position stabilization capabilities, and can only 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 for robot joints are much higher than those for rudders, and as a commercial product, the quality requirements for servos are much higher than those for DIY players.

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电机知多少

851779592 posted on 2024-6-3 09:47 The OP put "What is the difference between a brushless motor and a brushed motor? What is the difference between a synchronous motor and an asynchronous motor? Are all servo motors AC motors? ...

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