Most Commonly Used Actuator–Motor

Abstract

In energy-saving control, a large number of electric motors are used for load distribution and devices switching. A lot of rotary motions, linear motion or other forms of mechanical motion are mostly driven by motors. It can be said that motors are the most commonly used actuators in the field of electrical engineering and automation. After being connected to a suitable power supply, the motor generates rotary motion, and the linear motor produce linear motion.

In energy-saving control, a large number of electric motors are used for load distribution and devices switching. A lot of rotary motions, linear motion or other forms of mechanical motion are mostly driven by motors. It can be said that motors are the most commonly used actuators in the field of electrical engineering and automation. After being connected to a suitable power supply, the motor generates rotary motion, and the linear motor produce linear motion.

6.1 Three-Phase AC Motor

Three-phase AC motor is connected to three-phase AC to generate rotation. It is one of the most widely used motors at present. There are many types of three-phase AC motors, including squirrel-cage three-phase AC asynchronous motors and wound rotor three-phase AC asynchronous motors, three-phase AC synchronous motor, three-phase AC permanent magnet synchronous motor, variable frequency motor, etc. The appearance of the three-phase AC motor is shown in Fig. 6.1.

Fig. 6.1

Three-phase AC motor

6.1.1 Basic Principle of Three-Phase AC Asynchronous Motor

Knowing the basic knowledge of three-phase AC motors can better understand the control methods of three-phase AC motors.

At the beginning of the nineteenth century, the British scientist Faraday used a small magnetic rod to move through a closed-circuit coil, and found that there was a current in the coil. Since then, humans have discovered the phenomenon of electromagnetic induction. This phenomenon shows that mechanical energy and electrical energy can be transformed into each other. It also shows that Inspired by the principle of mutual conversion between electricity and magnetism, generators and motors finally stepped onto the stage of mankind, revealing the arrival of the electrical age of mankind. Based on this principle, the American inventor Tesla invented the AC motor at the end of the nineteenth century.

There are two such experiments in middle school physics, one is as shown in Fig. 6.2. The U-shaped magnet is rotated clockwise by hand, and the two magnetic poles, N pole and S pole, form a magnetic field that also rotates at the same time. At this time, the aluminum in the middle of the magnet, the frame also turns in the same direction, and the faster the hand turns, the faster the aluminum frame turns.

Fig. 6.2

Converting magnetic field rotation into mechanical rotation

The second experiment is shown in Fig. 6.3. UVW is three identical coils. The three coils are placed at 120° to each other. There is a rotatable aluminum frame in the middle of them. When the three coils are connected to three-phase AC, it can be seen that the aluminum frame rotates, which means that the three coils with three-phase alternating current also generate a rotating magnetic field, so the aluminum frame rotates.

Fig. 6.3

Rotation principle of three-phase AC motor

So, why do three coils with three-phase alternating current generate a rotating magnetic field? Let us start with the characteristics of the three-phase alternating current, the expressions of the three-phase alternating current are as

$${i}_{U}={I}_{m}cos\omega t$$

(6.1)

$${i}_{V}={I}_{m}\text{cos}(\omega t-{120}^{0})$$

(6.2)

$${i}_{w}={I}_{m}\text{cos}(\omega t-{240}^{0})$$

(6.3)

where I_m represents the maximum peak value of the current, ωt represents the electrical angle changing with time. i_U represents the current flowing through the U-phase coil, and i_V represents the current flowing through the V-phase coil, i_W represents the current flowing through the W-phase coil. Equations (6.1), (6.2) and (6.3) show that the UVW three-phase current has a time difference of 120° Angle, the waveform diagram of three-phase current is shown in 6–4.

Assume that the head end of the U-phase coil is U₁, the tail end of the U-phase coil is U₂. The first end of the V-phase coil is V₁, the end of the V-phase coil is V₂. The first end of the W-phase coil is W₁, and the end of the W-phase coil is W₂. When the U-phase current i_U is positive, it means that it flows in from the head end U₁ and flows out from the end U₂; when the U-phase current i_U is negative, it means it flows in from U₂ and flows out from U₁. Similarly, when the V-phase current i_V is positive, it means that it flows in from V₁ and flows out from V₂; when the V-phase current i_V is negative, it means that it flows in from V₂ and flows out from V₁. When the W-phase current i_W is positive, it means that it flows in from W₁ and flows out from W₂; when the W-phase current i_W is negative, it means it flows in from W₂ and flows out from W₁. When the current flows in, it is represented by ⊕ (similar to when we see the tail of an arrow, and the arrow is moving away from us). When the current flows out, it is represented by ⊙ (similar to the arrow we see when the arrow is coming towards us).

Taking the moment of ωt = 0° and ωt = 60° as an example, analyze the current flow direction in the three UVW coils and the change of the magnetic field caused by the current flow direction.

Draw the flow direction of the current in the three UVW coils when ωt = 0°, as shown in Fig. 6.5.

According to Fig. 6.4, when ωt = 0°, the U-phase current is positive (and the maximum value), the V-phase current is negative, and the W-phase current is negative, so in Fig. 6.5, the U coil current flows from U₁, flows out from U₂; the V coil current flows in from V₂ and flows out from V₁; the W coil current flows in from W₂ and flows out from W₁. According to the right-hand spiral rule, the composite magnetic field formed by the three-phase winding is a two-pole magnetic field, and the direction of the magnetic field is from top to bottom, with the N pole at the top and the S pole at the bottom.

Fig. 6.4

Waveform diagram of three-phase current

Fig. 6.5

Magnetic field position where ωt = 0°

When ωt = 60°, draw the flow direction of the current in the three UVW coils, as shown in Fig. 6.6.

Fig. 6.6

Magnetic field position where ωt = 60°

It can be seen from Figs. 6.5 and 6.6 that although the three UVW coils do not move, the magnetic field formed after they are supplied with alternating current is rotating counterclockwise, which is equivalent to artificially turning the U-shaped magnet in Fig. 6.2, the magnetic field formed by the rotation of N pole and one S pole (1 pair of magnetic poles). This is the principle of the rotating magnetic field of a three-phase AC motor with the number of pole pairs equal to 1.

The rotation speed n0 (also called synchronous speed) of the stator magnetic field of the three-phase AC motor is

$${n}_{0}=\frac{60\times f}{p}$$

(6.4)

where f is the frequency of the power supply of the three-phase AC motor, p is the number of pole pairs of the three-phase AC motor, and the number of pole pairs generally ranges from 1 to 5.

6.1.2 Several External and Internal Wiring Methods of Three-Phase AC Motors

1. The simplest and most common external wiring method of a three-phase AC motor is to leave 2 taps for each phase winding, the head end and the tail end, U₁, U₂, V₁, V₂, W₁, W₂, total 6 taps. Connect U₂, V₂, and W₂, 3 taps together to form a neutral point N, lead out U₁, V₁, W₁ to connect to power supply U, V, W to form a “wye (Y) connection”. As shown in Fig. 6.7.

Fig. 6.7

Wye (Y) connection

Connect U₁ and W₂, 2 taps to together, U₁ is connected to the U-phase power supply. V₁ and U₂, 2 taps are connected together, V₁ is connected to the V-phase power supply. W₁ and V₂, 2 taps are connected together, and W₁ is connected to the W-phase power supply, thus forming a “delta connection”. As shown in Fig. 6.8, “wye (Y) connection” is represented by the symbol Y, and “delta connection” is represented by the symbol △.

Fig. 6.8

Delta connection

Y-connection and △-connection correspond to different rated working voltages of motors, the rated working voltage of Y-connection is high, and the rated working voltage of △-connection is low. When working at the same voltage, the output power of the Y connection is low. Using the AC contactor to switch the two connection methods can also realize the Y-△ step-down start of the high-power motor.

The line voltage of the three-phase AC motor: it refers to the voltage between the two terminals of the three-phase power supply. Generally, the three-phase power supply voltage we refer to refers to the line voltage.

The phase voltage of the three-phase AC motor: it refers to the voltage on each phase winding. If the Y-shaped connection is adopted, the phase voltage U_U of the U-phase winding is equal to the voltage between the U terminal and the neutral point N, which is the line voltage 1/√3; If the △-shaped connection is adopted, the phase voltage U_U of the U-phase winding is equal to the line voltage.

The line current of the three-phase AC motor: it refers to the current flowing through each power line, such as the U line current I_U.

The phase current of the three-phase AC motor: it refers to the current flowing through each phase winding, such as the phase current I'_U of the U-phase winding, I_U = I'_U in the Y-shaped connection method, and I_U = I’u × √3 in the ∆-shaped connection method.

2. Multi-speed three-phase AC motors that can change the number of pole pairs have different numbers of shifting gears, and the number of terminals and wiring methods are also different.

Take the 2-speed variable speed motor as an example to illustrate the corresponding △/2Y speed change method. Each phase winding of this motor is composed of two sets of coils in series, and a terminal 2U, 2 V and 2W is drawn out at the middle connection point of the two sets of coils. In order to keep the speed direction of the motor unchanged after the pole change, the 2U and 2W leads are reversed. The ends of the three-phase windings are connected in pairs and 3 terminals 1U, 1 V and 1W are drawn out, and there are 6 terminals in total. When the motor is connected in △ shape, 1U, 1 V, and 1W are connected to the power supply U, V, W. At this time, the number of pole pairs of the motor is large and the speed is low, as shown in Fig. 6.9. When the motor is converted to 2Y connection, the poles of the motor as the logarithm becomes smaller, the motor speed increases, as shown in Fig. 6.10.

Fig. 6.9

Delta connection with large number of pole pairs

Fig. 6.10

2Y connection with small number of pole pairs

6.1.3 Calculation of Rated Torque of Three-Phase AC Motor

The rated torque T_e of the three-phase AC motor must be greater than or equal to the torque required by the load to ensure the normal operation of the equipment. The starting torque T_q of the three-phase AC motor must be greater than the starting torque required by the load to ensure the equipment. It can start normally. When the power supply frequency f is constant, the rated torque T_e, starting torque T_q and maximum torque T_M of the motor are proportional to the power supply voltage U, that is:

$${T}_{e}\propto {U}^{2}$$

(6.5)

$${T}_{q}\propto {U}^{2}$$

(6.6)

$${T}_{M}\propto {U}^{2}$$

(6.7)

For this reason, in the application of frequency converter, if the driven motor cannot start or run by dragging the load, the way of increasing the output voltage is often used to increase the starting torque or increase the running torque.

The method of calculating the rated torque T_e according to the rated power P_e and the rated speed n_e is as follows:

$${T}_{e}=9550\times \frac{{P}_{e}}{{n}_{e}}$$

(6.8)

where P is kilowatts (Kw), n_e is rotation speed per minute (rpm), and T_e is N.m.

6.1.4 Three-Phase Permanent Magnet Synchronous AC Motor

If the aluminum frame is replaced with a permanent magnet with high magnetic field strength, as shown in Fig. 6.11, it becomes a three-phase permanent magnet synchronous AC motor. Because the rotor itself has electromagnetic force with the stator magnetic field, which is no longer like an asynchronous motor that requires the relative motion of the rotor and the stator's rotating magnetic field to generate electromagnetic force. So as long as the torque of the AC motor dragging the external load does not exceed the rated value, the rotational speed of the permanent magnet rotor and the speed of the stator's rotating magnetic field will maintain a more precise synchronization. In many occasions, using this kind of motor, the speed of the motor can be precisely controlled only by changing the power supply frequency, and speed measurement and feedback can often be omitted.

Fig. 6.11

Three-phase permanent magnet synchronous AC motor

6.1.5 Three-Phase AC Synchronous Motor

If the above permanent magnet is replaced with an electromagnet formed by introducing direct current with a slip ring and a brush, as shown in Fig. 6.12. This becomes a three-phase AC synchronous motor, which is the same as a permanent magnet synchronous AC motor. Because the rotor itself is an electromagnet with fixed magnetic poles do not require relative motion between the rotor and the stator's rotating magnetic field to generate electromagnetic force. Therefore, as long as the external load torque of this synchronous AC motor does not exceed the rated value, the rotational speed of the rotor is the same as the speed of the stator's rotating magnetic field. Just stay in sync.

Fig. 6.12

Three-phase AC synchronous motor

6.1.6 Three-Phase AC Asynchronous Motor with Wound Rotor

If the aluminum frame is replaced with a winding that can be used to lead out the end of the slip ring, as shown in Fig. 6.13, this becomes a three-phase AC asynchronous motor with a wound rotor. Since the starting current of a high-power three-phase AC motor is very large, it is affected due to the limitation of the development of power electronic devices, it was difficult to start and adjust the speed of the early high-power cage type three-phase AC motor. But the three-phase wound AC asynchronous motor can be adjusted by adjusting the size of the rotor series resistance R or the series connection electromotive potential can better solve the problem of starting and speed regulation.

Fig. 6.13

Wound rotor three-phase AC asynchronous motor

6.1.7 Three-Phase Frequency Conversion Speed Regulation Motor

With the society's increasingly urgent requirements for energy saving and environmental protection, frequency converters for speed regulation of three-phase AC motors are widely used. If the motor is required to run at low speed for a long time, the current in the motor has to be reduced, that is, the torque output by the motor is reduced, which is also called reducing the load capacity of the motor. For this problem, the fan driven by the rotor of the motor can be replaced by a fan powered by an independent external power supply, so that the cooling fan can always run at high speed; the cooling area and cooling capacity of the three-phase AC motor shell can be increased. Another problem is that the starting torque of ordinary three-phase AC motors (when the slip rate is large) is small. In order to increase the starting torque, the rotor structure of the motor is redesigned to increase the starting torque. After these improvements, the ordinary three-phase AC The motor becomes a three-phase variable-frequency speed-regulating motor. The operating frequency of the specially designed variable-frequency motor is higher than that of the ordinary three-phase AC motor. The fan part of the motor is large and has an independent terminal box, and some variable frequency motors are equipped with a rotary encoder or resolver to measure the rotor speed and position, so some variable frequency motors have three terminal boxes. Three-phase variable frequency speed regulating motors are shown in Fig. 6.14.

Fig. 6.14

Three-phase variable frequency speed regulating motor

6.2 Single-Phase AC Motors

Single-phase AC motors are the most widely used in the civilian field. Almost all motors in household appliances are single-phase AC motors. In the industrial field, they are only used in some occasions that require small output power of the motor, such as cooling fans of control cabinets and control equipment.

A single-phase AC motor adopts the principle of phase separation. There are main winding AX and auxiliary winding AY on the stator. The main winding and auxiliary winding are 90° apart in space. The main and auxiliary windings are connected in parallel, and a suitable starting capacitor C is connected in series on the auxiliary winding. Make the current i_b phase of the auxiliary winding lead the current i_a phase of the main winding, as shown in Fig. 6.15, so that a rotating magnetic field is generated on the stator, and the rotor of the motor can rotate. This kind of motor is called a capacitor single-phase motor.

Fig. 6.15

Split-phase single-phase AC motor

At t = 0, i_a = 0, according to the left-hand rule, the magnetic field is up N down S. At t = 0.25, i_b = 0, according to the left-hand rule, the magnetic field is right N left S. The magnetic field rotates clockwise 90°. The analysis of other points is the same.

Another single-phase AC asynchronous motor adopts the principle of shaded poles, which divides the stator poles into two parts. The large part is the main magnetic field, and the small part (1/5–1/3) of the magnetic poles is surrounded by short-circuit copper rings, as shown in the Fig. 6.16. When the stator coil is fed with single-phase alternating current, the magnetic field in the magnetic pole part covered by the short-circuit copper ring will produce hysteresis, which is equivalent to covering the magnetic pole of this part. The magnetic field lags behind the magnetic pole of the part without the short-circuit ring Part of the magnetic field causes the magnetic field in the stator poles to produce a rotating effect. This type of motor is called a single-phase shaded pole motor.

Fig. 6.16

Shaded pole single-phase AC asynchronous motor

6.3 DC Motors

DC motors are widely used in electric vehicles, automated production lines, printing, steel rolling, papermaking, vending machines, machine tools and other fields. DC motors have magnetic poles, armatures, mechanical commutators and brushes. The magnetic poles can be composed of permanent magnets. Such a DC motor is called a permanent magnet DC motor. It can also use the field winding to form the magnetic poles. The brushes and the mechanical commutator are in sliding contact. The rotation principle of the DC motor is shown in Fig. 6.17.

Fig. 6.17

Rotation principle of DC motor

In Fig. 6.17, brush g and brush h are connected to DC power supply U, brush g and brush h are slidingly connected with commutator segment e and commutator segment f, and conductor ab and conductor cd on the armature are energized, to generate current i, according to the left-hand rule, the conductor ab under the N pole will produce a leftward force F1, the conductor cd under the S pole will produce a rightward force F2, and the armature will produce counterclockwise rotation. When the conductor ab and When the corresponding commutator piece e turns to contact with the brush h, the conductor cd and the corresponding commutator piece f turn to contact with the brush g, the current direction of the conductor ab and the conductor cd is reversed, and the conductor in contact with the brush g The current direction of cd is inward, and the current direction of the conductor ab in contact with the brush h is outward. According to the left-hand rule, the conductor cd under the N pole produces a force to the left, and the conductor cd under the S pole produces a force to the right. The armature still rotates counterclockwise, and the motor rotates continuously.

The function of the mechanical commutator in the DC motor is to keep the direction of the current in the armature coil under the magnetic pole unchanged, that is, to keep the direction of the force on the armature coil unchanged, thereby producing continuous rotational motion.

According to the relationship between the field winding and the armature winding, there are other excitation, parallel (shunt) excitation, series excitation and compound excitation DC motors, as shown in Fig. 6.18.

Fig. 6.18

Separate excitation, parallel excitation, series excitation and compound excitation

The field winding of separate excited DC motor is powered by an independent power supply, the field winding of a shunt excited DC motor is connected in parallel with the armature winding and uses the same DC power supply. The field winding of a series excited DC motor is connected in series with the armature winding. One excitation winding of compound (combined) excitation DC motor is connected in series with armature winding, one excitation winding is connected in parallel with armature winding, powered by a DC power supply.

The shape of the DC motor is shown in Fig. 6.19.

Fig. 6.19

DC motor

6.4 Brushless DC Motor

DC brushless motors are widely used in the fields of electric bicycles, instruments, household appliances, computer peripherals, small rotating machinery, cooling fans, small water pumps, etc., its principle is basically the same as that of the DC motor mentioned above, except that there are no brushes and mechanical commutators, and position sensors and power electronic devices are used for commutation, so they are also called commutator-free motors and commutator-free DC motors. Since there are no vulnerable parts such as sliding brushes, it is easy to maintain and there is no spark.

The brushless DC motor uses a position sensor to detect the position of the rotor, and uses a power electronic switching circuit to change the current direction of the stator winding, it can also achieve the same current direction of the winding under the fixed magnetic pole, thereby forming a rotational motion, as shown in Fig. 6.20.

Fig. 6.20

DC brushless motor

There is one N pole and one S pole on the rotor. When the Hall element A detects that the N pole is under the stator winding A1, it controls b2 to energize the stator winding B2. The N pole of the motor generates attraction, and the rotor rotates 90° clockwise. When the Hall element B detects that the N pole is under the stator winding B2, control a2 to energize the stator winding A2. After the stator winding A2 is energized, it will appear as an S pole on the side close to the rotor, attracts the N pole on the rotor, and the rotor rotates 90° clockwise. If this continues, the brushless DC motor will rotate clockwise. Changing the order of electrification of the stator winding can change the rotation direction of the brushless DC motor. Use the sensor to detect the position of the rotor rotation before changing the energization of the stator coil, so it will not appear out of step phenomenon, and it is a synchronous working mode.

In order to improve the utilization rate of the stator windings, the three stator windings are connected in a three-phase symmetrical star connection, it is similar to the connection method of the AC motor. In such a DC brushless motor, the forward and reverse directions of the stator windings can be used It can form NS changing magnetic field, use Hall element to detect the rotor position, and also obtain the rotational motion of the permanent magnet rotor. As shown in Fig. 6.21. Since this working method is very similar to the working method of the frequency converter, it is sometimes called a DC frequency conversion, but its frequency change is not active, it is controlled by a position sensor.

Fig. 6.21

DC frequency conversion

The common brushless DC motor is a permanent magnet brushless DC motor, which can adopt a structure with a permanent magnet rotor inside or a permanent rotor outside. The structure of a brushless DC motor with an internal permanent magnet rotor is shown in Fig. 6.22, the rotor is a permanent magnet rotor, and the stator winding can change the direction of the current, that is, the direction of the magnetic field. The sensor detects the position of the permanent magnet rotor, and the magnetic field direction of the stator winding is changed by the power electronic switching circuit, forming a continuous rotating traction on the rotor.

Fig. 6.22

Permanent magnet brushless DC motor

The permanent magnet materials of permanent magnet brushless DC motors are mostly rare earth permanent magnet materials. The structure of the permanent magnet rotor includes surface magnetic poles, embedded magnetic poles and ring magnetic poles, etc., as shown in Fig. 6.23.

Fig. 6.23

Rotor forms of surface poles, embedded poles and ring poles

The position sensor can be a Hall element that detects the magnetic field, or a proximity switch that measures the metal boss on the rotor, or a photoelectric switch that detects a gap, a rotary encoder, or a resolver. The position sensor detects the position of the rotor, and then Control the current direction in the stator winding so that the magnetic field of the stator always forms a rotating force on the rotor.

The brushless DC motor with an external permanent magnet rotor fixes the stator winding inside, and the external permanent magnet rotor rotates. The sensor detects the position of the outer rotor and accordingly changes the direction of the current on the internal stator winding, that is, changes the direction of the magnetic field. The magnetic rotor forms a continuous rotating traction torque, as shown in Fig. 6.24.

Fig. 6.24

Brushless DC motor with external permanent magnet rotor

Brushless DC motors can also be made into thin disk structures. The circular stator windings and the circular permanent magnet rotor are placed opposite each other. Sensors detect the position of the permanent magnet rotor and change the direction of the magnetic field on the stator windings accordingly. The same principle can be used for permanent magnet rotors. The magnetic rotor forms a continuous rotating traction torque, as shown in Fig. 6.25.

Fig. 6.25

Disk type brushless DC motor

There are many types of brushless DC motors, and there are also many shapes. Figure 6.26 shows the appearances of several brushless DC motors.

Fig. 6.26

Brushless DC motor

6.5 Stepping Motors

The stepping motor rotates step by step. Every time a pulse signal is input, the stepping motor will advance one step, so it is sometimes called a pulse motor. The stepping motor has no cumulative error, and several windings of the stepping motor are energized in a certain sequence. Direct current can form a rotating magnetic field on the stator of the stepping motor, and the rotor rotates under the action of electromagnetic force. The higher the frequency of sequential power supply, the faster the rotation speed of the magnetic field of the stepping motor. The function of the stepper motor driver is to convert the input electrical pulse signal into a corresponding stepper motor winding energization sequence, thereby changing the rotation angle of the stepper motor.

Stepper motors are divided into permanent magnet type, reactive type and hybrid type according to the excitation method. Reactive and hybrid stepping motors are divided into 3 phases, 4 phases, 5 phases, 6 phases, and 8 phases according to the number of phases of the stator windings.

Taking the permanent magnet stepping motor as an example, its stator is composed of 2-phase or multi-phase windings. After each phase winding is fed with direct current, K magnetic poles are formed in the circumferential direction. The rotor is a star-shaped permanent magnet composed of multiple magnetic poles. Along the circumferential direction, NS phases are arranged alternately, and the number of poles of the rotor is also equal to K. Take a 2-phase permanent magnet stepping motor as an example, as shown in Fig. 6.27.

Fig. 6.27

2-phase permanent magnet stepper motor

In Fig. 6.27, the A-phase stator winding has four magnetic poles 1, 3, 5, and 7, and the winding directions (or wiring directions) of the four magnetic pole coils are different to form four magnetic poles arranged alternately in NS; the B-phase stator winding has 4 magnetic poles 2, 4, 6, 8, different winding directions (or wiring directions) of the magnetic pole coils can also form 4 magnetic poles arranged alternately in NS; the rotor has 4 fixed magnetic poles 11, 12, 13, 14, and The NS poles are arranged alternately.

The movement of the rotor is divided into 4 steps, and then it runs repeatedly:

Step 1: A1 of phase A is connected to the + of the DC power supply, and A2 is connected to the—of the DC power supply. The polarity of the four magnetic poles of the A phase and the polarity and position of the rotor are shown in Fig. 6.27a. Due to the attraction of the magnetic poles and repulsion, the rotor remains in this position.

Step 2: Phase A is powered off, B1 of phase B is connected to + of DC power supply, B2 is connected to—of DC power supply. The polarity of the four magnetic poles of phase B and the polarity and position of the rotor are shown in Fig. 6.27b. Due to the attraction and repulsion of the magnetic poles, the rotor rotates 45° clockwise and remains in this position.

Step 3: Phase B is powered off, A1 of phase A is connected to—of the DC power supply, A2 is connected to + of the DC power supply. The polarity of the 4 magnetic poles of phase A and the polarity and position of the rotor are shown in Fig. 6.27c. Due to the attraction and repulsion of the magnetic poles, the rotor rotates 45° clockwise and remains in this position.

Step 4: The negative power supply of phase A is powered off, B1 of phase B is connected to—of DC power supply, B2 is connected to + of DC power supply. The polarity of the four magnetic poles of phase B and the polarity and position of the rotor are shown in Fig. 6.27d. Due to the attraction and repulsion of the magnetic poles, the rotor rotates 45° clockwise and remains in this position.

Step 5: Repeat the first step, the B-phase negative power supply is powered off, the A1 of the A-phase is connected to the + of the DC power supply, and A2 is connected to the—of the DC power supply, and the rotor rotates 45° clockwise and maintain this position.

In this way, the stepping motor starts to rotate. Since the original energization sequence is repeated after 4 energization sequences, it is called 4-beat operation mode; since only one phase winding is energized each time, it is also called single-phase operation mode; The motor has a step angle (angle of rotation per step) of 45°.

The power-on sequence of phase A and phase B power supply is: A, B, (–A), (–B), A…, and the voltage waveform of each phase is similar to that of alternating current, as shown in Fig. 6.28.

Fig. 6.28

Voltage waveforms of A and B phases

In fact, for the 2-phase stepping motor, it is also possible to use a single-phase and two-phase mixed power-on mode. 8-beat operation mode: A, AB, B, B(–A), (–A), (–A) (–B), (–B), (–B) A. The step angle of this energization method is half of the original one, which is 22.5°.

Take A, AB, B three-step power-on sequence as an example to illustrate the rotation angle and direction of the stepping motor, as shown in Fig. 6.29.

Fig. 6.29

Rotation of the stepping motor

Step 1: A1 of phase A is connected to the positive of the DC power supply, and A2 is connected to the negative of the DC power supply. The polarity of the four magnetic poles of phase A and the polarity and position of the rotor are shown in Fig. 6.29a. Due to the attraction and repulsion of the magnetic poles, the rotor remains in this position.

Step 2: A1 of phase A is connected to + of DC power supply, A2 is connected to—of DC power supply. B1 of phase B is connected to + of DC power supply, B2 is connected to—of DC power supply. The polarity of the 8 magnetic poles of phase A and phase B And the polarity and position of the rotor are shown in Fig. 6.29b. Due to the attraction and repulsion of the magnetic poles, the rotor rotates 22.5° clockwise and remains in this position.

Step 3: Phase A is powered off, B1 of B phase is connected to + of DC power supply, B2 is connected to—of DC power supply. The polarity of the 4 magnetic poles of phase B and the polarity and position of the rotor are shown in Fig. 6.29c. shows that due to the attraction and repulsion of the magnetic poles, the rotor rotates 22.5° clockwise and remains in this position.

The rotor rotates 22.5° clockwise per beat, so the same stepper motor has different power-on methods, and its step angle is also different. The step angle of many stepper motors is expressed in the form of y°/0.5y° for this reason.

The principle of a single-three-beat reactive (reluctance type) stepper motor is shown in Fig. 6.30. The rotor is a silicon steel sheet. The stator winding energizes, the rotor produces magnetism. When the A5-A2 windings are energized, the rotor turns to the smallest reluctance position. The rotor 1–3 is aligned with the A5-A2 winding, when the B5-B2 winding is energized, the rotor turns to the position where the magnetic resistance is the smallest, the rotor turns 30°. The rotor 2–4 is aligned with the B5-B2 winding, C5-C2 When the winding is energized, the rotor turns to the position with the least reluctance, and the rotor rotates through 30°. The 1–3 of the rotor aligns with the C5-C2 winding, so reciprocating, the rotor rotates.

Fig. 6.30

Single three-beat reactive stepping motor

The shape of the stepping motor is shown in Fig. 6.31.

Fig. 6.31

Stepping motor

Stepper motors can realize open-loop positioning control without encoder feedback, no cumulative error, simple structure, and will not burn the motor if it is blocked, but the torque is small at high speeds. Stepper motors are used in CNC machine tools, valve control, automatic winding machines, it is widely used in fields such as medical equipment, bank terminals, computer peripherals, cameras and quartz clocks.

6.6 Servo Motor

Servo motors include AC servo motors and DC servo motors. The principle of servo motors is basically the same as that of DC motors and AC motors. Servo in English means “slave” in Greek. Servo motors are mainly for fast and high-precision positioning control, the servo motor can withstand high overload torque. In order to achieve these purposes, the structure of the servo motor has been specially designed. In order to obtain high starting torque, it is made of non-permanent magnet materials. The rotor of the AC servo motor has a large impedance. In order to obtain a fast motor, most of them have a slender structure with low inertia. In order to obtain an accurate position signal, the rotor generally has an encoder or a resolver for measuring the angular position. Some servo motors also need an external encoder, and the position signal of the encoder is fed back to the servo drive to achieve the angular position required by the command. The drive that controls the servo motor can receive position signals (such as pulses and rotation directions) and speed control signals (Mostly positive and negative voltage signals). In most cases, the servo drive can only achieve its high performance by driving the matching servo motor, which is very different from the frequency converter. The frequency converter is more versatile, it can drive any three-phase AC motor that does not exceed the rated current of the frequency converter. The main purpose of the frequency converter is to regulate the speed of the motor, and is not required that the rotor of the motor must have an encoder.

At present, with the rapid development of AC frequency conversion technology, some inverters already have servo functions, and the control accuracy is not significantly different from that of traditional AC servos. Therefore, there is a trend of gradual integration of inverters and AC servos.

Due to the rapid development of power electronics technology and control technology, AC servo motors have gradually replaced DC servo motors and are called the mainstream of servo motors.

AC servo motor is divided into AC permanent magnet synchronous servo motor and AC asynchronous servo motor. The rotor of AC permanent magnet synchronous servo motor is composed of permanent magnets, and the stator coil forms a rotating magnetic field. As long as the load does not exceed the synchronous torque, the permanent magnet rotor will rotates synchronously with the rotating magnetic field, which is basically similar to the AC permanent magnet synchronous motor mentioned in the previous section. For the AC asynchronous servo motor whose rotor is a hollow cup or squirrel-cage structure, its principle is similar to that of a single-phase split-phase motor. Its stator winding is composed of an excitation winding and a control winding with two phases placed at a 90° difference in space. The AC phase difference between the excitation winding and the control winding has a certain angle, so that an elliptical rotating magnetic field is generated on the stator, and the rotor cuts the magnetic field line and rotates under the traction of the electromagnetic force. Changing the power frequency of the excitation winding and the control winding can change the speed of the servo motor, changing the power supply voltage of the control winding can also change the output speed of the servo motor. When the voltage of the control winding is reversed, the servo motor will rotate in the opposite direction.

The structure of the permanent magnet AC servo motor is shown in Fig. 6.32.

Fig. 6.32

Permanent magnet AC servo motor

The price of the servo motor is relatively high, and the power is not too large. At present, it is mainly used in occasions such as large speed range, accurate positioning, fast tracking, low speed and high torque, such as precision machine tools, packaging machines, printing machinery, manipulators and other fields. In these systems, the application of AC permanent magnet synchronous servo motor is more common.

The shape of the servo motor is shown in Fig. 6.33.

Fig. 6.33

Servo motor

6.7 Linear Motors

The principle of the linear motor is equivalent to cutting the rotor and stator of the AC or DC motor, the rotor and the stator are unfolded on a plane, and the rotor moves linearly along the direction of expansion. It can also be considered that the linear motor is an AC or DC motor with an infinite diameter. The outer surface of the rotor and the inner surface of the stator become planes, and the rotor moves in a straight line along the same direction as the stator arrangement direction.

Taking the linear stepper motor as an example, its principle is the same as that of the rotary stepper motor. Figure 6.34 is a schematic diagram of a 5-phase linear stepper motor. The mover of the linear motor consists of 5 n-shaped iron cores, the adjacent n-shaped iron core and the stator teeth are staggered by 1/5 tooth pitch. There are two oppositely connected coils on the two poles of each n-shaped iron core. The magnetic field formed by the two coils makes the n-shaped iron core one pole N and one pole S, and the magnetic flux does not enter other n-shaped iron cores. When phase A is energized, the n-shaped iron core on the A-phase coil is aligned with the teeth on the stator core, then phase A is de-energized, and phase B is energized. The mover moves 1/5 pitch to the left. Similarly, when phase B is de-energized and phase C is energized, the mover moves 1/5 pitch to the left. This is the 5-phase 5-beat operation mode. If the power-on mode is A -AB-B-BC-…, 5-phase 10-step operation mode, the mover moves 1/10 of the pitch in each step.

Fig. 6.34

Principle of 5-phase linear stepper motor

For linear motors with other principles, the analysis method is the same as above. The stator and rotor are also cut apart, and a linear motion magnetic field is applied on the winding side (or called primary), so that the mover (or called secondary) is forced to move along a straight line. As shown in Fig. 6.35.

Fig. 6.35

Stator and rotor cut

The mover can be a permanent magnet or a coil, and the stator can be a permanent magnet or a coil. The linear motor with a permanent magnet as the mover is shown in Fig. 6.36. The base is provided with a linear guide rail and a stator coil. A sliding table is installed on the linear guide rail. A mover magnet is installed below the sliding table. The mover magnet is opposite to the stator coil. The stator coil is connected to the power supply to form a linear motion magnetic field, and the mover magnet will produce a linear motion.

Fig. 6.36

Linear motor with permanent magnets

When the automation equipment needs to do linear motion, the linear motor can omit the conversion mechanism such as the screw. The linear servo motor has small inertia, high speed, high acceleration, unlimited length, and high positioning accuracy. Linear motors are used in CNC machine tools, electronic device manufacturing, electronic patch equipment, manipulators, hardware processing, solar cell manufacturing, maglev trains, aircraft ejection, elevators and other fields have many applications. The shape of the linear motor is shown in Fig. 6.37.

Fig. 6.37

Linear motor

6.8 Switched Reluctance Motors

The switched reluctance motor is made according to the principle of minimum reluctance. It is a new type of speed-regulating motor. Its structure is similar to that of a reactive stepping motor. Every time the energizing combination of the winding changes, the rotor takes a step. The operation of the switched reluctance motor also needs to use sensors to detect the position of the rotor. Using technology that can accurately predict the rotor position, it is also possible without position sensors. According to the position of the rotor, the sequence and time of energizing the stator windings are determined. Therefore, the operation mode is similar to that of a brushless DC motor, and the rotor position detector is installed in the motor.

The poles of the stator and the rotor of the switched reluctance motor are salient pole structures, and the poles are protruding, and the stator and the rotor are made of laminated silicon steel sheets. There are no windings and permanent magnets on the rotor, and there are windings on the stator. Two 180° opposite windings are connected together as “one phase”. Take a 4-phase switched reluctance motor (8/6) with 8 windings and 6 poles as an example. The structure is shown in Fig. 6.38.

Fig. 6.38

4-phase switched reluctance motor

In Fig. 6.38, 1–1’ of the rotor is aligned with the CC’ winding of the stator, and then the CC’ winding is de-energized, and the D-D’ winding is energized. According to the “minimum reluctance principle”, the magnetic flux always take the path with the least reluctance. The D-D’ winding will attract 2–2’ of the rotor to coincide with itself, the rotor runs counterclockwise, when the poles of the winding and the poles of the rotor coincide, the inductance on the winding is the largest. But the winding no longer has traction torque on the rotor. Detect the position of the rotor, and then energize the A-A’ winding in a timely manner, and then energize the B-B’, following the sequence of D-A-B-C-D, the rotor will rotate counterclockwise. If the energization sequence Change it to B-A-D-C, then the rotor rotates clockwise.

Take the A-phase winding as an example, when A + and A- are high, T1 and T2 are turned on, and the A-phase winding is energized. When A + and A- are low, T1 and T2 are turned off, and the current in the A-phase winding passes through D1 and D2 regenerates and feeds back to the power supply U, the system efficiency is higher. It also speeds up the turn-off speed of the A-phase winding. In the H-bridge circuit, the upper and lower transistors are turned on at the same time and there is a fatal problem of short circuit.

The switched reluctance motor has simple structure, low cost, large starting torque, low starting current, high-speed operation, one-phase winding failure can still reduce power operation, frequent start and stop or forward and reverse, and high efficiency. The internal structure of the switched reluctance motor is shown in Fig. 6.39.

Fig. 6.39

Internal structure of switched reluctance motor

The appearance of the switched reluctance motor is shown in Fig. 6.40.

Fig. 6.40

Switched reluctance motor

6.9 Power Supply Voltage and Operating Voltage of Electrical Device

The power supply voltage in the United States is not the same as the design working voltage of electrical device. The power supply voltage is the voltage that is provided by the power supply company. Due to line loss, when the power is delivered to the device, the voltage will drop. The electrical device must be able to work within a certain voltage range. For example, for an AC120V power supply, the designed operating voltage of electrical device may be AC110-120 V.

6.9.1 Power Supply and Structure in the United States

The three-phase power supplies in the United States are: AC208V, AC240V and AC480V.

There are delta connection and wye connection for three-phase power supply, as shown in Figs. 6.41, 6.42 and 6.43. The delta configuration also derives different power supply system due to the different layouts of the ground wire and the neutral wire.

Fig. 6.41

Four-wire Wye Configuration

Fig. 6.42

Three-Wire Delta Configuration (Ground)

Fig. 6.43

Four-Wire Delta Configuration (Neutral)

Due to the different voltages and structures of the three-phase power supplies, there are many types of power supply voltages derived from them. The most common single-phase power supply is AC120V.

The three-phase AC208V star can generate AC120V and AC208V power supply voltages, and the single-phase AC120V power supply is commonly used in residential buildings, as shown in Fig. 6.44.

Fig. 6.44

AC120V and AC208V

The three-phase AC240V delta connection can also derive the power supply voltage of AC120V and AC208V, as shown in Fig. 6.45. In the same way, the three-phase AC480V delta connection can derive the power supply voltage of AC240V and AC415V.

Fig. 6.45

AC120, AC240V and AC208V

Three-phase AC480V wye connection can derive single-phase AC277V power supply voltage, as shown in Fig. 6.46.

Fig. 6.46

AC277V and AC480V

Fig. 6.47

AC120V/15A wall socket

6.9.2 Design Working Voltage of Electrical Device

Considering that the power transmission between the power supply and the electrical device must have line loss, the design operating voltage of the electrical device is lower than the voltage of the power supply, and the tolerance factor must also be considered.

In the three-phase AC480V power supply system, for example, the rated voltage of the electrical device is designed according to AC460V, considering the tolerance, the working voltage range is AC440-480; in the single-phase AC120V power supply system, the operating voltage range of the electrical device is AC110-120 V.

6.9.3 Power Sockets

The most commonly used wall sockets are AC120V/15A sockets, and the shapes of the two sockets are shown in Fig. 6.47. In Fig. 6.47, the hole on the left (long) of the socket is N (Neutral), the hole on the right (short) is L (Line), and the semicircle hole on the bottom is G (Ground).

Three-phase sockets generally have 3 holes or 4 holes, and if the system ground and equipment ground are separated, there are 5 holes.

Due to the influence of many factors such as current size, voltage level, number of phases, whether there is a ground wire, whether there is a neutral wire, and the configuration of the power supply, there are many types of sockets, which will not be listed here.

6.9.4 Power Supply and Structure in China

In China, the most common low-voltage power supply voltages are: single-phase AC220V and three-phase AC380V. Three-phase AC power mostly uses wye connection and three-phase four-wire power supply, as shown in Fig. 6.48.

Fig. 6.48

Single-phase AC220V and three-phase AC380V

The neutral line is usually connected to the earth, and the grounded neutral point is called the zero point, and the grounded neutral line is called the zero line.

The voltage between terminal wires is called line voltage, AC380V. The voltage between the terminal line and the neutral line is called the phase voltage, AC220V. They have the following relationship.

$$\sqrt{3}*220=380$$

(6.9)