Speed Regulation Method in Energy Saving System

Abstract

In the energy-saving control system, in many occasions, it is necessary to use the method of speed regulation to distribute the load of the equipment and to start and stop the equipment. These speed regulation methods are both mechanical and electrical. Due to the rapid development of high-voltage and high-current electronic devices, the application of frequency converter speed regulation of AC motors is becoming more and more common.

In the energy-saving control system, in many occasions, it is necessary to use the method of speed regulation to distribute the load of the equipment and to start and stop the equipment. These speed regulation methods are both mechanical and electrical. Due to the rapid development of high-voltage and high-current electronic devices, the application of frequency converter speed regulation of AC motors is becoming more and more common.

7.1 Electromagnetic Slip Clutch

The magnetic powder clutch mentioned in the last chapter can realize the speed of the load on the output shaft side by controlling the magnitude of the input voltage. A similar method also has the electromagnetic slip clutch to be mentioned below.

The basic principle of the electromagnetic slip clutch is shown in Fig. 7.1. The motor (1) rotates at a constant speed. The armature (2) composed of the motor (1) and the cast steel cylinder is rigidly connected through the rotating shaft. The motor (1) drives the armature (2) rotate, the excitation winding (3) on the magnetic pole (4) has a DC voltage U_f through the slip ring brush. The current in the excitation winding (3) causes the magnetic poles (4) to establish a magnetic field. The rotating armature (2) cuts the magnetic field and induces an electromotive force. The induced electromotive force generates eddy currents in the winding. Eddy currents are generated between the armature (2) and the magnetic poles (4), and the eddy currents interact with the magnetic field to generate electromagnetic force. The direction of the electromagnetic force hinders the relative movement between the armature (2) and the magnetic poles (4). According to the characteristics of the action force and reaction force, the magnetic pole (4) rotates with the armature (2), so that the motor (1) and the load (6) are in a “connected” state. When the DC voltage U_f is zero, the electromagnetic force in the armature (2) disappears. The magnetic pole (4) does not follow the rotation of the motor (1), and the motor (1) and the load (6) are in a state of “disengagement”.

Fig. 7.1

Electromagnetic slip clutch

Changing the excitation voltage U_f can change the magnitude of the eddy current in the armature (2), which also changes the magnitude of the electromagnetic force in the armature (2) and the rotational speed of the magnetic pole (4). The speed of the load (6) changes accordingly. This speed regulation method has simple structure, reliable operation and convenient control.

This kind of speed regulating device is sometimes called electromagnetic slip speed governor, and the combination of speed regulating device and electric motor is called electromagnetic speed regulating motor. In this speed regulation method, the maximum speed on the load side is lower than the rated speed on the motor side. The general speed range is 10–80%. The disadvantage of this method is that there are eddy currents in the armature. The heavier the load, the greater the electromagnetic force required and the greater the eddy current, so considerable heat will be generated in the armature. At low speeds, the transmission efficiency is very low. The torque is T_M, then the efficiency η_m of the electromagnetic clutch is as Eq. (7.1).

$${\eta }_{m}=\frac{{9550}{T}_{M}n}{{9550}{T}_{M}{n}_{0}}=\frac{n}{{n}_{0}}$$

(7.1)

7.2 Hydraulic Coupling

In the early days, it was inconvenient to adjust the speed of some engines, and the power electronic devices were very expensive. A mechanical speed control device was connected in series between the engine and the load to adjust the speed of the load. The hydraulic coupling is one of such speed control devices.

The principle of the hydraulic coupling is shown in Fig. 7.2. The motor (1) rotates at a constant speed. The motor (1) is rigidly connected to the turbine (2) through the rotating shaft. The motor (1) drives the turbine (2) to rotate, and the turbine (2) inside Filled with a certain volume of liquid (3), the liquid (3) can be vegetable oil or water. The rotation of the turbine (2) makes the liquid (3) in it thrown out from the outer edge due to centrifugal force, and the liquid (3) enters the turbine (4). The impingement turbine (4) rotates in the same direction as the turbine (2), the turbine (4) is coaxially connected with the load (5), and the load (5) rotates accordingly. Adjusting the capacity of the liquid (3) can change the turbine (2) The force acting on the turbine (4), while changing the rotational speed.

Fig. 7.2

Hydraulic coupling

This speed regulation method uses liquid to transmit kinetic energy and pressure energy, and the control method is simple and convenient. The disadvantage is that due to the speed regulation by changing the liquid volume, the speed response is slow, the internal liquid generates considerable heat, and at low speeds, the transmission efficiency is very low. The maximum speed of the load side achieved by this speed regulation method is smaller than the speed of the motor side. Assuming that the load torque is T_M, the efficiency η_m of the hydraulic coupling is as in Eq. (7.2). The range of speed regulation is about 20–97%, and the load cannot run at 100% the rated speed of the motor.

$${\eta }_{m}=\frac{{9550}{T}_{M}n}{{9550}{T}_{M}{n}_{0}}=\frac{n}{{n}_{0}}$$

(7.2)

7.3 Fluid Viscous Clutch

The working principle of the liquid viscous clutch is shown in Fig. 7.3. The motor (1) rotates at a constant speed, the motor (1) is connected with the active friction plate (2) through the rotating shaft, and the motor (1) drives the active The friction plate (2) rotates, and the oil medium (3) is filled between the active friction plate (2) and the driven friction plate (4). Due to the action of friction, the rotation of the active friction plate (2) makes the oil medium (3) Rotation in the same direction also occurs. Due to the transmission of friction force, the rotation of the oil medium (3) drives the driven friction plate (4) to rotate in the same direction. The moving friction plate (4) is coaxially connected with the load (5), and the load (5) rotates, the driven friction plate (4) can move left and right through the hydraulic oil. The closer the distance between the driven friction plate (4) and the active friction plate (2) is, the smaller the gap is, the driven friction plate (4) and the oil medium (3) between the active friction plate (2). The greater the friction force, the higher the speed of the driven friction plate (4), and the higher the speed of the load (5); on the contrary, the farther the distance between the plate (4) and the active friction plate (2), that is, the larger the gap, the smaller the friction force transmitted by the oil medium (3). The lower the speed of the driven friction plate (4), the lower the speed of the load (5). In this way, the adjustment of the load speed is realized.

Fig. 7.3

Fluid-viscous clutch

This speed regulation method uses the viscosity of the fluid to transfer energy. The control is simple and convenient. The speed regulation range is about 20–100%. When the distance between the driven friction plate and the active friction plate is zero, the load side of this speed regulation method is the highest. The rotational speed may be equal to the speed on the motor side. The disadvantage is that the speed response is slow due to the use of changing the mechanical clearance for speed regulation, and the internal medium oil generates considerable heat, and at low speeds, the transmission efficiency is very low. Assuming the load torque is T_M, the hydraulic viscous speed regulation clutch efficiency η_m is as follows.

$${\eta }_{m}=\frac{{9550}{T}_{M}n}{{9550}{T}_{M}{n}_{0}}=\frac{n}{{n}_{0}}$$

(7.3)

7.4 Mechanical Governor

There are many methods of mechanical speed regulation, but it is not the focus of this book. Figure 7.4 is a method of speed regulation by using belt variable diameter, and Fig. 7.5 is a method of speed regulation by using turntable variable diameter. There are also many ways to achieve mechanical speed regulation, which will not be described in detail here. The stepless speed changer of many small electric motors uses these speed regulation methods. The speed of the motor is constant, there is a speed change device on the output shaft of the motor, and the speed change of the output shaft can be changed by adjusting the handwheel or lever on the speed change device.

Fig. 7.4

Belt variable diameter speed regulation method

Fig. 7.5

Turntable variable diameter speed regulation method

In Fig. 7.4, the motor (1) drags the trapezoidal wheel (2) that can change the width of the T shape. The trapezoidal wheel (2) drives the trapezoidal wheel (4) that can change the width of the T shape through the T belt (3). The wheel (4) drives the load (5) to rotate. Synchronously adjusts the width of the T-shaped grooves in the trapezoidal wheel (2) and the trapezoidal wheel (4), so that the transmission ratio between the trapezoidal wheel (2) and the trapezoidal wheel (4) can be changed. At the same time the speed adjustment of the load (5) is also realized. As shown by the arrow in the figure, make the T-shaped groove of the trapezoidal wheel (2) wider. The position of the T-shaped belt (3) in the trapezoidal wheel (2) will fall and be closer to the axis, which is equivalent to a smaller diameter of the effective transmission wheel. For the same motor rotation speed n₀, the linear velocity of the T-shaped belt (3) becomes lower. In order not to change the axis positions of the trapezoidal wheel (2) and the trapezoidal wheel (4), make the T-shaped groove of the trapezoidal wheel (4) narrowing. The position of the T-shaped belt (3) in the trapezoidal wheel (4) is raised, away from the axis, which is equivalent to the increase of the effective transmission wheel diameter, so the speed on the load (5) is finally reduced. If it needs to be raised the rotation speed of the load (5), then the adjustment direction is opposite.

In Fig. 7.5, the motor drives a turntable (1) with a large friction coefficient to rotate at speed n0. The spherical runner (2) is installed on the rotating shaft (3) and can move left and right. The surface of the spherical friction wheel (2) is in contact with the surface of the turntable (1) is in direct contact. Due to the effect of friction, the spherical friction wheel (2) rotates. The rotating speed of the spherical friction wheel (2) depends on the linear velocity of the contact point with the turntable (1) and the spherical friction wheel (2). The spherical friction wheel (2) drives the load (4) to rotate, the linear velocity of the turntable (1) is larger on the outside and smaller on the inside. When the spherical wheel (2) moves outward, the rotational speed of the spherical friction wheel (2) increases, when the spherical friction wheel (2) moves inward, the rotational speed of the spherical friction wheel (2) decreases, thus changing the rotational speed n₁ of the load (4).

The speed range of the mechanical speed control method can be very wide, and the speed of the load can exceed the speed of the motor. The transmission efficiency of these methods depends on the transmission form and structure.

7.5 Stepper Motor and Stepper Motor Driver

Stepper motors and stepper motor drivers are widely used in some situations that do not require too high precision and too high dynamic performance. It can directly realize synchronous control and positioning control without feedback signals like encoders. The main parameters of the motor are step angle, working torque, holding torque, positioning torque, no-load starting frequency, maximum operating speed, control mode, power supply voltage, etc.

The minimum step angle of the stepper motor determines the open-loop control accuracy of the stepper motor, so the step angle is one of the most important parameters of the stepper motor. Since the control method of the stepping motor has the number of phases, the stepping motor and the stepping motor driver should be used together. The wiring of a general stepping motor is shown in Fig. 7.6.

Fig. 7.6

Wiring of the stepping motor

In Fig. 7.6, the power input of the stepper motor driver is either AC power supply (AC 60 V, AC 100 V, AC 220 V, etc.) or DC power supply (DC 24 V, DC 12 V, DC 36 V, etc.), and the wiring of the stepper motor can be It is the A + , A-, B + , B- mode in the figure, and it can also be U, V, W or A, B, C, D, E and other wiring modes. The pulse command input is used to control the number of steps of the stepping motor. The direction control input is used to control the rotation direction of the stepper motor (forward and reverse). The offline signal input makes the stepper motor in a free state. The appearance of the stepping motor and the stepping motor driver is shown in Fig. 7.7.

Fig. 7.7

Appearance of stepping motor and stepping motor driver

7.6 AC Servo Motor Driver

The use of AC servo motors and servo drives is very similar to frequency converters, and many of the parameter settings are also very similar. The main difference between servo drives and ordinary frequency converters is that they have higher control precision and faster control speed. It has the input port of the motor encoder (currently some frequency converters also provide the encoder input port), and the servo drive generally has a pair of encoder pulse input port and encoder pulse output port for interconnection between the servo drives. Which can be conveniently Realize the precise proportional synchronization of the speeds of the servo drives. So there is a synchronization factor on the servo drive, one is used to input the pulse number of the tracked axis, and one is used to input the corresponding tracking pulse number of the servo motor. The servo drive It can be used for positioning control, and the number and direction of pulses taken by the motor are determined by the input signal. The servo driver can control the speed and start and stop through the panel, and can also be controlled by communication. The wiring of the general servo driver is shown in Fig. 7.8.

Fig. 7.8

Main wiring of the servo drive

In Fig. 7.8, R, S, T are connected to three-phase AC power supply (some are also connected to single-phase power supply AC 220 V), U, V, W are connected to AC servo motor. The speed control input (0- ± 10 V) terminal is used to control the speed of the servo motor. The enable signal controls whether the servo driver is working, and the pulse command input terminal is connected to the encoder of the previous motor or drive shaft to be tracked. It can be connected to the encoder command output of other controllers. The pulse command output is used to send the position of the servo motor to the next-level servo driver as a tracking command. The encoder feedback is connected to the encoder on the servo motor. The communication port is used to communicate with other controllers. Controllers, such as PLC, are used for data transmission and control. The servo drive is mainly selected according to output torque, maximum speed, encoder resolution, power supply, installation method, etc. The main parameters that need to be set for the servo driver are operation control mode (speed mode, position mode or torque mode), control method, maximum torque, maximum speed, etc.

When the workpiece movement requires a large acceleration (2 ~ 10 g) and precision movement without mechanical clearance, it is necessary to use a small linear motor and a servo driver. The appearance of the servo driver is shown in Fig. 7.9.

Fig. 7.9

The appearance of the servo driver

7.7 Speed Regulation Method of DC Motor

Before the large-scale application of frequency converters, DC motors and DC motor governors have always been the protagonists of speed control in the field of motor drive, and their application time can be traced back to a long time ago. The expression of the speed n₁ of the DC motor is:

$${n}_{1}=K(U-Ir-2\Delta U)/{\varphi }_{1}$$

(7.4)

In the Eq. (7.4), n1 is the output speed of the DC motor, K is a constant, U is the armature voltage, I is the armature current, r is the internal resistance of the armature. △U is the voltage drop on a brush, φ₁ is the excitation magnetic flux generated by the excitation coil.

It can be seen from Eq. (7.4) that there are two main speed regulation methods of DC motors: one is to change the voltage on the armature winding, and the other is to change the excitation current on the excitation winding. In general, the stepless speed regulation of the motor is realized by changing the armature voltage below the rated power of the motor, and the constant power speed regulation is realized by weakening the excitation when the motor speed exceeds the rated power. The main parameters of the DC speed controller are as follows: rated voltage, rated current, rated power, rated speed, speed up and down time of the controlled motor, regulator parameters of the speed control loop, PI adjustment parameters of the current control loop, maximum allowable current value, encoder parameters, etc. The parameters of the DC motor governor can be modified according to the manufacturer's instructions through the buttons on the control panel and the display screen. The wiring of the DC governor is shown in Fig. 7.10.

Fig. 7.10

Main wiring of the DC speed controller

In Fig. 7.10, R, S, T are connected to three-phase AC power supply (some are also connected to single-phase power supply AC 220 V), U + and U- are connected to the armature winding, Uf + and Uf- are connected to the excitation winding of the motor, and the speed of the motor And the start and stop can be controlled by the panel or by external analog signal (0 ~  ± 10 V) and switch signal. In closed-loop control, the speed and position signal are input through the encoder.

The appearance of the DC speed controller is shown in Fig. 7.11.

Fig. 7.11

Appearances of several DC speed regulators

7.8 Rotational Speed of AC Motors

AC motors are the most widely used power equipment in the industrial field.

From the previous chapter, the basic principle of the three-phase AC motor we know that the rotational speed n₀ (also called synchronous speed) of the stator magnetic field of the three-phase AC motor is expressed as:

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

(7.5)

In the Eq. (7.5), f is the frequency of the power supply of the three-phase AC motor, and p is the number of pole pairs of the three-phase AC motor.

The expression of the rotor output speed n of the three-phase AC motor is:

$$n=\left(1-s\right)\times {n}_{0}=\left(1-s\right)\times \frac{60\times f}{p}$$

(7.6)

In the equation, n is the three-phase AC motor rotor speed rpm (revolutions per minute); s is the slip rate, and s represents the difference between the output rotation speed of the three-phase AC motor rotor and the magnetic field rotation speed on the stator. The slip rate of the three-phase synchronous AC motor is s = 0, that is, the rotation speed of the rotor output is equal to the rotation speed of the magnetic field on the stator. The slip rate of the three-phase asynchronous AC motor is s > 0, and the expression of the slip rate s is:

$$s=\frac{{n}_{0}-n}{{n}_{0}}$$

(7.7)

7.9 Efficiency of AC Motors

The efficiency η expression of a three-phase AC motor is:

$$\eta =\frac{{P}_{2}}{{P}_{1}}=\frac{{P}_{2}}{{P}_{2}+{p}_{cu1}+{p}_{Fe1}+{p}_{cu2}+{p}_{Fe2}+{p}_{f}+{p}_{ad}}\%$$

(7.8)

In the Eq. (7.8), P₁ is the total input power of the three-phase AC motor, P₂ is the mechanical output power of the motor, p_cu1 is the copper loss caused by the current flowing through the copper wire resistance in the stator, and p_Fe1 is the stator iron core turn-on the variable magnetic field leads to the iron loss caused by the existence of eddy current, etc., p_cu2 is the copper loss of the rotor, p_Fe2 is the iron loss on the rotor. p_f is the mechanical loss caused by the friction of the rotor bearing, etc., and p_ad is the additional loss caused by the transverse current in the rotor. Since the rotor rotates with the rotating magnetic field, the frequency of the alternating magnetic field in the rotor is zero during synchronous operation, and only 6–3 Hz during asynchronous rated operation. So, the iron loss p_Fe2 on the rotor is generally very small, and circuit analysis is often ignored.

The electromagnetic power transmitted from the stator to the rotor of a three-phase AC motor is P_M. P_M is equal to the total input power P₁ minus the stator iron loss p_Fe1 and the stator copper loss p_cu1. The electromagnetic torque T_M of the motor is generated by the interaction between the rotor current and the stator rotating magnetic field. The stator rotating magnetic field speed n₀, such as Eq. (7.9).

$${P}_{M}={P}_{1}-{p}_{cu1}-{p}_{Fe1}={9550}{T}_{M}{n}_{0}$$

(7.9)

The electromagnetic torque T_M is transmitted to the motor rotor, the speed of the motor rotor is n. The total mechanical power on the motor rotor is P_m, P_m is equal to the electromagnetic power P_M minus the copper loss p_cu2 and iron loss p_Fe2 on the rotor, p_Fe2 is negligible, as Eq. (7.10). The power transmission efficiency η_m between rotor and stator is in the following Eq. (7.10.1)

$${P}_{m}={P}_{M}{=p}_{cu2}={9550}{T}_{M}n$$

(7.10)

$${\eta }_{m}=\frac{{P}_{m}}{{P}_{M}}=\frac{n}{{n}_{0}}$$

(7.10.1)

The copper loss p_cu2 on the rotor is equal to the heat loss of the rotor current i₂ on the rotor internal resistance r₂, and it is also equal to the product of the slip rate s and the electromagnetic power P_M.

The mechanical output power P₂ of the motor is equal to the total mechanical power P_m on the rotor minus the mechanical loss p_f on the rotor and the additional loss, as shown in Eq. (7.11).

$$ p_{cu2} = 3i_{2}^{2} r_{2} = P_{M} P_{m} P_{M} \left( {\frac{{P_{M} - P_{m} }}{{P_{M} }}} \right)P_{M} \left( {\frac{{n_{0} - n}}{{n_{0} }}} \right) = sP_{M} $$

(7.11)

$$ P_{2} = P_{m} - p_{f} - p_{ad} $$

(7.12)

The efficiency η of the three-phase AC motor can also be regarded as the product of the stator efficiency η₁ and the rotor efficiency η₂, such as Eq. (7.13).

$$\eta =\frac{{P}_{2}}{{P}_{1}}=\frac{{P}_{M}}{{P}_{1}}\times \frac{{P}_{2}}{{P}_{M}}={\eta }_{1}\times {\eta }_{2}$$

(7.13)

The iron loss of the motor is basically unchanged, so it is also called constant loss. The copper loss of the motor increases with the increase of current (that is, the load rate increases), so it is also called variable loss. The three-phase AC motor stator and when the constant loss of the rotor is equal to the variable loss, the efficiency η reaches the maximum η_M. In order to use the same metal material to drag a large load as much as possible, when the three-phase AC motor is manufactured, the rated working state is generally designed to be greater than the maximum efficiency when the load rate is above. The load rate at the highest efficiency is generally 70–90%.

The relationship between the power consumption of each part of the three-phase AC motor and the total input power P₁ and mechanical output power P₂ is shown in Fig. 7.12.

Fig. 7.12

Energy distribution of the motor

7.10 Speed Regulation Method of AC Motor

From Eq. (7.6), it can be seen that there are no more than three ways to adjust the speed of a three-phase AC motor. One is to change the frequency f of the three-phase power supply, the other is to change the number of pole pairs p of the three-phase AC motor, and the third is to change the slip ratio s.

7.10.1 The Speed Regulation Method of Changing the Number of Pairs of Poles

This is the simplest method for adjusting the speed of three-phase AC motors. It is widely used on the spindles of machine tools such as boring machines and grinding machines and centrifuges. This speed adjustment method requires that the structure of the three-phase AC motor itself must be able to change poles. This speed regulation method changes the number of poles of the three-phase AC motor by changing the wiring mode of the stator coil inside the motor through the AC contactor or manual switch. There is no other intermediate link, so there is no efficiency loss of the intermediate link. The energy efficiency of this speed regulation method is the highest, of course, this does not refer to the efficiency of the pole-changing motor itself.

For example, when a three-phase AC motor is converted from a 4-pole (2 pole pair) wiring mode to a 2-pole (1 pole pair) wiring mode, the output speed of the three-phase AC motor can be doubled. When switching from 2-pole to 4-pole wiring, the speed is reduced by half. However, because the number of pole pairs p has only a few levels, this speed regulation method has steps, and cannot realize the continuous regulation of the speed of the three-phase AC motor.

The number of pole pairs of the cage rotor can be automatically changed through electromagnetic induction with the change of the number of pole pairs of the stator, while it is difficult for the wound rotor to change the number of pole pairs by changing the wiring. Therefore, the pole-changing speed regulation method is mainly used for cage-type three-phase AC motors.

7.10.2 Nine Speed Regulation Methods to Change the Slip S

There are many speed regulation methods to change the slip s, and this section gives 9 of them.

7.10.2.1 Rotor Series Resistance Speed Regulation of Wound Rotor Motor

The rotor structure of the wound rotor three-phase AC motor is shown in Fig. 7.13. In addition to the terminals of the stator winding, this motor also leads the three rotor windings out of the motor through slip rings and brushes. The leads are generally installed on the motor. At the shaft end, change the resistance value of the external resistor on each phase winding of the rotor to realize the adjustment of the motor speed.

Fig. 7.13

Rotor series resistance speed regulation

The slip rate s of the wound rotor after the series resistance, the rated slip rate s_e before the series resistance, the self-resistance r₂ of each phase of the rotor, the series resistance R of each phase of the rotor. The electromagnetic torque T before the series resistance, after the series resistance the relationship between the electromagnetic torque T’ is as follows:

$$R=\left(\frac{sT}{{s}_{e}{T}{\prime}}-1\right)\times {r}_{2}$$

(7.14)

The slip rate s of the wire-wound rotor after series resistors is obtained by the Eq. (7.7), and the rated slip rate s_e before series resistors is obtained by the following equation:

$${s}_{e}=\frac{{n}_{0}-{n}_{e}}{{n}_{0}}$$

(7.15)

According to Eq. (7.14), (7.15) and (7.7), the relationship between the speed n and the resistance R of each phase of the rotor connected in series is as follows:

$$n=\frac{{n}_{0}\left(T-{T}{\prime}\right)+{n}_{e}{T}{\prime}-\frac{R}{{r}_{2}}\left({n}_{0}-{n}_{e}\right){T}{\prime}}{T}$$

(7.16)

If the resistance of each phase of the rotor connected in series is switched in stages, as shown in Fig. 7.14. The control of the closure of KM3, KM2, and KM1 contacts in the figure is equivalent to connecting different resistors in series in the rotor. This method the speed adjustment is graded, and the series resistors are generally composed of multiple high-power metal resistors.

Fig. 7.14

Speed regulation by step-by-step switching of series connected resistors

In order to realize the smooth adjustment of the speed, it is necessary to uniformly change the resistance value of the series resistance of each phase of the rotor. A liquid resistance governor uses sodium bicarbonate aqueous solution as the resistance liquid, and changes the interval between the moving plate and the static plate in the liquid. The size and length of the surface can evenly adjust the resistance value, so that the speed can be adjusted steplessly, as shown in Fig. 7.15, the motor drives the screw to rotate, the screw drives the screw nut to move up and down. The screw nut drives the moving plate to the electric resistance liquid moves up and down, and when the moving plate moves downward, the distance between the static plate and the moving plate decreases, and the liquid resistance decreases. When the moving plate moves upward, the distance between the static plate and the moving plate increases. As the distance becomes larger, the resistance of the liquid increases, so that the continuous adjustment of the motor speed can be realized.

Fig. 7.15

Liquid resistance stepless speed regulator

Another way to continuously change the series resistance of the rotor is to lead the wound rotor out and connect a three-phase bridge rectifier, connect a fixed resistor R at the DC terminal of the rectifier. A thyristor (GTO) or high-power transistor in parallel at both ends of the fixed resistor. The structure is shown in Fig. 7.16.

Fig. 7.16

Continuous adjustment of series resistance with thyristor

When the GTO is turned on, it is equivalent to the external resistance is zero. When the GTO is turned off, it is equivalent to the external resistance is R, and the ratio of the on-time T₁ to the off-time (T-T₁) of the GTO in a cycle T is controlled. You can change the equivalent series resistance R_d, such as Eq. (7.17).

$$ R_{d} = \left( {\frac{{T - T_{1} }}{T}} \right)R $$

(7.17)

The rotor circuit series resistance speed regulation method is simple, convenient, and easy to implement, but when the output speed of this method is low, the slip rate s is large. According to the Eq. (7.11), the copper loss P_cu2 on the rotor will be proportional to the slip rate s bigger, less efficient. Assuming that the electromagnetic torque of the stator to the rotor is T_M, the power transfer efficiency η_m between the rotor and the stator is as in Eq. (7.18). Note that it does not refer to the total efficiency of the speed regulating motor, and the overall efficiency also needs to consider the stator efficiency and the rotor mechanical efficiency.

$${\eta }_{m}=\frac{{9550}{T}_{M}n}{{9550}{T}_{M}{n}_{0}}=\frac{n}{{n}_{0}}$$

(7.18)

This type of governor is limited to the use of wound rotor motors with brushes for speed regulation. Since the speed regulation is achieved by increasing the slip rate, the lower the speed, the lower the efficiency. It should avoid working at low speed, and generally control the speed within the range of 50–100%. However, if this kind of governor works near the rated speed, the operating efficiency is also very high. Because after all, its highest efficiency is almost 100%, which cannot be achieved by frequency converters anyway. At light load and no load, changing the resistance of the rotor in series does not change much, so this method is suitable for heavy load speed regulation. The cold state value and hot state value of the resistance will change, this method is not suitable for occasions requiring fast response and precise speed regulation.

7.10.2.2 Cascade Speed Regulation Method of Wound Rotor Motor

A resistor R is connected in series in each phase circuit of the wound rotor to regulate the speed of the AC motor, as shown in Fig. 7.17. In fact, the speed regulation is achieved by reducing the current of the rotor circuit. The electromotive force induced by the rotor of the three-phase AC motor is E₂, internal resistance r₂, inductive reactance X₂, current i₂, series resistance R, and the heat loss i₂²R on the external resistance are wasted in vain.

Fig. 7.17

Connecting resistors in series reduces rotor current

People seek other methods to adjust the current of the rotor loop to avoid the generation of i₂²R. According to the knowledge of electricity, there are many ways to change the current in a loop. A three-phase bridge rectifier is connected externally to the lead-out end of the wound rotor. Charging a DC power supply E₃ at the terminal can also change the current value of the rotor so that the current of the rotor is i₂, as shown in Fig. 7.18.

Fig. 7.18

DC power supply connected in series to reduce rotor current

In Fig. 7.18, the DC power supply E₃ can be used to excite the DC motor, and the DC motor is coaxially connected with the speed-regulated AC motor, as shown in Fig. 7.19. Changing the excitation and polarity of the DC motor can change E₃ Value, thereby changing the output speed of the rotor of the three-phase AC motor. Increasing the excitation E_a of the DC motor, the counter electromotive force E₃ of the DC motor increases, the armature current of the DC motor decreases, and the current of the rectifier circuit decreases. The current i₂ of the AC motor rotor decreases, the electromagnetic torque of the AC motor decreases, and the speed n decreases. Similarly, if the excitation E_a of the DC motor is reduced, the speed n increases. The output power of the DC motor works by dragging the load, so that the slip power loss is recovered. Figure 7.19 is a schematic diagram of the DC motor feedback cascade speed regulation principle.

Fig. 7.19

Principle of DC motor feedback cascade speed regulation

Replace the above DC motor with an inverter, the voltage E₃ is converted into AC through the inverter and fed back to the AC grid, so that the slip power loss can also be recovered, as shown in Fig. 7.20. The electromotive force is connected in series to the rotor In the loop, this is the principle of thyristor cascade speed regulation. This speed regulation method can realize the output regulation of the motor speed lower than the synchronous speed. This speed regulation method has high efficiency and is suitable for occasions where the speed regulation range is not large. Because the inverter part of this method is only responsible for converting the slip power, so the power of the equipment is low, and the cost is lower than the method of directly adjusting the frequency of the stator by the frequency converter. Power factors are generally low.

Fig. 7.20

The principle of thyristor cascade speed regulation

In order to improve the power factor of the cascade speed regulation, a turn-off thyristor is incorporated in the DC circuit, and the motor speed is adjusted by controlling the ratio of the thyristor on and off (that is, the duty cycle). The lead angle of the inverter is adjusted, fixed and take the minimum value, so that the reactive power required by the inverter from the grid side can be reduced, and the power factor of the speed control system can be improved, as shown in Fig. 7.21, which is the schematic diagram of the thyristor cascade speed control system with chopper.

Fig. 7.21

Thyristor cascade speed regulation system with chopper

The cascade speed regulation method adopts the rectification and thyristor inverter feedback mode, and the grid interference is relatively large. Due to the feedback of slip power, the operating efficiency is very high. At the rated speed, the efficiency is close to 100%, which is higher than the highest efficiency of the inverter. The speed response of this method is also faster, and the speed regulation range is generally 50–100%. The power device of this speed regulation method only needs to meet the power requirements of the slip power part, so the total power capacity of the device is small and the cost is low, but the motor must be a brush-wound motor.

7.10.2.3 Double-Fed Speed Regulation Method of Wound Rotor Motor

Connecting the AC power supply E₁ in series with each phase of the rotor winding can also change the current value of the rotor so that the current of the rotor is i₂, as shown in Fig. 7.22.

Fig. 7.22

AC power connected in series to the rotor winding

Use a low-power bidirectional frequency converter to supply power to the three-phase rotor coil of a three-phase AC wound motor, and change the size and phase of the rotor winding current, as shown in Fig. 7.23.

Fig. 7.23

Double-fed motor speed control system

In Fig. 7.23, when the thyristor connected to the rotor winding acts as a rectifier bridge and the thyristor connected to the grid transformer acts as an inverter, it is equivalent to the above cascade speed regulation and feeds power to the grid; when the thyristor connected to the rotor winding acts as an inverter, the thyristor connected to the grid transformer rectifies, the grid supplies power to the rotor. This speed regulation method is called double-fed motor speed regulation system. In this speed regulation system, the energy of the grid is rectified and reversed to the rotor winding. The frequency, phase, and amplitude of the power supply can be adjusted. According to the superposition relationship between the positive and negative of the power supply frequency and the stator frequency, the output speed of the motor is higher or lower than the synchronous speed. Because this method can realize super-synchronous operation, so this system is also called super synchronous cascade speed regulation system.

7.10.2.4 Speed Regulation Method of Brushless Doubly-Fed Motor

If the rotor winding is replaced by a cage structure, two sets of windings with different pole pairs are installed on the stator. One set is the power winding, the number of pole pairs is p₁, connected to the three-phase industrial frequency power supply, the frequency is f₁. The other is the control winding, the number of pole pairs is p₂, connected to the inverter whose frequency f₂ can be adjusted, it is required that p₁ > (p₂ + 1), as shown in Fig. 7.24.

Fig. 7.24

Brushless double-fed speed control system

When the number of pole pairs is fixed, a set of rotating magnetic fields with fixed speed will be formed after the power winding is energized, and the speed of the rotating magnetic field of the control winding can be adjusted. The two groups of rotating magnetic fields work together to form a rotating magnetic field that can change the speed. This synthetic magnetic field when it acts on the rotor, the output speed can be adjusted. In short, the frequency f₂ of the power supply on the control winding can be changed by the frequency converter, and the speed n of the motor can be adjusted. This system is called brushless double-fed speed control system.

The speed n of the brushless double-fed adjustable-speed three-phase AC motor is:

$$ n = \frac{{60 \times \left( {f_{1} \pm f_{2} } \right)}}{{p_{1} + p_{2} }} $$

(7.19)

7.10.2.5 Using a Chopper Tube to Realize the Speed Regulation Method of Stator Winding Voltage Regulation

Disassemble the three stator windings of the three-phase AC motor, and then connect it to a three-phase rectifier bridge, and connect a chopper GTO to the DC side of the rectifier bridge, as shown in Fig. 7.25. When the GTO is turned on, it is equivalent to the full voltage of the three-phase stator winding. When the GTO is turned off, it is equivalent to the voltage of the three-phase stator winding is zero. The duty cycle and switching frequency of the GTO on and off can change the average working voltage of the motor stator.

Fig. 7.25

Single thyristor speed regulation method

This is a speed regulation method proposed by the author in the last century. Because this method is extremely simple, it has attracted the attention of many people in the industry after the article was published. Due to job changes and property rights restrictions, this method has not been studied.

This method is suitable for the speed regulation of the motor driven by the pump fan. The motor is connected in Y shape. The working pressure of the pump fan station is close to the rated head (rated pressure), so the range of motor speed adjustment is also close to the rated speed. In this case, this method’s efficiency is also very high. When the speed is close to the rated speed, the operating efficiency is close to 100%, and the efficiency is low at low speed.

In order to utilize the current at the moment when the stator winding is turned off, when the GTO is turned off, the current will charge the capacitor C through the reactor; DC motor D outputs power to the load, as shown in Fig. 7.26.

Fig. 7.26

DC motor feedback energy

If the DC motor D in Fig. 7.26 is replaced by an inverter, part of the energy can also be fed back to the grid, as shown in Fig. 7.27.

Fig. 7.27

Energy feedback grid

7.10.2.6 Speed Regulation Method Using Thyristor to Regulate Stator Winding Voltage

Connect three sets of thyristors in series to the power wiring of the stator winding, as shown in Fig. 7.28. Adjust the effective voltage applied to the stator winding of the three-phase AC motor by changing the firing angle of the thyristors, thereby changing the output speed of the three-phase AC motor. This is similar to the principle of dimming table lamps in our home, but the dimming table lamps use a single-phase power supply, and the brightness adjustment is realized by adjusting the trigger angle of the thyristor and changing the effective voltage on the bulb.

Fig. 7.28

Thyristor voltage regulation and speed regulation method

The speed regulation method of adjusting the stator winding voltage is neither constant torque speed regulation nor constant power speed regulation, and is suitable for water pump fan loads whose torque decreases with the speed. When the speed is close to the rated speed, the operating efficiency is close to 100%, and the efficiency becomes low at low speed. This method adopts thyristor trigger adjustment, and the grid interference is large, but the speed response is fast. The speed adjustment range is generally 80–100%, cage type Both AC motors and wound rotor motors can be used.

7.10.2.7 Speed Regulation Method of Stator Winding Connected in Series with Saturated Reactor to Adjust Voltage

This method is similar to the speed regulation method of a household electric fan, changing the saturated reactance connected in series to the stator winding, and due to the voltage division effect of the reactance, the voltage value applied to the stator winding can be changed, thereby controlling the speed of the three-phase AC motor, as shown in Fig. 7.29. When the speed is close to the rated speed, the operating efficiency is close to 100%, and the efficiency becomes lower at low speeds. This method has almost no interference to the power grid.

Fig. 7.29

Series reactance voltage regulation and speed regulation method

7.10.2.8 Speed Regulation Method Using Three-Phase Autotransformer to Regulate Stator Winding Voltage

Use the three-phase autotransformer to adjust the working voltage applied to the stator winding, so as to adjust the speed of the three-phase AC motor, as shown in Fig. 7.30.

Fig. 7.30

Autotransformer voltage regulation and speed regulation method

This speed regulation method, when the speed is close to the rated speed, the operating efficiency is close to 100%, and the efficiency becomes lower at low speed, but this method has almost no interference to the power grid.

7.10.2.9 Speed Regulation Method Using Stator Winding Series Resistance to Adjust Voltage

Use the voltage division function of the resistor to adjust the voltage applied to the stator winding, so as to adjust the speed of the three-phase AC motor, as shown in Fig. 7.31. If the power of the motor is large and the heat generated by the resistor is large, this kind of series resistance voltage division speed regulation method consumes a lot of energy. This speed regulation method, when the speed is close to the rated speed, the operating efficiency is close to 100%, and the efficiency becomes lower at low speed, but this method has almost no interference to the power grid.

Fig. 7.31

Method of voltage regulation and speed regulation with series resistors

7.10.3 Speed Regulation Method of Changing the Frequency

In the early stage, the frequency conversion technology was limited by the limitations of power electronic technology devices. With the rapid development and price reduction of power devices and computing devices, the frequency conversion AC speed regulation technology and products developed rapidly. Frequency conversion technology can be applied to both asynchronous AC motors and synchronous AC motors, it can drive squirrel-cage AC motors and wound AC motors also. Frequency converters are used to provide variable-frequency power to three-phase AC motors. The stepless speed regulation of the AC motor is realized, and the high-efficiency operating area is relatively wide when operating in a full range. The rated operating efficiency is about 94–98%. When the rated frequency is output and there is a certain load, it is about 2–6% more wasteful than direct power frequency operation.

7.10.3.1 Voltage Type Inverter

At present, a large number of low-voltage inverters are voltage-type inverters. This is the most widely used inverter in the industrial field, so it is also called a general-purpose inverter. Its structure is shown in Fig. 7.32. It adopts AC–DC–AC structure, mostly used in low-voltage frequency converters. The three-phase AC power supply RST is connected to a three-phase rectifier bridge composed of diodes, and the AC power is first converted into DC power, level V₊, level V_-. The DC power passes through a large capacity Capacitor C, capacitor C stores electric energy and filters it, keeps the DC voltage UD basically unchanged, which is equivalent to a voltage source (so it is called voltage type). Then the DC power passes through the inverter to become a three-phase power supply that can change the frequency and voltage. When V₁ is turned on and V₂ is turned off, the U phase is connected to V₊, and when V₁ is turned off and V₂ is turned on, the U phase is connected to V_-. The output voltage of U phase is a rectangular wave, it has two voltage levels. The situation of V and W phases is the same. So, such a frequency converter is also called a two-level frequency converter. Due to the influence of the inductance in the motor, the rising speed of the current lags behind the voltage. When the U-phase output V₊ appears and the U-phase current is negative, D₁ is turned on, and the current flows back to the DC side, and D₂ has the same effect. RST three-phase AC power supply, each phase power supply has 2 peaks, 1 positive peak and 1 negative peak in 1 sine wave cycle, 3 phases have 6 peaks in total, and the difference between the peaks is 60°. After passing through the three-phase rectifier bridge After rectification, it becomes 6 positive DC peaks, so it is also called 6-pulse rectification.

Fig. 7.32

Voltage type inverter

At present, most of the inverters of inverters with this structure are composed of IGBTs (insulated gate bipolar transistors). Since the sine wave output by the inverter is formed by changing the pulse width of a rectangular square wave, the harmonic component is large. When the motor is far away from the frequency converter, the distributed capacitance between the line and the ground becomes larger, and the high-order harmonics easily flow into the ground through the distributed capacitance, forming a leakage current. Affecting the nearby video signal, and tripping the leakage switch. Some measures need to be taken to solve this problem, and we will talk about these methods later. Generally, the speed range of the frequency converter is very wide, about 5–100%, the speed response is fast, and more precise speed control can be realized.

7.10.3.2 Current-Mode Frequency Converter

The structure of the current-type inverter is shown in Fig. 7.33. The three-phase AC power supply is connected to a fully-controlled three-phase bridge, and the AC power becomes DC. The DC current flows through the large-capacity reactor L, and the reactor L stores magnetic field energy and filters the current, keep the DC current I_D flowing through the reactor L unchanged, which is equivalent to a current source (so it is called current type). Then the DC is converted into a three-phase AC current with variable frequency through the full-controlled bridge to drive the three-phase AC motor.

Fig. 7.33

Current-type inverter

The two links of rectification and inverter from power input to motor output of this kind of frequency converter are symmetrical, so by changing the trigger angle of the controllable device, it can run in reverse. The electric energy generated by the motor in the power generation state can be used as the power supply. The original inverter bridge is controlled as a rectifier bridge, and the original rectifier bridge becomes an inverter bridge, which feeds the electric energy generated by the motor back to the grid to avoid waste of electric energy.

The GTO operating frequency of this kind of frequency converter should not be too high. After the variable frequency power supply is output to the motor, the noise of the motor will be relatively large, so it is rarely used in low-power three-phase AC motors. Due to the high withstand voltage and high current characteristics of turning off the GTO, this kind of frequency converter is mostly used in the occasion of driving high-voltage and high-power three-phase AC motors.

7.10.3.3 Three-Level Inverter

The structure of this inverter is shown in Fig. 7.34. The three-phase AC power supply RST is divided into two groups of power outputs that are isolated from each other and have a certain electrical angle difference through the phase-shifting transformer B. The two sets of power outputs are respectively connected to two groups of three-phase rectifier bridges, Q₁ and Q₂. The positive terminal of rectifier bridge Q₂ is connected to the negative terminal of Q₁ to form 0 level V₀, the positive terminal of rectifier bridge Q₁ forms + level V₊, and the negative terminal of rectifier bridge Q₂ forms -level V_-. So that the AC becomes a direct current with an intermediate 0 level, and the direct current is respectively filtered by two sets of large-capacity capacitors C₁ and C₂ to keep the direct current voltage basically unchanged. Then the direct current is converted into a variable frequency alternating current by the inverter to control the three-phase AC motor.

Fig. 7.34

Three-level inverter

When transistor V₁ is turned on and V₂ is turned on, the U-phase level is V₊; when transistor V₁ is turned off and V₂ is turned on, D₁ is turned on, and the U-phase level is V₀; when transistor V₃ is turned on, and V₄ is turned on, U-phase The level is V_-, when the transistor V₄ is turned off and V₃ is turned on, D₂ is turned on, and the U-phase level is V₀. Because the DC side and the frequency conversion output side of this inverter have three levels of V₊, V₀, and V_-, so this kind of inverter is called a three-level inverter. The inverter with this structure is also a voltage type inverter. It is currently mainly used in rolling mills, locomotive traction, hoists and other fields. The output waveform of the three-level inverter is closer to the sine wave, so the harmonic component on the output side of the three-level inverter is smaller than that of the two-level inverter. The phase-shifting transformer adopts a set of △ primary side, two sets of Y and △ secondary sides, the phase difference of the two sets of secondary side power supplies is 30°, and the 12 DC peaks (12 pulse waves or pulse), the difference between the peaks is 30°, which is more uniform and smoother than the six DC peaks of the single rectifier bridge, so that the current wave on the grid side is closer to the sine wave and the harmonic pollution is smaller.

7.10.3.4 Multilevel Frequency Converter

At present, the medium and high voltage inverters widely used in the industry are mainly multi-level inverters. The power input side of this inverter uses a phase-shifting transformer to convert the high voltage of the power grid into multiple groups of low voltage and low voltage groups that are isolated from each other. The number and the voltage value of each group are directly related to the working voltage level of the driven three-phase AC motor. In order to make the current on the power supply side closer to a sine wave, different phase shift angles are used for each group of low voltages. The magnitude of the phase shift angle is directly related to the working voltage level of the AC motor.

The purpose of phase separation is to stagger the voltage peaks (or valleys) of each group of low-voltage outputs as much as possible, so that the rectified output DC waveform of each group of low-voltage power supplies after phase shifting has more dispersed and more uniform peaks and valleys, and the peaks and valleys of the current, so that the current waveform integrated is closer to a sine wave, and the harmonic interference to the grid is smaller.

Send each low-voltage three-phase AC RST to the rectifier bridge of a single power module. A single power module consists of 6 diodes to form a full-wave rectifier bridge. The H-type single-phase inverter bridge, the structure of a single power module is shown in Fig. 7.35. Multiple low-voltage power modules are connected in series to form a higher output voltage.

Fig. 7.35

Structure of a single power module

In Fig. 7.35, V₁ and V₄ are turned on, V₂ and V₃ are turned off, then the voltage V_UV between U and V is equal to + U_D, V₃ and V₂ are turned on, V₁ and V₄ are turned off, then the voltage V_UV between U and V Equal to -U_D, V₁ and V₃ are turned on, V₂ and V₄ are turned off, then the voltage V_UV between U and V is equal to 0 V, V₂ and V₄ are turned on, V₁ and V₃ are turned off, then the voltage V_UV between U and V is also equal to 0 V. When the switch K is closed, V_UV is equal to 0 V. This function can ensure that the inverter continues to run with reduced capacity when the power module fails, which is very important for occasions with high safety requirements.

Taking a 3000 V-4160 V inverter as an example, the main structure of a multilevel inverter composed of a phase-shifting transformer and multiple power modules is shown in Fig. 7.36. The phase-shifting transformer has 12 low-voltage secondary windings, which are divided into 4 groups. Each group is composed of 3 three-phase windings with the same phase. The phase angle difference between groups is 15°. One three-phase winding is selected in each group. A total of 4 three-phase windings provide power to the 4 U-phase power modules. For a 3000 V inverter, the voltage of each three-phase winding is 430 V, 4 of which form a U-phase supply voltage of 1720 V in total, and the line voltage is 2979 V. For a 4160 V inverter, the voltage of each three-phase winding is 600 V, and the V_UV of power modules 1, 4, 7, and 10 are connected in series to form U-phase output power. The V_UV of power modules 2, 5, 8, and 11 are connected in series to form V-phase output power. The V_UV of modules 3, 6, 9, and 12 are connected in series to form W-phase output power.

Fig. 7.36

Perfect Harmonic Frequency Converter

In Fig. 7.36, the U terminals of power modules 1, 2, and 3 are connected together, which is equivalent to a neutral point to form a reference voltage. According to different conduction conditions of the power modules, the voltage output to the three-phase AC motor is determined: positive and negative and voltage amplitude. The output voltage of this inverter adopts multi-level superposition, which is closer to a sine wave, the harmonic component is very small, and no output reactor is needed, so some people call this inverter a perfect harmonic-free inverter.

This kind of frequency converter adopts the power module inverter side series connection method, which is equivalent to connecting small DC power supplies in series to form a higher voltage. The output voltage of each power module inverter is irrelevant, so each power module can be realized by mature low-voltage frequency conversion technology. The power modules are identical, with good interchangeability and convenient maintenance.

7.10.3.5 Direct Series Connection of Medium and High Voltage Inverters Without Using Input Transformer Power Devices

In fact, the three-level inverter mentioned above is also an inverter composed of power devices in series. This can be seen from the connection method of power devices in each bridge in Fig. 7.35. The output voltage level of the inverter, the higher it is, the more power devices need to be connected in series, as shown in Fig. 7.37. This kind of inverter is the same as the main circuit topology of the voltage-type inverter mentioned above. But after the devices are connected in series, in order to make each device withstand the withstand voltage in a balanced manner, to avoid damage to local devices due to excessive pressure, certain measures need to be taken.

Fig. 7.37

Direct series connection of medium and high voltage inverters

7.10.3.6 High-Low–High Frequency Converter

Use a step-down transformer to change the high and medium voltage into low voltage, use a low-voltage frequency converter to realize variable frequency output, and then pass the three-phase power output of the variable frequency output through a step-up transformer. This inverter adopts a high-voltage-low-voltage-high-voltage structure, as shown in Fig. 7.38. The advantage of this method is that it can use mature low-voltage frequency converter technology to realize the speed control of high- and medium-voltage motors without technical obstacles. However, since this method requires two-stage voltage transformation, the operating efficiency will drop a little.

Fig. 7.38

High-low–high frequency converter