Methods of speed control of three phase induction motor

How to control the speed of a three-phase induction motor? The speed control method includes: changing the number of poles, stator voltage control, stator frequency change, cascade speed control, double-feed speed control, hydraulic coupler, electromagnetic slip clutch, etc.

The actual speed of the three-phase asynchronous motor is given by n = ns (1 - s) = 120f / p (1-s). It can be seen from the formula that the speed of a three-phase induction motor can be changed by changing the number of poles “p” of the induction motor, the slip “s” and the frequency of the power supply “f”.

The actual speed of 3-phase induction motor

Pole-variable speed controller

As shown in the formula ns = 120f / p, you can change the synchronous speed of the motor by changing the number of poles of the stator coil, thereby changing the running speed. Pole-shift speed control is most commonly used in squirrel-wheel induction motors. The pole-changing speed controller has the following characteristics:

Heavier mechanical properties and good stability

No-slip loss and high efficiency

Easy wiring, convenient control and low price

However, due to the large degree of difference, speed control cannot be achieved smoothly with this method. Therefore, it can be used with voltage speed control and electromagnetic slip clutch to obtain a more efficient smooth speed control characteristic.

This method is for step-less speed control manufacturing machines such as metal cutting machines, hoists, cranes, fans, water pumps and so on.

three-phase induction motor

Variable sliding speed control

1. Change the stator voltage

The torque of the induction motor is proportional to the square of the stator voltage. That is, changing the stator voltage can change the mechanical properties and torque of the motor.

This method is not suitable for a conventional squirrel-wheel motor because the rotor resistance is very small and the current increases rapidly at low speed.

But for a wound-type induction motor, it can be used in the rotor circuit using a series resistor or a common varistor to reduce the heat of the motor.

2. Change the rotor resistance

This speed control method is only applicable to the winding motor. In the rotor circuit of an induction motor with a series of resistors when the load is fixed, the higher the resistance, the lower the motor speed. The lower the resistance, the higher the speed.

This method is simple, easy to manage, and has low initial investment. However, the sliding energy is heated by the resistance. It also has soft mechanical properties.

3. Cascade speed control

Currently, cascade speed control uses the SCR inverter cascade control circuit and has the following advantages: stronger mechanical characteristics, low rectifier voltage drop, small space, no rotating part, low noise, easy maintenance. This is one of the speed control methods of the wound-type motor.

This also has its drawbacks. That is, the rotor circuit is equipped with a filter reactor, so the power factor is low.

Variable frequency speed control

According to the induction motor speed formula, it can be seen that when the slip s is constant, the speed n of the motor is essentially proportional to the power frequency f. Therefore, changing the frequency f can control the speed of the induction motor smoothly. Changing the power supply frequency is an economical speed control method and is one of the most popular ways to control the speed of an induction motor.

Variable frequency control allows you to change the power frequency of the motor stator and then change its synchronous speed. The main equipment of the variable frequency speed control system is the frequency converter or the variable frequency converter (VFD), which provides a frequency change to the power supply. Variable frequency drives can be divided into two categories: AC-DC-AC VFD and AC-AC VFD.

Three-phase induction motor and VFD

Today, widely used VFDs use digital technology and tend to be miniaturized, highly reliable, and highly accurate. It not only provides significant energy-saving performance for applications but also has the following performance:

 

• High precision, smooth speed control.
• Full protection function that can diagnose the fault with self-diagnosis and easy maintenance.
• Starting on a direct line, with high starting torque and low starting current, which reduce the impact on the electrical network and equipment and are provided with lifting torque, thus saving the soft starting device.
• High power factor and save the capacitor compensation device.

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Working Principle of Three Phase Induction Motor

How does a three-phase induction motor work? In short, it works based on the principle of electromagnetic induction. When the stator windings are supplied with three-phase alternating current, a rotating magnetic field is generated between the stator and the rotor. The rotating magnetic field cuts the rotor windings to generate induced electromotive force and current in the rotor circuit. The current in the rotor conductor forces the rotor to rotate under the effect of the rotating magnetic field. Below, we will specifically analyze the generation of the rotating magnetic field, its direction and speed, as well as the slip.

How does it generate a rotating magnetic field?

For three-phase induction motors, U / V / W windings with the same three-phase structure are placed in the stator core. Each phase of the winding differs spatially from an electrical angle of 120 degrees, as shown below, and the three-phase windings are provided with symmetrical three-phase AC, as shown in Figures (b) and (c) below. Here, take a 2-pole induction motor as an example to illustrate the location of the magnetic field in space like the current at different times.

three-phase winding in the star connection of the induction motor As shown in Figure (b) above, it is assumed that when the instantaneous current value is positive, it flows from the first ends of each winding and flows through the ends of the tail. On the contrary, it is when the current is a negative value.

As Figure (c) shows, when ωt = 0, iu = 0, the value of iv is negative and iw is positive. Then, the current of phase V flows from V2 and leaves V1, while the current of phase W flows from W1 and W2. According to the Ampere right-handed rule, the direction of the composite magnetic field produced by the three-phase current can be confirmed at the time ωt = 0, as shown in figure (d) ① below. It can be seen that the composite magnetic field is a pair of poles and the direction of the magnetic field is consistent with the direction of the longitudinal axis, that is, the top is the North Pole and the bottom is the South Pole.

symmetric three-phase current waveform diagrams ωt = π / 2, after a quarter cycle, the value of u changes from zero to maximum, and current flows from the first end U1 and exits the end U2. The iv value is still negative; therefore, the current direction of phase V is the same as shown in figure ①. iw also becomes negative and therefore the current of phase W is input and output W2. The direction of the compound magnetic field is shown in figure (d) ② that the direction of the magnetic field rotates clockwise by 90 ° compared to when ωt = 0.

Using the same analytical method, magnetic fields can be plotted when ωt = π, ωt = 2/3 * π and ωt = 2, as shown in (d) ③ ④ ⑤ respectively. Obviously, it is seen by the figure that the direction of the magnetic field gradually rotates in a clockwise direction, fully 360 °, that is, a rotation cycle.

2-pole winding rotating magnetic field diagram

It can be concluded as follows: the three-phase windings are placed in the stators of the three-phase motor in the same structure, but in the spatial position with an electrical angle difference of 120 degrees between them. As they are supplied separately with the three-phase AC, the composite magnetic field generated between the stator and the rotor rotates along the stator's inner circle, called the rotating magnetic field.

The direction of the rotating magnetic field

It is shown in the figure above that three-phase AC changes in the sequence of U-V-W phases, so the generated rotating magnetic field rotates clockwise in space. If the current phase sequence of the two-phase motor windings, such as U-W-V, is arbitrarily switched, it is practically proven that the rotating magnetic field generated must rotate counterclockwise. In conclusion, the direction of the rotating magnetic field depends on the phase sequence of the three-phase AC power supply in the winding. As long as the motor phase sequence is switched arbitrarily, the direction of the rotating magnetic field can be changed.

Magnetic field rotation speed and sliding

The example above is based on the 2-pole motor for illustration. If you want to obtain a 4-pole magnetic field, the number of coils will be doubled, as shown in figures (a) and (b) below. According to the analytical method above, the 4-pole rotating magnetic field diagram in space is shown in figure (c). Comparing the speed of rotation of the magnetic field in figure (c) with that of figure (d), as mentioned above, it is not difficult to discover that the speed of the magnetic field is not only related to the frequency of the power but also the number of poles.

Rotating magnetic field of 4-pole induction motor

Therefore, the speed of the rotating magnetic field is calculated by n1 = 120f1 / P, where it is:

n1 - the speed of the rotating magnetic field in rev / min

f1 - the frequency of the three-phase AC supply in Hertz

P - the number of poles

The speed of rotation of the magnetic field (n1) is also known as synchronous speed. The rotor speed of the three-phase induction motor (n) will not be accelerated to the speed of the rotating magnetic field (n1). Only in this way will there be a relative movement between the magnetic field of the winding and the rotating one to cut the magnetic lines. Thus, the induced electromotive force and current can be generated in the winding conductor of the rotor, producing the electromagnetic torque to make the rotor rotate continuously along with the direction of the rotating magnetic field. It can be seen that n ≠ n1 and n <n1 are a necessary condition for the operation of the induction motor, from which the name “asynchronous motor” derives. The difference between them is called "slip", which is expressed by the ratio between the difference and the synchronous speed: s = (n1-n) / n1.

Difference between Synchronous Motor and Induction Motor

AC motors are divided into two types, synchronous motors and asynchronous motors also called induction motors. The biggest difference between synchronous and asynchronous motors (induction motors) is whether the speed of the rotor is consistent with the speed of the rotating magnetic field in the stator. If the rotor speed of rotation and the stator field speed are the same, this is called a synchronous motor; otherwise, it is an asynchronous motor. In addition, there are major differences specific to the performance and application parameters between the two.

Difference in construction

The stator windings of the synchronous and induction motors are similar, and the main difference is in the rotor structure. There are DC field windings in the rotor of the synchronous motor, which must be supplied with the external excitation power introduced through the slip ring. However, the rotor windings of the induction motor are short-circuited, which produce current by electromagnetic induction. On the other hand, synchronous motors are more complex and expensive.

Stator

The components of the synchronous motor stator are basically the same as those of induction motors, playing a role in receiving, producing electrical energy and producing rotating magnetic fields. There is not much difference in the form of the result. The stators of the synchronous motor and the induction motor are made of the magnetic stator core, conductive three-phase AC windings, the base of the fixing core, the terminal cover etc.

Rotor

• Synchronous motor: the core of the rotor pole is laminated by steel sheets perforated by steel sheets. The pole core is placed by excitation windings that are wound with insulated copper wires. Structure of the PM synchronous motor For permanent magnet synchronous motors, the permanent magnet on the rotor is the key factor to distinguish it from other motors.
• Induction motor: the rotor consists of iron core and windings, is made of laminated steel sheets and is installed on the rotary shaft. There are two types of rotor: squirrel cage and wound type. The wound type induction motor is also equipped with a slip ring and a brush mechanism.

Difference at work

1. Synchronous motor

The synchronous motor rotates for the interaction between the rotating magnetic field produced by the drive stator windings and the magnetic field generated by the rotor. For the PM synchronous motor, it rotates due to the driving torque generated by the interaction between the rotating magnetic field of the stator and the secondary magnetic field of the rotor. As for the rotor winding, it does not induce current during the normal rotation of the motor and also does not participate in the work. It only serves to start the engine.

During the steady state operation of the synchronous motor, there is a constant relationship between the rotor rotation speed and the grid frequency:

N = Ns = 120f / p

f - the network frequency, p - the number of the motor pole, Ns - synchronous speed.

2. Induction motor

The stator core of the three-phase induction motor is incorporated with symmetrical three-phase windings. After the connection, between the stator and the rotor produces a rotating magnetic field that rotates at a synchronous speed. The rotor bar is cut by the rotating magnetic field in which it produces the induced current. The rotor drive bar is subject to electromagnetic force in the rotating magnetic field, therefore, the rotor overcomes the rotation of the load torque and accelerates its rotation. When the electromagnetic torque is equal to the load torque, the motor rotates at a constant speed.

The rotation speed of the induction motor (stator speed) is slower than the speed of the magnetic field of rotation and this difference is called "slip", expressed by the percentage of synchronous speed:

S = (Ns-N) / Ns.

S - slip, Ns - the speed of the magnetic field, N - the speed of the rotor.

Difference in applications

• Synchronous motors are used mainly in large generators, while induction motors are almost used as motors to drive machines.
• For synchronous motors, their power factor can be flexibly adjusted by excitation. However, the power factor of the induction motor is not adjustable; therefore, in some large factories, for the most applied induction motors, a synchronous motor can be added as a phase modifier, to adjust the power factors of the factory and network interface. However, due to the high cost of synchronous motors and a lot of maintenance, capacitors are now commonly used to compensate for the power factor.
• The operation of the synchronous motor is not as easy as the induction motor because the synchronous motor has the excitation winding and the slip ring in need of a high-level control of the excitation. In addition, compared to the maintenance-free induction motor, the work to keep the motor synchronous is great. So, like an engine, the engine

Construction Of Three Phase Induction Motor

The induction motor, also known as asynchronous motor, is a type of AC electric motor. According to the different power phase, it can be divided into single-phase and three-phase. The main construction of the induction motor consists of two parts - stator and rotor. In addition, there are end bells, bearings, engine structure and other components. Below, more details on the main structure of the three-phase induction motor or the asynchronous motor will be provided.

construction of induction motors

1. Stator

The stator is a fixed part of the induction motor, consisting of an iron core, windings and motor structure.

Stator iron core

As part of the motor's magnetic circuit, it is installed inside the motor frame. It is a hollow cylinder, the outer wall of which is connected to the engine structure. And the stator windings are placed in the groove of the iron core inside. To reduce the loss of the iron core, the iron core of the stator is stacked with 0.5 mm thick silicon steel sheets.

Stator winding

It is a part of the electrical circuit of the motor, generating the rotating magnetic field by inducing three-phase alternating current. The stator windings are wound with insulated copper wires and embedded in the stator slot, which is separated by insulating material between the windings and the slot.

For the methods of connecting the stator windings of the three-phase induction motor, not all of them are connected to the star connection (connection Y). But only under the circumstance of high capacity and high voltage will they connect in this way. In general, as in the low-capacity, low-voltage induction motor, six ends of the three-phase stator winding wire are pulled to connect to the delta connection (Δ connection) or star connection (Y connection). In this way, the motor can be applied to two different levels of supply voltage, for example, the star connection is inserted into the 380V power supply and the delta connection used for the 220V power supply, which can satisfy the need of departure. In other words, it is designed as the delta connection for the 380V power supply and switched to the star connection at startup to achieve the reduced voltage starting objective. Wiring diagram of the three-phase induction motor stator winding

Motor frame

It fixes the stator core and windings and supports the rotor with two bells at the ends. In the meantime, it protects the electromagnet part of the entire engine and dissipates the heat generated during engine operation. The frame is usually made of iron or aluminium.

2. Rotor

The rotor is a rotating part of the induction motor, including iron core, windings and shaft etc.

Rotor iron core

It is also part of the magnetic circuit, usually stacked by silicone steels and fixed to the shaft.

Axis

It plays a role of torque conversion and supports the rotor. It is usually made of medium carbon steel or alloy steel.

Rotor winding

It produces induced current by cutting the magnetic field of the stator and, under the effect of the rotating magnetic field, forces the rotor to rotate. According to the different structure, it can be divided into two types: a squirrel cage rotor and rolled rotor.

The winding rotor windings can be connected to the star or delta connection. In general, the small capacity rotor is connected to the delta while the large and medium capacity rotor is connected to the star. These three ends of the winding wires are connected to three slip rings fixed to the shaft by an electric brush assembly. It can connect the external resistor to the rotor winding circuit. The purpose of rope resistance is to improve the characteristics of the motor or to adjust the speed of rotation.

The structure of the squirrel cage rotor winding is quite different from the structure of the stator. There are slots in the iron core of the rotor with a bar in each slot. Two ends of the iron core that connect all the bars to the external grooves, respectively, form a short circuit. If the iron core is removed from the stator, the shape of the winding is like a squirrel cage. Some bars are made of copper and others are aluminium. If the winding is made of copper, the prepared bare copper bar will be inserted into the crack in the iron core and then covered with copper rings at both ends, followed by welding; if the winding is made of aluminium, the molten aluminium liquid is directly melted into the slits of the iron core of the rotor, melted with rings and fan blades at the same time.

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Describe the construction of the starters used to start a three-phase slip ring induction motor.

The starting methods of the three-phase induction motor are usually direct online starting, low voltage starting and soft starting.

Direct online match

This type of starting mode is the most basic and simplest when starting the engine. The method is characterized by less investment, simple equipment and a small amount. Although the starting time is short, the torque is lower when starting and the current is large, which is suitable for starting small capacity motors.

Starting with reduced voltage

The reduced voltage starting method can be introduced in medium and large size induction motors to restrict the starting current. When the engine finishes starting, it will run again at full pressure. However, the result of the reduced voltage start will decrease the starting torque. Therefore, the reduced voltage starter is only suitable for starting the motor in unloaded or lightly loaded conditions. Some common methods of starting low voltage are as follows.

Stator circuit series resistance start

A three-phase electrical ballast is inserted into the motor stator winding circuit. The electric ballast can simply be considered as a coil, which can produce induced electromotive force to reduce the direct input power frequency voltage.

Star-delta match

In normal operation, the three-phase induction motor whose stator winding is designed to connect the delta connection can be started in star during the start, to reduce the voltage of each phase of the motor and then reduce the starting current. After finishing the game, he is connected to the delta.

Delta starter is widely used due to its advantages, including simple starter equipment, low cost, more reliable operation and easy maintenance.

Starting the autotransformer

The reduced voltage start of the autotransformer refers that the reduced voltage of the mains energy is connected to the motor stator windings until the speed approaches a constant value and then the motor is connected to the mains.

When starting, the switch is pulled to the "start" position, and the autotransformer is connected to the network, followed by connection to the motor stator windings to obtain a reduced voltage start. When the speed of rotation approaches the nominal value, the key will be pulled to the "operational" position and the motor will directly access the grid under full pressure operation, cutting the autotransformer.

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Starting the three-phase induction motor autotransformer

The reduced voltage start of the autotransformer is introduced in the star connection for the large capacity motor or normal operation with the start of a certain load. Depending on the load, the transformer tap is chosen according to the required starting voltage and starting torque. At this point, the starting torque is still weakened, but it is not reduced by a third (compared to the reduced voltage start of the star triangle). However, the autotransformer is large and light in size, with the high cost and inconvenient maintenance, which cannot be moved frequently.

Smooth start

The soft starter is a new type control device whose main advantages include soft start, light load and energy-saving and speed. One of the most important features is that the electronic circuit is conducted in the silicon-controlled rectifier of the motor under the tandem connection of the power supply. Using the soft starter to connect the power supply to the motor and different methods to control the driving angle on the silicon-controlled rectifier can cause the motor input voltage to gradually increase from zero and transfer all voltage to the motor from the beginning at the end, what is called a soft start. When starting in this way, the motor torque will gradually increase with the optimized speed. In fact, the soft starter is a voltage regulator that only changes the voltage without changing the frequency at startup.